A DIY Receiver for GPS and GLONASS Satellites

VHFCOMMUNICATIONS 1/94

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A DIY Receiver for GPS andGLONASS Satellites

Part-l

I.I\ACK G RO U;\1l INFOR ~IATIO;\ 0:'0 (;PS xxnGLO-';ASS

Next to the amateur radio satel lites, themost irucresting sate lfncs arc weathersatel lites . Rad io amateurs have alreadysucceeded in build ing equipment toreceive Images from all blown ....eethcrsatellites and for all transmi ssion standards in use, and it was also radioamateurs who .....ere the first to receiveTV satel lite signals. using particularlysmall aeria ls. MJmC lime before thesebecame mass-produced articl es.

It is already more than ;\0 years sincethe first navi gation sate llites werelau nched into the cos mos. But it is onlyin the last few years that satellitenav igation and posi tioning have

become really popular. with the i ll 'troduction of more reliable. more: accurate and more user -friend ly systems.s uc h as th e Ame r ica n G lo ba lPos itioning System « iPS) and theRue..ian GLObal NAvi~ation SatelliteSystem (GLO:"lASS) .

Eac h "y,"cm will evcntuall y replace awhole ran!!e of ground-..upported navigaricnal aids. As a useful by-prod uct.they give anyone with the right equi pment a very preci se lime base (l OOns )and a very accurate frequency (10 12),

Ongtna uy both systems. UPS andGI.O;'>lASS, were designed for militarypurposes. Bur since then there havebeen considerably more civilian thanmi litary users .

G PS navigati on receiv ers (soon to bejo ined by combined GPS/G LONASSreceivers ) can be manufact ured 10 be ashandy, user -friendly and favourably

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VHF COMMUNICATIONS 1/94

2.1. Ra d io navigation background

Like all areas of electronics and radi oeng ine e ring, rad io nav igation I S

developing very fast.

The basis on which a ll rad io naviga tionsystems operate is that exte nsiveresea rch has been done into the propaga tion mecha nisms of radio wave s,and that the propa gation speed of radiowaves is normal ly c lose to the speed oflight in free space. Systems working.with radio waves normally have asufficiently extens ive range to make itmeaningful to lise them for detennininglocations. speeds and positio ns.

In the end, all measurements involvingradio waves, whether we 're talk ingabout position finding, and thereforedirect ional search, measuring runningtime, phase measurement or the measurement of the Doppler frequency shift,can he carried out - on the user 's side.al least - using simp le and reasonablypriced technical aids .

Ea rlier radio navigat ion systems madeuse of the directional effect of thereceiver aerial, the transmitter aerial , orboth. In both kinds of system, the maincause of measurement errors was thelack of precision in the alignmentcharacteristic of the aerial. Since themeasured variable consists of an angle ,the positional erro r increases linearlywith the distance of the USCT from theposition of the navigationa l reference.These systems arc therefore very much

2.n ESCRIPTION OF GPS &GLONASS SYSTEMS

Finally. this system C,HI also he of uscin the positioning. and alignment ofhigh -gain microwave aerials.

Th is article will be split into severalpart s: I sha ll begin by describing the.sate llites themselves and their radiosignals. Next will come II description ofthe way II (I PS or OLONASS receiverworks . This will he followed by assem bly instruction s for a DIY GPS orGLONASS rec eiver. together with anexplanation of the operationa l software.

These receivers can be built in twoversions - as independent portable recei vers with a small keyboard and aliquid crys tal display screen. or asadd-ons with plug-in modules for theDSP computer ( I) (2).

priced as modem portab le radio sets.

These pieces of equipment can measurethei r three-d imensiona l position with anaccuracy of app. 50 met res at any pointon the Earth's surface . So these sets aTC

of interest. not only to leisure-timepilots, lorry dri vers or mountaineers.but to radio amateurs too!

Apart from the c hallenge o f buildingyour own satellite receiver. radio amateurs can also put (I PS and GLONI\SSsignals to other uses. Probably thesimplest applicat ion for a <irs orGLONASS receiver is 10 usc it as ahigh-precis ion frequency source. Exactliming and synchro nisation can be used.for example. for modem transmissiontechniques. or for precise experimentsconcerning the propagation path andthe propagation mechanisms of radiowaves.

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limited with regard to their capabilityof usc - in their range or precis ion. soto speak. They are. of course . outstanding ly suitable for one application getting the user to a specific point. forexample guiding an aircraft onto therunway by means of an instrumentlanding system (ILS).

Time and frequency are definitely thephysical variables which can he meas ured with the greates t accuracy. If thepropagation speed and propagation conditions of specific radio waves areknown, the distance can be calcu latedin the simplest way by measuring the

running time. The abso lute precision ofsuch distance measure ments just docsnot depend on the order of magnitudeof the distance to be measured. irres pective of the uncertainties of the propagation speed of the radio waves usedon this path.

Por this reason, all high-precision radionavigation systems which arc suitab lefor long distances are based on meas urements of running time or pathdifference and/or on the derivatives ofthese variables in relation to time. alsoknown as the Doppler frequency shift.

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VHF COMMUNICATIONS 1/94(~! - - - - - - - - - - -----'-"'--'=== = '-""=The simplest way \0 determine thedistance LO a known locat ion is toinstall a converter there. transmi t asignal and measure the runn ing limeuntil the response signe t is received.Although systems of this kind areindeed in usc (c .g. D!\.1E for civi laircraft), they do have their limitations.smcc eac h user has 10 have atransmitter as well as a receiver.

In the civilia n sector. this system hasthe addit ional disadvantage that theequipment has to be licensed , and themilitary try and avoid transmission asmuch as possible. so as not to giveaway their position. which must be keptsecret. But the higgt'st handicap is thatonly a limited number of people canusc this system. for the simple reasonthat they ca n only use it one at a time.

As regards the user side. we could dowithout the transmission equipment ifwe could usc some other means 10achieve and main ta in the synchronisation of lite two sides.

For example. both sides. the user andthe nav igation transmitter, could beequ ipped with high-precision time standards. e.g. an atomic clock . The userwould merely have to synchronise hisor her clock at a known location. andthen lake this clock to the unknownlocation as an aid 10 measuring therunning time.

Hut since atomic c locks are a littleclumsy and expens ive. we need 10 finda considerab ly simpler sotuuon, whichneeds nothing bUI a receiver.

Such a system must co nsist of a wholeseries of synchronised transmitters. asshown in Fig.l .

However. since the preci se time is notknown on the receiver side, we can notmeasure either the delays or the d istances dl , d2. dj, etc. to Ihe transmit tersTX t . TX2. TX3 . etc . directly. We ca nmeasure only the varying arri val limesof the different transmission signa ls.These lime differences directly correspond to the differences in distance.

TIle multi tude of points for a give ndistance difference for two preset pointsproduce a hyperbola (loo ked at in twodimensions) or a hyperbolo id (loo ked atin three dimensions). Here the twotransmitters arc in the foci of thehyperbo la (or the hyperboloid).

l-or two-dimensiona l navigati on (10

cation-findi ng), signals mu st he received from at least three ..ynchroniscdtransmitters. ....or exa mple. the hype rbola dl • d2 = const. 12 can be plotte ddirectly on a map from the d ifference inthe running times measured for TX Iand TX2. The hyperbo la d2 - d3 =c on s t . 23 can b e plott e dcorrespondingly on the map from thedifference in the running times mea sured for TX2 and TXJ. The intersectionpoint of the two hyperbo las is theunknown locat ion of the user!

For three-dimensional nav igat ion .signals must he received from at leastfour synchronised transmitters . Th ethree differen t runnin g time differencesthen give three hyperbolo ids. Thesurface.. of two hypcrboloids intersectin a curved line. which intersects withthe surface of the third hyperboloid al asingle point. Th is correspond s 10 thethree -dimen sional position of the user.

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Should even more transmitters beavailab le, we can sed : out the three orfour transmitters which give hyperbolas(hyperboloids) which intersect a lmost atright angles. The remaining transmitterscan then be brought in to check forpossible errors or ambiguous solutions,since curved lines and surfaces canproduce more than one point of irucrseetten.

Hyperbolic naviga tion systems wereoriginally structured as ground-supported systems in the medium andlong-wave ranges. e.g . Loran. Decca orOmega. Rut since these systems ope ratefrom the ground upwards. no threedimensional position-finding can becarried out - only a reliable determi nation of geographical latitude andlongitude. For it to be possible tomea sure th e heigh t as well , atransmitter must he as far above orbelow the receiver as possible. or atleast outside the horizontal plane of theuser.

Radio navigation systems insta lled onthe ground use relatively low frequencies of the radio spectrum to obtainas wide a range as possible. andsimultaneously to avoid undefinedpropagation through space waves. Forexample, Omega uses a frequency range between 10 and 14 kHz, and thus,using only eight transmitters, covers theentire surface of the Earth!

Longwave navigation systems weredevel oped at a time when digita lcomputers were not easily available.And navigation on two planes, usingtransmitters at fixed locations, requiresonly a minimum of calculation from theuser, Moreover. the multip licity of

,""-'= ="'-"""'--"''''--------- - --'f'.- \ ..hyperbolas for each transm itter pair.including the necessary corrections forpropagation anomalies. ca n be plotteddirectly for use on corresponding maps.

One of the first applications (or artificia l satellites was radio navigation.Naturally, for their pan , artificial satellites needed rad io naviga tion 100. inorder to estimate the power of thecarrier rockets and determine the satellite ' s final orbit. Moreover. space is anidea l locatio n for na vi ga ti o naltransmitters . firstly. an enor mous rangeis available for VHF and for the higherfrequencies. and secondly the propagation of the radio waves is calc ulableand the influence of the continuouslychang ing ionosphere is insignificant.fi nally. the locations of the navigationtransmitters in space ca n he selected insuch a way that three-dimensionalposition-finding is possible all over theworld.

Since originally satellites could he usedonly in ncar-Earth orbits. the firstnavigation satellites . such as the Amencan Transit satel lites or their Sov ietcounterpart, Cicada. were launched iraolow Polar orbits (abo ut 1.000 km. up).Since a satellite in a near-Earth orbittravels a long its path very quickly. evena single satellite can be made use of forposition-finding. The accuracy of aquartz watch is sufficient to measurethe few minutes required by a satellitefor an overflight. The change in thesate llite 's position roughly correspondsto a quantity of transmitters at variouspoints along its night path.

In practise, we measure the Dopplershifl of the satellite signal for a certaintime and then use the satellite path data

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Fig,2: Eq uat ions for the Calculation of Running Times and Doppler Shin

to calculate our own unknown position.Although only a single satellite isneeded to determine a position. thesesystems usually consist of from sixsatellites (Transit) up to twelve, inorder to cover the surface - after all, asatellite in a low orbit is visible fromthe Earth' s surface only for a certainperiod of lime. And since theionosphere has a certain influence on

radio waves in the VHf and UH~

ranges, both systems - the Americanand the Soviet satellites - operate ontwo frequencies. at 150 MHz and at400 MIll.. The actual frequencies arc inthe exact ratio 3/8 and the transmiucrsarc kepi phase-synchronised, so that theinfluence of the ionosphere can be

balanced out.

The zero point of the co-ordinationsystem nonnall y lies in the centre orthe Earth. The 7. axis corresponds to therotational axis of the Eart h. and pointsto the North. '1l1C X and Y axes arc inthe plane or the Equator. with the 7.axis pointing in the direction of theGreenwich meridian, whil st the Y axisis orientated in such a way that aright-handed orthogonal co-ordinatesystem is produced

Naturally it is also possihle to convertthe data into a more popular coordinate system. c.g . into degrees oflatitude and longitude. plus hcif.htabove sea level (height above surfaceor an ellipsoid). These conversion procedures are always based on the finalresult. since most or the calculationsrequired for position-delennining innavigation receivers can be carried outconsiderably more easily in a Cartesianco-ordinate system.

Finally. we should not leave out or ourreckoning the fact that there are variousco-ordinate systems in use with thesame basie definition . Meanwhile. SNSs

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The most serious disadvantage of navigation satellites in low orbit is the factthat we have to wait for a satellite: tofly over, and we then need severalminutes for the measurement. Finally.ou r own speed and course must beknown precisely. so that they can betaken into account in determining ourposition. Several sate llites are neededfor a position 10 be determined veryrapidly. U at least four satellites arevisible at various points in the sky . ourown location call he dete rmined immediately a.. regards longitude. latitudeand height. without having to wait Iorthe satellites 10 move through the ~ L:y.

To keep the number of satellites required as low as possible. they must beput into higher orbits. The AmericanGPS satelli te navigation system and theSoviet GLONASS system arc intendedeventually to cover the entire surface orthe Earth. each having 24 sate llites.including reserve satellites in space.AI least four of the satel lites fromeither system should be visible anywhere on Earth. distributed over the sl yin such a way as to make threedimellSionalnavigation possible.

Nor should we forget the enormousamount or calculations required to carryout three-dimensional position-findingusing satellites. The fact that the satcllite's position is constantly changingmeans a computer must be used.

Perhaps this explains why satellitenavigation is only now becoming popular. Suitable satellites have indeedbeen available for thirty years - butreasonably priced computers have not.

VHF COMMUNICATIONS 1/94~~~"=~~~---------(~2.2. Eq uations Ior sate llite nevlgatlon

In order to understand satellite nav igation systems (SNSs). we: must firstlook at the mathematical background tosatellite navigation. First we mustdefine a co-o rdination system . Mostsatellite navigation systems operate bymeans of a right-handed Cartesia n co ordinate system. like the one shown infig.2. The co-ordinate system is rigidlyconnected to the Earth and is thus arotating co-ordinate system. and thusdeviates hom the inertia co -ordinatesystem for Kepler elements used formost satellites.

VHF COMMUNICATIONS1/94

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Fig.3: Hefght and Angle of Inclination or Or bits of GPS and GLONASSSatellite s

have improved the absolute positioningaccuracy until it is now within onemetre. as a result of which the smalldiscrepancies in the different localgeographical co-ordinate systems become noticeab le. Thus GPS uses theWGS·84 co -ordi nate system and

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GI.ONASS u se s SGS -S5 . Thedifferences between the two systemsadd up to a difference in position ofabout to m. in the East-West directionand (he same in the North-South direclion for the author' s location in CentralEurope.

Apart from the visibi lity problem, thenavigation equations impose additionalrestrict ions and requirements on theorbital paths of the naviga tion satellites.The accuracy with which the loca tion

2. If the user' s speed vector is known,or if the user' s speed is ' .CTO (instationary operation), then the location can be determined using threeindependent equations for calculating the Doppler frequency shift.

3. Various combinations of the equalions for running time difference andDoppler frequency shifts are possible.

1. If the localion is a lready knownfrom the equations for calculatingthe running time difference, theuser' s speed vec tor ca n hedetermined using three independentequat ions to calculate the Dopplerfrequency shift.

If we use a vector representation.navigational equations can be expressedmuch more simply. Using a threedime nsional Cartesian co-ordinate system, it can easil y be understood that anindividual vector describes three independent variables.

Th e nav iga ti ona l equ at io n fordifferences in running time consistsmerely of tbc area vectors which givethe positions of the transmit ters [satcllites) and the receivers (users). Thedifferences between users and satellitesare calculated as absolute values fromthe area vector differences. On theother side of the equatio n stands thedifference in running time measured,multiplied by the speed of propagationof radio waves (c).

If the user' s location is unknown, whichmeans the area vector is too, threevariab les of the scale are missing, andthree independent runn in g t imedifference equations have to be solvedto ca lcu late them. AI least fourtransmitters (and thus four visible sate llites) arc needed \0 solve these threeequations. The absolute value of avector is a non-linear function, which isdetermined by ca lculating ..quares androots. These equations are thus solved,either by numerical iteration or ana lytically (3).

I\. navigation equation for calculatingthe Doppler shift d ifferential containsboth area vectors and speed vectors.The speed differential for the Dopplerfrequency shift must be calculated first,so that the project ion of the speeddifferentia l vector onto the direction ofprcpagauon of the radio waves can thenbe calculated. Vector projections are

VHF COMMUNICATIONS1/94,=""""'''''-''''''''-''''''''-'-''''-- ----- - - '~- \ . .

calculated by determining the scalarproduct of two vectors .

On the other side of the equation. wehave the Doppler frequency shift as adependent variable. the absolute va lueof which is obtained by dividing by thecarrier frequency ro' The relative frequency diffe rential can then he coovencd into speed values by multiplyingthe variable by the speed of prop-agation (c).

The Doppler frequency shift navigationequations contain both the positiona lvector and the speed vector of the user.Th is can mean up 10 six unknownsca lars. Rut since we normally do nothave six independent equations. thefoll owing rou te can he taken:

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VHF COMMUNICATIONS 1/94(t ,------------"-"-""'=="-'==or speed is finally determined dependson the construction of the equationsystem .

If the equation system is badly selected,every measuremen t error appears enlarged even in the final result. In termsof geometry. a badly selected equationsystem is tantamount to having intcrscctions of lines and surfaces at veryshallow ang les.

The impairment of accuracy due to anunsuitable equat ion system is referredto 3." UDOP (geomet rical dilution ofprecision). Naturally . satellite orbitsmu st be se lected in such a way that theGooP is as smal l as possible for thebiggest possible number of users. Hutsince we arc dealing with non-linearequations. the GlX)P alieni with thelocation of the user. Users must therefore select four satellites which arcfavourable for them. It is certainlyabso lutely possible that several satellites will be "visible", possibly even a!a rather high elevation. which alsoincreases the GOO P.

The most remarkable case of a largeGDOP when running time differenceequa tions arc used occurs when twonavigation satellites arc close to eachother in the sky. A more common caseis when all four satellites arc in almostthe same plane. For the same reason, ageos tationary orbit is also unfavourablefor navigation satellites. A furtherdisadvantage would seem 10 be the lowrelative speed of the satellite. since the

equations for calculating the Dopplerfrequency shifts are not designed forcases where the positional vector ismultiplied hy very small numbers.

2,3. The GPS & Gl,ONASS s.alellilesystems

UPS and GI.ONASS are the first satellite systems which require the simult aneous operation of severa l satellites.Other systems are already operatingwith one satellite, and each one improves the system further .

In the GPS and GLONASS systems. thesatellites have to he sync hro nised andcan in all cases operate only in sds of81 least four salellites visible to theuser. The requirements for a low GIXWshould not be forgotten here .

UPS and GLONASS satellites havebeen put into similar orbits. Fig.3compares the orbits of (iPS and(; I,ONASS satellites with other knownsatellite orbits, such as the geostationary or near-Eart h contra-rotating Sunsynchronised orbits.

(iPS and GLONASS satellites havecircular orbits, with an inclination of 55- 65 degrees and an orbital period in theorder of 12 hours. which corresponds to8 height of app. 20,000 km. (about I IhEarth diameters).

(To he conlinul"d)

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Part-2

2.3 . crs & G LONASS SalcllilcSystem s

GPS and GLONASS arc the firstsatellite systems that require thesimultaneous operation of a number ofsate llit es. In other satel lite systems,includ ing curl ier navigat ion systems,the operation of every single satellitewas almost autonomous and anyaddit ional satellites only improved thecapacity of the system.

Tn <ii 'S or n J,oN'i\SS the satellitesneed 10 be synchronised and can onlyperform as a constellation of at leastfour visible satellites for every possibleuser location without forgetting theGl'Op requirement! Both GPS andGLONASS satelli tes arc launched intosimilar orbits . A comparison amongGPS, GLONASS and more popularsatellite orbits like the geostationaryorb it or the retrograde sun-synchronousLow-Earth Orbit (LEO) is made on thescale drawing on Fig.3. Both GPS andGLONASS satellites are launched into

circular orbits with the inclinat ionranging between 55 and 65 degrees andthe orbita I period in the order of 12hours, which corresponds to an altitudeof around 20000kin (one and a halfEarth diameters).

The GPS system was initially plannedto \LSe three different orbital planes withan inclination of 63 degrees and theascending nodes equally spaced at 120degrees around the equator. Eachorbital plane would accommoda te Requally spaced satellites with an orbitalperiod of 11 hours and 58 minutes,synchronised with the Earth' s rotationrate [4].

During a 10-year test period from 1978to 1988 only 10 such "Hlock rsatellites were successfully launched inorbital planes "A" and "C" as shown inFigA. The GPS specification waschanged afterwards [5] and the new"Block II" satellites are being launchedin 55-degrees i nclination orbits in sixdifferent orbital planes A, B, C, D E

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VHF COMMUNICATIONS 2194

and F, with the ascending nodesequally spaced at 60 degreesaround the equator. The new GPSconstella tion should also include24 sate llites, having fOUT sa tellitesin each orbital plane. includingactive in -orbit spares. The orbitalperi od of the GPS satellitessho uld be increased to 12 hours toavoid repeat-tra ck orbits andresona nces with the Ea rth' sgravity field. fina lly, the new"Bl ock II" satellites 3 1.~O include anasty feature called "Se lectiveAvailability" (SA): the on-boardhard w are may. on gro undcommand. intentionally degradethe accuracy of the navigationsignal s for civilian users whilem ilitary users still have access tothe full system accuracy.

Bcgmning in L988 and up toMarch 1993. 9 GPS "Blod : II"and 10 new «rs " Block I1A"satellites have heCII launchedusing "Delta' rockets. The SAmode is currently turned on anddegrades the accuracy to be twee n50 and l(XlM.

The G LONASS system is plannedto usc three different orbitalplanes with an incl ination of 64 .8degrees and the ascending nodesequall y spaced at 120 degreesaround the equator. Each orbitalplane would accommodate 8 (or12) equally spaced sate llites withan orbita l period of II hours, 15minut es and 44 seconds. so thateach sa te llite repeats its groundtrack after exactly 17 revolutionsor 8 day s 16).

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Sate llile La unc h Orbit PRN# Deeom missioned

GPS BI-Ol 7R 20 I\. c-t 4 luI 85(iPS BI-02 7R 47 A I\.-? 7 lui 81GPS BI-03 7R 93 A A -? 6 May 92OI'S Bl-04 78 112 A c -? 8 Oct 89GPS BI-05 8011/\ c.t 5 Nov 83GPS 8 1-06 80 32 A s-: 9 Mar 91UPS RI-m Launch fai lure(iPS 131-08 IB n A C-3 II May 93(i PS Bl -09 84 59 A C- l 13GPS 131-10 84 97 A A-I 12GPS Ill- II 85 93 A C-4 3

(iPS Oll-Ol 8913 A E-I 14(i PS BlI-02 89 44 A R-J 2(i PS HIl-03 89 (~ A E-3 16(i PS BII-04 89 85 /\ A-4 19(;pS BII...Q5 &997 A 0 -3 17UPS RII-06 90 8 A F-3 18(i PS RIl-07 90 25 A 8-2 20( ;1'8 BII-08 90 68 A [\ -2 21UPS BIT-09 9088 A 0 -2 15

(i PS BIIA- I()l)O IOl A E-4 23(i PS O[lA-119147 I\. D-I 24GPS BIIA-1292 9 1\ A-2 25UPS BITA-1392 19 I\. C-2 28GPS HlIA·1492 39 A f -2 26UPS BIIA-I592 58 A A-3 27(i PS HIIA-I69 2 79 /\ f -I I wa s #.12GPS HIIA·1792 89 A F-4 29(i PS BIIA-1893 7 A 0 -1 22UPS BIIA-1993 17 A C-I 31(IPS 8111\.-2093 32 A C-4 7GPS 8 11A-2193 42 A A- I 9UPS BIIA-2293 54 A 8-4 5

Fig. 4: Published GPS Sat ellite Ope ration

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(/:' VHF COMMUNICATIONS 2/94

Satellite Launch Orbit CHN# Decommissioned F ig. 5:

Glonass 34 8843A 1-8 ? ?Recently observed

Glonass 36 88 43C 1-1 24 ? GLONASS Satellite

Glonass 39 88 85C 3-1 8 10 Jan 92 Operation

G lonass 40 89 Ii\. 1-2 9 Rep laced Mar 93Glonass 41 89 IB 1-3 6 Rep laced Feb 92Gl onass 44 90 45A 3-17 24 Formerl y #21G tonass 45 90 45B 3-19 3 Mar 93Glonass 46 90 45C 3-20 15 Sep 92Gtonass 47 90UOA 1-4 4Glonas s 48 90110B 1-7 13Glonas s 49 90 110C 1-5 23 Formerly # 19Gtonass 50 9 125A 3-21 20 Jan 92Glonass St 9125D 3-22 11Glonass 52 9125C 3-24 14 Feb 92Glonass 53 925A 1-3 22 Jan 93G laoass 54 92 SB 1-8 2Glonass 55 925C 1-1 23 Formerl y # 17Olonass 56 9247A 3-24 1Glonass 57 92 47H 3-21 24 Formerly 3-18Gfonass 58 92 47C 3·20 8 Former ly 3-2 1Gtonass 59 93 lOA 1-3 12Gloua ss 60 93 lOB 1-2 5 ",,,0>'Glonass 61 93WC 1-6 22 Formerly #23 , oot"

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Fi2.6: Block Diagram of GPS and GLONASS Satellites

68

Much smaller and lightwe ight rubid iumatomic clocks are used as a backup inthe case the main time/frequencystandard fail s, a lthough rubidiumatomic cl ocks ar e an orde r ofmagnitude less accurate . Due to thestable space enviro nment these atomicclocks usually perform better than theirground-based counterparts and anytong-term drifts or offsets can he eas ilycompensated by upload ing the requiredcorrection coef ficients ill the on-boardcomp uter,

The output of the atomic time/frequency standard drives a frequencysynthcsiser so that a ll the carri erfrequencie s and modulation rates are

2.·t GPS & GLO~ASS Sat elliteO n-hoard Equipm ent

Since the two systems are similar, GPSand G LONASS sate llites carry almostthe same on-board equipment as shownin Fig.6.

Since the beginning of the GLONASSprogram a large number of satellite shave been launched into GLONASSorbital planes 1 and 3, the orbital plane2 has not been used yet Some satellitesnever transmitted any radio signa ls,since the GLONASS system alsoinclu de s passive "EtaIon" satelli tes usedas optica l reflectors for accurate orbi tdetermination.

GLONASS satellites arc launched threeat a time with a sing le "Proton" rocket.Due to this constraint all three satellitescan only he launched in the sallieorbital plane. Recently observedGLONASS satellite operation is shown0 11 Hg.S. The observed lifetime ofGLONASS satellites seems to beshorter than that of American GPScounterparts.

A crit ica l piece of on-hoard equipmentare the atomic clocks required forsystem synchronisation. Although eachsatelli te carries redundant rubid ium andcaes ium clocks. these caused the fai lureof many GPS and GLONASS satelli tes.In add ition to this, GLO~ASS satelliteshave had problems with the on-hoardcomputer , Unfortunately, the (iPS orGLONASS orbit altitude is actually inthe worst ionising-radiation zone, Ihesame radiation thai already destroyedthe AMS A'j"-OSCAR-I0 compute rmemory .

'~======='--- - - - - - -----\ ..For the navigat ion function alone. thesatellite s could be muc h simpler.carry ing a simple linear transponderlike on civilian communicat io nssatellites. The required navigationsignals could be generated andsynchronised by a network of groundstations. However, roth GPS andGLONASS aTe primari ly intended asmilitary systems.

Uplinks arc undesired since they can beeasily jammed and a network of groundstations can be eas ily destroyed.Therefore, hoth UPS lind ULONASSsatellites arc des igned for completelyautonomous operation and generation ofthe required signals. Synchron isation ismaintained by on-hoard atomic d ocksthat arc on ly periodica lly updated bythe ground stations .

Both (iPS and GLONA&'<'; satellitescarry a caesium atomic clock as theirprimary time/frequency standard, withthe accur acy ranging between 10-12and 10-13,

69

(f'!-----------=~~~-""-"'''''-'"

derived coherently from the samereference frequency. The on-boardcom puter generates the so-callednav igati on da ta . These in cludeinformation about the exact location ofthe satellite, also called precis ionephemeris, information about the offsetand dr ift o f the on-hoard atomic clockand information about other satellites inthe system, also called almanac. Thefirst two arc used directly by the user'scomputer to assemble the navigationequations.

The almanac data can be used topredict visible satellites and avoidattempting to use dead, malfunctioning

or non-existent satellites, thus speed ingup the acquisition of four valid satel litesignals with a reasonable Glx)P.Be s id e s t he transm itt ers forbroadcasting naviga tion signals, GPSand GLONASS satellites also havetclccommand and te lemetry radio links .

In particula r, the tclecommand link isused by the command stations to

Satellite Channel LI • Carri er 1.2 • Ca rrier

1575.420 MHz 1227.600 MHz

1602.000 MHz 1246.000 MH,1602 .5625 MHz 1246.43 75 MHz1603.125 MHz 1246,&75 Mllz1603.6785 MHz 1247.3125 MHz1604.250 MH, 1247,750 Mllz,1604 .8125 MHz 124&.1&75 MJ£7,1605.375 MH, 124&.625 MJh,1605.9375 MH, 1249.0625 MJIi'.1606.500 MHz 1249.5(X) MHz1607.0625 MHz J249.9375 MHz1607.625 MHz J250375 MHz1608.1875 MHz 1250.& 125 MHz1608.750 MHz 1251.250 \'1Hz16093125 MHz 1251.6875 MHz1609.&75 MH, 1252.125 MHz1610.4375 MHz 1252.5625 MH,1611.fXlO MHz 1253.000 MH,1611.5625 MHz 1253.4375 MHz1612.125 MHz 1253,875 MH,1612.6785 MH, 1254.3125 :MHz1613.250 Mfu 1254.750 :MHz.1613.8 125 MH, 1255.1&75 MHz1614.375 MH, 1255.625 MHz1614.9375 MH, 1256.0625 MHz1615 .500 MH, 1256.500 MHz

GPS (all Satelli tes)

GLONASS Channel 0GLONASS Channel JGLONASS Channel 2ULONASS Channel 3(iLONASS Channel 4GLONASS Channel 5GI .oNASS Cha nnel 6GLONASS Channel 7GI, ONASS Channel 8GLONASS Channel 9G I.oNASS Channel 10GLONASS Channel 11GLO NASS Channel 12ULONASS Channel 13GLONASS Channcl14GLONASS Channel ISGLONASS Cha nnel 16GLONASS Channel 17GLONASS Channel 18GLONASS Channel 19GLONASS Channel 20GLONASS Channel 21GLONASS Channel 22GLONASS Chann el 23GLONASS Channel 24

Fig.7: Carrier F requencies for GPS and G LO!'lASS Satellites

70

VHF COMMUNICATIONS 2194 '~======="----- --------- -\ .

,..,,"" .. ..,

£ ~ .....

1. .. S<">_.., Gl,

----j'" .""."s.".., .I' . ~ , 5e>_O,"'" G;>", ' .

~0 ' " " "

,

-

--<:1:= (]:: -

l , , .

C/A·Code

Ausgang

••"o.""0 ' _50010 00 '

S53MV

Fig.S: The GPS CIA Code Genera tor

regularly upload fresh navigat ion datainto th e on-hoard computer. Usuallythis is done once per day. although theon-hoard computer memory can storeenough data for several weeks inadvance .

In add ition to dedicated telemetry links.part of the telemetry data is alsoinserted in the navigation data stream.

2.5. GPS & G LO J\ASS SatelliteTransmission"

(iPS and GLONASS satellites use themicrowave Lband (0 broad cast threeseparate radio-navigation signals ontwo separate RF channels usually calledLI (around 1.6 GlIz) and L2 (around1.2 G Hz)- These frequenc ies werechosen as a compromise between therequired satellite transmitter power and

ionospheric errors. The influence of theionosphere decreases with the square ofthe carrier frequency and is very smallabove 1 (1Hz.

However, in a precision navigationsystem it still induces a position errorof about 50m at the L1 frequencydur ing daylight and medium solaractivity. On the other hand. GPS andGLOKASS were designed to work witho mn id ire c t io na l . hemi s ph eri c a lcoverage receiving antennas. Thecapture area of an antenna with adefined radiation pattern decreases withthe square of the operatin g frequency.so the power of the on-boardtransmitter has to be increased by thesame amo unt.

Both GPS and GLONASS broadcasttwo di ffer ent signa ls: a CoarselAcquisition (CIA ) signal and Precision

71


so it has the same repetit ion period asthe GPS CIA-code. The P-codc istransmitted at 10 times the speed of theCIA-code: 10.23 Mbps for GPS and5 .11 Mbps for GLON ASS. Th etransmitter power level for the P-codeon Ll is 3dB below the LI C/A-codcand the P-code on 1.2 is 6dB below theLI C/A-c odc. The P-code repetitionpe riod is very long. making anautonomous search for synchronisationnot practical. All Pccode rece ivers firstacqui re lock on the CIA-transmission.which also carries information thatallows a qu id; P-code loc k. Both CIAand P-codes are generated by digitalshi ft -r eg ist ers with th e fee dbackselecte d to obtain pseudo-ran domcodes. The navigat ion data is modulo-Zadded to the pseudo-random codes.Since the navigation-data rate is vcrylow, only 50 bps. il docs not affe ctsignificantly the randomncw prope rt iesof the codes used.

The navigation data at 50 bps Issynchronised to the C/A-eode period toresolve the timing ambiguity caused bythe relatively short I ms CIA-coderepetition period . GPS " Block II"satellites may encrypt the publishedP-codc into the secre t Y-codc. Thisprocess is called "Anti-Spoofing" (AS) .Its purpose is 10 prevent an enemy fromjamming the GPS with false ( JPS-likesignals.

Details of the .GLO:--JASS Picode arcnot published. In fact. the GLONA SSP-code is even not mentioned in [6l.although these transmissions can beeasily obse rved on a spectrum analyser.The GPS and GI.ONA SS RF channe lcarrier frequencies are shown on Fig.7.

The GPS CIA-code is 1023 bits longand is transmitted at 1.023 Mbps. TheCIA -code repe tition period is therefore1 rns. The GLONASS C/A~eodc is 5 11bits long and is transmitted at 5 11 kbps.

72

(,,---------~~~~~~

(P) signal. The CIA-signal is onlytransmitted on the higher frequency(LI) while the Pcslgnal is transmitted ontwo widely-separated RF channels (L1and L2).

Since the frequency dependence ofionospheric errors is known. theabsolute error on each carrier frequencycan be computed from the measureddifference bet wee n the two Ptransmissions on LI and L2 carriers.

The LI CIA - and Pccarricrs are inquad rature 10 enable a single po....cramplifier to be used for both signals. asshown on Fig.6. The LI and 1.2transmi tter outputs arc combin ed in apassive network and feed an array ofhelix antenn as. These produce a shapedbeam covering the whole visiblehemisphere from the (JPS/GI.o~ASS

orbit with the same signul strength.

All t hr ee GPS or GLO NASStran sm iss io n s are con tin uo us.stra igh tfo rward BI' SK mo dulatedcarriers. Pulse modulalion is not used.The riming information is transmitted inthe modulation: the user' s receivermea sures the time of arrival of adefi ned hit pattern. which is a knowncode. If desired, the modulat ion codephase can he rel ated to the carrierphase in the receiver to produc e evenmore accurate measurements , since boththe carrier frequency and the code rateare derived coherent ly from the samerefere nce frequency on-board thesatel lite.

Fig.9 : (irS CiA Codes a nd the treoeretation wllh th e Regtsters

5dks6 ctks7 elL:s8 d ks

17 dh18 elks

139 dks140 ells141 ells25 1 elks252 elks2.~ ctks255 el ls256 clh257 elks258e1b469 el ks470 c1l;..471 elh472 elks473 elks474 d b509 db5 12 c1h5lJ e1l;s514 elks515 elks5 16 c1k'i859 elks8fJO elks861 elks862 db

G2Clock C~dcs

2 & 63 & 7' &85 &91&92 s: 101 &82&93 & 102 & 33&45 & 66& 77 & 88&99& 101 & 42& 5.1 &64 & 75& 86& 91& 34& 65& 76 & 87 & 98 & 101 & 62 &7

3&'4&9

G2RC21..ter Taps

CIACod e l'\umher

GPS PR N IGPS PRJ\ 2GPS PR:,\ 3UPS PR:--l" 4GPS PRN 5GPS PRN 6GPS PR~ 7GPS PR!\ 8U I'S PR." 9GPS PR N 10GPS PR N IIUPS PRN 12GPS PR~ 13G PS PR X 14GPS PRN 15G PS PRN 16GPS PRN 17UPS PRt\ 18(iPS PRr-; 19GPS PRX 20GPS PRN 21GPS PR N 22GPS PRN 23GPS PR t\ 24GPS PR~ 25(iPS PRN 26GPS PRN 27G PS PRr-; 28GPS PRX 29GPS PRN 30GPS PRN 31GPS PRN 32

VH.!:..!~~~~~ '~- \ . ~

All GPS satelmes transmit on thesame LI a nd L2 carrie rfreq uencies: 1575.42 Mfll and1227.6 Mill. which are he ld inthe exact ratio 77/60 and areintege r mu lli p lcs o f th efu n da mental G P S doc kfrequency of t0.23 Mil l .

Every GPS satellite transmits itsown set of CIA · and P-ro<ks thathave good cro ss-corre tarionproperties with tbc codes Uk-d byother GPS satellite" . Since a GPSr e c e rv m g a n t e n n a i somnidirect ional and receivesmany satellites al the same lime.the rece iver is using Code Di vis io n Multipl e Access(COMA) techniques 10 separate..igna ls co ming from differentsatelhtes,

GPS sate lli te,", arc thereforeident ified hy th e Pseudo Random -Noise code number(PRN#). ",be GLONASS 'ItcllilNuse 2S d ifferent Rf< channel s.Channel 0 is reserved (ur testingspare satellites while channels 1to 24 are dedicated 10 operationalGf ,ONASS satellites.

All GLON'ASS satellites transm itthe same C/A-rodc and arcusually identified by the ChannelNumber (CHN# )'Ibe 1.1 and 1.2carrier frequencies arc in theexact ratio 9{7 and the channelspacing is 562.5 kHz at Ll and437.5 kHz at 1.2. Although thereexi st civilian Pccodc receivers.the majority of civilian GPS orGLONASS recei vers are CIA.on ly receive rs , Si nce th e


73


de

T8kl Einurl ~ @ll e 'l

'"511 kHz9 BIt Schiebe' e<j1 5te' G

OMA

" " " "'" " or " "I I I I I I

L.<CIA-Co

S53MVAusgang

FIJt.l0: The GLONASS CiA Code Ge nerator

advanta ges of using the Iccodc arclimited, especially with SA. AS or bothact ive, only the C/A-code transmissionwill be discussed in detail here.

2.6. GPS C/A-T ransmls.<;;ion Fo rmat

GPS satellite s use co de -d iv is ionmultiplexing on both CIA· and p,transmissions. Since CIA-codes arerelatively short sequences (only 1023bits), the codes have to be carefullyselected for good cross-correlationproperties. GPS C/A' l.-OOes are Goldcodes (named after their inventorRobert Gold) that can be generated as amodulo-Z sum of two maximum-lengthshift-register sequences. The GPS C/A-code generator is shown on Fig.S.

II includes two lO-bit shift registers Gland G2, both clocked at 1.023 MHz.each with a separate feedback networkmade of exclusive-or gates. Bothfeedback net works are selected so thaiboth generated sequences have themaximal length of 1023 bits. Both shift

registers arc started in the "all-ones'state and since both sequences have thesame length, the shifl registers maintainthe synchronisation throughout theoperation of the circuit.

Gold codes arc obtained by a moduto-zsum (another exclusive-or operation) ofthe outputs of the two shift registers (11and G2. Different codes can beobtained hy changing the relative phaseof the two shift registers. Instead ofdesynchrcnisiug the shift registers it iseasier to delay the output of one ofthem (G2). This variable delay isachieved with yet another modulo-Zsum (exclusive-or) of two G2 registertaps.

Exclusive-or feedback shift-registersequences have the property that amodulo-2 addition of a sequence withits delayed replica produces the samesequence. but delayed by a differentnumber of clock cycles. Choosing twoG2 register taps , 4, different delays canbe generated yielding 45 different Goldcodes with good auto-correlation and

74

2.7. GLONASS C/A-T ra nsmis.<;ionFormat

GLONASS satellites usc the morecon venti ona l fr equency -d iv i s ionmultiplexing at least for the CII\ -codetransmissions. All GI.ONASS satellitesuse the same CIA-code, generated by a9-bit shift register G as shown onFig.10. The GLONI\ SS C/A~ode is amaximum-length sequence and thus hasan ideal auto-correlation function.

The allocation of the single data wordsis completely described in (5j. Mostnumerical parameters are 8-, 16-, 24- or32-bit integers , either unsigned orsigned in the two's complement format.Angular values that can range from 0 to360 degrees are usually expressed insemi-circles to make better use of theavailable bits. GPS is also using its owntime scale. The units are seconds lindweeks. one week bas 604800 secondsand the week co1I1I1 is incrementedbetween Saturday and Sunday. Gi'Stime starts on the midnight of January5/6. 1980.

Prequcncy-civiston multiplexing allowsa better channe l separation than code division multiplexing. The separationbetween two adjacent GLONASSchannel s should be better (han -48dB. A

GPS time is a continuous time andtherefore it diffcrs by an integer numberof leap seconds from UTe. 'thedifference between In C anti (IPS timeis included in the almanac mes~age .

The first suhframe in the frame containsthe on -board clock data: offset, driftetc. The second and third subframcscont ain the precision ephemeris data inthe fonn of kcplerien clements withsevera l correct ion coefficients (0

accurately describe the sate llite's orbit.Fina lly, thc fourth and fifth subframcsconta in almanac data that is not

cross-correlation properti es. Out ofthese 45 possible codes, 32 areallocated 10 GPS satellites as shown onFig.9.

The cross-correlation propert ies of GPSCIA-c odes guarantee a cros s-talksmalle r than ·2 1.6dB between thedesired and undesired satellite signals.'The 50bps navigation data stream issync hro nised with the CIA-codegenera tor so that hit transitions coincidewith th e if all-ones" state of both shiftregisters GI and G2. Al 50hps one databit corresponds to 20 C/A -cooe periods.

The nav igation data is formatted intoword s, subframcs and frames . Wordsare 30 hits long including 24 data hitsand 6 parity bits computed over the 24data bits and the last two bits of theprevious word. Parity bits are used tochec k the received data for errors andto reso lve the polarity am biguity of thenpSK demodulator. 10 words (300 bits)form a subframe which always includesa subframc sync patten! " 10001011"and a time code called "Time-OfWeek " (TOW). One subframe istransmitted every 6 seconds. Fivesubfrarnes form one frame (1500 bits)that contains all of the informa tionreq uired to usc the navigation signals.One frame is transmitted every 30seconds.

"""-''''''''''= ''''-''''''''-'=-- -------- '~\ ,required immedia tely and aresuhco mmutated in 25 consecutiveframes. so that the whole almanac istransmitted in 12.5 minutes.


75

large channel separation is useful whenthe signal h om one satellite is muchwea ker because of reflected wavesand/or holes in the receiving antennaradiat ion pattern .

On the other hand , the GLONASSsatelli tes requ ire a wider RF spectrumand a GLONASS C/A-rc('civ{'r isnecessarily mere complex than a GPSCIA-receiver.

'111e GLONASS navigation data streamis synchronised with the C/A-coocgenerator so that level transitionscoincide with the "a ll-ones" state of theshift register. 'Inc navigation datastrea m is formatt ed into lines of thedura tion of 2 seconds. Each lineco nta ins 85 i nforma t ion bit s .transmi tted at 50hps for 1.7 secondsand a "ti me mark" sync pauer n" 11111000 1101 1101010000 10010110",which is a pseudo-random sct.jucnce of30 hits transmitted at 100hp~ for theremainin g 0.3 seconds.

The 85 information data bits alwaysstart with a leading "0" , followed by7 6 h il s cont a in in g n av ig a ti o ninformation and 8 pan ty -che cking hits,co mputed according to the (85. 77)Hammi ng code. After computing theparity hits. all of the 85 hits arcd ifferent ially encoded to resolve thephase ambiguity in [he receiver .

Finally , the 85 differenti ally-encodedbits arc Manchester encoded, so thai a" 10" pattern corresponds to a logical"one" and a "01" paUcrn corresponds toa log ical "zero ". The additionaltransition in the midd le of the data hitsintroduced by the Manchester encodingspeeds-up the synchronisation of therece iver. 15 navigation data lines fonn

76


one frame of the durat ion of 30seconds. The allocation of the sing ledata bits in the frame is completelydescri bed in (61. The first four lines ofa frame contain the time code, on-boardclock offset and dri ft and precis ionepheme ris data of the satellite orbit illthe form of a state vector (positionvector and velocity vector). To simpli fythe computations in the user' s rece iver.the corrections for the Sun- and Moongravity forces are also supplied. Thealm anac data is transmitted in theremaining I I lines of the frame.

Almana c satellite ephemeris is in theform of kcplcriau elements and istransmi tted in two consecutive lines III

a frame. The whole a lmanac istransmitted in five co nsecutive framesalso called a superframe of the durationof 2.5 minutes . The various numerica lparameters arc transmitted as differentsize, either unsigned or signed integers.Signed integers arc transmitted in theform of a sign bit followed by allunsigned integer re presenting theabsol ute value 01' the num ber (this isdifferent from the two 's complementnotation!). Angular values are usua llyexpressed in semi-circles.

T he GL O NA SS ti me is ke p tsynchroni sed 10 U'l'C. GLONASS usesmore conventional time units like days.hours, min ute s and seconds. The daycount begins with a leap year (cnrrently1992) and counts up to 1461 daysbefore returning back to zero.

(fa be co ntinued)

(References overleaf)

VHF COMMUNICATIONS2/9.

7.REFERENCES

( I I M atjaz Vidma r: "Digital SignalProcessing Techniques for RadioA mateurs. Pan-2: Desi gn of a nspCom puter for Radio Ama teu rApplicat ions"; VIJfCommunica tions IJ91.pp 2-24

12} Matjaz Vidmar: "Digital SignalProcessi ng Techniques for RadioAmateurs. Part -S: Construct ionand usc of the DSP Computer",VIIF..communicaliom 2/89, pp74-94

[31 Jonathan S. Abd. Ja mes W.C ha ffee: "Ex istence andU niqueness o f G PS Solutions" ,pages 952-95616-91. VOL.27.IEEE TRA~S. O~ AEROSPACeAND ELEC I"RO:'/TC SYSTE',1S.

14) "Interface Co ntrol Doc umentMHO~"()(xx}2-400. rev- E", (84pages), August 7th. 1975.Roc kwell Interna tionalCorporalion. Spa ce Divi..ion.12214 Lakewood Boulevard.Do wney, Cali fornia 9024 1, USA.

[51 "Inter face Control Doc umentG PS-200", (102 pa ges ), November20th, 1981. Roc kwe llIn te rnational. Space Operationsand Satellite Sys tems Division.12214 Lakewood Boulevard,Dow ney, Ca lifornia 9024 1, USA.

16 J "Global Sate llite NavigationSystem GLONASS InterfaceCo ntrol Docume nt" (46 pages).1988, Resea rch-and-Productio nAssociatio n of Applied Mechanics,

Inst itute of Space Dev iceEng ineering. GLAVKOSMOS.USSR.

171 Raben C. Dixon : "SpreadSpectrum Systems". (422 pages).1984, Second Edit ion , John Wiley& Sons, Ne w Yore, USA.

18J Matjaz Vidmar: "Di~ita l SignalProc-essing Techn iques for RadioAmateu rs••Theoretical Part".v lrlt-Communications 2/88, pp76-97

19) P. Mattos: "Global Positioning bySatellite", (16 rages), lnmos,Technical note 65. Ju ly 1989 .

1101 J. n. Th omas: "FunctionalDescription o f Signa l Processingin the Rogue (iPS Receiver" . (49pa ges), June I, 1988. Je tPro pulsion Laboratory. Cali forn iaInstitute of Technology, Pasadena.Californ ia.. USA.

1111 Char les C. Kilgus: "Shaped Co nica l Radiatio n Pattern o f (heBackfire Quadri filaTHe lix" ,(pages 392-397). IEEETransaction s on Amennas andPro pagation. May 1975 .

11 2J Matjaz Vidma r: " A VeryLow-Noise Aeria l Amp lifier forthe I.-Band"; VHfCommunications 2/92, pp 90-96

11 31 Matjaz Vidmar: "Radio-amateurapplications o f ( a 'S/GLONASSsatellites: Using GPS/G I.oNASSsatellites as an acc uratefrequency/l ime standa rd", strani186· I9O/Scripl1lm dcr vortraege.37. Weinheimer UKW Tagung,19.·20. September 1992

77


Marjaz Vidmar S53MV (ex Yll 3 lIMV, YT 3 M)


Part-3

3.THEORY OF OPERATlOl\O F ( iPS AND GLO NASSRECEIVF:RS

s.r. GPS/G LO NASS ReceiverPr inciples of Op eration

Since the signals transmitted by GPSand GLONASS satellites arc similar,the re ceiver design [or any of thesesystems follows the same guidelines.The pri nciple block diagram of a (IPSor GLO;.TASS receiver is shown inFig.l l . Only a single channel recei veris shown for simplicity. The problem ofsimultaneously receiving more than onesigna l (like the CIA-signal and both Psignals from four or more satellites)will be discussed later.

Since the user's pos ition, velocity and

attitude are unknown in a navigationproblem, satellite navigation receiversgene rally IISC eithe r one or more omnidirectional antennas. All satel lite navigation sig nals arc circularly polarised(usually RHel') to allow the user'sreceiver to further attenuate any reflected waves, since circu larly polarisedwaves change their sense of polarisation on each reflection. Reflected wavesare a major nuisance in precisionnavigation systems: they represent anunpredictable propagat ion anomalywhich is a major source of measuremcnr errors.

The radio signa ls collected by anomnidirectional receiving antenna arcweak. A low-noise amplifier will prevent any further degradation of thesignal-to-noise ratio, but it can 110treduce the thermal noise collected bythe antenna nor unwanted naviga tion

151

VHF COMMUNICATIONS 3194C/o,-----------'.!:!!:.-""""'""""''''"''''~'''''"--~~

SS3MY

rlg...... n.n"

"''''

Cod. Cod.S400Cl r_.

L....:..------'-""'-"""'------1-'----'---JCPU

Fig:.l1 : Pr inciple Block Diagram of it GPS/<i l ,O NASS Receiver

satellite transmissions on the samefrequency. OI'S and GLONASS satellitesignals arc widcband, ranging from IMHz (GI.ONASS CIA-code) to 20MHz (UPS p-coa c). lind the satellitetransmitter power is limited 10 around25dBW EIRP (Ll e lk -code for bothGPS and ULONASS) Of even less thanthis (Pctrausnussions). making the signal usually weaker than the thermalnoise co llected by the antenna.

Although buried in thermal noise andinterference. these signals can still beused. since the given bandwidth andmegabits-per-second rates apply to aknow n code and not 10 the informationbandwidth. which is smaller than I ltlzfor hoth timing and Doppler shiftmeasurements and the navigation datatransmitted at 50hps. In other words,GPS and GLONASS signals are directsequence spread-spectrum signa Is, using:Code-Di vis ion Mult iple Acc ess(CDMA) techniques [7).

A GPS or GLO~ASS receiver will firstdownconvert the signals 10 a suita ble IFand amp lify the m before furtherprocessing. At this stage a wide IFfilter, corresponding: to the com pleteoriginal signal bandwidth, can he usedto improve the dynamic range of thereceiver . T he downconvcrtcr may bema de tunea ble if widely separatedchannels arc to he received, like the(1J.ONI\ SS C/I\ -transmissiom .

The wideband IF signal is then multi plied by (mixed with) a Ioceny-gencrercd sate llite signal replica, modulatedhy thc same code . If the locallygene rated code is synchronised to thesatel lite transmission. the band width ofthe desired mixing product will collapsedown to almost zero, since two identica l Oli gO-degrees BPSK modu lationprocesses exactly cancel each other, onthe other hand" the bandwidth of allunwanted signals, like noise or interfercncc. will be further expanded by thisope ration to a double bandwidth.

152

VHF COMMUNICATiONS 3/94

Since the bandwidth of the desiredsignal collapses, this operation is usually caIled signal spectrum desprcading .The des ired signal can now be filteredout with a narrow IF filter having abandwidth ranging from 100 Hz 10 10kl Iz in a GPS or GLONASS receiver.After the narrow IF filter, the signalto-noise ratio finally achiev es usablevalu es and typically reaches 20dH.

The filtered IF signal is then used forsevera l purposes. First, it is used (0

acq uire and maintain synchroni sation ofthe locally generated code. Ditheringthe locally-generated code hack andfort h by a fraction of the bit periodge nerat es an amplitude modula tion onthe filt ered signal. The phase of thismodulation cont a ins the informationrequired to keep the synchronisation ofthe loca l code generator.

The fi ltered IF signal is also fed to aBPSK demodulator (usually a squaringI'LL or a Costas PLL) to extract the50bps navigation message data. TheBPSK demodulator also provides aregenerated carrier that is used forDoppler-shift measurements. On theother hand, the code -tim ing informationis obtained from the local code generator. A ll three signals, code timing ,Doppler shift and 50hps nav igation dataarc fed to the receiver CPU to computeth e user position, velocity, accurat etime etc.

For Earth-located" slowly-mov ing users, the Doppler shift on the satellit esignals is mainly due to the satellit emotion and amount s up to +/- 5 kHz onthe LI frequency. In most cases somefine tuning will be required to compensate the Doppler shift in front of the

153

VHF COMMUNICATIONS 3194(to: - - ------- ---''-''--'=====navi gation data and synchronise the irlocal Pccode generator to the CIA-codetransmission first. Since the Pccode rateis only 10 times the CIA -code rate,there arc very few possible Pccodcphases left 10 be tested 10 lock on thePctransmission.

GPS and GLONASS have been designed 10 supply liming codes. the userposition be ing computed from themea sured propa gat ion time differences.Add itio nally. the user velocity can becomputed from the already known posilion and the measured Doppler-shiftdifferences on the signal carriers.

Although the Doppler shift can also bemeasured on the code rates, this rucasurcmcnr is usually vcry noi sy. On theoth er hand , no abs olute delay diffe renceca n be measured tin the ca rrier. sincethe carrier phase becomes ambiguousafter 360 degrees.

Finally, relating the carrier phase to thecode phase may produce excelle nt results, hut requires un accura te compcnsauo n of ionospheric efforts, whichhave opposite signs : the ionospheredela ys Ihe modulat ion and al the sametime advances the carrier phase!

Besides the described principle of operation of a GPS or CiLO NASS receiver, there arc some other possibilities. Por example, the CIA-code synccould he obtained muc h fasta using ananalogue (SA W) or digital (rFf) corrclutor. To evaluate Ionospheric errors,codeless reception techniques can heused to receive both Pctrausmissions onLl and L2 frequencies without evenknowing the codes used .

3 .2. DigilllJ Signal Processing (DSP)In G PS/GLONASS receivers

After the princ iples of opera tion andthe required funct ions of an electronicci rcuit are know n. one has to decideabout the techn ology to practicallyimplem ent the ci rcuit. III most ca sesUPS or (JJ .O NASS receivers are mobileunits installed on vehicles or eve nportable handhel d units. The recei verweight. size and power-con sumption arca ll importa nt. While every <iPS orGLONASS receiver must have an antcnna. a RF front -cud and a d i~ital

co mpute r to solve the navi)!alion cq uattcns. the IP signa l prot'cssillg mayinclude just a single channel in a simpleCIA-only receiver or more than 10channels in a full-spec Ll & 1.2 It-coderecei ver .

when the same circuit function needsto I:'C duplicated several times. like theII' proces sing channels in a radionavigation receiver, it is usually COIl

vcuicnt to use Digita l Signal Processing(I) SP) techniq ues. An impo rtant advanrage of nsp over analogue ci rcuits isthat d uplicated cha nnels are completelyidentica l a nd require no tuning orcalibrat ion to accura tely measure thedifference in the time of arriv al orDop pler shift of radio-navigation signals. A single nsp circuit can also heeasily multiplexed among severa l signals. since the inter nal variables of af)SP circuit like a PL! . veo frequencyor phase can be stored in a compute rmeOlory and recalled and Updated whenneeded again.

Th e bandwidth of the navigation sarellite signals is several MHl and this is arather large figure for nsp. Implement-

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ing the whole IF signal processing of aUPS/G LONASS receiver completely insoftware (like described in the introduction to nsp techniques in [liD isdifficu lt although it has been done 19]for the GPS CIA-code using powerfulmicrocomputers. Most GPS/GLONASSreceivers usc a com bination of dedicated DSP hardware and software for lfsignal processing. Dedicated DSP hardware is only used where the bandwidthis large and the functions arc relativelysimple , like the local satellite signalreplica generation and the signal despreadin g, while all other functions,including all feedback loops, arc implemented in software.

When designing a DSP circuit and inparticular when designing dedica tedDSP hardware it is essential to know,bes ides the signal bandwidth or sampling frequency, also the resolution ornumber of bits per sample requi red torepresent the signals involved [10]. A

GPS or GLONASS signal is a constantamplitude signal and limiting is there fore not harmful. However, after thewide If filter in the rece iver there is amix of many satellite signals of diffe rent strength and lots of thermal noise aswell. If such a mix of signals is limited,the resulting intcrmodularion distortiondegrades the signal-to-noise ratio byaround 2dB.

Since navigation satellite signals arcpseudo-random sequences, all undesiredsignals and all inrcrmodulatlon product sonly af fect the desired signal in thesame way as thermal noise. The refore,in a GPSjGLONASS rece iver, very fewhits arc required to represent the wideband IF signal. Most GPS/GLONASSreceiv ers simply limit the widcband IFsignal, thus accepting the 2dB scnsirlvity degradation and represe nting eachsample with just two quantisation levelsor one single bit. Increasing the numberof bits per sample only increases the

155

VHF COM MUNICATIONS 3/94

Of course both NCOs have to heaccura tely steered to the required frequency and phase to maintain lock onthe incoming signal. The feedback

formed in softwa re, since an interruptrate of only I kTIL can be accepted byany microproce ssor. The accumulateddata in the integrator has a resolution of12 to 14 bits. so any further softwareprocessing can he done without anyloss of quantisat ion accuracy norproce ssing speed of a general-purpose16-bit microprocessor.

Dedi ca ted hardwa re is also required forthe generation of the local signal replica. Ca rriers or rates are convenientlygenerated in Numerically Controlledoscillators (:--leOs). An 1\CO inc ludes adigital adder and an accumulator. Inevery d ock cycle , a constant representing the desired output or rate is add edto the accumulator. If an ana logueou tput were desired, the accumulatorcontent could be fed to a RO M contai ning a sine table and then to a D/1\converter, forming a direct digita l frequency synthcsiscr.

In a I-hit nsr navigat ion-receiver thesine table and Df" converter arc notrequired. Since the nsr hardware operates with l -bit data. it is sufficien t totake the MS!l of the NCO accumulatoras the frequency output Two NCOs arcrequired: on for the carrier frequencyand another for the code rate. The

code-rate NCO supplies the clock to acode generator like the ones shown all

Hg.S or 10. The output of the codegenerator i ~ exclusive-or gated with theoutput of the carrier NCO to produce aBPSK-modulated satellite signal replica.

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DSP hardware complexity while bringing marg ina l sensitivity improvements,so that no known receiver des ign usesmore than 3 bits per sample (8-levelquantisation).

On the other hand, an I -bit/sample DSPGPS/ULONAS S receiver may have areally simp le IF signa l processing asshown 011 the principle block diagra mon Fig.12. The IF signa l is limited, sono AGe is required. Signal samp lingan d A(D convers ion is performed by asing le D type flip-flop. Signal despreading or mult iplication with the locallygenerated signal replica is accomplished with an exclusive-or gate . Sincethe narrow IF can be selecte d close tozero, the narro w IF bandpass filter maybe replaced by a Jowpass filter or anintegrator. In the case of I -bit samp les,the latter is simply a counter with thed ock set to the sample rate and gatedby the input signal .

However. after the narrow If filteringthe resulting signal can no longer berepresented with a sing le bit per sample , i f the sample rate of the narrowhand signal is significantly reduced. Ina CIA-code receiver, the integrator isread and then reset each millis econd, tomatch the perio d of either GPS orGLONASS CIA-codes, since the autoand cross-correl ation properti es of thesecodes are only main tained over aninteger number of code periods. Anintegration period of 1 m s correspondsto a narrow IF bandwidth of +1· 500 lIzaround the centre frequency. The latteris a very good choice for a GPS orGLONASS receiver.

Any furthe r signal processing after theintegration can be conven iently per-

156

~~=~~~----------(I:,VHF COMMUNICATIONS 3/94

funct io n can be performed hy themicroprocessor, since the feedbackspeed is vel)' low: a 100 Hz update rateis usually fast enough. Fina lly. the NCO

frequency can be ea sily steered modifying the addition constant and the NCOphase can be easily steered modifyingthe accumulator content. In a timemultiplexed W cha nnel. both can beeas ily stored by the microprocessor andrecall ed when th e channel ha rdware isswitched hack to the same satellitesignal.

Prom the technology point of view, aDSP IF channel can he built on an"Eurocard" size printed circuit hoardusing just bare 74TICxxx logic. I\. singleIP channel may also be programmed ina programmahie-logic int egrated circuit.f ina lly , the comple te If signal process ing with 6 or S indep endent ch an nelsmay be integrated ill a single customintegrat ed circuit. Commercia l satell itenavigation receivers usc custom integrated circuits essentially to preventunaut horised duplication. On the otherhand, hare 74HCxxx logic is preferredfor an amateur, home-made rece iver.IIopefu lly programma ble-logic Ies willsome day become standardis ed and thenecessa ry programming tools cheapcnough to allow amateur applications.

3.3. Multi-channel reception ofnaviga tion signals

A sate llite navigation receiver shouldbe able to rece ive the signals from fouror more satellites at the same time, tobe able to measure time and Dopplerdifferences. When the GPS specifications were published back in 1975 [4] ,the digi tal computer was the largest and

most complex part of a satellite navigation receiver. Both GPS and GLONASSreceivers were initially intended to havesevera l analogue IP processing channels, one per each signal type persatellite . Since these receivers wereintended for military vehicles likefighter aircraft, tanks or battle ships, theprice and complexity of several analogue IF processing channels wa s almost unimportant.

Early civilian ( ;PS receivers also usedanalogue If processing, although initially limited to the C/I\.-code and oneor tw o time-multiplexed IF channel s.Time-multiplexing is difficult w ith analogue IF channels, since the latter have10 reacquire lock each time the sate llites arc changed. Lock Acquisitionmay take 15 to 20 seconds, so tha t theme asureme nt loop through four or moresatellites takes severa l minutes . Thesereceivers were only suitab le for sta tionary or slowly-moving users.

The introduction of Dxl' technique s andinexpensive computers allowed muchfaster multiplexing. Since the variablesof a DSP circuit call be stored andrecalled, a nsp IF channel docs notneed 10 reacquire lock each time it isswitched to anothe r satellite signal. ADSP IF channe l is typically switchedamong satellite siguals around a hundred times per second makin g thewhole loop among all required signals afew ten times per second. I Icwcvcr,because of the avail able signal-to-noiseratio, the navigation solut ion in aCIA-code receiver only needs to becomputed about once per second.

AH current commercial GPS and GLONASS receivers lise DSP IF process ing .

157

VHF COMMUNICATIONS 3/94C/'·- - - - - - --- -----'-"'-== = "'-"= =Sma ll handheld C/A-code receivershave one, two or three time-multiplexedIF channels. Mobile C/A-codc receivershave 5. 6 or eve n 8 independentchannels so tha t no multiple xing isrequired . TIme multiple xing males thecarrier lock and Doppler measurementsdi fficult and unrel iable. so it is undesired in mobile receivers.

Unfortu natel y. multi-channel 01.0NASS rece ive..-rs require a wider rawsigna l IF and a milch higher sa mplingrate due 10 the wide FD:\1A channelspaci ng. On the other hand. UPS rccciv C!S require the saute raw IF ba ndw idthreg ardle ss of the number of channelsthanks to CIJMA. The higher samplingrates required for nI.O~ASS ace a littleim pract ical with currently avail able integ rated circuit s. Maybe this is anotherreason why (J I'S rece ivers are morepo pular and ( jI.ONASS is almost unkno wn. Since faster Ie s will ce rtaintybe available in the future. one can

expect that com bined GPS/GLONASSreceivers will become stand ard.

In this article I am going to descr ibe asingte-channel C/A-only recei ver usingfast time mu ltiplexing . Th is receiverca n be built in two versions: GPS orGLO:-JASS. Although hoth versions usethe same modules as much as possible,this is not a combined GPSJ{jLONAS Sreceiver yet. The main limitation o f asingle IF channel. tirnc -mulriplcxcd reccivcr is that the maximum number ofsim ultaneously tracked satellites is limited to four or five. so that a com binedGPS/GLO:"JASS receiver docs not makemuch sense.

3.4. Praclica l (j PS receiver design

Th e block d iagram of the described(i PS receiver is shown on Fig..!3. In themicrowave frequency range, at Lcband ,the antenna needs a direc t v isibility o fthe satellites . Therefore it has to be

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installed outdoor, 011 the vehicle roo f oron top of a portable receive r. Due to itsex cellent performance, a half-tum quadrifila r helix is used as a circularlypolarised, hemi sphe rica l-coverage antenna,

A freq uency of 6139 k il l . was selectedas ma ster cry stal oscillator frequency o fthe de scribed riPS rece iver. since thebest TeXOs arc usually avai lable forthe frequency range betwee n 5 MHzand 10 MIT/..

The output of Ihe 6139 kl lz masteroscillator is used both as the samplingfreq ue ncy for the I}-, AID conve rsionan d as an input to a cha in of multiplierstages to supply a ll of the frequenciesrequired in the do wnconvcrrcr. Limiting

The LNA is install ed directly under theantenna. Using two ine xpensive GaAs ~ETs it achieves 30dB of gain makingany following (reasonable) cable lossa lmost unimportant.

'111C Gl'S receiv er includes a fixed tuned downconvertcr to a suitable IF,an IF amplifier and limiter, a ded icatednSf' ha rdware. a MC68010 based microcomputcr with a sma ll keyboa rd anda LCD display and a single mastercrys tal oscillator for all frequency con ve rsions and s':llnp l i ll~ rate s. The downconve rsion from the C PS 1,1 frequency(1575 .42 MHz) is made in two steps forconve nient image filteri ng. '111C firstwide IF is in the 102 MHz range andthe second wide IF is ill the 10 MHzran~e. The wide IF bandwidth is set toaround 2 MfJ/.. The actual va lue of thewide IF bandwidth is not critical, sincefiltering is only required to preventspec tru m aliasing ill the signal samplingcircuit.

==== = =='---- - - --- --- -';;.- , -the temperature range from 0 10 30degrees C. as encountered during normal receiver ope ration, the TCXO wasreplaced by a much less expensivecon ventional crysta l osc illator in all ofthe prototypes built.

Sampling the 10 Mill. wide IP signa lwith 6139 kH7. produces a th ird down conversion \0 a 230 3 kl lz nomina lcentre frequency. TIle Inter is the finalcarrier frequency that needs to herege nerated in the dedica ted DSr hard ware . The dedicat ed nsp hardware i.sdcsigucd as a mic roproc essor peri pheralwith read and writ e registers and isinterrupting the MC6RO[O CPO onceevery millisecond to match the UJ'SCIA-code period .

In the portable, stand-alone (fI'S rece iver. the operating soft ware is storedin a com pressed form in a 32l:hYlcEPROM. After powe r-oil reset. thesoftware is decompressed ill 12RkhYlcso r battery-bac ked CM OS RAM, whic his also used to store the sys tem almanacand other data to speed-up the acquisition of four va lid sate llites. For thesame reason the CPU also has tlCCCSS toa sma ll battery-backed real-time clockchip.

A smal l 8-key key hoard is used toselect the va rious menus or the opcratlng software and manuall y set somereceiver param eters if so desired. Theportable version of the (iPS receiver isusing a I .eD mod ule with integrateddriving elect ronics and two rows or 40alphanumeric (ASCII ) ch aracters eac h.to display thc receiver status . the almanac data or the rcsuhs of the navigat ioncomputations.

159

VHF COMMU NICATIONS 3/94

fixed. . n this case the only contributionto group-delay variations across theGLONASS Ll frequency range arc thetuned circuits at 1.6 GUz. Group-delayvariations introduce errors in the mcasured time difference... so they immediately affect the accuracy of a navigation receiver. This problem does norexist in a GPS receiver. since all GPSsatelljtos transmit on the same carrierfrequency and any signal flltcn ng produces the same group delay on allsatellite signals that exactly cancels-outwhen com puting the differences.

Both wide . Ps are fixed tuned at II R.7Mllz and 10.7 Ml T/, respectively. Toavoid any gm up-dclay variation~ in thewide , FIi. the frequency symhcsiscrsteps must accurately match the channelspacing so that all signals are co nvertedto the same , F values. Finally. the , Flimite r should not introd uce a variable

The downconversicn from the nLONASS Ll frequency range (1({)1 to1615.5 MHz) is made in two steps forconvenient image filtering. To reducegroup-de lay variations, the first conversion is made tuneable and the second is

(~, - - - - - - - ------'""--''''''"=== '-''=3.5 . Practical GLONASS rece iver

design

The block diagram of the describedGLONASS receiver is shown in Fig.l4.The GLONASS receiver uses the sametype of antenna and LNA and the samededicated DSP hardware and microcomputer as its GPS co unterpart. TIlemain di fference between the two receivers is in the dow ncouverter. TheGLONASS receiver includes a tuneabledownconvcrter, otherwise the wideH ) MI\ cha n nel spacing would requireImpractically high sampling rates in thededicated DSP hardware.

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del ay as the input signal strength ischanged when switching amo ng chanriels. The second wide IF signal at 10.7MIll. is sampled with 4500 kHz produc ing a third downconverslon to a nominal centre frequency of 1687.5 kllz,

T here are several d ifficult-to-meet requireme nts for the freq uency synthesiscr supplying the signet for tile firstconversion. This synthcslscr has toprovide a clean signal in the frequencyrange from 1483 to 1497 MHz in stepsof 562.5 kHz. Its phase noise should helow enough to allow carrier lock and50b ps nav igation data demodu lation: itsspectral Iincwidth should he abo ut totimes JI3 lTOwer than required ill a voiceSSB receiver . Fina lly, ill a time-multiplexed, single -chann el rece iver the syntheslscr should he able 10 switch andsett le to another frequency ill less thanI ms the CiT.ONi\SS CIA-code per iod,to avoid increasing the switching: deadlime.

The freq uency synrhesiser is a PI J .with a frequency convener in thefeedbac k loop, 10 decrement the dividermodulo, increase the loop gain, speedup the sett ling and improve tbe outputphase noise performance. The feedback.signal is downconvertcd to the frequency range 25 10 38 MIIz, so that ave ry low programma ble loop-dividermodu lo between 45 and 69 is required.The comparison freq uency is sci to562.5 kHz.

A well -designed PLI. will settle in 100to 200 cloc k periods of the comparisonfrequency and the described PLLachieves this performance with a set tling time between 200 and 300 microseconds .

The described GLONASS rece ive r isusing a master crystal oscilla tor at18.000 MH/.. This frequency is multiplied by 6 to obtain the 108 MHz signa lrequired for the second conversion andby 81 10 obtain the 1458 MHz signa lrequired for the PLI. feedback-loopconvers ion, The master osci llator frequency is divided by 4 to obtain the4500 KHz sampling frequency and by32 to ohtain the 562.5 kHz PLI.reference frequency. Like in the (i PSreceiver, in place of an expensiveTCXO conventional crystal oscillatorswere used in all of the prototypes built,limithig somewhat the operating temperature range.

In the described GI.ONASS recei ver,the microcomputer has one functionmore . Besides controlling the dedicatedDsp hardware, keyhoard and LCDdisplay, all identical to the UPS courtterparts. the mic rocomputer has to setthe frequency synrhcsiscr when switching among chan nels. The ope rat ingsoftware is very sim ilar to that ill tbc(iPS receiver and has the same hard wa re requirement s: 32khyles o fEPRO M, 128kbytes of battery-backedCMOS RAM and a batte ry-backedreal-time clock.

3.6. ( ' I'S/( , I.ONASS dedicat ed nspha rdware design

Although the theory of ope ration of anI -bit USP (I PS or GLONASS receiverhas already been discusse d, the practical imp lementation still offers ma nydifferent choices and some additionalproblems to be solved. For example,from the theoretical point-of-view it isuni mportant whether the code lock or

161

(~ - - ---- - -----'-'-"-'== = "-'==the carrier lock is achieved first. Inpractice . the code lock should beach ieved first and should be completelyindepend ent from the ca rrier lock, bothto spee d-up the initia l signal acqulslrlonand to avoid loosing lock at short signaldropouts (obstructions, fading) or rccciver frequency refer ence instabilit ies.

The block diagram of the practicall yimplemented GPS/GLONASS dedicatedDSP hardware is shown on fi~ . 1 5 .

Allhotlg.h the imple mented hardware isintended for a single channel, timemult iple xed operation. it dirk rs siguificanny from the theoretical block diagra m shown on Fig.12. The mai nd iffe rence is that there arc four signa ldcsprcadin g mixers (multipliers. ex-orgates) and four integ rators (counters)for one single channel.

In practice, two separate signal-dcspread ing mixers arc required whendownconvcrting 10 a narro w IF ofalmost zero. The mixers arc driven withthe same satellite signal replica, moduIatcd with the same code, but with thecarriers in quadrature . In th is way noinformation is lost aft er signal despreading, downconversion and integration. The code lock can he madecompletely independent from the carrierlock, since the narrow If signal amplitude-can he com puted out of the J andQ integration sums withou t knowing thecarrier phase. The same 1 and Qintegration sums arc used in a differentway to achieve carri er lock and extrac tthe Subps navigation data . Due to thelow sample rate (1 1.: 1Lt,) the biter areconveniently performed in software.

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Although code lock may be maintainedby dithering the locally generated signal replica, two separate narrow IFs foran "early" and a "late" local signalreplica provide a 3dB improvement inthe signa l-to -noise ratio on time-delaymeasurements. The former solution,code dithering, is usually used inreceivers with an analo gue narrow IF,since it is difficult to build two identica l circuits in analog ue technology. Thelatter solution is used in receivers witha nsp narrow IF, since nsp circuitsperform the sum c nume rical operationsand arc therefore mathema tically identica l.

To achieve the 3dB signal-to-noiseimprovement, two separate sets of I andQ signal-processing cha ins for thenearly" and "l ate" signal replicas needto he used . This brings the total numberof signal-desprcading mixers and inte grators to four. Of course the localsignal replica genera tion includes thegeneration of four different signals:I·EARLY, Q-EARLY, I-LATE andQ-LAlTI. All these signals can heobtained from a single carrier and codegenerator, since they arc merely delayed versions of the same signal: eitherthe carrier or the code or both aredelayed . In nsp, dcluys can he easi lyobtained with shift registers.

On the other hand, the local satell itesignal replica generation can be simplified with a look-up table. Since theintegration period is I ms and the inputsample rate is 6139 kHz (GPS) or 4500kHz (GLONASS), there are only 6139or 4500 different hits to he stored in thelook-up table for each dcspreadingmixer and integrator. The look-up table

sampli ng rate or any odd multiple ofthis value: 1536 kHz for GPS or1125 kHz for GLONASS.

In pract ice 613 9 kHz was selected asthe sampling rate for the GPS rece iverto avoid interference with the GPSCIA-code rate (1023 kIlz), since thedescribed look-up table generator maintains a fixed phase relationship betweenthe code transitions and sampling rate.Considering the various conversion frequencies obtained from the samesource, a n IF of 23m kl Iz resulted aftersignal sampling.

In the GLON I\SS receiver. any intcrferencc betwee n the sampli ng rate andcode rate are unimportan t since allsatellit es use the same C/A-<:oue . Thesampling rate of 4500 kl lz was chosenfor convenience. Considering the operation of the frequency syn thcsiscr. thefinal wide IF value could he chosen in562.5 kHz steps. The value of 1687.5k Hz was selected 10 avo id some spurious frequencies generated in the synthcsiscr.

Finally. the described dedicated DSPhardware always requires the support ofa microcomputer. TIle latter shouldcompute and load the look-up tablesfirst. After each interrupt request (every

164


millisecond) the microco mputer readsall four integrated sums. From the I andQ components it computes the earlyand late magnitudes used to search andmaintain code lock. The code phaserequired to maintain lock is at the sametime the result of a time-delay measurement, referenced to the rece iver clock.The difference of two such measurements is a parameter of a navigationeq uation.

On the other hand. the T average li nd Qaverage are supplied to a Costas-loopdemodulator to recover the carrier anddemodulate the .)Ohp~ nav igation datahils. Then the subframe or line sync isdetected 10 fonnnt the data stream andcheck the parity bits before the navigat ion data is used in the computations.

10.LIT ERATURE

i l /1 literature refcrences in this articleare 10 he found on IXl?,f'- 77 of issue2/94

(To he cont inued)

Very low noise aort al amplifier for th eI. -band as per the YT3MV article on pag90 o f VHF Communications 2/92.Kit complete with housing An No. 6358DM 69. Orders 10 KM Publications at theaddress shown on the inside cover, or toUKw -Bcricme d irect.

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Quadrifilar Backfire HelixAntenna

Al1hou~h lon l.: r a nge, prt'CI ~ 101l na vluat lon systems like (iPS or GLONASS were dl's i~ nl'd to he lnucpcnd c n t us much HS possib le or the perIor mn nce or eith er tran smlulnn orreeelvt ng lInll'nnlls. the antennas IISfiIsHU have so me influen ce on t hesystem per for ma nce.

I.ANTENNA REQUIREMENTS

The tra nsmitting antennas installed onthe spacecra ft have a shaped beam tosupply any Eart h-located users with thesame signal strength and usc the onboard transmi tter power more efficiently .

Maintniniug till ' same ~ iJl: ll a l strength isespeci ally impo rtant ill CnMt\, sincethe GPS C/A--codes are too .\II(m tooffer a very g-ood crosstalk performalice. The ideal receiving- antennashould have a he mispherica l radia tionpau cm. offering the same signalstrength from a satel lite at zenith and(mill another sate llite just above horizon. Furthe-r, the receiv ing antennashould match the transmitter polarisalion (RHCP) i ll a ll va lid directi ons .

Hnally, the receiving antenna shou ldattenua te an)' signals coming from undesire d d irecti ons, like dguals comingfrom negative elevarious, since thesearc certainly reflect ed waves and thelatter arc a major source or measurement errors due 10 their unknownpropa gation path .

197

2.THE TURNSTILEANTENNA

Although a turns tile antenna(two crosse d dipole s fed inquadrature) with or without areflector is frequently usedfor satellite reception, thisantenna is-not very suitablefor satell ite navigat ion forseveral reasons . The polariselion of a turnstile antenna iscircular only in the zenithdirection and is complete lylinear in the horizon plane.Therefore, a turnstile antennaoffers no discrimination be'w een the desired RHCP dircct wave and tile unwantedt He]> reflected wave, sincecir cularly polarised wave schanged thei r sen se of po lari sati on on cuch reflection. Reflected waves cause severemeasurement er rors and arelatively slow and deep signal fading, so that the reccrvcr even looses lock onthe signal.


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198

Fig.I: The Quadrifilar Backfire Antenna withRight-Hand Circular Polar isation

="""''''''''''"''''-'''''=''''-----------(~.VHF COMMUNICATIONS 4/94

nator pro vides a useful rad iation patternwith a reasonably circular pola risa tionover a wide nl11ge of eleva tions. Un fortunatel y the radia t ion pattern o f :I

microstrip antenna fall.. down to zero inthe ho rizon plane. Micros trip antennasarc usu ally used when a simple, lowprofi le antenna is requ ired, usually 10he insta lled 011 u vehicle roof. Sincelow-elevation satcllitc,s can not be rece ived , a mlcrostrip antenna usuallylimits the available ntX}P.

4.TH E Q UADRIF ILARUACKFIRE AN'I'ENNA

Th e be st antenna for satellite na vigati onand o ther applic at ions requir ing hcmispherica l eovc l';l ge see ms to he thequudrifilar back fire hel ix (als o ca lled a"vol ute" anten na) . Sud , an antennaprov ides a shaped conical beam. '!11l'beam shaping and cone aperture can he

co ntrolle d by ad justing the hel ix radius.turns p itch dist ance and number ofturns as desc ribed in II 1], By the W:lY,the same type o f an tenna is frequen tlyused o n low-Earth orbit satellites, likethe ~Oi\A weat her satel lite s.

As till' GPS and G I.O:.rASS satellill'sa lready prov ide a const ant signalstrengt h for Earth -located users regardless o f the satellite elevatio n, no par·ticular beam shapi ng is required for there c e iv ing antenn a . T he opt imumnumber of turns of a quadrifila r hackfire hel ix used as a (iPS or GLOr\AS Sreceiving antenna seems to he between

1.5 and 3. Making a qna drifiJar backfirehel ix longer hy incre asing the nu mberof turns docs not have much effect onthe gain or the bea m-cone apertu re, hutit improves the bea m shaping a ndfurther attenuates the undesired lobe inthe opposite di rection (downwards).

Although the best GPS receivers usesuch a quadri filar heli x with 1.5 or 2turns. such :111 antenna is di fficult tomanufacture and teet. In particula r, thefour helica l wire s have 10 he fed inquadrature and there is very lill ie Spaceon top of suc h an antenna to inst all 111(:.feeding net work. Further, a z- turn hackfire helix is rat her large (Zucm high) fora porta ble receiver. If its improvedpattern per formance is to he fullyexploited. the direction o f its axi..should 1101 d eviate too much fromvertical and this is 110 t a very practi calrequirement for a porta ble receiver .

Most UPS/( H.O NASS receivers therefore lise a sim pler ante nna . usually ashort one -hal f 111 m backfi re hel ix likeshown 0 11 Fig . 16. Making (he q uadrifilar heli x shorter resonance e ffects canbe used to feed the four helical wireswith the prope r sig na l phase.... Tn pal'·neuter. one pair of wires is ma deshorter to mckc its impedance ca peci ti vc at the operating frequency and theot her pai r of wires is made longer 10make irs impedance inductive at theoperating frequenc y.

To obtain Rlle p a con ventional endfire helix ha~ 10 be wound like arig ht-hand screw, The back fire hel ix isjus t opposite: 10 obtain R1ICP thebackfire hel ix has to be wound as aleft -hand scre w, bes ides the properpha sing of the four hel ical wires, of

199

VHF COMM UNICATIONS 4/94

hand, the performa nce of the antenna isnot degraded much if no balun is usedas shown on Fig. 1.

In the prac tical construct ion of a ha lftum quadrifilar helix it is espec iallyimportant to respect the exact lengthsof the helical wires, since the antennauses resonance effects and is rathernarrowband. The dimensions shown onf ig. 16 arc for the GPS L1 frequency(1575.42 MHz). A GLONASS LI antenna sho uld be approximately 37"sma ller. Fina lly. an antenna for both(IPS and GLONASS Ll channels canhe built by des igning it for the ave rageof the two frequency bands.

T he fccdpoint impedance is in the 500range, symmetrica l. A good matc h to500. is usually sacrificed for the radia tion pattern wh ich is much more important. Usually one o f the [om helicalwires is replaced by a semi-rigid coaxial cab le of the same miter diameterto fonn an " infin ite ba lun". 011 the other

(~--------- - - ---'-"'--==== ==course! Further, the back fire hel ix requires no reflector. The four helicalwires arc fed at one end of the helixand shorted together at the other end o fthe hel ix. Since the main (desired)rad iatio n lobe is directed towa rds thefeedpo int and away from the shortedend, such an antenna is called abackfire antenna .

IMPROVEMENTS CHANGES

Sup pression 01' in terfer ence in 70 ·cmATV mod e us tng high ly selectivenotch IiIter, by E.B erbe rich j 1/94pp.45-55.

Some errors crep t into fig . t 2 on p. 52.so here' s the circ uit again.-H."I Sm ool~ l ll ~

x~;r :: :®"Ik-T ''l 1

Im provements and additions to theSpect r um Analyse r by Dr.L jlrmann,DBiNV

Some points were not clear rega rdingthc structure of the spec trum ana lyserand need correcting :

200

I. Printed circuit board 00 7 (LOfPU .):Circuit dia gram and components diugram gave different values for resistance of 17, pin-2: the version with a56k resist ance to earth is correct .

The capacitor at pin-a o f 11 (l\ E 551 4)has a purely blocking function. It co uldbe given a value o f. for example. O. l ufo.

2. Printed circuit hoard 009 (run-offcontrol) : The tendency of the emitterfollower to oscil late did not becomeapparent until the layout had beencompleted . It can be remed ied bymeans o f a l nl- (not lnl-) ceramiccapacitor on the foil side.

C ircuit diagram and components dia gram gave dif ferent values for resistance of 12, pin 2 to eart h. The correctvalue here is 150k; at 39 k, the tuningd iode in the second La would have abias voltage in the conducting direction.

"-"-==='-'='-='''-=------------(~,

Matjaz Vidmar , S53MV


Part-4

<,

In this part of lhe ser ies completeconstruction details will he shown forthe (i PS RF Module Hod lhe IFConver ter. lhe GLO;'llASS RF Module Hod IF CURve-TIer Mod the <;1..,0·~ASS PLL Symhcslscr Conver ter .

4,The RF and IF Stages of theGPS and GLONASSConver ters.

4.1 The Anlenna

111e Quadrifilar Backfire Antenna usedby the Author for this project wasdescribed in part-Sb of this project inVHF Communications 4/ 1994, pp.197 .ZOO,

4.2. Low-Noise Amplifier

This unit, which is common to both theGPS and the GLONASS receive convertcrs was described fully in VHFCommunications 211992. pr . 90 - 96and is available as a separate kit fromKM Publ ications (see the advertisementon page 17 of this issue).

4.3. GPS RF Module

The (IPS receiver only requires 3

single-frequency (1575.42 MHz) downconverter and its design is relativelystraightforward. The (i PS downconverier includes two modules: a RFmodule built in microstrip technologyand an IF strip built on a simple,single-sided printed circuit board.

The circuit diagram of the UPS RFmodule is shown on Fig.l9. The GPSRF module includes three RF amplifier

35

VHF COMMUNICATIONS 1/95(~ -----------"="'-""''''''''''''-'''''~'''-''-'''

L. L12

MIlf511

1.57GHz La

<' L3 L' L9 INPUT73""M1

IN PUT u " L8

'" 2. HP2%Ol8A481 l

>-"i "'"' :L2• OUTPUT

220 1·' '" .p 220 .," l<l2t'Hl,

. 12',' 55 JMV

Fig. 19: (i PS RF Module

stages and the first downconvcrsinnmi xer. The amplifier stages arc identi cal and usc silicon MRF571 transistors .

Much of the gain provided hy thesetrans istors is lost in the microstripfilters, since the latter arc etched on alossy but inexpensive glass fibre-epoxylaminate.

Th e first dcwnconvcrsion to 102 MHzis performed by a harmo nic mix er usingtw o anti-parallel Sc hottky d iod esHP2()OO. 13 1\4X\ or simil ar. Such a

m ixer has a higher noise figure than

convcnriona l diode mixers, especiallywhen using the suggested low Froqucncy diodes . On the other hand, therequired local oscill ator signal is at736 MHz. only h;Jl f of the frequencyrequired for the downconvcrsion(1473 MHz).

The Rf modulo circuit includes anetwork to supp ly with +12V the <';al\sFET preamplifier through the RF cab le.On the other hand. the + l 2V supplyvoltage for the RIo' module itself istaken out of the If.' convert er, after

Fig.20:GPS RF Module,upper side(top view)

o 0 0

II ill.Ii .'-"-'""36

VHF COMMUNiCATIONS 1/95

= "- --157 ~'1l

fiF IN

".

II':102 HH'1 IIF aUT! !

Fi~.21: GPS RF :\tlndule Component Overlay

"""11716 HHl! !L O IN

4 . 20~ 4 ·20~ J6834MH:

;Jt "VIr. ...I· m BfX 89 .,f4<h. r .n, u BFX89- 8tX89 '"'il. G35"1 Hl

'''' " u ~

Hi" hor I1H .ltHHI """ .,., ,.., 2. ll pi n"

" n" ,.,. ,,"

Fig.22: GPS IF Converter 1\11111iplier and Mixer

'"BFX89

55 3MV

37

VHF COMMUNICATIONS 1/95'/'\ . - - - - --- - - - - - ----""'--'= === = ="

'"

'''~ ,. ,.. ".n9~1 ,~ ,~.. '"

. 11~

,~.. "" '"\'tt,Ifl •t~( 1wr

BFX69 ~.~lKhtr

SS 3MV m HH, .1?V

'" '"cu OUTPUT

Fig.23: <IPS IF Converter Loca l Osdllato r

bein g filtered by a choke and a IOOuFcapacitor. The RF module is built inmicrostrip techno logy on a douhie-sidedhoard made of O.79mm thick glassfibre -epoxy. The lIpper side is shown onrig.20 while the lower side is notetched . '111c loca tion of the compo nentsis shown on Fig.21. Befo re insta llingthe components. 1.3, I,5, L7. 1.9 andLJ 2 should he grounded by solderingsmall Unshaped pieces o f wire ill themarked local ions.

r.r. 104. Ili, L8 and Lt l are quarterwavelength chokes. These arc madefrom about OCIll of O.15mm thickcop per enamelled wire. tinned for about5mm at each end. The rema ining wireis wound on a lmm inner diameter andthe finished chokes are small sel fsupporting coils. on the other hand. L2is a commercial IOOuH "moulded"choke.

It is recommended to use thin Tefloncoa x like RG· 1811 for the internal RF

wiring of the ( iPS receiver. Th e bra idof the cable shou ld he soldered directlyto the rnicrostri p grouudptanc while thece ntral conductor reaches the uppertracks through a hole in the printed ci rcuit board. To avoid shorts. thecopper platin g around this hole on thegroundplanc side should be carefullyremoved using a much larger (.1mm)drill tip,

The (i PS Rl- mod ule needs some adju stments of the striplines and these arcbest performed a ncr aII of the receiverhardware is assembled. 1.3, L5, L7 and1.9 usua lly need to he trimmed shorterby about I mm at the open end 10

achieve the maximum gain at 1575MIl,.. On the other hand, LI D and Ll 2may need some smaII pieces of copperfo il (abo ut 7mmx 7mm) at d ifferentlocations along these striplinee 10

achieve the best noise figure from thediodes actually used in the mixer.

38

VHF COMMUNICATIONS 1/95'-""-'''"'''''"'''"'''"''''=-'!E- (f':

Fig.24:GPS IF Converter,bottom view

"136MHzl:LO OUT I I

""

I ',11102MHz: : I F INI I

lQMHzZF OUT

..12V

6139kHzeLK OtT

Fig.25: GPS IF Converter Component Overlay

39

VHFCOMMUNICATIONS 1/95

the GPS CIA-code signal bandwidth (2MHz) ln fact, LI 1 already requi resdam ping resistors to achieve the required bandwidth .

Th e GPS IF converter is built on asingle -sided board as shown Oil Fig.24.The location of the com ponents isshown on Fig.25. Due to the limit edspacc all o f the resistors are installedvertically. The capacitors arc convcntional ceramic discs (except for toOuF)with a pin spacing of 5mmCapacitivetrimmers arc pla stic foil types of 7.5mllldiameter: green 4-20p1' and yellow2-10pF. There is also a wire jumpermarked with "A".

The GPS IF converter includes severalind uctors. Most of them are air-wound.self-supporting coils wound with copperenamelled wire of either O.5mm orl mm diameter. The turns of the se coilsarc not spaced and the leads go straightthrough the printed-circuit board with om any additional bending or forming.In these way the coils themselve s haveabout 1/4 of a turn less than speci fiedin the following para graph.

L1 and L2 have 3 turn s each of 1millwire wound on a 4mm inner diameter.L3 has 5 turns of O.5mm wire wound ona 3mm inner diam eter. L4 and L5 havetwo turns each of l rnm wire wound on

Th e BFX89 is used as an universal RFtransistor in this module and has n1<lnypossible repla cements: TIFY90. TIFW.10etc. The four leads of the BF981MOSFET are bent so that the device isinsert ed in the printed-circuit hoardwith the marking towa rds the board.The 7805 regulator docs not require aheat sink provided that it is-a TO-220version.

The GPS If strip includes a seconddownconvcrsion to 10 MHz. signalamplification and limiti ng at 10 MHzand the generat ion of all required localoscill ator and clock signals from asingle m aster frequency reference.

The second downconversiou to 10 MHzand the LO frequency generation isincl uded in the GPS IF convertermodule shown on fig .22 and Fig.23.The GPS IF converter module includesa 61 39 kHz crystal oscillator (Fig: .21).This frequency is used both for signalsampling and suitably multiplied forboth downconvcrsions . Since the rcquircd short term stab ility is vcry high,in the J.E-9 HlllgC. to be ah le todemodul ate the 50 HI'S I'SK navigationdata. the crystal oscillator has its ownsupply regulator no:; :.I1lU is followedby tw o buffer stages.

TIlt' crystal oscillator output frequenc yis first multiplied by five to obtain 30.7MH z and then by three to ob tain the 92MHz required for the second downconversion. Three additiona l Frequencydoubler stage s <Ire required to obtainthe first downconvc rsion signal at 736MHz from the availa ble 92 MHz signa l.The design of ali mult iplier stages issimil ar and is using two tuned cir cuitsin each stage except for the first stage,where three tuned circ uits arc necessarydue to the higher multiplication factor.

The 102 MHz IF signal is first amplified (BFXR9) and then filtered (L9 andLlO ). The second mixer is a simpledual -gate MOSFET mixer (BF98 l). Theselectivity provided hy the tuned circuits at 102 W Iz (L9 and LI O) and at10 NITIZ (Ll ) is already comparable to

40

(~ -------------'-"-"-'''''''''=~~~

4.4. GPS TF Converter


3 311111I inner diameter . L6 has ::l turnsof 0.511I 01 wire wound on 301m innerdiameter. 1.7 and L8 have one slugfcturn (or "U" loop) of imam wire with a3mm inner diameter. L9 and U O have5 turns each of O.5mm wire 0 11 a 4mminner diameter. l-inalty . L13 and 1.14have 6 turn s each of O.5mm wirewound (1 11 .:1 4mm inner diameter. 1.13has all add itional coupling loop of onesing le tum around the mai n winding.

Lt I ,1.I 5. 1.16, 1.17 and LI S arc woundon standard cores for IF trans for mc,TS

(To ko or Mitsumi) with the externald imensions of lO111m x lOmm. Llishould have about 4.5uH and in pra\:rice this means 15 turns of O. 15mmdiameter copper enamelled wire 0 11 a10.7 MIII. IF transformer core setinclud ing. a fixed central ferrite core, alladjustable ferrite (,.·up, various plast icsupport parte and a me tal shieldi ng. can.Ll 5, 1.1 6 and L17 should have aboutDAuB and in practice have 6 turns (If0. 15111 111 diameter copper enamelledwire 011 u 36 MH7. IF trans forme r coreset including a plastic suppo rt with acentral adjustable ferrite screw. a plastic e;) p and a meta l shielding call .

The exact value of LI S de pends on thecrysta l used and the frequency required .In all of the prototypes built inexpen sive computer crys tals designed for6 144 kll z were used. These requirequite ::I large iml uctivity to be pulled 5kl-lz down to about 6139 kH2". Aninductivuy around 40u H is requ ired forthis shift. The exact value dependsmuch on the crys ta l used and theparasitic capacita nces of the circ uit.Since the performance of the (IPSreceiver depends on the stabitiry of this

master crystal osci lla tor. also 1.18 need sto be very stable. Therefore :I 36 MII:f.IF transformer co re set is recom mendedand the latter requires about 60 turns ofO.08mm diamet er copper enamelle dwire .

Finally, Ll 2 IS a 100 ull "moulded"choke .

The GPS IF convener has severa lconnections. The two coa x ca bles canying IF and LO signals to the RI; moduleand the +12V supply wire for the Rftmod ule are all so ldered directl y to thebottom side of the IF conv erter module.The 10 MHz IF output, the 6 139 kl-lzclock output and the +l 2V supplyvoltage are availa ble on a 'z-pin counce tor obtaine d from a piece of a good

qua lity Ie socket with round contacts.

The (;l 'S IF module req uires seve ra ladjustme nts, but the crysta l osci llatorshould he adjusted first III roughly 6139kflz. T hen the multiplier chain sho uldbe adjus ted. Each mu ltiplier stageshould he adjusted to provide themuximutn signal at the required Irequcncy to the next stegc . The levels ofthe Rf signals can be easily mon itoredwith a rx:: vo ltmeter, sincc they arcrectified by the BE junction of the nextstage . Without any Rr input, the rx~

volta ge is set to about O.7V acros s theBE junction. When the multiplier chainis operating correct ly, this vo ltageshould decrease down to about zero andmay even become negat ive

If the transistor base !l0e.s more negative than -O.5V, RP transistors may be

damaged and th is shou ld he avoided bydecreasi ng the values of the couplingca paci tors.

41

VHF COMMUNICATIONS 1/95(,/" - - - - - ---- ------'-'-"--'==""'-='-"-'"

Of COlJrSe, the voltmeter required forthese adjustments should only be COIl

nectcd through a RF choke to avoiddisturbing the RF ci rcuit. A lOkohmresistor may also be used as a RPchoke. In this way all of the multiplie rstages can be adjusted except the lastone to 736 MHz, SblCC 110 BE junctionfollows this stage. The level of the 736Ml Iz signal is monitored ill a differentway. by connecting a DC ohmmeter tothe IF output Of the mixer. The higherthe LO signal level . the lower theresis tance measured by the ohmmeter.

the signal circuits is best performed ona real GPS signa l ob tained from adirectiona l antenna (a 15 tum helix or asmall dish) pointed to a GPS satelli te .

f inally, the crystal oscillator shoul d headjusted to the exact freq uency requ iredby the software. f or the current versionV l 22 the exalt frequency is 6139 .050kl lz, but this may change in the future.The exact frequency is specified in theprogram listing.

4.5. GLONASS RF ModuleThe signal circuits (L9, LlO and LI l)arc best adjus ted after the receiver iscompletely assembled, since the following I f ampl ifier has a Scmctcr output. Agrid-dip meter can he used 3S a signalsource at 102 MIlz. The trimmers inparallel to 1.9 and L 10 tunc almost tothe ir maximum capac ity and I.10 maysometime, require an additional capaci tor in parallel. The Fina l adjustme nt o f

The GLONASS receiver requires atuneab le dcwncouvcrrcr across all ofthe 25 Gr.ONASS channels spacingfrom 1602 MHZ to 1615.5 MHz. therefore its design is more complicated thanthe GrS coun terpart. Thl~ GLONASSdownconvcrtcr is d ivided into fourmodules for shielding purposes anddiffere nces in the construction technology: au RIo" module and a PLL synthe-

MRF 511

""'''''

Ii'(5M.ll1)

" " ..l\

co

'" '" " 01490M HI ,. '00 '0; 1483.:112 .

. 1496.6'1211H~

HF it! no,'"1602 . " no 'Ok llFR91

. \61S.SMHl MRFS71

",no, ,~,

tt SS 3MV

, 11

'"12{l~ H

I 120~H.12V

Fig .26: GLONASS RF Module

42

The GLONASS HF mod ule only needsfew adjus tments, mainly to the yeofeedba ck network. To cover the desiredfrequency range. the centra l fingerusua lly needs to be trimmed shorter byseveral mra. The two side fingers-mayneed adjustments if the veo stopsoscillating at band edges.

slscr converter buill in microsmp technology :md an IF converter and synthcslser logic built on simple. single-sidedprinted ci rcuit boards.

111e circu it diagram of the GLONASSRF module is shown on Fi~.26. TheGLONASS RF module includes twoselect ive RF amplifier stages. the firstreceiver mixer to the first (fixed) I~ of118.7 MH1.. and a ym followed by II

buffer stage. The two RF amplifierstages are identical and use MRF57 Itransistors. Since the GLONASS Rr:modu le is built on a lossy. bUI thickerlaminate thun (IPS, the losses in the Rffilt ers arc lower and two amplifierstages provide eno ugh ga in.

The veo inclu de s an am plifie r(fiFR9 1) and a highly-selective interdigital filter feed back network . Such II

veo can only cover a very limitedfrequency rolnge (abont 10% around thecentral frequency). hut its phase noiseill v\."I)' low. The veo is tuned by annlO; varicap in the central finger ofIhe irucrdigual feedback network.

The Veo is followed by a buffer slagewith another BFR91A microstrip coupler takes part of the y e O ompntsignal to drive the PLL circui ts. Theyeo and Rf signals arc then combinedin an inlcrdigiu l filler network to feedthe mixe r d iode I JP2900 or BA48 1.

The GI.ONASS RF module circuitincludes a network to supply with + 12Vthe GaAs rET preamplifier through theRr cable .

The GLONASS RF mod ule i.'i built inmicroerip technology on II double-sidedboard made of 1.57mm thicl.: glassfibre-epoxy. The upper side is shown on

VHF COMMUNICATIONS 1195 'f>.= ===== = '---- - - ----- - -\ .Rg.27 while the lower side Is no tetched . The locat ion of the com ponentsis sho wn on Fig.28. Before installingthe components, the resonators of L3.1.5. L6 and L7 should be grounded bysoldering short pieces of Imm diametercopper wire at the marked locations.Tbe transistors and diodes are installedin 6mm dia meter holes in the printedcircuit boa rd.

L1. L4. 1.8 and L12 arc quarter wavelength chokes. These arc madefrom about ecm of O.15mm thickcopper enamelled wire, tinned forAbout 5mm at each cud. The remaining:wire is woun d Oil a I mill inlier diameterand the finished chokes arc sma llself-supporting coils . On the other hand,L2. L9 and 1.11 are commercia l 120u1l"moulded" chokes.

Rr interconnections inside the GL().NASS receiver are made wilh thinTe non coax like RG· 188, installed justlike in the UPS receiver fronl end. Onthe other hand, GI .ONASS mlcmstripmodules include Iconbrongh capacitorsto save spaC'C 011 the priraed-circunboards. The fecdtbrough ca pacitors aresoldered to the mtcrostnp groundplancfrom the bottom side. Some compo·nears. like chokes and resistors in thesupply network , are also installed onthe bottom side of the microstriphoards.

43

•1===::'••The rCJnilll11ng lnrcrdig uul filters usual ly d,) not need any adjustments toprovide the best performance i ll thedesired frequency range . If the veo isoperating correctly, the mixer diodewill provide a rccrifc-d voltage o f about-OAV across the 1500hm resistor.

: vee OUTI I U ti 3 J ' 4 97~l

""


Fig.27:GLONASS RFModule. upp er side(lop view)

•

4.6. GLONASS IF Conve r te r

The GLON/\SS If strip includes asecond downconvcrsion 10 IO.? MII:t.signal amplification and limiting al 10.7Mlh and the generation of the requiredlocal osci llator and d ock signals from asingle master frequency reference.

I f OUT118.71'11'11

•

55 3HV

"w '"as

MRFS71

ea,~

' lOp

HF IN1602/ 160!i.511M1

Fig.28: GLONASS RF Module Component Over lay

44


The second downconversion to 10.7MHz and the LO frequency generationis included in the GLONASS IF convener module shown on H g.29 . TheGLONAS S IF conve rter module inetudes an 1S MHZ master crystalosci lla tor. T his Frequency is used, di vided by four , for signal samp ling,divided hy 32 ;JS the PLL referencefrequency and suitably multip lied lorthe second signa l downconvcrsi un andfor the PLi . downconvcrsion. The Ci-LONI\ SS IF module only inclu des theoscillator and some mul tiplier stages.The dividers are 10C;Jl ed in the PLLsynthcsiscr log ic module and the lastfrequenc y m ultiplier is ill the PLLsynthcsiser conv erter. Like ill the GPSreceiver, the required short rerm staoitiry is very high. in the I-E-9 ran ge. tohe able to demodulate the 50 BPS PSKnavigation data Therefore the cry stalosc illator has its own suppl y regulator?SOS and is followed hy two bufferstages just like in the C;PS If convenermodule.

The crystal oscill ator output frequenc yis first multiplied by three to obtain S4MHz. This signa l is then doubled tolOS MITz for the second down convcrsion and multiplied hy three to obtain162 MHz to drive lite I'LL synth cslscrconverte r. using two sep arate multiplierstages fed by the same 54 MHz signal.Th e 162 Mil l. signal is further amplified in a buffer stage (BI'R 96) to dr ivethe SRD multiplier in the Pl.L synthesiscr converter.

Since the des cr ibed GI DNASS receiv erinclu des a more complicated RP frontend than GPS, mo re filtering is req uiredin all multiplier stages to avoid spuriousfrequencies. Therefore multiplier stages

may have three or even more limedcirc uits on the ir outputs. The 118.7:M1 h; If< signal is filtered (L9, U 0 andLJ 1) and amplifie d (BFXS9). T he second mixer is a simple dua l-gate MOS :rET mix er rnrcs I ). The selectivitypro vid ed by the tuned c ircuits at II R.7MITz (L9 , UO and LlI) and at 10.7MITz (L12) is al ready compara ble to theGl .DNASS CIA-code signal bandwidth(1.2 MHz). In fact, Ll 2 already requiresdampi ng resisto rs to ach ieve the re quired bandw idth.

The GLONASS IF converter is built ona single-sided board as shown onJo'ig.10. The location of the componentsis sho wn on Fig.J I. Due to till' limitedspace all of the resistors arc installedvertically. The capacitors arc convcn

tiona! ceramic discs (except for 100ur)with a pin spacing o f Sr nn-Capacitivctrimme rs 4~20pF arc a plastic foil typeof 7,Smm diameter. marked with agreen body , There is also a wire jum permarked with "A",

The IH'X :'l9 is used as an universal RFtransistor as in the (i PS If converte r.Also the n f<9S 1 is installed just like inthe (i PS lf convener module and a1'0 -220 case 7805 reg ulator is recommended so that no heat sink is req uired.

The (JL ONASS IF conven er incl udessevera l inductors . Most of them arcair-wound. self-supp orting coils woundwith copper enamel led wire o f 0.5H1mdiam eter. The turns of these coils arcnot spaced and the leads go straightthrough the printed-circuit hoard with out any additional bend ing or forming.In these way the coil s themselves haveabout 1/4 of a turn less than specifiedin the following paragraph.

45

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46

'"~~~~~~--------'~- \ .VHF COMMUNICATIONS 1/95

Fi2.30:GLONASS IFConverter, bottomview

10.7MHlZF OUTPUT

18MHlel K OUTPUT

118.7MHl

ZF INPUT

1112 MHl

LO OUTPUT

Fig.31: GL ONASS IF Converter Component Overlay

47

VHF COMMUNICATIONS 1/95(~ --'.'2.C~~~~~

L5. L6 and L7 have 4 turns each woundon a Jmm inn er diam eter. L9. L10 andLl l have 4 turns wound on a 4mminner d iameter. 1.13 and L 14 have 5turns wound on 11 4mm inner diamet er.Ll4 has an additional coupling loop ofone single tum around the ma in winding. r.r.L2. D , lA and Ll2 arc woundon standard cores for IF tranefon nc re(Toko or Mu sumi) with t il l' externald imensions of Imm x IOmm.

1.12 should have about 4.5ul1 and inpractice this means ISlum s ofO .15mmdiameter copper en amelled wire Oil a10.7 Mllz IF tran sformer core sciincl uding. a fixed central ferrit e core, <I ll

adjust able ferrite cup, various plasticsupport parts and a metal shielding can.L2. 1.3 and L4 should have aboutO.D ull and in practice have 3 turns ofO.Jmm diameter co pper enamelled wire011 a 36 MHz If transformer core setIncluding a plastic support with acentra l adjustable ferrite screw, a plastic cap and a metal shielding can,

The exact value of 1.1 depend s 0 11 thecrystal used and the frequency required.In all of the prototypes bu ilt inexpensive computer crysta ls desig ned for18000 kl lz, ser ies resonance, wereused. These require a small induetivityin ser ies to compensate for the feedbackcapacit ors of the oscillator network. Inpract ice about 211 1I were required , correspond ing to 16 turns of O, 15mmdiameter copper enamell ed wire on a 36MHz If trans former core set.

Finally, Ul is a VK200 "six -hole"ferrite cho ke and LI 5 is a 100u1l"moulded" cboke.

The GLONASS IF conve rter modulehas several con nect ions. The two ca bles

carry ing the 118.7 Mi ll If from the RFmodule and the 162 MH z LO to thePLL synthesiser co nvener are all so ldered directly to the M ilam side of theIf converte r modu le. Th e to.7 MH:l Ifoutpu t, the 18 MIl l. clock outp ut andthe +Il V supply voltage are available011 a 7-pin connector obtained from apiece of a good-qual ity Ie socket withround contacts.

In the GI.ONASS If converter modulethe multiplier stages should be alignedfirst, just like in the similar GPSmodule . However , only the output ofthe first mnlrip hcr stage to :; 4 MIIz canbe monitored as a d ip of the followingstage base volta ge. The output of thel OX MHz mult iplie r may be observed asu dip in the drain voltage of the HF9RImixer. while the output of the 162 MII/.multiplier ma y be measu red av therectified voltage by the SRO multiplierin the PI.L synthcsiscr converte r.

The signal circui ts (LQ, L10, Ll I and1.12) an ' best adjusted after the receiv eris completely assemble d. since thefollowing IF amplifi er has a S-mdcroutput. 1\ grid-dip meter can used as asignal SO\1Tee at 118.7 Mill'. The trimme rs in parallel to 1.9, 1.10 and Ll Itunc a lmost to their maxi mum capacity.The final adjustment of the signalcircuits is best performed O il a realGLO;.lI\SS signal obtained from a direct ional arucrma (a 15 tum helix or asmall dish) pointed to a GLO;.lASSsatellite.

fina lly, the crystal osci llator should beadjusted 10 the exact frequency requi redby the software. For the current versionV39 the exact frequen cy is 18000.000kj-lz, but th is may change in the future.

48


'11V

u

VOlIN?lJl

14aB1 'l,. 1496.61'l MH,

rc I.. PUT

162l1l1l

1H41l.6

.. 1pS

••

m , ... ,"-r UGA

'"to " '"l

1PS .......

S5 JMV

"

Fig.32: (; J,ONASS PLL S.\'nthesiscr Converte r

The exact freq uency is speci fied in theprogram li "lin~"

4.7. C:; LO NASS I'LL S-",nth.;siscrConver ter

A sing le -channe l GLONASS receiverrequires :I Iast-scufing frequ ency synthcsiscr, since the receiver is continuously switching among diffe rent frequency chan nels . Besides this require-

ment the symhcsiscr should have a lowphase noise. To limit gro up-de lay variatio ns the synrbe-iscr should supply avariahlc frequency already to the firstdowntonv erter. AI[ these requirementsask for a PJ.I. synthcsiscr with afrequency downconvcrtcr in the feedback loo p, to decrease the dividermodulo and increase the loop gain .Therefore. the GLONASS I'I ,L syntbc slscr includes a veo in the R io' mod ule ,

••- •

•

Fi~.;l;l :

(jLONASS PLLSynthcsiserConvcrtcr, upperside (to p view)

49

(~

55 3MV

~. "(" . 21. •lS •

V' If lo •I UJp

:~."","," ,

" ""' " ••""'" , _ ~ na,nl~~J<1

'"6f ~'IO


(0 ' NPUT'U ~~ '

.~ere ,~PUT

,..Il""1~",

Fig .~''': (iLON ASS PLL Syntbcstscr Converter Component Overlay

a downconvertcr and conventiona l PIJ .sYlll hc~i~n log ic like va riable modulodi viders and a frequency/phase cOlllparater.

'illC circuit diagram o f H l~' CiT .oNi\SSPT.], synrhesis e r COIlVCI1cr i~ show n onPig.n . The circuit inc ludes anotherbuffer s l;J~c for the y e O sig na l around141Xl MIIz. a step-recovery diode(SRI)) frequency multiplie r hy 9. 10 gel145R ~lIz from the available 1 6~

Ml lz, a mixer diode and an fro ampli4.lcr stage. The veo buffer sta ~c

(BrR90) is requ ired 10 avoid getting:my unwanted spur ious signals hack inthe Gl .ONASS RF mod ule.

The SRI> mult iplier uses a w ry lncfficient silicon I'N-jnoction diod e IK4 14&.Ot he r diode s like VIIF T V tune r bandsw uc hing diodes (Oi\ 182 or UA482)provide an lip to 20dB stronger signalar 1458 MH z in the sam e ci rcu it, hut ahigher signal level is not required hereand it is even harmful, since it may getin the Rf module and cause unwantedmix ing products. In practice it is thu sconvenient to keep the 1458 MHzsignal level low and drive the mixer

50

diode into the non- linear reg ion withthe 1490 MH z veo signal.

To avoid any spurious generat ion a llsignal levels are kept low. Even thebuffered y eO signal amou nts to onty afew hundred mY on the mixer diod eIIl '2900 (or Hi\ 4S I) while the 14.'lXMl lz sij!llal level is much lower. Toope rate e fficie ntly at low signal levelsthe mixer diode receives a IX: biasCUITent.

The PLL IF signal then needs muchamplification III reach the m . levelrequired hy the va riable -mod ulo l' OUlI

rcr. Till' first I'LL l f ampli fier stage(fl FRQO) is built in the I' lL convertermodule. The following PLL IF cmplific r stages arc located in the PI.I.synthcsiscr lo!!ic module for shield ill!!purpo ses, si nce harmonics o f the PU .rr fall in the first IF ( 11 8.7 MI L,,)frequency runge of the described 0 1.0:--lASS receiver.

The ClLON ASS PLL symn csiwr conve rier is built in mic rostri p technologyon a double -sided board made of15 7mm thick glass fibre-epox y. '111C

uppe r side is shown on Fig..33 while the

The microstrip filters in the GLONi\ SSI'LL synthcslscr conven er usually donot require any trimming. The lOkoluntrimmer for the SRI) bias current isusually set 10 Skohm'lhc SRI) multiplicr will operate correctly if the rectilied IX.: voltage hy the IN4148 d iodeamounts to about 2V.

lower side is not etched. The locat ionof the components is shown on Fig.34.Before installing the components, theresonators of 1.I . 13 lind L4 should I'ICgrounded by soldering short pieces of1mm diameter copper wire at themarked locations. The transistors anddiodes are installed in onun diameterholes in the printed circuit hoard.

1.2. L5. L7 ami U: :lTC quarter-wavelength chokes. These are made fromabout 6cm of 0.15nuII thick copperenamelled wire. tinned for about Smmlit each end. The remaining wire is

VHF COMMUNICATIONS 1/95""'-"""""""'~~'-'-""~----:-------(t·

wound on a lmm inner diameter lindthe Fin ished chokes are small selfsupporting coils. L6 is a self-supportingcoil with 3 turns of O.5mm diametercopper enamelled wire wound on aSmm inner diameter.

/lP" APAIMllf

VHF COMMUNICATIONSTHE COMPLETE INDEX

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51

VHF COMMUNICATIONS 2/95(~, ---------~~~~~~

Matjaz Vidmar, S53MV


Part-S

This part or the ser ies conttn ucs wil ltth e co nst ruct ion details for theGLONASS PL L Synthcstser, SecondIf' Amp lilier and lite W'S/( iLONASSUSI' Hardware.

4.8 GLONASS I'L L SynthesiserL~ic

To convert the frequency range1602 Mill. to 1615.5 Mllz down to118.675 Ml Iz the veo must operate inthe freq uenc y flJng C 1483.3125 MlT... to1496.8125 MIll',. Subtract ing 1458MHz in the PI. I. syr ahesiscr converter,thls frequency range is downconverted10 2'U125 MHz to 38.8125 MHz. TheJailer frequency range corresponds tointeger multiples ranging between 45and 69 of the ( iLONASS channelspacing of 562.5 lllz.

The design of the PIJ . synthcsiscr logicj.'i therefore straightforward and thecorresponding circuit diagram is showni ll f ig.36. The I'LL synthesiser logicincludes a PLL IE a fixed divider hy32 to obtain the 562.5 kllz referenceand a frequency/phase comparator.

The PLL IF signal in the range from 25MHz to 39 MHz is first amplified to aTIl . level in a two stage amplifier. Thegain of this amplifier is set higher thanrequired to have a considerable safetymargin. The base bias resistor of thesecond stage may need some trimmingalthough the suggested value of 2.21dJwill usually work.

The programmahie modulus counter isbuilt from two synchrono us counters: a74F l61 and a 74IIC 161. A 74Fxxxcounter is required in the first stage tooperate rel iably at the hi ghes tfrequ ency, since the PLI. IF may besevera l tens of MHz above 39 MHz inthe unlocked state!

The counter feedback network includesan inverter and a 74HC157 multiplexer.The modulus of the sec ond counter(74HC16l) is programmed direct ly.The modulus of the first counter(74F161) is set to 10, except during thelast state of the second counter, whenthe 74HC157 switches the 74Pl61preset inputs 10 the 4094 outputs. Inthis way setting the modulus of the

78

VHF COMMUNICATIONS 2/95 (+.,

" ".,'" ·w'-if- .,.~ .' ,.

.,. , ffi ''..

...... ". ., "",,,",~, t •.1-,, . , • •

OIl' .... ' -", , .....

.,.~.,......... ."-, '..'. ,",. • " It""" '" • ,",

",,, ," • .1('

' 0 f 'lO ."O.,._o-J~I ""' .. '"'" ' I" ' ..•• " ".... -

'· ~ 1." .", ...

"" ."...... 55 3MV "."" "..,. .,.

Fig.3!': GI.ONASS Synthcslser Logic

second counter changes the wholedivider modulus in steps of 10. whilesetting the modulus of the first counterduring the last cycle only provides thesingle-count steps.

The 4094 is an g-bit shift register withoutput latches, It is used as a serialto-para llel interface driven by thecomputer. Of the eight available outputlines, four are used to control themodulus of the 74HC161 counter andthe other four to control the modulus ofthe 74[-161 during the last cycle of the7411Cl 61. One should he especiallycareful when programming the modulusof the divider: the data is inverted andthe first divider modulus should neverbe set too low to allow for the delays inthe slower 74Hcxxx logic!

The 18 MHz master referencefrequency is also amplified to TfLlevel in a single -stage amplifier(2N2369), A 74HC393 counter divides

this frequency by four to obtain the4500 klfz sampling frequency and by32 to obtain the PI.L referencefrequency, The 4500 kl lz signal isattenuated with a resistor network tolimit spurious radiations. It is thenamplified back. to TIL level in thededicated DSP hardware module.

The frequency/phase comparator is acharge-pump circuit. including two Dtype flip-flops (741lC74), a feedbacknetwork with a NAND gate and chargepump switches with Schouky diodes.The backlash problem is solved bymaking the charge-pump circuit fasterthan the feedback. network. This doesnot make the phase detector linear yet,but provides a stable locking point forthe PLL with no dead zone. andproduces a very clean synthcsiser signalspectrum. An additional NAND gateprovides a LOCK signal for testpurposes,

79


Flg.36:PCB Layout torthe GLO~ASSSynlhesiser Logic

(~-----------'-"'--===='-'''-'=

The GLONi\SS PLL receiver logic isbuilt on a single-sided primed circuitboard as shown in Fig.36. The locationof the components is shown in Fig.37.Th ere arc three wire jumpers on thishoard and two of them arc installedbelo w the 74IICI5 7 multip lexer.A complex single-sided board alsoplaces some constraints on theinstallation of the resistors: those with aIOmm hole spacing are installedhorizontally, whilst the others are

installed vertically to save space. All ofthe capacitors arc ceramic with a Smmpin spa cing , inclu ding th e )uF(multilayer).

L1 is a commercial 1(J(}flH mouldedcho ke. The module has th reeconnectors with 3, 4 and 7 contacts,made from pieces of good quality Iesockets. The integrated circuits shouldhe soldered directly to the board exceptfor the 4094. It is recommended toinstall this Ie on a socket, so that it can

'"DATAsra'"0

Pll IF~l9_

INPUT

~.llJo,:+..

~. rr----G" o0,m ~"- - ,

:;; II::; I Iz "

L.:.:..1"1216~ {I .().1OIJ

""o~~ "'0 0 0'",..;!. l~ T,"1I)k -<>- 0 '; g

' H " ,~ :: " ':t lk-o-lOO ~

iec- H~'>«I 2~21 '-e- -0- T 'Kt'n i '000 -0- :-it- {I ~S(H1000 S ~ 1_ 220 ~10 1001

~ , SMlil CUT

' t M ~ I IN

."",«PLL SI.

F ig.37: Component Layout for the GLONASS Synthesiscr Logic

80

VHF CO MMUNICATIONS 2/95

. 12V

TEST

Hz ZF

TPUT

.I 120p 1,1,1,1"lI : WO~

,I,

/' ( A3089,~'""" ~ k5 " I( A3189) nop OU, ,

I"-1-' L,-p ~ 1 t;;~' I"UF '"o Bf~

22 r"" 1''' WO k

tn~.

'" "Inn In n553MV

~O M H I ZF lC

; ~PUT I gnl

Fig.38: GPS.!GI.ONASS Second IF Ampli fier

he removed from the circ uit andreplaced by wire jumpers, to he able totest the synthcsiscr witho ut th ecomputer runnin g.

T he GLONASS synthcsiser logic mayrequire a single adjustment: the biasresistor for the second I'LL IF amplifierstage . This may be adjusted if theo utput DC voltage dev iates much from1.3V (with 110 input signa l) or if theprogrammable counte r does not operatereliably .

4.9 GPSiGLONASS Second IFAmplifier

Both GPS and GI,ONASS receiversrequire a limiting IF amplifier at thefinal 11' frequency around 10 MHz. Thecircuit d iagram of this amplifie r isshown in Fig.38. The GPS/GLONASSsecond If amplifier includes a firststage with a bipolar transistor, n r X89,and a sec ond stage using the integratedci rcuit CA3089.

T he ga in of the first stage is limited bythe 220 resistor in the emi tter circuit.The firs t stage is followed by a tuned

circuit (Ll) to limit broadband noiseand avo id amplifying var ious spur ioussigna ls from the ma ny osci lla tors insidea GPS or ULONASS receiver . T hedamping resistor in parallel with LIsets the bandwidth of this tuned circui tto he comparable wit h (iPS orGI DNASS signa l bandwidth.

The second stage uses a popular FM IFstrip integrated circu it. The latterprovides widehand amplification andlimiting, while the disc riminator sectionof th is integrated circuit is not usedhere. The limited JF output is availableon pin -S and the signal level amount toa few hundred millivolts at 10 MHz.111is is not enough to drive thefollowing TIl. logic direc tly and theremaining gain is built inside theded icated DSP hardware module .

The CA3089 inte grated circuit includesan S-meter outp ut. This is of little useduring actual rec eiver operation, sincethe sa t elli te signa l l evel s arecomparabl e to noise in the widcbandIf. In the case o f a GPS receiver, theS-me ter output can on ly show the sumof all the signa ls present. On the other

81

hand. the S-meter output is very usefulduring receiver testing and alignment ofthe RP and the first and second IFtuned circuits.

n oth GPS and GLONASS receiversinclude an S-meter function inside thenarrowband IF process ing. Since thelatter is done in software, the realrece iver Svmeter as displayed on theI .CD is just another software functionand is NOT related to the hardwareScmctcr output of the IF strip.

T he GPS/GI.ONASS se co nd IFamplifier is built 0 11 a single-sidedprimed circui t board as shown inPig..]\), with the com ponent locationlayout shown in fig. 40. Due to limitedspace all the resistors arc mountedvc rt tca tty . Th e capaci to rs ar econventiona l disc ceramics (except forthe 22jlf) with a pin spacing of Smm.

LI is wound on a 10.7 Mill. Iftransformer set. including a fixed

VHF COMMUNICATION$ 2/95

Flg.39:PCB La yout for the GPS/( iLONASSSecond IF Amplifier

central ferrite core. an adjustable ferritecup. various plastic support parts and ameta l shield ing can. The primary(resonant) winding of LI has 10 turnsof O.I Smm diameter enamelle d copperwire. corresponding to an induct ivity ofabou t 2JlH. The secondary (link)winding has two turns of the same wire.1.2 is a l OO~LII moulded choke.

The second U; amplifier module hastwo connectors; a 2-pin connector forthe input and a 5-pin for the output andsupply voltage. both obtained frompieces of good quality Ie socket withround contacts.

The tuned circuit with Ll is bestadjusted after the GPS or (;LONASSreceiver is complet ely assembled,finding the maximum IX~ voltage onthe test S-meter output in the same Ifmodu le. The final adjustment of allsigna l circuits is best performed on area l satellite signal obtained from a

1k 100k 2ZjJ-0- .l .L .L 41

220 22n 22n to r 2iT or -<>-:=:1====10NHz ZF:=1,=:1=::: ' 1211INPUT ¢ -n-- -n- -n- -o- 1k TEST

21 1k5~ l 1100' I+o=<> 9)f( r ~089 ~ i :=+~= bOU~~zUTZF

lOOp _11_6FX89 -0. - tl-- 11 -11- 220p

6&k 1'2'OD 22n

Fi~..w: Component Layout for (he GPS/GLO NASS Second IF Amplilier

82


...

• • ··; , e , , I; ,• • •0 0 0 0 - •

. . . ,

- · !r- • .~

il

>>:~ ..-.. .~

'~V>

? ~ I.;:.r-.. . , .. -

.,

~I.1

~ I

"'

I::

~I., ' il " 91

~ .' ::-

T . '"

~ ~ " ~ ~0 _ .... _

... !;' .

··

, ., ,,

83

t- - -"I'

2•

•. ,

lr.l:< r:;:


...,

.. ,.,.j

~,..- ,-."! .• I

•

ft--t----1':

(~

•a : , . , : e•. • . . .• J[.,I .~ '"

,

• - ,•,

r-r-

-~"!t••1:•-

-...."E

I.o ~ "'''' ~

c o n nnu o n.,.j JJ

L..c ~ ? .¢.• '1

" ILY,< I

.... 1:.~--

'II

,. -c

"e .• • , ,· 0 •

j· ~

I · n• :I •· · - : . C' __ _ 01 .... ~

i ~· · .~ · <>OOco <> o o

e • • ; • •• • a~ · U>III ... ~n." l0 c c c •

J JJ

84


directional antenna (a I5-tum helix or asmall dish) pointed at an operatingnavigational satellite. A directionalantenna should provide a higher thanusual signal-to-noise (SNR) ratio ofmore than IW O already available inthe wideband IF. A high SNR isrequired to tune the circuits to thesignal peak and not to noise or somespurious signals while observing thevoltage on the test Scmctc r output.

4.10 GPS/GLONASS DSPHardware

The theo rv of operation of thedcdica ted UPS /GL O NASS nsr:hardware has already been discussed in3 .6 . There fo re t h e p ract ica limplementation only will be describedin the following section.

The dedicated GPS/GLOXASS DSPhardware is built as a peripheral plug-inmodule for the 1)S1' computer (1), (2).The circuit diagram of the GPS/GLONASS hardware IS shown In

Pig's.a t and 42. The nsp hardwaremodule includes two amplifiers for thelimited If signal and master d ockcoming from the analogue part of theGPS or GLO~ASS receiver, a look-uptahle RAM. four signal dcsprcadingmixers, four correlator counters. all ofthe timing logic to scan the look-uptable and generate interrupts to theCPU and all of the interface circuitsnecessary for the DSP computer bus.

The input signal amplifiers are builtwith 74HC04 inverters to amplify theinput signals of a few hundredmillivolts up to TfL levels. In this waythe signal levels in the analogue part of

The look-up tables are stored in a 32k x8 static RAM. The RAM area isdivided into 8 separate areas of 4kbytcs each, selectable through amicroprocessor output port. Tn this waythe look-up table does not need to herewritten when switching 10 anothersatellite. The receiver is usually timemultip lexed alllong four differentsatellites and all four different look-uptables are stored in the RAM. Whenswitching to another satellite. thehardware is simply switched to anotherlook-up table and this only requiresexecuting a few instructions instead ofrewriting the whole 4 kbytc table.

The 4 kbytes of each look-up table arcwritten as bytes by the microprocessor.The microprocessor writes all of thebytes to the same location, since theaddress co unter is inc reme ntedau tomat icall y a fte r ea ch writeoperation. In read mode the look-uptable is scanned hy the same hardwarecounter (74ITC4040) clocked at half thesampling frequency (3069.5 kl'Iz forGPS or 2250 kHz for GLONASS). Thebyte data is latched (74HC273) andthen multiplexed to 4 bits (74HCI57)

85

VHF COMMUNICATIONS 2/95(~ - --- - - - - - - --'-"'-'== = -"==

Fi g.44: 1)(:1\ lA,yoUI for tbe {~ rS/(j LO:\ASS J)sr Hardware (lop view)

to get all of the four required localsignal replicas to he multiplied (EXO R7411C86) wi th each input signa lsample, Each look-up tab le may he rhusup to 8192 samples long. The unusedsamples need not he writt en since theyarc not used by the hardware.

The dedicated DSP hardware requiressix programm able counters: fourcorre lation accumulators. a samplingfrequency divider and a variable delaycount er. all contained in two PD7 1O.54

(82C54) integ rated circuits . EachP0 7 1054 contai ns t hree a lmos tindependent 16·bit counters that ca ll heprogrammed in di fferent ways. forexa mple. the four co rrelat ion cou ntersarc clocked with the same signa lsampling frequency and the signals arefed to the GAn~ inputs, which areprogrammed as dock ena bles,

The signal sampling frequency (6139 or4500 kHz) is divided down 10 1 kHz tomatch the CIA-code period (1 ms).

86


Flg.44: PCB Layout for lhe GPS/GLONASS IlSP Hardware (bou om view)

This signal is also used 10 requestInterrupts from the CPU. since thecorrelation counters need to be readeac h millisecond. After receiving aninterrup t request. the CPU will latch thecontents of all counters in a single busopera tion and then read the latchedcontent of every single counter inseparate bus operations. The sampiingclock divider is also latched and readand its content is used as an accuratetiming reference.

Every interrupt request sets a flip-flopthat needs to be reset by themicroprocessor after the interrupt hasbeen serviced. Interrupt ann (cnable)and reset (disable) is performed throughone (04) of the eight output port bitsprovided by the 74HC259 addressablelatch. Of the remaining 7 bits, three(Ql , Q2 and Q3) are used 10 select oneof the eight look-up tables in the RAMand another bit (QO) is used to selecteither write or read mode for thelook-up table logic. The lasl three bits

87

VHFCOMMUNICAliONS 2/95(~------------"-"-===-"==

GND ~~~~~ H~HsraDATA f".' -!I- lOOn

'" 00(ilQNASS '" f;l~~LDU lO ~G ~ l!J

ooo

+lOOn I

w~[i]OO~

o HS(H~~01

8 - 10k

HC245

-'-T 100n

I 553NV

I I

Fi~..a5: Component Lecauon Ior the GPS/GLONASS I)SP Hardware

(Q5 , Q6 and Q7) arc used to control theGI.ONASS PI.L modulus or as sparesin a GPS recei ver.

The bus interface \0 the DSP computerincludes a hi-directional data -bus buffer(74 HC"245) and an add ress se lectionlo g ic (tw o 7 4 11<": 138 and o ne74HC245). The bus interface docs notrequest any wait states from theMC..68010 CPU. The address decodingfo r the PO? )054 programmab lecounter s must allow simultaneous writeoperations to both control registers ofboth peripherals. to be able to latch thecontents of all of the counters atexactly the same time. Finally. theRESET signal is fed to the 74IK.'259addressable latch esscnnau y to prevent

BB

any interrupts or other unintendedoperations before the whole DSPhardware is correctly initialised.

'I'he bus addresses are assigned asshown in Tahlc-l . However, one shouldnotice that the remaining addn:sscs inthe range (rom SEO(X)O to SfFFFf arcnot fully or correctly decoded, althoughthe module will ackn owledge theaccess to the MC6801O. Accessingother addresses in thi s range (citherreading from, or writing to) willprobably cause an erratic operation of(he whole module. Tbe 74I1C2.'i9addressable latch is programmed bywriting to the specified locations. Sinceonly the address is important and thedata is ignored. G..R.B instructions are

The bus addresses are assigned as follows:

$EOOO I$EOOO3

$EOOO9$EtXXlBSEU041$E0043$E0049$EOO4B

$E0081

$EOO83$EOO82

$EOOR9$r.omm$EOOC 1$EOOC3$E<XIC9$EOOCB

$E8IX lH

$E8041$E8043$E8049$E804B

$ESOS !$EROX3SE8U89SE808B

SE800 1

Disable look-up table write modeEnable look-up tab le write mode

Look-up tab le address A2 resetLook-up table address A2 setLook -up table address A I resetLook-up table address A l setLook-up table address AO resetLook-up table address AD set

Reset and disable l ms interruptEnable 1ms interruptReset and enable l ms interrupt. long transfer!

GLOKASS PL!. modulus STROBE resetULONASS Pl.T. modulus STROBE setGLOKASS PLL modulus DATA resetGLONASS PLL modulu s DATA setGLOr-;'ASS PI.L modul us CI.OCK resetGLONASS PLI . mod ulus Cl .OCK set

Common write to both 7 1054 command registers

71054 #1 C rRO - data 0.4 accumulator71054 #1 CTRI - data 1.5 accumulator71054 #1 CTR2 · data 3.7 accumulator71054 #1 command register

71054 #2 CTRO - variable CIA-code delay7 1054 #2 CTRO - (iPS 16139 or GLONASS 14500 dock71054 #2 CTRO - data 2.6 accu mulator71054 #2 CfRO - command register

WRITE byte to look-up table

used 10 write to single bytes and aCLiU . instruction is used to reset andarm the interrupt flip-flo p.

The dedicated DSP hardware board isbuilt on a double-sided printed circuitboard as shown in Fig's,43 and 44, withthe co mponent overlay shown inFig,45. The single resistors. diodes andthe 100 J.111 cho ke are inst all edhorizonta lly. The eight lOkU resistors

arc in a single 9-pin SIL package. Thecapacitors are ceramic except thel OOfW tantalum. and all have a spacingof 5mm.

Thc 74HC4040 sho uld NOT bereplaced by the standard 4040 device,since the latter is too slow for correctoperation in this circuit. To allo w fortroubleshooting it is recommended thatat least the two 7 1054 counters and the

89


PLL modulus contro l and an 8-pi nsocket for the interrupt select ion. All ofthese are made from parts of goodquality Ie sockets.

The dedica ted DSP hardware modulerequires no ali gnment or sett ing up.

(to be continued)

Details of kits and PCR's fo r thisproject appear on page 127 of this issue

(t-. -!.!l!.-"""==-"""'~

43256 RAM are installed in goodqualit y sockets. The speed of the AM isunimportant, since even the slowest150m static Ram devices arc fastenough for this project.

The ded icated DSP hardware modu le isinserted in the DSP computer bus witha 64-pole Eurocard A+C connect or.The remaining connectors include a5-pin socket for the IF input signal andd ock , a a-pin socket for the Gl.ONASS

VHF COMMUNICATIONS PROJECTS

KITS AVAILAHILITY UPDATEKit Issue Description Item No. Pricc

DC8UG·PA 3/94 SW PA for 13cm 06938 £266.00Com plete kit with LP's

DJ8ES -OI9 4,193 281144 'Fransverter 06385 £ 139.00

DJ 8 1 ~S -201 2,194 13cm FM AT V Exciter 06 3lUl t: 70,m

J)JI6NT-OOI 4f) ] Widcband Measuring Amplifier 06382 £ 65.rXI

KM Publications. 5 Ware Orchard, Barby, Rugby, CV23 SUI"Tel : (0)1788 8911365 Fax: (1I)1788 891883

Email: [email protected]

90

Mmju : Vidmar. S53MV


Part-6

Th is pa ri or th e ~ries continues wilhlhe cons truction derails tor the G PS!ClLO NASS Pnrhtblc Receiver CPU.the 8.kcy Keyboard. th e LCD DisplayModule and the Power Supply a ndReset cireuilry. SUj:!. j:!.cstcd mount ingar ra ngements for the var ious modules arc also discussed .

·t tl C; PSfGL01\'ASS Portabl eReceiver CPU

The n!'s or OI.ONI\ SS receiv er described in this series can be built as aninterface for the nspcomputer l t l and121, or as a stand-alone portable reccivcr. In the tarter case the receiverneeds its own microcomputer with akeyboard and an I.CD display, Afterconsidering severa l possible ahcm arives. the siruplee t solution resulted inusing a suitably modifi ed CPU board nsdescribed in [I] ami [21 as the microcomputer.

The circuit diagram of the modifiedCPU board is shown in Pip!'> ,46 and 47 .Since 11 C;PS or GLO;.;rASS receiver is a

portable unit. the power consumption i.san importa nt factor and consequently74IICxx logic devices should he usedeverywhere, This allows for the omis sion of three 3.3 kD pull-up resistor!'> ,The original VSP com puter CPU hoardrequires the following modifications:

The pad!'> below the EPROM socketshould he connected so that pin-27receives the " 14 sil!-nal requi red hythe 27(: 256 EPRO M. Origina lly thispin is connected 10 +5V when usin gthe 27128 EPROM.

2 Th e RA~ should he Increased from64 kbytes up to 12R kbytcs . This isachieved by piggy -bac k solder ingIWO addit io nal 43256 RA M chips onlop of the e xivting IWo RA M chipson the CPU board . /\ 11 pins of theadditiona l RAM chips are connectedin parallel with the existing RAMchi p pins, except for pin-20 (chipselect). The IW I) chip-select pins ofthe additional RAM chips arc thenwired 10 pin-ll (Q4) of the middle74JIC138 address decoder.

153

VHF COMMU NICAl iONS 3195(~ - - - - - - - - - -----"-"'-=== = ==,. .. 4 I~., ~ '~

I', ",

:=.-li Fr ~~a~; ~

--•t:~

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~154


~,.

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,"•

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155


F'i~.48 : GPS/GLONASS CPU. upper side (lop "jew)

3 An HSCH I00 1 Schottky diodeshould be conn ected between pin-l I(<")4) of the tor 74Hf:13 S addressdecoder and 'h e DTI\C K signa l(pirJ-12 of the 74IIC05), to acknowledge the additional RI\M chips 10

the CPU.

The keyboard connection remains unchanged. Th e total /partial reset switchhas a new function with the GPS orGI,ONI\SS software. In the case of astand-alone porta ble receiver this inputshould al~ay~ be left open (+5V)!

The parallel output port (PD71055channel-B) is now used to comm andthe HD44780 LCD controller. Sincethere are only S output bits available,the HD44780 is driven in the 4-bitmode - write only. The real-time clockchip PD4990 is required by the GPS or

156

GI,ONASS softwa re and must remainin place. The IN'I7 jumper must remainin place for keyboard Interrupt requestswhile the INTI jumper is no longerneeded, although il may left in place.

The printed circuit hoard is not modilied, as shown in Hg.~.48 and 49, theaddit ions and modificat ions arc fullyvisible on the component location planin Pig ..'iO. The connections of the S·keykeyboard and the LCD controller arcalso shown in Fig.50.

It i ~ recommended thai the CPU hoardi ~ tested first, if possible, in a DSPcomputer. and then modified as described above only when it is fullytested and working 100%. In parti cular,the CPU board should be tested athigher clock: freq uencies to find anydefect ive components. A 10 MIll. ver-


••Fig,-l9: (; rS!G LO~ASS CPU. lower side (bou om view)

sion of the MC68010 will u_~u3 I1y worklip 10 a clock frequency of 15 Mil, OIl

room tempera ture. so a 12 MHl clockcrysta l is a safe choice. The (iPS or(i LONASS software docs not requ ire..uch a high clock frequency. till! theaccuracy of some measurements ishigher and (he updating of the displayis faster OI l higher c1IK:k rates .

-l.12 M.kry Keyboard

A portable GPSj( iLO!'lASS receiverrequires OJ small keyboard 10 issuecommands to the com puter. Since a fullASCII keyboard is impract ical for aportable piece of equipment , a smallx-kcy keyboard was developed forportable receivers.

The circuit diagram of the 8-key key-

board is shown in Fip:.5I , The 8 keysclose towards ground. otherwise theinput lines arc held high by pull-upresistors. A priority encoder (74HC I48)is used to encode the Kkeys into .3 bits.A double moncaaole (74HC45.38) isused to generate the strobe pulse after akey is depressed.

The s-kcy keyboard is assembled on asmall single-sided printed circuit boardas shown in Hg.52. This printed circuitboard was designed to fit on the frontpanel of the rec eiver and carry eightsqllarc (12 x 12.7mm) push-buttons.The location of the components isshown in H p:.5J . Due to the spaceconstraints all of the componentsshould have a low profile and smal ldime nsions. The capacitors have a2.5mm pin sJ"'3cin l and the resistors are

157

VHF COMMUNICATIONS 3/95(~ -------------'-"'--""=="'-'-"=~

"....~ ...... "i'~~"JCI" '-'

Hr."4~iI~ I I

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FiR50: GPS/GLONASS CPU. Component Ov~r18Y

01nt00

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t,OO" LI ,,0< r -•~e-

I 74HC4')18l' «." r ,.,

•~ " n"!. ,

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FI~.51 : C1rcuil orthe S. li:ey Keyboard

158


Fi~.52 :

g-key KeyboardPCB (bot tom view)

installed horizontally . The 8 x 10 kOresistor network in a SIT , packageshould be first soldered in place andthen bent towards the circuit boardTh efi-pin connector is installed on the back- so lder side of the hoard!

The GPS/GLONASS receiver softwareonly accepts ASCII characters from $30to $37 as commands, corresponding tonumerals 0 to 7. These can be generatcd by a standard ASCII keyhoardwith a parallel output. or by suitablywir ing the 8-key keyboard described. Inparticular. to obtain the codes between$30 and $37. the outputs DO. Dl andD2 should be wired to the correspond.ing inputs on the CPU board. Inaddition. D3, D6 and D7 should heconnected to ground and D4 and DS to+5V. All these connect ions, includingthe supply rails and the strobe signal,were already shown in Fig.5O.

4.13 LCD Displ"y Module

The only practical for a portable GPS/GLONASS receiver is an LCD modul ewith built-in drivers. Such modules arcavailable in many different shapes and

sizes, and mayor may not he equippedwith a display controller. LCD moduleswith a built-in contro ller are easy tousc, since the interfacing to any microprocesw r is very simple, it is identica lto a parallel I/O port.

Most small dot-matrix alphanumericLCD modules use the Hitachi HD44780LCD controller. This integrated circuithas an on-board, extend ed ASCn character generator, a display area RAMinclud ing up to two rows of 40 characters eac h and all of the liming circuitsrequired to drive the LCD. Modulesu~jn~ the 11044780 controlle r may havea different number of characters perrow, or rows displayed. the totalnumber of characters is, however, limited hy the internal RAM to RO. Sincethe HD44780 SMD flat package hasonly RO pins. addit ional l .Cl r driverchips (lID44 ICX) are used in most LCDmodu les.

AN LCD module with two lines of 40ASCII charact ers each was selected forthis project. Such an LCD moduleincludes the liquid crystal display itself,one HD44780 LCD cont roller and fourlI D44100 LCD drivers. Further. the

3» rr ~3(lH Is, / " I" /'0~ ~ mHm ~.

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55 l MV

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159

VHF COMMUN ICATIONS 3/95(j- ------- - - ----"-"-"-""""""""''''''''''''''''''

...12V120~

, 470~

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III TRIG 2 10kTRE 6

S53MVOND

9/2NV1

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Fig.::i4: LCD Raeklight Power Supply

MSB data

LSB data , 4-hit nus

5S3MV

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-ir,20p -l~ ,o p

Ground+5V supplyLCD vo ltage. wiper oftile contrast potRegister Select,o= instruc tion, I = dataRead/Write, () = write,1 = readEnable, 0 = inactive,1 = act iveLSB data , x-hit bus

Pin-l Vssl'io-2 VddPln-J Vo

Pin -4 RS

Pin -5 RfW

Pin-6 E

Pin -7 D130Pin-R DBlPin -9 Dfi2Pin -IO DnJPin-II DB4Pin -l2 DB5Pin-I) DB6Pin -14 DB?

display module may include some formof illuminating the LCD, either EL foilor LED. The tarter is recommended ifthe GPS /GLONASS receiv er is to beused at night as well .

Alth ough such display modules areavailable from severa l different manufac turcrs, tile print ed circuit boards onwhich they arc mounted all have thesame dimensions: IR2mm wide x 33.5high x 1.1mm thick and all have thesame 14-pin electrical connector. Thepin numbers are usually marked on theprinted circ uit board and the pin allocations arc as follows :

Fig.55: LCn Backlight Power SupplyPCB (bottom view)

Fig56: LCD Backlight Power SupplyComponent Overlay

160

VHFCOMMUNICATIONS 3/95=-======""-------------(~

1NIB1

C' SV

<CMOS

1k1

. <_ NjCd,- :=::: 3.6V

-I 60mAh

3k3 1k5

; vv' 100IlH

100 H

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<121,1

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L __~~~_ 81,12

6k61--,-- - '" pEn

1k2 /1::~B(~ aa

1k2 238 47k

F ig.57: Power Supply and Reset Circuit

The EL or LED back-light may havetwo additional pins on the connector orsolder pads on the printed circuit board.

When LCDs are driven in time multiple x the adjustment of the voltageapplied to the I .CD is crit ical to obtaina good contr ast. A con trast controlpotentiometer is usually provided toadjust the nest available contrast for agiven viewing angle . Thi s potcntiomctcr provides the Yo voltage to theLCD module . Modem LCD modulesrequire operating voltages of less than:'iV, so the resistor termi nals ca n beconveniently connected to ground andto +5 Y, whilst the wiper is connectedto U1e V o input.

Due to the inte rna l circuits o f the LCDcontroller and driver chips, the internalI.CD ground is vee (+5V). The volta geacross the LC D equals the potenti aldi fference between Ydd and vo. Thecurrent thro ugh the Yo te rmin al Is verysmall. so a 10 kG linear potentiomete r

is suffic ient for the LC D contrastcontrol.

If usin g an LCD module with an ELfoil hack -light, a sui table power supplyneed s to he built. The J::L foil usuallyrequires a suppl y voltage o f approximate ly 1I 0 Y at around 500 lI/" Th(~ ELfoil behaves electrically as a lossycapacitor. T he vo ltage across its tcrminals affects the amount of light pro duc ed, whil st the frequency affe cts theCOIOllf o f the light.

Fig.58: Power Supply and Rese t PCB(bottom view)

161

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,,-: -

Fi~59:

Power Supp ly andRN:l CtreuuComponent O"crlay

,

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G"" HF. Tfl l• GPS< ZF +Tell, 0~ ~N.N>

An..,( ht von untl'n

Fil!_60:Portable (IPS Receiver:\1odlllt" Location

162

VHF COMMUN ICATIONS 3/95

A suitab le power supply is shown inFig.54 and includes a power oscillator(555), a step-up transformer and a fewEMI filtering components. For 500 Hzoperation a conventional mains transformer with a laminated core can beused, either 220V19V or 220V/6V, ofcourse with the primary and secondarywindings interchanged . A 1.2W or

Oraufs i(h t

GLO NASSLoqik

,, ,, ,Synt hesizer :(untpnl I,,,

'---+- - - --~

,-- - 1-- --,,Stromvers rgung Iund RES T :l unten ) I,

1 LCD Module

fr ontplc tte

rcu -e ccro(ob ~n l

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-

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GlONASS Pll Synt hesizer

Gl ON ASSZF - Teil

Abs(hlrmung

GLON ASSZF - 'lail

Fig.61:Portable GLONASSReceiver ModuleLocation

163

the load. Since the osc illator feedbackis taken from the output (pi n-1 of the5:~5 ) , some 555 IC:s may not operate ina stable way in this circuit. In the latterease the solut ion is to increase theoscillator frequency hy decreas ing the100nF capacitor.

Of course, an LED back-light is mucheasier to me and usually only requires a5C DC supply.

VHF COMMUNICAn ONS 3i95

01.101 Power Sup ply e nd ResetC irc uit

The GI'S/G I ,O~ASS rece iver is intend ed to be operated from a 12Vbattery, with the negative grounded.This supply Voltage is common 10 allportable and mobile equipment.

The ana logue circuits of the rece iverarc already designed 10 operate from a

_ .MENU KEYS . DISPLAY FUNCT ION

KEY # 4 KEY # 5 KEY # 6 KEY # 7

lCODE $ ~" J (CODE $ 35) (CODE S 35, rccoe S 37;

SHOW ax S"O\'/ SHOVi ""cUiMENU , CHANNEl 1"''''EUIATf A" . HAGfU su -cs-

OATA NA ~ DAI A "'A~ o"' ~ " '.."""A·~

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PRE SSI'Ia

ACTI ON ' 'I CRto Mf~ I'"" IAFTE R Ill( CHANNH ' 0 vo ' .. '0011:1. <REPEAH O NUMBI' A ACTION A C 1 I C ~ , r r l L IT ~ ;

PRESSING { ", 3 , ~ 1,2 , A. VA' . AC

<, /' /' , I .:/

sr- A" L

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P'E;, I ' VE; '

I -WI- «ev DECREMENt SH LOti "......•

sr - V A"W " CHANNEL IF

DEGAEESiDECIMA.. FRACTIONS"'-ELLITE

~ (CODE ) S E LE C ~

< ' " 11 ' Hz STEPSIOISPl./l.Y <OA"' AT

~' C [)E

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( ;~F )GAUSS ~ 'l :)GE 'l

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( f .Hz STepS! DISPLAY FORMA~ IoIODE

164

wez«Io>«~

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II-

eZZ[«(>';••62,~ GPS/GLO!'OASS

Receive r Status

+12VC supply rail , since thi s voltagerepresent s a convenient choice .

Of course, digita l circuits req uire a +5Vsupply voltage, but bes ides the +5Vthere are other requirements. A GPS orGJD NASS receiver should include areal- time clock that operates even whenthe receiver is powered down . Similarly, the almanac data including infor mation about the available satelliteorbits should he stored in the compute rmemory when the receiver is powereddown.

Finally , since the +SV power drainam ounts to about one hal f o f the totalpower drain of a C;!.lS or GLONASSreceiver , the +5V supply re gul atorshould also have a good effici ency,especially in a portable receiver .

The requirements for the mic roco mputer power suppl y are therefore thesam e as for the DSP computer published in l1J and l2 1. The original DSPcomputer power supp ly is howeverabout 10 times too large for thisappl icati on. so a scaled-down version isshown in Fig.57. The latt er includes aswit ching regulator from 12V to 5V, amemory (clod) backup batt ery and avery rel iable RES ET circuit.

The microcom puter pQlVer snpply isbuilt on a single-sided prin ted circuithoard as shown in f ig5?; . The correspo nd ing component location overlay isshewn in fig.59 . Ail of the resistors,diodes and chokes arc insta lled hori zonta lly . All of the capacitors have aSmm pin lead spa cing.

Several mounting holes and relatedpads are provided for different styleNiCad batt er ies.

4.15 (, PS/GLO NASS receiverModule L oc al inn

A (iPS or (jI.ONJ\ SS rec eiv er includesborh low-level Rf signal arnplif k-ationand processin g and very nois y di~dtal

ci rcuits. so the module location has tobe selected carefully and some shielding is required in any r use.

In the case of a (iPS or CiLON AS S

receiver operating as a peri pheral forthe DSP computer, the Rf part o f thereceiver shou ld be built in its ownenclosure, whi le the dedicated DS!'hardware module is plugged into thecom puter bus. Of co ur se, it is assumedthat the computer already has its ownshield ed enclo sure.

The GPS receiver Rf unit needs noadd itional internal shields among thethr ee modules: RF, IF converter and 1lamplifier. The GLONASS receiver RF

165

VHF COMM UNICATIONS 3i95(~ ------------'-'-"-'~"""'''''-'-'''''''''-'''''''

unit is more rornpliru tcd and rcquircv.~OIll C shielding . i ll particular. the ( ;1.0 ·Ni\ SS 1'1.1. synthcciscr log it, needs 10

be wel l shie lded from the rema iningmod ules: RF. IF co nverter. IF amplifiera nd 1'1.1 . convener.

In t il t' ca cc o f a stand-a tone porta blec;flS or (iI.O NASS rccc i..-cr it is ofcrutrs c' dcvirable (0 ha ve th e com pleterece iver packa ged in one single cnclosure . It i.~ suggested that th is encl osureid fabrica ted from unpainted aluminiumshee t to have a good electrica l cont actamong the various parts. Such a con .lain cr is made o f a rect angular frameand two ( 'OVI,.'rs installed with wlflocld ng S(' f l' WS .

The (ram " has an addit iona l inte rna lpla te that div ides 11K' inte rna l volumeinto two sections, shielded beth fromthe outside and each other. One of nrcsect ions is used for the noisy digit;l!circui ts and the other lor the low-levelRr stages .

Non-I nterfering modules, such as thekeyboard and the power supplies, mayhe install ed in e ither sect ion.

TIK' s ll1!gr st ~'d modu le locat ion for il

portable (i PS rece iver is shown infig.60, The suggested dimensions arc2rXlmm wide x ]oommdeep x 80m mhigh,

T he interna l plate is insta lled at aheight o f 30mm, so that the digitalsect ion has a volume of 200m m x160mm x 50mm (top) and the analoguesection a volume o f 200mm x 160mm x30mm (bottom), The internal plate tsscrewed onto the frame on all foursides with many screws to ensure agood elect rical shielding.

The two modules with (14-pin liurrx-atdcon nec tors arc insla lk d on :1 .\ 11011 husmother board with juct two fem ale con ncct ors with the correspon ding pins tiedtogether. The hns can he made hy" ull ing a piece o f the DSI' com puterbus hoard. or hy "imply i n sl ;l lh n l~ twoconnectors on a piece o f a univ ersa lhoa rd with hole s in a uniform 0 .1 pitch.

The connections between the anal ogueami di)!it:ll uni ts do nOI requ ireIeodtbrough capacitors if they arcrouted ca refully . away from the scnslt i v (' or V ( ' ry noisy com ponents. SOIllCtimes it is ;11.'0 o f b enefit to addiuonall yground the coaxial cable sbic klv whencrossing the internal scree n.

Th e suggested module 1IIl'31ion (or aporta ble (TJ -ONi\ SS receiv er is .•hewnin Fi!!.6 1. T he sugpcstcd dimensionsare 24(hnm wide x ]60mlll deep xxnmm high.

Th e internal p late is insta lled at aheight of 30mrn, so that the digi ta lsection has a vo lume o f 240rn m x160rnm x 50rnm (top) and the analo~lI e

section 240mm x )(j()mm x 10lllm(bouom). As with ti ll' GPS receiv er theinternal screening plate is screwed ontothe frame on a ll four sides .

In add it ion, there is a small shie ldbetween the RF module and the remaining module, in the analogue S~T

t ion . On the other hand, in the (TI.ONASS receiver Ihe computer po wersupply is installe d in the digita l section,toget her with the PLL synthcsisc r logic.

A list of PCBs and kia for this projectcan hi' found on page-l v t of this issue

(References for this projec t can hefound overl eaf)

166


7.REFERENCES

[I) Matjaz Vidma r: "Oip: ilal Si!'nalProcess ing Techniques for Rad ioAmateurs , Pan -:!: Design of a nspCompute r for Radio AmateurApplicaliom";VHf Commu nications 1/9 1.pp 2-24

121 Matjaz Vidmar: ~t)jJ! i la l SignatPr rcc....in!! Techniques fOT RadioAmateurs. Part -S: Construc tion anduse of the nspCo mpute r";VflF Communications 2/Hf) ,fir 74 .94

131 Jonathan S. Ahd. James W.Chat fcc : "Ex iste nce andUniqueness o f CPS Solut ions";ra pes 952-9SfJlf)-9 I , Vol. 27 IEEET ram. o n Aerospace andElectronic Systems

(4) "Interface Control Docu mentMII0 8.f lOfIU2-4{)fI. rov-E". (84pages). Aup:U<;f 7th. 1975.Rod.well Internationa l Co rporalion. Space Division. 12214La kewood Bo ulevard. Downey .California 90241. liSA

151 "lmerrecc L'omro l DocumentGl'S-2 IJO", (102 pa~e~) , I\uj!u~t

71h, 1975,Roc kwel l Internat ional Co rporalion, Space O pcra tiom andSate llite Systems Div ision, 12214Lakewood Boulevard, Do wney,Californ ia 9024 1. lJSA

16J ~( ; I oha l Satel lite Navigat ionSystem ( if .O NI\ SS InterfaceContro l D ocument" (46 pages).

191UI. Research-and-Product ionAs~iation of Applied mechanics,Institute of Space DeviceEngineering. GI.nkmnux , Russ ia

(7] Robert C. Oixon: "SpreadSpec1J1Jm SYQcms", (4 22 pages),1984, Second Edition, John Wiley& Sons, New Yori:, USA

181 Malj:17. Vidmar: "(li ll.iu l SignalProcessmg Techniques for RadioAmateurs, 'Ih coreucal Part ";VI Tf Com munications 2/88pp 76-97

[9] P. Mattos: "(i lohal Pos itio ning bySate llite", (16 pages ), lnmos.Tec hnica l note 6.5, July 1989

IIOJ J. D. Thomas: "funct iona lDescriptio n o f Sif!: na l Processing inthe Rogue UPS Receiver". (49pages), June I, 1988, JetPropulsion Labora tory , Ca liforn ialnstirurc o f Tec hnology. Pasadena,Ca lifornia, USA

1111 Chou b C. Kilgus: "Sha f'l.-'\IConica l Radiat ion Pattern of theBackfire Quadrifitar Helix ", (page."391·397), IEEE Tra nsactions onAntenna.. and Propagatio n, May 75

(121 Matj;u: Vid mar. "II. Very LowNoise Amplifier for the I.-nand ";VIIF Communica tions 2/92pp 90-%

(13] M3tja/. Vidmar: "Rad io Ama teu rApplications o f G PSKiI .ONI\SSSa telli tes: Using GI'S,K;LONASSSa te llites as an AccurateFrequency / Time Standa rd";(pages 186· 190). Scriptum del'vonraege, 37 We inhe imer UKWTagung , 19/20 Se ptember 1992

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VHF COMMUN ICATIONS 4/95(~ - ----- - ------""--==='-'="-==

Matjaz Vidmar, S53MV


Part-7 (conclusion)

In l his the Final part of th is projectthe Receiver Software is desc ribed,d<'1 l1il ing t he Real-Time 11lSk", MainProgra m-Loop tasks and the Sortware Menus and Commands.

S.(;J'S & (;I.ONASS RECEIVERSOFTWARE

s.t GPS/( ; I,O ;-.JASS ReceiverSoftware Overview

Satel lite nav igation is one of the firstapplications that totally depends on theava ilability of suitable com puters andthe corresponding softwa re. Althoughini tially the digita l compute r was onlyintended to solve the nav igation equation s, other tasks were being graduallyadded to simplify the hardware in frontof and behind the comput er itself. Inthe GPSjGLONASS receiver described

194

even most o f the signa! pruccssi ng isperformed in soft ware, just to keep theanalogue front-end and dedicated DSPhardware as simple as possiblc .

The softw are running in a GPS orGLO:'-lASS receiver is therefore verycomplex and includes a variety of verydifferent functions . For exam ple. dig italsigna l processing requires qu ick hutsimple integer urlthrnctic, while so lvingthe navigation equat ions requir es higha(TUraey floating-point arithmetic . Thelatter docs nut need to he as quick asthe forme r signa! processing, hUI aconsiderable number of ope rations stillneed to he performed in a limitedamount of time.

To ma ke a fair comparis on one shou ldconsider the development time for thehardw are and for the softwa re.

In the cas e o f the navi gation receiverdescribed here, the software requiredbetween tw ice and three time s as muchtime to develop than the hardwa re!


t .ntortu narel y. it Is much more diffi cult10 descri be the software down to thesmallest detai l than it is the hardware ,For the hardw are one can draw theci rcu it diagram s and prepare det ailedparts lists, On thc other hand, detaileddescriptions of the software tend tobecome boring and minor detail s tendto hide the real problem being. solved .

Therefore, only the major functionsperformed by the so ftware will bedescribed in th is article. These includesigna l acquie uion and proce ss ing. almanac and precision ephem eris data co llccnon, time and frequency measnrc mcnts, soJvi n~ the nuvigarion eq uationsand data display in a suita ble format forthe m er. At the e nd the user inter face .displ ay me nus and use r commands willhe descri bed in det ail.

The ove rall software is writte n indifferent language s due to the differingfunctions to be performed : M('MW \ Oavwmbly language, DSP comput erhigh-level langua ge and even directlyin machine rodc.

Thc digi tal signal pron'~smg softwa reis wriucn in the MC68010 assemblylanguage . Till' COlTcspe nding fill' hasthe ex tcnsiou .ASM. This file is firstcompiled into mac hine code and theninto hexadecimal forma t, <;,0 that it canbe eas jly inserted in the DSP com pute rhig h-le vcl Ianguugc.

The orbital mech anics and nav igationequat ion pan i ~ written in the ])S Pcomp ute I high-l evel lang uage . The latter supponc :I floating -point forma t witha 32- t'li t mantissa and l o-bit exponent.A 12-hit mantissa is gene rally sufficientconsidering the accu racy of the da taobtained from the GPS or G1.0~ASS

sate llites . The corresponding file has anex tension ,SRC and ca n be compiledinto a .EXE file and executed on a I1SPcomp ute r equip/X'd with the describeddedicat ed DSP hard ware board, hutwith an unmod ified CPU board!

In a portable GPS/G I,ONASS rece iverall of the software is stored in a27C25 6 EPRO :..1. T he latter incl udes astart ing program. the high- level tan gnage compiler and a version of the,SRe file with all of the comments andUlilll'cl's sary symbols remov ed .

When the portable rece iver is turn ed 0 11

the program is compil ed in the RA M,Th is operation takes around 10 secondsand is necessary to save EPROM space,since the compiled program in 1I11'RA \.1 take s around JOO kbyt cs.

The present discussion app lies to thecu rren t software vers ions v l n (Gl'S)or V39 (GLONASS) , When r unning theso ftware on a DSP computer the type o fd isp lay may he selected by the·r U r AI. /PART IAI, RI ~SI ~T switchwhile starti ng the program : switch 0 (lC ll

(TOTAL RE SET) selec ts the LC D,while switch d osed (PAR T IAL RJ ~

SET) selects the C RT display . Ofcourse, the LC D can on ly be se lected ina stand-alone portable receiver. and thecorresponding input 011 the CPU hoardMUST HE LEFr OPEN !

S,2 RCHI-Ti mc Tasks: SignalAcq uisitio n lind Processing

The signal acquisition and process ingtasks run under the I "Hz interruptsrequested by the dedicated DSP hardware module .

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VHF COMMUNICATIONS 4/95(~ --------------'-"-"-"~~~~~

T he tasks inclu de :

- Multiplex ing of the single-channelhard ware among four different satcllites,

- C/A -rode synchronisat ion acqu isition and track ing,

- Carr ier lock acquisition and track ing,

- T he 50bps navigation-data dem odulation.

Bit synchronisa tion and frame synchronisat ion with purity check

- Averagtng of the me asured codephase, code rate and carrier Irequent'y,

Aft er au interru pt request is receivedfro m the ded icated DSP hardwa re. theinrcrrupr-scrv icing routine will firstlatch the contents of all the counters inthe ded icated hard ware . It will thenread the latched content and reset andarm the interrupt request flip. flop, Thehard ware counters are never reset. Theactual integ rated value is computedfrom the difference betwee n [he actualcounter content and the previous ....mpic content. The four difference s arcfurther normalised using the result fromthe re ference I ms div ider, Finally, theinte rrupt -servicing routine also incrcrnem s :I 32-bit mil lisecond counter.which is later used ro relate the measurements to the 50 bps navigation data .

Th e single-channel hardware is multiplexed tlmong all of the sate llitesreceived. However, due to the lim ita-_no ns of the hardw are. switching. toanother sate llite will corrupt one milliseco nd of data . T herefore. the basic

multiplex ing period incl udes one millisecond to sw itch the hardware fo llo wedhy 8 milli seconds to co llect the datafrom a given sate llite , A fter th is l)

milli second period the hardware isswitched to another satelli te. The multi plexing rate is therefore 111 hops persecond

The mul tiplexing rate and especia llythe mu ltip lexing sequence have to be

chosen carefully. The navigation data istransmitted at a speed of 5Obps. so onebit is 20Ins long and lasts exactly 20interrupt periods, If the navigation datais to be co llected from a given sate llite,thcn this sa tellite should get at least afew l rns samples o f data from each20ms bit period. Further, the multip lexing period should not he an integersubmultiple of the hit per iod, so thatthe hit transit ions can be detected .

Considering the limitations o f the singlc-channel hard ware, the mult iplex ingsequence can not allow the collectingof navigati on dat a Irom more than twosatelli tes at a time. In practice, sincefour satelli tes need to he received for anavigation solution. the navigation datacan o nly be co llected from a singlesate llite at a time usinp: half of thesingle-c hannel hardwar e time. The remaining hardwa re time is split among

the remaining three satellites. Theprivileged satellite that gets more hardware time of cours e needs to heperiodically exchanged to allo w co llecting o f the navi gation data from all foursate llites.

There is yet another constraint on themultiplex ing seque nce. If a certainsate llite on ly pets a few sam plingperiods. then fa lse locks of the carrier

196


recov ery loop become vel)' likely . Inorder to avoid this, the fo llowingmultiplexin g sequenc e is used in theC;PSjGLO NASS rccc rvcrs llJ thisproject (1 = privi legcd satellite; 2, 3 &4 = others):

121213141412131314

121213141412131314

T he complete multiplexing sequencetherefore repe ats after IS mult iplexingper iods or 162 milliseconds .

Thc CjA-code synchronisation is always obtained from the signa l magnitude obta ined from the ded icated hard ware . T he signal phase information isinte ntionally not used for this purpos esince ~he carrier phase lock is a muchmore cr itical operat ion . Therefore, forthe CIA-code synchro nisat ion, the earlyand late magnitudes arc computed fromthe related I and Q sum s for every lITI Saccumu lation period. T hese sums arcthen averaged over the R milliseco ndscontaining valid data in a 9 millisecondmu ltiplexing period .

The initial state of the receiver isunlocked and the CjA~code synchronisat ion has to be obtained first. Thehardwa re variable delay will thereforebe scanned through all possible C/A-code phases (1023 for GPS and 511for GLONASS) by increment ing thevariahie -delay counter in suit able steps(6 for GPS and 9 for GLOKASS).W hen a signal magnitude abovethreshold is detec ted, the so ftwareswitches to the synchronisation main-

197

VHF COM MUNICATIONS 4/95(~--- - - - - - -----'"""-""""""""""'''-'''''=following multiplex period. accountingfor all of the time spent by thehardware processing the signals fromthe other satellites. In a navigationreceiver the code phase and the carr ierfrequency are the main parameters tobe measured , and these arc supplied bythe corresponding phased-locked loops.In addition to this, the code rate is alsoused by the software to compute arough approximation for the carrierfrequency and eliminate the ambiguitycaused by the I kfIz signal samplingrate . Before further processing, thecode phase. code rate and carrierfreque ncy arc averaged over 16 mult iplexing per iods correspond ing to a timespan of 2XKms (privileged satellite) orS64ms (other satellites) . The averagedmeasurements are placed in a FIFOmcmory together with I ms time tags tobe read hy the main program.

The last task performed by the interruptrout ine is navigation data processing.The latter includes yet another I'LL forhit synchronisation. This I'LL locks onthe transitions in thc data stream . Thedemodu lated l ms samples containingthe transitions arc rejected, while all ofthe other availab le samples for a givensate llite arc acc umulated into hits(GPS) or half-bits (the GLONASSManchester phase is nor known yet).

The following navigation data processing depends on the data format and thisis slightly different between GPS andGLONASS . The GpS data is formattedinto 30-bit words containing 24 truedata bits and 6 parity-check bits. Theword synchronisation is obtained bychecking the parit y bits, including thelast two bits of the previous word, for

any possible word phase. The BPSKpolarity ambiguity is also resolved bythe parity bits. The synchronised andchecked GPS data words are placed inanother FIFO memory together withl ms time tags, to be read by the mainprogram.

The GLONASS data is formatted intolines with 85 data bits in Manchesterform at and a non-Manchester syncpattern, for a total dura tion corresponding to 100 bits. The sync pattern is notused in the GLONASS receiver in thisproject. The synchronisation is obtainedby checking the 8 parity bits for anypossible half-bit phase (200 possiblephases), to resolve the Manchesterphase ambiguity as well. Since the databits are different ially encoded, there isno polarity ambiguity to be resolved.Like in the GPS receiver, the correctlyreceived data lines are placed in an-,other FIFO memory, together with 1mstime tags, to be read by the mainprogram.

S.:\ Main Program Loop Tasks

Since most of the functions performedby the main program loop requirehigh-accuracy floating-point arithmetic,the main program is mainly written inthe DSP computer high-level language.Of course, all of the interfaces to theinterrup t routine and ( 0 the variousperipherals (initialisa tion of the dedicated DSP hardware, LCD drive andthe real-time dock chip) arc at leastpart ially written directl y In theMC6ROIO machine code and arc inserted in hexadecimal format in themain program source code,

198


The ma in program loop executes oncefor every new set of averaged meas ureddata. The latter is available every R64milliseconds for the three satell ites thatgel less hardware time. The privilegedchannel supplies three separate set s ofave raged data in the same time period,but the excess data is not used by themain program loop.

The ma in program loop also updatesthe l.CD or writes a new line on a CRTdisplay. The internal operat ion of theprogram is howev er independent of theselected menu on the display . Themen u on ly affects the keyboard functions and some comp utations closelyrelated to the format of the displayeddata, like coord inate conversions.

The Fi rst task of the main program loopis to write the look-up tables in thededicated DSP hardware memory. Thisoperation is done at rece iver power-lip,when changing satellites. when adjust mg the carrier frequency (in 1 kHzsteps) or when switching the privilegedsatell ite . The satellites can he selectedma nually, but usua lly the software isset to automatically selec t v isible satellites.

When a given satellite is selected, thereceiver requires some time to lock onits signal. The software will first lookfor all possible CIA-code phases . If thelock is not achieved, the main programloop will change the hardware look-uptable frequency in 1 kHz steps in agiven frequency range (20 kHz in theGPS rece iver, or 25 kHz in the GLO NASS receiver). Of course, the look-uptable freque ncies for all four sate llitescan also be preset manually.

199

VHF COMMUNICATIONS 4/95(f'.--------- - --""--"""= = = =sample to relate all of the data 10 asing le time point using linear interpo lation. This simplifies the following computations, since the positions and veloci ties of all four satellites need to heca lcula ted for a single point in time.

'Inc measured data and the satellitepositions and velociti es (comp uted fromthe precision epheme ris data) are thenassembled into the navigation equations. A set of three time-differencenavigation equations is obtained fromcode-phase d iffere nces and another setof three Doppler-difference navigationequations is obtained from carrierfrequency differences.

The time-difference navigation equa tions arc solved first, using the Newtonsmethod. The start ing poi nt is taken inthe Ea rths ce ntre (x=y=z=O). f rom thisstarting point the Newtons methodn..squire s octween three and fou r iterations to converge to the final result fora user located on the Earths surface.The result in Cartesian coordinatesx,y,z is then converted to longitude,latitude and height.

The position obtained may now he

co rrected for the propagation anoma liesin the ionosphere and troposphere. Theprese nt software does not apply anycorrection for the ionosphere . The navigation equations arc only corrected forthe troposphere at the calculated heigh tand the Newtons method is iteratedonce aga in to obtain the final result.

Since the pos ition is already avail ablefrom the time-d ifference navigationeq uations, the Doppler-difference navigat ion equations are solved ' to obtainthe velocity of the user. Solving theDoppler-difference equations for tbe

200

velocity does not require a numericaliterative method, since the equationsresult linear for this unknown . Thecomputed ve locity vecto r is convertedinto magnitude. azimuth and elevationon the display .

The accuracy of the navigation solutiondepends on the geometry of the satellites. In place of the GOOP the software only computes the determinant ofthe linea rised system of equations at thecalcu lated position. This dete rminant isa dimensionless quantity. The higherthe determinan t, the more accura te thesolution. If the determi nant is too low,an error conditi on is signalled. TheDopple r-difference equations have thesame determinant if solved for velocity.

The main program loop also performsdata averaging. Roth position and velocity arc averaged. Only good datawith no erro r I signalled is added to theaverage . The determi nant of the systemof equations is used as a weight foreach new data set added ( 0 the average .Of course, the averaging buffer may bemanually reset if desired.

The display includes several differentmenus and I W O of them arc devoted tothe immediate and averaged data. Theposition may be displayed in differentformats: degrees on ly, deg rees, min utesand secon ds or Gauss-Krueger rectangu lar grid . Other menus are used toshow the receiver status and the almanac data.

Finally, the main program loop usuallyalso performs an automa tic satelliteselect ion. This function is triggered ifan error condition is signalled continuous ly for a certa in pcriod of time (100main loops). The software then uses the

almanac data and the real-time clock tofind the visible sate llites at the ave raged user location. The rece iver is thenprogrammed for the four visible sarellites with the highest elevations in thesky. Although this procedure docs notyield the best GooP its operation isfoolproof.

5A SohW3Tt" Menu.s andCommands

Since a portable GPS/GI.ONASS receiver only has a small keyboard with afew keys and a smal! alphanumericdisplay. the various user commandsneed to be arranged into several different menus.

The keyboard has eight 'different keys,four o f them (eorrcspondmg to ASC IIcharacters 4, 5, 6 and 7) arc used tose lect the four main menus. Depressingthese keys only changes the content ofthe di splay and the functions of theother keys, but does not affect theinterna l operation of the (;I 'S /(jLO NASS rec eiver. Some of these keyshave additional functions if depres sedmore than once. Depressing key 4cyclica lly shows all four virtual reccivcr channels (satellites) on the display. Depressing key 7 cyclica lly showsthe genera! receiver status and thealmanac data for all currently visiblesate llites,

The remaining keys (ASCn 0, I , 2 and3) arc ca lled parameter keys. Depressing these keys affects the interna loperation of the GPS/GLONASS rece iver as a function of the currentmenu. The mode of operation of thesekeys is a lso dependent on the actua l

In order to understand (he commands ofa GPS/GLONASS receiver it is necessary to understand the internal operation of the latter. 1\ GPS/CiJ.ONASSreceiver includes a non-volatile RAMto store the almanac data and a realtime clock that arc a lways powered bya small internal Ni(:d battery. Thenon-volatile RAM is used to store thealmanac data and the approximate userposition 3 !O a ' result of a previousreceiver operation. At power-up thisdata is used together with the rea l-rimeclock data to find all v isible satellitesand speed-up the acquisition of fourusable satellite signals.

When a GPS Of GLONASS rece iver isfirst powered lip, all of the non-volati leRAM conta ins random data ami a tota lreset is required. The total reset erasesall a lmanac data and pUIS the receiverin the mannaI satellite select mode. Allreceiver virtual channels are set to acentra l carrier frequency and a GPSPRN# 16 or GI.ONA SS ClI N#13. Themenu 4 is selected to show the virtualchannel #1 data ,

The initial satell ite signal acquisitionwithout any almanac data may take alarge amount of time. especially in asingle-channel rece iver. l b c rece iver

20 1

VHF COMMUNiCATiONS 4/95(~ ------------""--"""""""="""'~CH#: GLONASS satellite RF channel

number

CH#: GLONASS Rf> channel numberEJ.: elevation (degrees)AZ: azimu th (degr ees)DF: Doppler frequency shift (lIz,

po larity as in the IF)

If the rec eiver has been turned on forthe first l ime, or has not been used for aconsiderable period of time (more thanone week), the best thing to do is tocollect the complete almanac data first.To speed-up the almanac data collection it is recommended thai the remaining three receiver virtua l channels areset to the same satellite and to the sameIF frequency as the channel that alreadyacquired a satellite. After all fourvirtual channels achi eve data lock, thea lmanac data collection takes 12.5minutes for (iPS or ;) minutes forGLONASS, since the (jUJNASS receiver doe s not make use of half o f thealmanac frames .

The accumulated almanac data can hechecked by repeatedly pressing key 7.The almanac data includes the satellitename/number plus the following information:

doc s not know which satellite to lookfor not its frequency offset caused bythe Doppler shift and by the unknownfrequency drift of the receiver itself.The current software for the UPSjGL ONASS receiver in this project isnot abl e to select dif ferent satellitesa utomatically witho ut any almanacdata, so this has (0 he done manually .

After manually selecti ng the satc llitc(s).the software is goin g to try to achieveCIA-code lock. If the latter docs notoccur on the given IF frequency, thereceiver is going to scan the expectedIF frequency range in 1 kl-lz steps bywrit ing the corre sponding look -up tabl e.On ly the privileged satellite IF isscanned in the range from 2310 kHz to2330 kllz (CPS) or from 1675 Id17- to1700 kHz (GLONASS).

While searching for the initial signalacquisition there is a small di fferencebetween the errs and ( il.ONASS reccivcrs. The errs constel lation is nowcomplete and more than four visi blesatellites can he found at any time, sothe ( ;PS receiver is only going toswitch the privileged channel after asate llite signal is acquired . On the otherhand, the (;LONASS constella tion isnot complete and someti mes there isjust one visible satellit e, so the GLONASS receive r is going to try adiffe rent virtual channel with a di ffer cut satell ite if the current privilegedsatellite was not acquired.

After a sate llite signal has been acquired. the key 4 menu shows the mostimportant receiver parameters :

RX: virtual channe l num berSV: GPS satellite PRN code

number

CF:

R:

S,SVH:URA:ASF

En:

AOE:

look-up table preset centra l IFfrequency (UII-)mea sured IF frequency (kIT:!""from code rate)signal level (S-meter)satellite health flag (O=OK)GPS mer range accuracy (m)GPS anti-spoofing flag(O=OFr)GI .ONASS ephemeris upload»sc (days)GI.ONASS ephemeris age (S)

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VHF COMMUNICATlONS 4/95

AlE: GPS almanac/precisionephemeris data

SVH: satellite almanac health flag(GPS O=OK;GLONASS l =OK)

ASP: GPS anti-spoofing andconfiguration flag

UTC: UTe dale and time

The elevat ion, az imuth and Dopplershift arc comp uted for the referenceuser position as obtained from theaveraged data , After a tota l reset thisreference pos ition is set in centra lEurope. Checking the almanac it isposs ible to lind out whe n the receivercan be switched to automat ic satelliteselectio n. In the automatic sate lliteselec tion mode , the program selects thefour sate llites with the highest elevations and sets the corresponding cen tralIF values in the hardware look-uptable s. This selection can be doneimmedi atel y hy pressing key I) in me nu7, or hy switch ing the prog ram to doth is automatically after a period of haddata (key 3 in menu 7).

After four satellites have been acquiredthe dis play may be switched 10 menu 5or 6 to show respectively the immediateor averaged data.

These menus show the fo llowing:

DET:

AVG:

LAT,LO'<,JI,

determ inant of the system ofequations (menu 5 only)averaging weight (menu 6only)velocity vector ma gnitude(kmJh), azimuth and elevat ionlatitude (degrees or n. north)longitude (degrees or m, east)height abov e elli psoid (m)

6.CONCLUSION

The GPS and GLONI\SS systems aremain ly intended for navigat ion, butthere are many other less adverti sed butnot less important , not less interestingapplica tions of these system s. Since

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VHF COMMU NICATIONS 4/95(~---------_.:=-===='-"=

these systems arc known and the technology 10 use them is available toanyone, we rad io amateurs should consider our own applications of thesesystems (131.

Although the nav igation itself is not ofmuch interest to radio amate urs, it

would probably make more sense totrans mit GPS or GLONASS coord inatesof a contest location, rather than theinaccurate EU or WW locato r, which isalready not accurate enough for seriousmicrowave or laser comm unications.By the way. (iPS and GLO NASS uscalmost the same coordinate system anda long time average shows differencesof the order of only 10m between thesystems.

A side product of both UPS andGLONASS is accurate time and frequency broadcast. In order to achievethe specified nav igation accuracy theliming measurements have to be performed to an accuracy of about 10m .The same requiremen t applies to theon-board satellite atomic clocks . Thefinal user time transfer ,lCt'uracy rangesbetween 30ns and lOOns, dependingalso on the knowledge of the exact userloca tion. Thus the user should alsocom pute his position even if he oulyneeds accurate tim e

Radio amateurs could use this timetransfer capability of both (iPS andGLONASS every time when accuratesynch ronisation is required. Coherentcomm unications are just an examp le,the accuracy of GPS or GIDNASSoffers more than this. f or example, theactual propag ation path of the radiosignal and the propagation mechanismcould be inve stigated in this way.

The frequency broadcast accuracy ofboth GPS and GLON ASS is in therange of 10-12, far better than can beachieved with HF or LF standardfrequency transmitt ers. The accuracy ofthe latter is limited to around 10-7 bythe propagation effects alone, and thisis not enough for serious microwavework. GPS and GLO~ASS arc alsoavailab le globally 24 hours per day andarc not limited by the transmitter range.

Finally, GPS and ULONASS representa step away from being just an operatorof black-box amateur radio equipment.Although there arc several ready-madeGPS receivers on the market we willprobably have to develop our ownrece ivers for om exper iments, both thehardware and the software . Buildingsuch a receiver may be an intere stingchallenge as well. The receiver shownin this art icle is perha ps just the firststep, other related projects or betterreceivers will follow soon.

Thi s completes this constructio nproject . although the editor[eels certainthat further articles on the subject ingeneral and this project in panicluormay appear in futur e issues.

11 complete list of all literature references fottows on the next page.

11 complete list of kits and printedcircuit boards fo r this project appearson page-255 oj this issue.

204

VHF COMMUN ICATIONS 4/95

7.REFERENCES

[1] Matjaz Vidmar: "Digital SignalProcessing Techniques for RadioAmateurs, Part-2: Design of aDSP Computer for Radio AmateurApplications";VHf Communications 1/91.pp 2-24

(2J Matjaz Vidmar: "Digital SignalProcessing Techniques for RadioAmateurs, Part-S: Constructionand use of the l)SP Computer";VIIF Communications 2/89,pp 74~94

(3J Jonathan S. Abel, James W.Chaffee: "Existence andUniqueness of GPS Solutions";pages 952·956/6.91. Vol.27 IEEETrans. on Aerospace andElectronic Systems

f41 "Interface Control DocumentMHOR-00002·400, rev.E ", (84pages), August 7th, 1975,Rockwell Internat ional Corpora.tion, Space Division, 12214Lakewood Boulevard, Downey,California 90241, USA

(5] "Interface Control DocumentGPS·200", (102 pages), August7th, 1975.Rockwell International Corporation, Space Operations andSatellite Systems Division, 12214Lakewood Boulevard, Downey,California 90241, USA

[6] "Global Satellite NavigationSystem GLONASS InterfaceControl Document" (46 pages),

205


Tips. Improvements and Corrections

DIY Construction of a Receiverfor GPS and GLONASSSatellites by Matjaz VidmarS53MV; Issues 1/94 to 4/95

For those having d ifficulty obtain ing asuitable display modu le, the PhilipsLH~ 244F-90 (with LED rea r illumination) can be used.

In diagrams 20 and 28 (1/95) someminor errors crept in and are correctedin the d iagrams overleaf.

In Fig.38 (2/95) the identification of L2is missing on the IOOJlII ind uctor nextto rc CA30IN .

In FigA2 (2/95) the identification o f thediode HSCG l OO l is missing, it leadstlway from pin- tv of the I>TACK Ie74HC245 .

10 Fig's.a l and 42 (2/95) and 46 and 47(3/95) the log ic gales are incorrectlydrawn. Corrected diagrams are shownoverleaf

Also in Fig,46 (3/95) the crystal isshown as 10 MHz instead of 12 \i llz.

" .~' 01. L.i"~'4U""

I ~..,.. e,

Flg .26 GLO:'llASS Rf Module

c.

Vt.CI O'Ir"" Pl~~HI.,,,,-.«,..tu.,..~ _

"" ~~ .a ~

T,,~

,.

187

•

~"..",

-~~i;i~ Below :Fig's:41 and 42 , )

'~~--,,- , ) , GLONASS nSf''''"iF- l!Jrv r Gl SI ts-l and 2- ,,.;,. --"'--"-~ A~1fA.~~{"H~ Hardware par

188

VHF COMMUNICATIONS 3/9 7

-,----

,

•••• ••·••••

.,•

....

.-.-

FigA6: CPU Roam Circuit () i:.lgrdm {pa r t-I)

..... ...., HU i 1JO,.<...t.-.u..- -• • .. .... w .'.

" •• ~ !ttQ

~u.,......

• •• • " .-. "•• III HD~~, •

~ •'. n (lllli) ""• •• "~

• ,mw' •

\10~~" ""'-1.. 1 ' n N { "1m- !I,v ..r• m (nMAl~ ""'

Fi2.47: CPU Hoard Circuit Diagram (part-2)

189

VHF COMMUNICATIONS 4/97(f' ----- - - - ---""'--""""= = """-"=

Motjaz Vidmar, S53..\IV

GPS/GLONASS ReceiverHardware and Software Update#1

Since the publication o r the series ofart icles about the Gl' S!(;!.ONASS receiver in UKW-RerichteIYHF-Communlcatio ns there have been a few modifications of the hard ware and softwar e of

these receivers. The original articlesdescri be the opera tion of the GPSsoftwa re Yi n and GLONASS softwareV39. The current update describes thenew DPS software V 125 and 0 1.0NASS software

V42. The new (, PS V1 25 and (lLONASS V42 inc lude the following mod ifications:

(1) Improved interna l ope ration of thesoftware. The new software is able tohandle the overflows of the hardwarecounters correctly thus almost eliminating the occurrence of the "T" error.

(2) Additions to the comma nd set:

(2.1) In menu #5, key #0 will shift theprivileged RX chann el.

(2.2) In menu #7, key #0 has a newfunction: in AUT mode it operates asbefore while in :MAN mode this com-

mand sets the carrier frequencies o f thecurrent ly selected satellites.

(2.3 ) In menu #7, key #1 has anaddi tional function : the receiver \vilJdisplay the Kcplcr ian elements of theGPSiG LONASS satellites as decod edfrom the almanac data before enteringthe total RESET sequence.

(3) 1\ simple b i-directiona l R. S-232 interface is included. requ iring only a fewadd itional hardware components to beinstalled in the recei ver.

(3.1) The RS-232 interface output circuit is shown on r ig . I. Because ofhardware limitations, the bit rate canon ly be set to 1000bps. The output dataformat is a serial async hrono us transmission inc luding a start bit, 8 data bits, noparity and one stop h it. The outputsignal level ranges from OV to +5Vonly, although these levels are usuallyaccepted by most RS-232 receive rs. Thesigna l polarity is inverted as usual inRS-23 2: OV represents a logical " 1"while +5V represents a logical "0" . Thedata output matches the LCD disp lay

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VHF COMMUNICATIONS 4/9 7

content, the display cle ar commandbeing replaced by a CR'LF combin ation.

(3,2) The RS-232 interface input circuitis shown on Fig,2, Because o f hardwarelimitations, the hit rate can only he setto IOObps or 10 limes slower than theoutput rate. The RS-232 input can beused to issue commands identical tothose co ming from the 8-key keyboard.Only A SCII characters "0" , " I", "2" ,"3", "4" , "Y', "6" and "1" are thereforeaccepted as valid commands. All othe rcodes are simply ignored. The dataformat is 8 b its. no parity. one or morestop bits. The signal po lar ity is invertedas usual in RS·232.

Since the PC.'6 input is now used for theRS·2 32 interface input, it can no longerbe used to select the display type, CRT

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A DIY Receiver for GPS and GLONASS Satellites

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