5_Navigation Systems-Final (5)

8/9/2019 5_Navigation Systems-Final (5)

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h. 5_NAVIGATION SYSTEMS

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5.0 Introduction 5.1 History

5.2 Basic Navigation

5.3 Radio Aids to Navigation

5.4 Radio Aids to Navigation Testing

5.5 Inertial Navigation Systes

5.! "o##ler Navigation Systes

5.$ %lo&al 'ositioning Syste

5.$.5 (t)er Satellite Systes *not covered+

5., Identi-ication riend or oe

5./ "ata ins *ilitary+ *not covered+

5.10 ongRange Navigation *not covered+

Re-erenceTet6 Test and 7valuation o- Aircra-t Avionics and 8ea#ons Systes9 Ro&ert7. cS)ea

Lecture Outline

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Navigational systems is a rather large subject area. T)is c)a#ter :ill cover &asic radio aids to navigation suc) as very)ig)-re;uency

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Test and Evaluation of Aircraft Avionics and Weapons Systems, Ro&ert 7. cS)ea Avionics Navigation Systems, 2nd edition, &y yron >ayton and 8alter R. ried *Ne:

?or6 @o)n 8iley Sons9 1//$+. T)e tet does a very good o& o- e#laining t)et)eory and o#eration o- navigational systes.

Anot)er tet :ort) reading is Integrated Navigation and Guidance Systems, &y "aniel@. BieCad *Reston9


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TON!IN GUL" VIETNAM

A#$% &f CV'#() &n the USS Tic&n+er&,- in ()/

5

Attac S;uadron


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N-0-l A0i-ti&n Ter1in&l&,2

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G CVW6 =arrier Air 8ing t)e s;uadrons t)at are de#loyed on t)e aircra-t

carrier. Several ty#es &y ission are listed &elo:6 ig)t Attac6 A49 A$9 A15

ediu Attac6 A!

ig)ter6 ,9 4. 149A1,

786 71,

AS86 S3

A786 72

(t)ers6 SAR )elico#ters9

taners9 etc.

G A&out $5 aircra-t


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*courtesy o- Boeing+

%&ein, $$$3$/$ c&c4it


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>ayton and ried de-ine navigation as Jt)e deterination o- t)e #osition and velocityo- a oving ve)icle.J

As :e sa: in t)e TS'I c)a#ter t)e #osition o- an o&ect in s#ace is really a ninestatevector co#rised o- latitude9 longitude9 altitude9 nort)eastdo:n velocity9 and nort)eastdo:n acceleration.

T)e et)ods o- o&taining t)is in-oration )ave steadily #rogressed t)roug) t)e years

:it) everincreasing accuracy. "ead reconing9 t)e original navigation et)od9 using tie9 s#eed9 and )eading is

still a &acu# ode on all current air&orne navigation systes.

As we also saw in the TSPI chapter , no single navigation system is capable of

directly measuring all nine states.

odern navigation systes ay use sensor -usion o- ulti#le navigation systes or

e#loy a navigation co#uter to calculate t)e reaining states not directlyeasured.

%ASI NAVIGATION

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1. Latitude2. Longitude

3. Altitude

4. North Velocity

5. East Velocity6. Don Velocity

7. North Acceleration

8. East Acceleration

!. Don Acceleration

Nine#St-te Vect&r 6-r-1eter

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It is t)e testerDs res#onsi&ility to understand not only t)e accuracy o- t)e syste9 &utalso t)e in)erent inaccuracies o- t)e easured states.

T)is conce#t is critical in evaluating odern systes designed -or use incongested airs#ace :)ere t)e re;uired navigation #er-orance to o#erate in t)isairs#ace is etreely restrictive.

any ties testers are at a loss to e--ectively #lan test issions &ecause t)ey do not

s#ea J#iloteeCe.J T)is is not a slig)t against testers9 as t)ey ay )ave never &een e#osed to aircra-t

-lig)t o#erations9 and ost sc)ools do not teac) &asic eart) coordinates anyore.

any testers are intiidated &y #ilots &ecause t)ey cannot e--ectively counicate:it) t)e.

T)e -ollo:ing tutorial is eant to e#lain soe o- t)e &asics o- navigation.

?ou :ill not &e a navigator a-ter t)is section9 &ut you :ill no: t)e di--erence &et:eena agnetic )eading and a true )eading.

%ASI NAVIGATION

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e!resher !rom TSPI" wee# $%. T)e -irst conce#t is t)at o- latitude and longitude.

T)e eart) is divided into grids &yiaginary lines o- latitude and longitude.

atitude lines are )oriContal &ands around t)e eart).

T)e e;uator is 0K latitude and t)e Nort) and Sout) 'oles are at /0K nort) andsout) latitude9 res#ectively.

ongitude lines are vertical &ands around t)e eart) and start at t)e 'rie eridian9:)ic) runs t)roug) %reen:ic)9 7ngland9 at 0K longitude.

T)e lines continue east and :est and eet at t)e International "ate ine9 :)ic) is at1,00 :est and east longitude.

Any position on the earth can be i&enti!ie& with a uni'ue latitu&e an&

longitu&e.

(igure )*+ shows these #ey points.

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6&iti&n


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6&iti&n

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e!resher !rom TSPI" wee# $%.

T)e distances &et:een any t:o #oints ay also &e deterined. 7ac) degree o- latitude traverses !0 n as you ove -ro #ole to #ole.

"egrees are -urt)er &roen do:n into inutes. T)ere are !0 in in a degree9 so eac)inute :ill traverse 1 n *!90,0 -t+.inutes are t)en &roen do:n into seconds9 and t)ere are !0 sec in eac) inute.

ore coonly9 inutes are resolved &y tent)s9 )undredt)s9 and t)ousandt)s o-

inutes. or ea#le9 0.001 in e;uals !.0, -t. 7ac) degree o- longitude traverses !0 n9 &ut only at t)e e;uator.

T)is is &ecause lines o- longitude converge at t)e #oles *see igure 51+.

As we get away !rom the e'uator" the &istance between each &egree o!

longitu&e is a !unction o! the cosine o! the latitu&e, +- o! longitu&e /- nm 0

cosine o! the latitu&e.

1ou can see that at the e'uator" the latitu&e is -2" an& the cosine o! 3ero is one. At the poles" the latitu&e is 4-2" an& the cosine o! 4- is 3ero 5lines o! longitu&e

converge6.

So i! you ever measure &istances on a chart" use the tic# mar#s between the

lines o! latitu&e" not the lines o! longitu&e" (igure )*%. (Handout)

6&iti&n

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All o! the charts that we use !or !light an& !light planning will be re!erence& totrue north.

All o! the groun&*base& ra&io ai&s to navigation are re!erence& to

magnetic north. Since the ra&io ai&s are in magnetic" pilots !ly magnetic.

:uring !light planning" we measure a hea&ing !rom the chart in true north" but

we convert it to magnetic north !or the pilots.

In or&er to accomplish this tas#" we must un&erstan& the concept o! magneticvariation an& how it a!!ects our measurements.

The plotter is place& with the bottom e&ge on the line &rawn between airport A

an& airport ; 5(igure )*76.

T)e #lotter is t)en #ositioned until t)e longitude line intersects t)e center )ole o- t)e#lotter.

The hea&ing is rea& on the outsi&e scale o! the plotter 5(igure )*86. I- a line o- latitude is used9 as in t)e case o- )eadings very nearly nort) or sout)9

t)e inside scale o- t)e #lotter :ill &e used. In our ea#le9 t)e )eading read on t)e#lotter scale is 250o or 0$0o.

7e-+in,

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7e-+in,

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7e-+in,

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T)ere are t:o ans:ers &ecause :e can &e going -ro A to B or vice versa *1,00 out+. Since:e are traveling 7N7 -ro A to B9 :e no: t)at t)e correctans:er is 0$0K.

This measurement is re!erence& to true north 5since it is rea& !rom the

chart6 an& pilots will note the hea&ing as -west value.

The magnetic variation o! Salt ?a#e City is +/o east. In our original e0ample 5(igure )*76" there is a 7o east variation at airport A" an& a 8o east variation at airport ;.

The only thing a tester nee&s to #now to correctly apply magnetic variation is @ast is

?east an& West is ;est.@ That is to say" in or&er to convert true hea&ing to magnetic

hea&ing we subtract east variation an& a&& west variation.

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I- our true )eading -ro air#ort A to air#ort B :as easured as 0$0o true9 t)eagnetic )eading at air#ort A :ould &e 0!$0 agnetic *0$0K 3K east variation+.

As :e a##roac) air#ort B9 aintaining t)e sae true )eading9 our agnetic )eading:ill c)ange to 0!!o agnetic. Because t)e agnetic variation )as c)anged as :e#rogressed along our route. 8e continue to -ly a straig)t line9 yet our agnetic)eading c)anges.

The &i!!erence between true an& magnetic hea&ing can be 'uite signi!icant. The true hea&ing !rom the center runway at Salt ?a#e City to the Salt ?a#e City

V= 5(igure )*)6 is roughly 7/-2 true. ;ut i! we apply the +/2 east variation" the

magnetic hea&ing becomes

7882.

Provi&ing the pilot with the correct hea&ing will &etermine i! he ever gets to the

point in space where he nee&s to be.

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#AND$%&

"IGURE 5#5 T2ic-l Secti&n-l

Ch-rt

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The ne0t variable we nee& to consi&er is velocity" or airspee&. Aircra!t travel is measure& in nautical miles per hour" or #nots.

?ou :ould su##ose t)at an aircra-t traveling at 300 nots :ould )ave traversed 300n in 1 )our *1 n M !0$!.12 -t M 1.150$, statute iles+.

T)is :ould &e true i- t)e #ilot :ere -lying true airs#eed *TAS+ in a no:ind condition.

Ln-ortunately #ilots usually -ly an indicated airs#eed *IAS+.

T)e #ilotDs airs#eed indicator is read in nots o- lAS.

T)e airs#eed is co#uted &y t)e aircra-tDs #itotstatic and central air data co#utersyste.

T)e syste easures i#act #ressure and co#ares it to a static #ressure t)egreater t)e di--erence &et:een t)e t:o9 t)e larger t)e IAS.

At sea level" IAS an& TAS are about the same.

As an aircra-t cli&s9 t)e #ressure decreases9 and so does t)e i#act #ressure ont)e #itot syste.

The IAS will &ecrease !or a constantly hel& TAS.

Airee+

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As t)e aircra-t con-iguration c)anges *angle o- attac due to c)anges in s#eed9altitude9 :eig)t9 and -la# or gear con-igurations+ t)e -lo: across t)e #itot syste :illc)ange9 t)us giving an erroneous reading o- #ressure.

In order to correct t)is error9 a loou# c)art is used to calculate a cali&ratedairs#eed *=AS+.

I- t)e aircra-t is -lying at s#eeds greater t)an a&out 200 nots *=AS+9 t)e air a)ead

o- t)e aircra-t :ill &ecoe co#ressed9 giving a )ig) reading on i#act #ressure. An airs#eed co#ressi&ility loou# c)art corrects -or t)is error and #roduces a

nots e;uivalent airs#eed *>7AS+. T)e di--erence &et:een


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7e-+in,

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Airee+

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T)e last varia&le :e need to address is altitude.

In test and evaluation9 :e use three different altitudes, or t!o altitudes and oneheight . igure 5$ s)o:s an aircra-t -lying over t)e eart).

8e can easure its altitude in t)e t)ree :ays t)at are de#icted.

Altitu+e8 ( &f )

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T)e -irst easureent o- altitude is a&ove ground level *A%+9 and is a directeasureent o- your altitude in -eet also re-erred to a )eig)t since it is aeasureent a&ove t)e terrain &elo:.

T)is easureent is acco#lis)ed :it) a radar altieter t)at utiliCes a -re;uencyodulated *+ :ave-or.

It can #rovide readouts to t)e nearest -oot9 and its accuracy ranges -ro 1 -t to 1

o- altitude. (igure )*E shows a typical aircra!t ra&ar altimeter. GAN:=HT

Allie& Signal ;en&i0>Ding

DA*+-A a&ar Altimeter 5courtesy o! NTPS.6

Altitu+e8 9 &f )

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The secon& measurement o! altitu&e is the height above mean sea level59S?6.

T)is easureent is o&tained :it) a &aroetric #ressure altieter *igure 5/+.

Aircra-t use t)e S easureent alost eclusively :)en navigating around t)eglo&e.

Geights o! airports" towers" mountains" an& navigational ai&s are all reporte& as

9S? altitu&es. 9S? is simply a comparison o! the pressure where the aircra!t is to the

stan&ar& &ay pressure at sea level.

A standard day is de-ined &y t)e air #ro#erties o- density9 s#eci-icvolue9 #ressure9 te#erature9 and viscosity.

A standard day :ould e)i&it a te#erature o- 5/K *15K=+ and a #ressure o- 14.$

#si *101.3 N2+. T)is #ressure :ould also corres#ond to 2/./2 inc)es o- ercury *inHg &aroetric

#ressure+.

Altitu+e8 : &f )

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Altitu+e8 ; &f )

3"


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T)e #ilotDs &aroetric altieter )as t)e ca#a&ility o- setting a #ressure -or t)e air

ass t)e aircra-t is -lying in. I- t)e aircra-t is sitting on t)e ground at an air#ort :)ose -ield elevation is 0 -t *sea

level+9 and i- it :ere a standard day9 t)e #ilot :ould insert 2/./2 as a setting -or#ressure9 and t)e aircra-t altieter :ould read 0 -t.

Since standard days are -e: and -ar &et:een9 t)is is rarely t)e case.

Su##ose t)at a )ig) #ressure syste is sitting over t)e air-ield and t)e #ressure is

30.45 inHg. I- t)e #ilot )ad 2/./2 set in )is altieter9 )e :ould &e reading a negative 530 -t on

)is instruent. As )e increases t)e setting to 30.459 t)e altitude :ill increase andread 0 -t :it) t)e correct setting.

?ou can calculate )o: -ar o-- you :ill &e &y erely su&tracting t)e t:o settings9 t)e)undredt)s #lace is :ort) 10 -t and t)e tent)s #lace is :ort) 100 -t6

Altitu+e8 5 &f )

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G T)e no& at t)e &otto rig)t o- t)e altieter in igure 5/ is :)ere t)e #ilotsets t)e #ressure setting o- t)e day.

G T)e :indo: ust to t)e le-t o- t)e no&9 called t)e >o)lsan :indo:9dis#lays t)e setting.

The !inal altitu&e is not really an altitu&e" but a height above an

ellipsoi&" PS height. This ellipsoi& is the earth mo&el in use" which was previously

&iscusse& in chapter %.

Altitu+e8 ) &f )

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8)en aircra-t -ly -ro #oint A to #oint B9 t)ey rarely -ly a straig)t line *visual -lig)trules *her li!e on the line.

(igure )*+- is a section o! the H.S. instrument !light rules 5I(6 en route high*

altitu&e !light in!ormation publication. 5Gan&out6

It de-ines t)e allo:a&le )ig):ays in t)e sy -or t)e :estern Lnited States -or aircra-t-lying under IR.

RADIO AIDS TO NAVIGATION

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Hsing (igure )*+-" weJll !ly !rom ?os Angeles International Airport to;a#ers!iel&.

?ou can see t)at :e :ould -ly t)e line identi-ied as K) to a #oint called ?AN:= andt)en #roceed on K) again to t)e S)a-ter


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urt)er along t)e route -ro S)a-ter t)ere is a little &o a&ove @5 :it) t)e nu&er 25in it. T)is is t)e distance in nautical iles -ro S)a-ter to AN"(.

I! we &eparte& Sha!ter on a magnetic hea&ing o! +%/o an& !lew !or %) nm" we

shoul& be over ?AN:=" rightL

This woul& be true only i! there was no win&.

At t)e center o- eac) co#ass rose t)ere is a sisided sy&ol. T)ese sy&ols

identi-y t)e radio aids to navigation t)at :e :ill tal a&out net. (igure )*++ 5Gan&out6 shows the types o! navigational ai&s i&enti!ie& by

symbols on the high*altitu&e chart.

8e :ill concern ourselves :it)


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The V= is a groun&*base&" short*&istance ra&io ai& provi&ing continuousa3imuth in!ormation in the !orm o! 7/- ra&ials emanating !rom the station.

The V= provi&es the bac#bone o! the civil airways structure throughout most

o! the worl&.

(igure )*+% provi&es a representation o! the concept o! ra&ials !rom a V=.

T)e triangle re#resents an aircra-t traveling nort) and east o- t)e


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V7" O1ni+irecti&n-l R-n,e


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3!

G (igure )*+7 s)o:s t:o ty#es o- dis#lays t)at are used to dis#lay


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ach &ot in the arc un&er the C:I nee&le represents a %2 &eviation !rom the&esire& course.

The white arrow in the right center portion o! the &isplay is the to>!rom in&icator

In this e0ample" we are going away" or !rom" the station.

T)e


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(nce entered9 as long as t)e


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V7" O1ni+irecti&n-l R-n,e


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The V= will provi&e the pilot with bearing in!ormation to a groun& re!erencebut will not provi&e a range.

It is #ossi&le to get a #ositional -i :it) t)e intersection o- t:o


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The :9 system wor#s in the HG( ban& between 4/% an& +%+7 9G3 an& is

capableo! replying to +-- airborne interrogations simultaneously.

T)is eans t)at eac) air&orne receiver :ill )ave to discriinate its re#ly -ro anyot)er aircra-t interrogations.

T)e discriination is acco#lis)ed &y integrating re#lies -ollo:ing eac) interrogation#ulse :it) res#ect to tie.

T)e tie &et:een transission and rece#tion o- t)e receiverDs re#ly :ill &e relativelyconstant over a sall nu&er o- s:ee#s9 :)ile all ot)er re#lies :ill occur at randotie intervals.

In or&er to assist in &iscrimination" the airborne e'uipment has

two mo&es. The !irst is the search mode" where pulse &iscrimination is

achieve&.

"uring t)is #eriod9 t)e transitter generates 150 #ulse #airssee to reduceac;uisition tie.

=nce the system is loc#e& onto the range" it enters a tracking mode an&

generates between ) an& %) pulse pairs>sec in or&er to prevent early saturation

o! the transpon&er.

45

Dit-nce Me-urin,

E>ui1ent # DME


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There are +%/ channels 5!re'uencies6 in use by the :9" an& to ensure thatthere is

no interaction between the transmitter an& receiver" transmit an& receive

!re'uencies are separate& by /7 9G3.

In ost civil a##lications9 t)e #ilot need only tune t)e navigation radios to t)e ui1ent # DME


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The TACAN system is an omni&irectional ra&io ai& to navigation &evelope& bythe military operating in the HG( ban& that provi&es continuous a3imuth

in!ormation in &egrees !rom the station an& slant range &istance via the :9

system.

The TACAN system has +%/ two*way channels *in reality 12! JPJ TA=AN and 12!J?J TA=AN+ o#erating in t)e -re;uency range o- 1025 to 1150 HC air to ground.

%round to air -re;uencies are in t)e ranges o- /!2 to 1024 HC and 1151 to 1213HC.

=ur original case o! Santa Catalina 5(igure )*+-6 shows in the in!ormation bo0

a!ter the three*letter i&enti!ier the number )+. This is the T!" channel

number .

Similar to V=" the TACAN system measures the time interval between the

arrivals o! two signals.

T:o &asic signals are #roduced &y t)e rotation o- t)e inner and outerre-lectors o- t)e central antenna.

(igure )*+8 shows the antenna array !or the TACAN groun& station.

T-ctic-l Air N-0i,-ti&n#

TA AN

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TA AN


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A 15 HC signal is #roduced once during eac) rotation o- t)e inner re-lector. A 135 HC signal is #roduced nine ties :it) eac) rotation o- t)e outer re-lector.

8)en t)e radio :ave lo&e o- t)e 15 HC signal #asses t)roug) agnetic east9 ase#arate onidirectional signal is transitted t)is is t)e ain re-erence signal.

8)en eac) o- t)e nine lo&es o- t)e 135 HC signal #asses t)roug) agnetic east9nine additional onidirectional signals are transitted and designated as auiliary

re-erence signals. (igure )*+) illustrates the main an& au0iliary re!erence signals o! the groun&

station.

To &etermine the aircra!t position in bearing !rom the station" a phase angle

must be electronically measure&.

This is &one between the main re!erence signal an& the +) G3

signal. T)is tie interval is converted to an angle :)ic) isolates one o- t)e 40K segents.

4!


TA AN


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5"


TA AN


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T)e tie interval &et:een t)e rece#tion o- t)e auiliary re-erence signal and t)eaia o- t)e 135 HC signal is easured :it)in t)at segent.

T)e angular di--erence is converted into degrees agnetic and #resented to t)e #ilot.

The range is &etermine& !rom the :9 system" which is incorporate& into the

TACAN system.

;ecause o! the basic construction o! the TACAN system 5eight au0iliary an&

one main re!erence pulse6" it is possible to have 8-2 loc#*on errors in a3imuth. 8)en t)e air&orne receiver is :oring correctly9 t)ese #ulses loc on to t)e air&orne

e;ui#ent :it) t)e ain re-erence at /0K *agnetic east+.

When the airborne receiver is wea#" the main re!erence pulse may sli&e over"

miss magnetic east" an& loc# on at one o! the au0iliary positions.

When this occurs" a3imuth in&ications will be 8-2" or some multiple o! 8-2" in

error.

51


TA AN


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The TACAN within the aircra!t also has an air*to*air mo&e that provi&es range toa cooperating aircra!t.

In this mo&e" the !irst aircra!t will select any TACAN channel 5+*+%/6 an& &irect

the cooperating aircra!t to select a TACAN channel /7 units away !rom aircra!t

+Js selection.

(or e0ample" i! aircra!t + selecte& TACAN channel %4 " aircra!t % woul& select

TACAN channel 4%. In this scenario" the :9 rea&out will be the &istancebetween the two aircra!t.

T)is s)ould &e a##arent i- :e recall t)at t)e "7 syste se#arates t)e transit andreceive -re;uencies &y !3 HC *section 5.3.3+.

Table )*% provi&es the transmit an& receive !re'uencies !or the +%/ channels o!

@@ TACAN systems. 5Gan&out6

In ne:er TA=AN systes9 t)e airtoair ode :ill #rovide &earing to a coo#erativetarget in addition to range.

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TA AN


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An N:; groun& installation combines a low*!re'uency transmitter with anantenna system provi&ing a non&irectional ra&iation pattern.

N:;s are a use!ul ra&io ai&" as they are relatively low cost an& low

maintenance.

As with V=" &ual receivers can provi&e a positional !i0 !or the aircra!t using

the intersection o! two N:; bearings.

N"B &eacons are assigned in t)e 1/0 to 535 HC range and vary in #o:er out#ut. As :it) t)e


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N&n+irecti&n-l %e-c&n


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Since the ra&io signal is non&irectional" a system must be installe& in the

aircra!t to &etermine bearing to the stationB range in!ormation is not available . The automatic &irection !in&er 5A:(6" sometimes calle& the automatic ra&io

compass 5AC6" is an aircra!t*installe& system that provi&es a bearing to the

N:;.

The A:( is a low*!re'uency receiver that operates in the +-- to +


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S*I+ urt)er rotation causes t)e signal strengt) to increase until it again reac)es its

aiu :)en t)e loo# is again #arallel to t)e transitted :ave.

T)e null #ro#erty o- t)e loo# can &e used to -ind t)e direction o- t)e transittingstation9 #rovided t)ere is a nondirectional antenna to solve t)e a&iguity o- t)e loo#.

T)e loo# alone can &e used to locate a transitting station9 &ut t)e o#erator )as no

:ay o- no:ing :)et)er t)e station is on a s#eci-ic &earing or its reci#rocal t)is isno:n as t)e 1,0K a&iguity o- t)e loo#.

8)en a signal -ro a nondirectional antenna is su#eri#osed on t)e signal -ro aloo# antenna9 only one null is received9 t)us solving t)e a&iguity.

(n ost ilitary aircra-t9 t)e A" antenna is -ied9 so t:o crossed loo# antennas areused and t)e null #osition is sensed inductively or ca#acitively :it) a gonioeter9

:)ic) is an instruent t)at easures angles or allo:s an o&ect to &e slaved to agiven angle.

56



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9o&ern aircra!t employ synchronous or coherent &etection in or&er to &iscernthe N:; signal !rom noise.

=o)erent detection systes internally generate a signal on t)e sae -re;uency ast)e tuned station.

T)e receiver t)en searc)es t)roug) t)e noise entering t)e receiver9 atte#ting toatc) t)e internally generated signal :it) t)e received signal.

(nce t)is is acco#lis)ed9 t)e syste is said to &e loced on. ilitary aircra-t also use direction -inding in t)e LH and


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Some o! the major contributing errors are, ;an# error.

◦ Ban error is ost #redoinant at altitude :)en t)e aircra-t is close to t)e station.

◦ 8)ile t)e aircra-t is &aning9 t)e &earing #ointer #oints do:n:ard to:ard t)estation9 t)us giving an inaccurate reading.

◦ T)e error is greatest on nose and tail &earings :)en &an is a##lied.

Thun&erstorm e!!ect.◦ Norally radio :aves are distorted &y electrical distur&ances

caused &y t)understors. T)ere ay &e erratic -luctuations o- t)e &earing #ointerin t)e direction o- t)e distur&ance.

◦ It is #ossi&le -or t)e &earing #ointer to )oe in on t)e t)understor.

Night e!!ect.

◦ Nig)t e--ect is caused &y t)e re-lection o- sy :aves -ro t)e ionos#)ere it is ostnoticea&le at sunrise and sunset9 :)en t)e )eig)t o- t)e ionos#)ere is c)anging.

◦ T)e re-lected sy :aves inter-ere :it) t)e rece#tion o- t)e ground :aves andcause t)e &earing #ointer to -luctuate.

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Shoreline e!!ect.◦ S)oreline e--ect or coastal re-raction occurs :)en radio :aves c)ange direction on

crossing a s)oreline.

◦ It is #ossi&le to )ave errors o- u# to 40K in &earing.

◦ T)e area o- aiu error is reac)ed :)en t)e &earing -ro t)e aircra-t to t)estation is less t)an 30K to t)e s)oreline.

9ountain e!!ect.◦ ulti#at) e--ects due to ountain re-lections :ill cause -luctuations

in t)e &earing #ointer.

linting or 'ua&rantal e!!ect.

◦ A ;uadrantal error is due to t)e glinting o- radio :aves o-- t)e sur-aces o- t)eaircra-t.

◦ T)is error is #riarily due to t)e location o- t)e antenna on t)e aircra-t.◦ True e--ects can &e calculated during antenna #attern testing.

The numerous sources o! errors in N:;s limit their use!ulness to general

aviation where &irectional accuracy is not critical.

5!



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a&io Ai&s to Navigation roun& Testing The !irst test in any systems evaluation is the static test.

It is very important to ensure that the system is !unctioning properly on the

groun& prior to e0pen&ing valuable !light resources.

T)ere are any -acilities and uc) test e;ui#ent availa&le to t)e evaluator to aeyour lives a little easier.

8e s)ould evaluate t)e need -or testing in t)e -irst #lace.

emember that i! we have a&&e&" mo&i!ie&" or move& antennas or e'uipment

on the aircra!t" we will nee& to per!orm 9I>9C testing 5previously &escribe&

in Chapter 8. I! we have a&&e&" mo&i!ie&" or move& antennas" we nee& to

per!orm antenna pattern testing 5a&&resse& in Chapter 86.

I! we are testing a V=" it woul& be a&vantageous to chec# the system against

a V= chec# ra&ial or V= test !acility 5V=T6.

A c)ec radial is a surveyed #oint on t)e air-ield t)at in-ors t)e #ilot t)at :it) t)ecorrect


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The V=T emits an omni&irectional magnetic north ra&ial plus aural

i&enti!ication consisting o! a series o! &ots. It is monitore& to a tolerance o! O

+o.

With the V= properly tune& to the V=T 5+-E.- 9G3 in the Hnite& States6" the

&isplay shoul& in&icate a 7/-o ra&ial in&ication on the bearing pointer. With 7/-o

set into the trac# selector win&ow" the trac# bar shoul& be centere& with a

@!rom@ in&ication. The allowable error is 82.

I! we are evaluating a TACAN" a TACAN test set is available.

The test set re'uires entries o! channel number" range" an& bearing.

With the aircra!t TACAN turne& on" enter the same channel number that has

been entere& into the test set. =nce tune&" the coc#pit in&ications o! range an&

bearing shoul& be the same as those entere& into the test set.

As with all avionics systems" controls" an& &isplays" built*in testing 5;IT6 an&

sel!* testing shoul& also be accomplishe&.

=ontrols and dis#lays testing are covered in c)a#ter ! and :ill not &e addressed)ere.

A sel!*test chec#s the electrical continuity o! the system out to the antennas.

A ;IT" i! available" chec#s the so!tware o! the system.

Indications9 results9 and #ossi&le logging o- errors :ill &e availa&le in t)e systesDanuals.

Tetin,

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Since all o- t)e systes :e )ave discussed #rovide us :it) aCiut) or range9t)ere are accuracy re;uireents associated :it) t)ese systes.

◦


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=ur aircra!tMs position at the time o! the bearing>range rea&ing is provi&e& by

TSPI&ata.

=ur altitu&e an& magnetic hea&ing is provi&e& by the telemetry stream or by

recor&ing the in!ormation in the coc#pit.

The V=>TACAN>N:; an& magnetic variation are #nown because the sight

&oes not move. We will &iscuss where this in!ormation can be !oun& in a

moment.

The coc#pit rea&ing is ta#en !rom the aircra!tMs instruments.

The geometry shoul& be set up as shown in (igure )*+/ .

In any avionics test9 :e try to ee# t)e easurand *t)e #araeter :e are evaluating+as constant as #ossi&le to reduce easureent errors.

(or bearing accuracy " we woul& !ly either inboun& or outboun& on a ra&ial an&

the bearing will stay constant. (or range accur acy" we woul& !ly an arc aroun& the site in or&er to #eep

the :9 constant.

8)en :e are ready to collect data9 :e :ould call a JarJ and note t)e re;uiredeasureents. A sample &ata car& !or this test is shown in Table )*8.

63

R-+i& Ai+ t& N-0i,-ti&n

"li,ht Tetin,


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64


"li,ht Tetin,


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65


"li,ht Tetin,



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The items in the top hal! o! the &ata car& can be !ille& out be!ore the !light.

The items in the lower hal! must be collecte& &uring the !light. The last two columns o! the lower hal! are calculate& post*!light. The

calculation is per!orme& by using a coor&inate conversion routine" converting

&elta 56 latitu&e" longitu&e" an& altitu&e to bearing an& slant range.

The error that is calculate& shoul& be put into absolute value terms.

emember that your initial answer will be a true bearing which must be

converte& to a magnetic bearing 5since the coc#pit rea&ing is in magnetic6"hence the nee& !or magnetic variation.

The magnetic variation that is applie& is always the magnetic variation at the

ra&io ai&.

The test appears straight!orwar& enoughB the only in!ormation that is missing

is the ra&io ai& &ata. This in!ormation is most easily obtaine& !rom an airport

!acility &irectory or en route supplement.A !acility &irectory is shown in (igure )*+


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67


"li,ht Tetin,


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A sample o! the in!ormation containe& in the !acility &irectory is shown in

(igure )*+E. To &emonstrate we will use a combination V=>TACAN station 5calle& a

V=TAC6 name& Gector which is locate& in Southern Cali!ornia about E- miles

east o! Palm&ale or


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6!


"li,ht Tetin,


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It is the pilotMs responsibility to chec# the N=TA9 !ile !or all !acilities along hisroute prior to !light so there will not be any surprises.

Testers also nee& to consult the N=TA9s prior to a test i! you plan on using a

ra&io ai& &uring the evaluation.

It woul& not be very bright to brie! the pilot that we are going to use Gector

V=TAC !or our test i! Gector is &own !or sche&ule& maintenance.

The !ar right corner tells us that we can !in& Gector on a ?os Angeles sectionalchart"

enroute high*altitu&e chart 8G" or enroute low*altitu&e chart 7C.

The secon& line starts out with an 5G6" which means that this ra&io ai& is a high*

altitu&e SSV.

The ne0t entry tells us that it is a V=TAC" the V= !re'uency is ++%.


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The ne0t section &eals with bloc#age areas aroun& Gector . Since Hector is situated in a valley &ordered &y ountains to t)e nort) and sout)9 line

o- sig)t *(S+ to t)e station :ill &e a consideration.

I- you are nort) o- t)e station you ust &e at or a&ove 109000 -t *:it)in 152, n+ or ator a&ove 149000 -t *&eyond 2, n+ to )ave radio (S.

This is another concern !or !light testers, 9a#e sure you can see the station

&uring the test. T)e -inal entry on t)e #age tells you :)o to contact -or -lig)t -ollo:ing *onitoring+.

In t)is case it is t)e Riverside -lig)t service station *SS+ and t)eir associated


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V=" TACAN" an& N:; systems are authori3e& !or nonprecision approaches. A nonprecision approach allows a pilot to per!orm the approach while in clou&s

as long as the ceiling an& visibility at the inten&e& air!iel& is above some

weather minimums.

In t)e Lnited States9 ost non#recision a##roac)es )ave :eat)er inius o- 500-t cloud ceiling and visi&ility over t)e run:ay o- 1 n. T)e a##roac) ay not &e -lo:ni- t)e o&servations are lo:er t)an t)e inius.

(t)er countries allo: #ilots to #er-or t)e a##roac) do:n to t)e inius even i-t)e o&servations are lo:er t)an t)e a##roac) inius.

72


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The basic approach plate 5(igure )*+4" Handout 6 provi&es the pilot with

in!ormation &e!ine& in terms o! range an& bearing !rom the ra&io ai& (which isnot always located at the airfield).

T)e large #icture in t)e center o- t)e #age is t)e J%odDs eyeJ vie: o- t)e a##roac)t)e saller #icture at t)e &otto rig)t is t)e vertical #ro-ile.

Belo: t)e vertical #ro-ile are t)e :eat)er inia6 straig)tin a##roac) *400 -t and114 n+ and circling a##roac) *500 -t and 114 n+.

An air-ield diagra is #ictured at t)e &otto le-t. T)e a##roac) is as -ollo:s6 over-ly t)e 'aldale


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T)ere are soe ot)er considerations in t)e evaluation o- radio aids integrations.T)ese considerations are listed &elo: in no #articular order o- i#ortance6

G Aircraft configuration.G Aircra-t con-igurations9 es#ecially on ilitary -ig)ters9 can

c)ange on a daytoday &asis9 and t)ese con-igurations can cause di--erent&locage areas -or rece#tion.

G I- antenna #attern testing is acco#lis)ed9 any o- t)ese concerns :ill &ealleviated.

G In -lig)t testing9 radio aids to navigation testing as :ell as counicationstesting :ill &e acco#lis)ed :it) t)e ost coon aircra-t con-iguration.

G Antenna s!itching .G


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G Statistical accurac y.

G In everyt)ing :e do in avionics testing :e are concerned :it) t)e con-idencelevel o- t)e data. Ideally :e :ould test range and &earing accuracy againstt)ree di--erent ground stations.

G 8e do t)is in case one o- t)e stations is out o- cali&ration and gives userroneous readings.

G 8e :ould only no: t)is i- t:o stations closely agree and t)e t)ird is not in t)e

&all #ar. Atmospheric effects.

As #reviously discussed9 N-./A-F testing :ill #rovide varied results &ased onatos#)eric conditions.

T)e tester is res#onsi&le -or collecting atos#)eric data during t)e test sosoe sense o- t)e data ay &e ade later.

+ropeller modulation. =are ust &e taen :)en evaluating antenna locations on #ro#eller driven and

rotary :ing aircra-t9 as t)e radio :ave :ill &e odulated :)en #assing t)roug)t)ese rotating devices.

76

Other &ni+er-ti&n in R-+i&

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&agnetic variation. &agnetic variation is al!ays applied at the ground station t)at is9 :e use t)e

lines o- agnetic variation closest to t)e ground station during our analysis.

0one of confusion. Above the groun& station there is a cone o! con!usion within which the

e'uipment receives only :9 in!ormation.

T)e cone can vary -ro !0K to 110K9 de#ending on t)e ty#e o- groundinstallation.

The most common is the #$% cone, and bearing information will be lost

when the &' reading is eual to the aircraft altitu&e in :9.

or ea#le9 an aircra-t -lying at 1,9000 -t :ill lose &earing in-oration:)en t)e "7 is e;ual to 3 n and :ill not regain &earing in-oration until t)e

"7 eceeds 3 n.

77

Other &ni+er-ti&n in R-+i&

N-0#Ai+ Tetin,


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All o- t)e #revious tests :ill &e valid to:ard a civil certi-ication9 )o:ever9 civilaut)orities re;uire soe additional testing as :ell as variations in et)odology.

These tests" which may be !oun& in the appropriate a&visory circulars 5ACs6"

are note& below,

1 Testing

Test ranges -ro t)e station -or &earing accuracy are eit)er 1!0 n or ,0 n

*de#ends on t)e aircra-t certi-ication altitude+. ongrange rece#tion s)ould &e accurate during 10K &an turns.


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"7 testing9 continued. A #enetration *)ig)altitude en route descent+ s)ould &e

acco#lis)ed -ro 359000-t do:n to 5000 -t9 5 to 10 n s)ort o- t)e -acility

A-F Testing

'ointer reversal s)ould &e tested to #rove t)at range e;uals t)e altitude -lo:n *cone

o- con-usion validation+.

Bearing accuracy is s)o:n &y -lying a iniu o- si )eadings over a no:nground #oint. Accuracy :ill &e s)o:n to &e :it)in 5K .

Indicator res#onse is a 1,0o c)ange :it)in 10 sec and accurate to :it)in 3K.

* roun& re!erence points must be at least one*hal! o! the service range o! the

station

5see Table )*7" below6.

7!

i0il ertific-ti&n &f R-+i&

Ai+

INERTIAL NAVIGATION


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In the earliest inertial navigation systems" gimbale& plat!orms isolate& theinstruments !rom the angular motions o! the vehicle.

T)e gyrosco#es acted as nullsensors9 driving gi&al servos t)at )eld t)egyrosco#es and acceleroeters at a -ied orientation relative to t)e 7art).

T)is #eritted t)e acceleroeter out#uts to &e integrated into velocity and #osition.

In t)e late 1/$0s and early 1/,0s9 t)e invention o- largedynaicrange gyrosco#es

and o- ore #o:er-ul air&orne co#uters #eritted t)e develo#ent o- Jstra#do:nJinertial systes in :)ic) t)e gyrosco#es and acceleroeters :ere ounted directlyon t)e ve)icle.

T)e gyrosco#es trac t)e rotation o- t)e ve)icle9 and algorit)s in t)e co#utertrans-or acceleroeter easureents -ro ve)icle coordinates to t)e navigationcoordinates :)ere t)ey can &e integrated.

In strap&own systems" the trans!ormation generate& by the computer per!orms

the angular*stabili3ation !unction o! the gimbal set in a plat!orm system.

In e!!ect" the attitu&e integration algorithms permit the construction o! an

@analytic@ plat!orm.

SYSTEMS #

T7E SYSTEM

8"

INERTIAL NAVIGATION


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(igure


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#AND$%&

Inerti-l N-0i,-t&r

82

INERTIAL NAVIGATION


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In a plat!orm system" the gimbal*isolate& structure" on which the gyroscopesan& accelerometers are mounte&" is calle& the stable element.

The gimbals 5(igure


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SYSTEMS

T7E SYSTEM

84

INERTIAL NAVIGATION


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When the inertial system is turne& on" it must be aligne& so that the computer

#nows the initial position an& groun&spee& o! the vehicle an& so that the plat*

!orm 5gimbale& or analytic6 has the correct initial orientation relative to the

arth.

The plat!orm is typically aligne& in such a way that its accelerometer input a0es

are hori3ontal. o!ten with one o! them pointe& north.

As the vehicle accelerates" maneuvers" an& cruises. the accelerometers

measure changes in velocity" an& the computer recor&s the position an&

velocity.

T)e inertial navigator also contains #o:er su##lies -or t)e instruents9 aco#uter9 o-ten a &attery to #rotect against #o:er transients9 and inter-acesto a dis#layandcontrol unit.

T)e syste ay &e #acaged in one or ore odules.

Ty#ical gi&aled systes in 1/!, :eig)ed 50 to $5 l& *ecluding

ca&les+9 o- :)ic) 20 l& :ere -or t)e #lat-or. Steadystate #o:er consu#tion :as a##roiately 200 8.

irstgeneration stra#do:n navigators *early 1/,0s+ :eig)ed 40 to 50 l& andconsued 100 to 150 8.

In 1//!9 stra#do:n systes :eig)ed 20 to 30 l& and consued a##roiately 30 8.

SYSTEMS

T7E SYSTEM

85

LTV A#$ C&r-ir II


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U.S. N-02 A#$E fr&1 Att-c4 S>u-+r&n ;)

?2, Secure voice

G. ANARN,4 digital TA=AN

G. ANA'P I ode 4

86


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A ty#ical #er-orance s#eci-ication -or an inertial syste contains t)e -ollo:ing6

1. SiCe and :eig)t.

2. =ooling andor )eating re;uireents.

3. 'o:er consu#tion during :aru# ands cruise. Increased servo #o:er re;uiredduring aneuvers *#lat-ors only+. 'o:er regulation *voltage and -re;uency+ andsusce#ti&ility to transients and oentary dro#outs.

4. aiu rates and accelerations along and around eac) ais.5.


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Inertial systes -or civil use are #acaged in accordance :it) ARIN= s#eci-ications

ilitary systes ust con-or to t)e a##lica&le "e#artent o- "e-ense standardsand s#eci-ications.

T)e e;ui#ent used to test #lat-ors or stra#do:n systes include t)e rate ta&le*t:o or t)reeais+ :)ic) a##lies angular rates a&out di--erent aes and t)e vi&rationta&le :)ic) su##lies oscillatory otion too t)e syste.

Inerti-l Secific-ti&n

88


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igure $.20 Ty#ical aircra-t inertial #lat-or *courtesy9 itton Systes+.Source6 Avionics Navigation Systes Second 7dition9 >ayton9 ried.

8!


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igure $.1, Ty#ical stra#do:n inertial navigatoror R% sensor asse&ly*courtesy9 Honey:ell Inc.+.

Source6 Avionics Navigation SystesSecond 7dition9 >ayton9 ried.

!"

ME 7ANI?ATION E@UATIONS#


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The mechani3ation e'uations calculate velocity an& position !rom the outputs

o! the hori3ontal accelerometers in a plat!orm or !rom the trans!orme&

accelerations in a strap&own system.

Several coor&inate !rames must be &e!ine& !or the purposes o! mechani3ing an

inertial navigator.

(igure


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OORDINATE "RAMES

!2


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The geographic coordinates in which the vehicle position is calculatedare labeled -.

(igure


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HANDOUT

(igure


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The navigation (or platform) coordinates " lie along the orthogonal

accelerometer input a0es in the case o! a gimbale& plat!orm.

In a strap&own system the " coor&inates are the a0es o! the analytic plat!orm

&e!ine& by the coor&inate trans!ormation matri0 or 'uaternion.

These a0es represent a set o! orthogonal accelerometers whose 0 an& y a0es

are level an& whose y *a0is ma#es an angle U west o! true north 5see (igure


96/357

OORDINATE "RAMES

!6


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In a strap&own system an a&&itional coor&inate trans!ormation relates the bo&y

!rame 5illustrate& in (igure


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urpose0 An accelerometer is a &evice that measures the !orce re'uire& to

accelerate a proo! massB thus" it measures the acceleration o! the vehicle

containing the accelerometer . (igure m p 3 a 4 - 1 & / m p 3 f *acceleroeter out#ut+ *$.1+

:)ere F is t)e -orce eerted on t)e #roo- ass &y t)e restoring s#ring or restoring

a#li-ier9 as s)o: in igure $.49 and F - is t)e un:anted distur&ing -orce caused &y-riction9 )ysteresis9 ec)anical da#ing and t)e lie.

I- t)e instruent is designed :it) negligi&le distur&ing -orces9 t)e restoring -orce is aeasure o- a G+ along t)e instruents in#ut ais.

◦ Accelerometers are use& to calculate the vehicleJs acceleration aB their

outputs must be correcte& !or gravitation - in the computer.

Acceler&1eter

!8

INERTIAL NAVIGATION SYSTEMS


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!!

Acceler&1eter



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The basic measurement instrument o! the inertial navigation system is theaccelerometer .

There are three accelerometers mounte& in the system,

◦ one to easure nort)sout) accelerations9

◦ one to easure east:est accelerations9

◦ and one to easure vertical acceleration.

There are two basic types o! accelerometers, a moving mass or a pen&ulum&evice.

The moving mass accelerometer is just a mass on a spring with a ruler

attache&.

The ruler may be an electromagnetic sensor that senses &istance.

When the vehicle accelerates" the mass moves an& the ruler measures the

movement. The system re'uires calibrate& springs" an& these are nearly impossible to

ma#e consistent.

1""

Acceler&1eter



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In a pen&ulum &evice" &ue to inertia" the pen&ulum will swing o!! the null

position when the aircra!t accelerates.

A signal #ico-- device tells )o: -ar t)e #endulu is o-- t)e null #osition.

T)e signal -ro t)is #ico-- device is sent to an a#li-ier and current-ro t)e a#li-ier is sent &ac into a tor;uer located in t)e acceleroeter.

A tor;ue is generated :)ic) restores t)e #endulu to t)e null #osition.

T)e aount o- current t)at is goes into t)e tor;uer is a -unction o- t)e accelerationt)e device is e#eriencing.

A !le0ure*pivote& accelerometer is shown in (igure


102/357

1"2

Acceler&1eter



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G "ue to t)e very sall ga#s ac)ieva&le &et:een t)e covers and t)e #roo-ass9

gas -il da#ing su##resses ec)anical resonances.G T)is #erits t)e acceleroeter to o#erate in )ig)-re;uency vi&ration environents.

1"3

Acceler&1eter

G2rSt-iliBe+ 6l-tf&r1


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In or&er to accurately measure the acceleration in a given a0is" the

accelerometer must be constantly aligne& to this a0is.

This is accomplishe& mechanically or computationally.

In the mechanical solution" the accelerometer is mounte& on a gimbal

assembly" commonly calle& a plat!orm.

The plat!orm" via the use o! gyroscopes 5gyros6" allows the aircra!t to

go through any attitu&e change yet maintain the accelerometers level.

(igure )*%+ shows a simple plat!orm structure. (igure )*%% shows how a gyro is use& to control the level o! the plat!orm.

The gyros an& the accelerometers are mounte& on a common gimbal.

8)en t)e gi&al is oved o-- o- t)e level #osition9 t)e s#in ais o- t)e gyro reains-ied t)e case o- t)e gyro is oved o-- level and t)e aount t)at t)e case is ti##ed:ill &e detected &y t)e signal #icu# in t)e gyro.

T)e signal is a#li-ied and sent to t)e gi&al drive otor9 :)ic) restores t)e gi&alto t)e level #osition.

1"4


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1"5



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1"6




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Since the accelerometer is always #ept level" it &oes not sense a component o!

gravity an& is only able to sense the hori3ontal accelerations o! the aircra!t as it

travels across the earth.

In or&er to #eep the accelerometers level with respect to the earth so that they

only sense hori3ontal accelerations" the !acts below must be accounte& !or.

◦ Wor#s !or a plat!orm that is !i0e& in space.

◦ Aircra!t !ly close to the earth" the earth is roun&" an& the earth is rotating.

arth rotation rate compensation is &epicte& in (igure )*%7.

T)e le-t side o- t)e -igure s)o:s :)at :ould )a##en to t)e #lat-or i- it did not account-or t)e eart)Ds rotation.

Without compensation the plat!orm maintains its same orientation in space" but

!rom the earthMs vantage point woul& appear to tip over every %8 hours.

To compensate !or this tipping" the plat!orm is !orce& to tilt in proportion to the

earthMs rotational rate.

With compensation" the o&Ms*eye view shows the plat!orm tipping over every %8

hours" but with respect to the earth, the platform remains level.

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The re'uire& rate compensation is a !unction o! the latitu&e o! the aircra!t

because the hori3ontal component o! the earth rotation rate sense& by the

gyros is a !unction o! latitu&e.

The ma0imum earth rotation rate is at the e'uator an& is +).-82>hr.

This value &ecreases as we move north or south" until it becomes 3ero at the

poles.

oveent o- t)e aircra-t around t)e eart) )as t)e sae e--ect on t)e #lat-or t)at:as caused &y t)e eart)Ds rotation.

T)is is due to t)e -act t)at t)e eart) is round and t)e aircra-t -lies an arc as it -ollo:st)e contour o- t)e eart).

T)e rate o- co#ensation is deterined &y using t)e aircra-tDs velocity.

7art) rotation rate and aircra-t oveent rate co#ensations are i#leented in t)esyste &y tor;uing t)e gyro.

The aircra!t movement rate an& the earth rotation rate terms are summe& an&

sent to a gyro tor'uer" which will always #eep the plat!orm level with respect to

the earth.

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*Si#+

A #endulu is said to &e at static rest only :)en its center o- gravity and its #ivotais are resting in t)e sae local vertical vector.

8)en a ve)icle carrying a #endulu accelerates9 t)e acceleration is introduced tot)e #endulu via t)e #ivot ais9 :)ic) ovesout o- t)e gravity vector t)e center o- gravity lags &e)ind.

T)e longitudinal ais o- t)e #endulu no: -ors soe angle ot)er t)an Cero :it) t)elocal gravity vector9 :)ic) in turn #roduces an angular acceleration o- t)e #endulu.

"uring constant velocity9 t)e #endulu sees to return to t)e vertical directly undert)e #ivot ais &ut continually overs)oots9 ani-esting itsel- as a #eriodic oscillation.

or a given ass9 t)e closer t)e #ivot ais is &roug)t to t)e center o- gravity t)e lo:ert)e #eriod o- oscillation and t)e -urt)er t)e center o- rotation.

I- t)e #ivot ais and t)e center o- gravity are &roug)t close enoug) toget)er9 t)ecenter o- turning can &e ade to coincide :it) t)e center o- t)e eart).

(nce t)is #endulu is &roug)t to static rest9 accelerations o- t)e #ivot ais cannotcause t)e #enduluDs longitudinal ais to -or any ot)er angle :it) t)e gravity vectorot)er t)an Cero.

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All )oriContal velocities :ill &e acco#anied &y t)e #ro#er angular velocities to

aintain constant alignent o- t)e #endulu to t)e rotating gravity vector. T)e #endulu :ill not oscillate &ecause o- )oriContal accelerations.

To prevent vehicular accelerations !rom causing an oscillation o! the stable

element in the INS" the plat!orm is mechani3e& to have an e'uivalent length o! a

pen&ulum e0ten&e& to the center o! the earth.

Any acceleration o- t)e #lat-or is a&out t)e eart)Ds center o- ass and t)at o- t)eec)aniCed #enduluDs center o- ass.

Any errors that intro&uce an o!!set in the plat!orm cause the e!!ective mass o!

the mechani3e& pen&ulum to be &isplace& an& intro&uce an oscillation with a

perio& o! E8.8 min.

This oscillation causes the plat!orm error to be average& out over a perio& o!

E8.8 min. This is calle& a Schuler perio& or cycle. The Schuler cycle is shown in

(igure )*7-. INS plat!orms that are Schuler tune& are use& to boun& any errors in the

system to acceptable limits so that they &o not continue to buil&.

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113


Schuler Cycle


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Two other !orces that must be consi&ere& are Coriolis an& centripetal e!!ects.

T)ese are considered as eternal or #)anto accelerations.

=oriolis -orces are a##arent &ecause t)e aircra-t is re-erenced to a rotating eart) anda##ear :)enever t)e aircra-t is -lying.

:ue to Coriolis !orces" an aircra!t moving to the north has an east acceleration"

an aircra!t moving east has an upwar& an& south acceleration" an& an aircra!t

moving initially upwar& has a west acceleration.

T)is eans t)at an aircra-t -lying -ro t)e e;uator to t)e Nort) 'ole :ill &e o&servedas -lying a curved #at) in s#ace9 even t)oug) it is -lying a straig)t line.

(igure )*%8 shows this phenomenon.

In or&er !or the aircra!t to maintain its northerly hea&ing" the accelerometer

signals must be correcte& accor&ing to aircra!t velocity an& present position.

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The oblateness o! the earth causes certain centripetal accelerations in certain

positions on the earth where the plumb line to the center o! the earth &oes note0actly match the true vertical.

The resulting element imbalance causes spurious centripetal accelerations.

Compensations !or this spurious acceleration must be ma&e be!ore the

acceleration is integrate& to compute velocity.

The oblate earth e!!ect is shown in (igure )*%).

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Since we are going to navigate with the INS" it is e+tremely important that we know

our origin and the orientation of true north and the center of the earth. The first step is to determine the gravity vector, which is a relatively simple tas#.

The pilot inserts the present position o! the aircra!t an& enters the !irst stage o!

alignment.

In t)is stage9 t)e gi&al drive otor oves t)e gi&als until t)e #endulu o- t)eacceleroeter *C ais +is aligned :it) t)e gravity vector.

At t)is #oint t)ere is Cero out#ut -ro t)e acceleroeter9 t)e co#uter sets t)e velocity toCero9 and attitude in-oration is availa&le.

A gyro syste used -or sta&iliCation *radar9 -or:ardlooing in-rared VIRW9 &acu#attitude syste+ or an attitude9 )eading9 re-erence syste *AHRS+ :ill cease at t)is #oint.

(igure )*%/ &epicts how the plat!orm orients itsel! to true north &uring the

alignment

processB this operation is calle& gyrocompassing. In t)e -ar le-t #osition9 t)e #lat-or is level and t)e co#uter assues t)at it is #ointing to

true nort).

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It only a##lies eart) rotation rate co#ensation to t)e east:est ':) gyro.

Since the plat!orm is misaligne&" the plat!orm will tilt o!! level as the earth rotates.

The tilt is &etecte& by the accelerometers an& a tor'uing signal is sent to the gyro

that controls the plat!orm in the hea&ing a0is.

The gyro control loop physically reorients the plat!orm towar& true north.

7ventually t)e #lat-or is oriented to:ard true nort) *-ar rig)t+9 t)e nort)sout) 'y)gyro re;uires no co#ensation9 and t)e #lat-or reains level.

As the aircra!t is !lown in the navigation mo&e" alignment o! the plat!orm is

maintaine& by the computer tor'uing the a3imuth gyro using a combination o!

earth rotation rate an& aircra!t movement rate.

The gyrocompass system 5ust described has one serious &isa&vantage, it cannot

fly in the polar regions.

I! the plat!orm were to be !lown &irectly over the poles" the plat!orm woul& have to

rotate +E-o at the instant it crosse& the pole. T)is is #)ysically i#ossi&le9 and in reality9 t)ese systes cannot &e o#erated :it)in a

-e: )undred iles o- t)e #oles &ecause o- t)e )ig) tor;uing rates t)at are re;uired toee# t)e #lat-or level to nort).

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This problem can be solve& with a wander angle inertial system.

The basic !un&amentals o! a wan&er angle system are the same as &escribe&

!or the gyrocompass system.

In or&er to allow the system to !ly in the polar regions" the plat!orm ta#es an

arbitrary angle with respect to true north &uring gyrocompassing.

This arbitrary angle is called the wander angle.

(igure )*%< shows the chronology o! events !or inclusion o! the wan&er angle.

In the !ar le!t o! the !igure" the plat!orm is levele&. As :it) t)e gyroco#ass alignent9 t)e #lat-or is level and all co#ensation is sent

to t)e east:est ':) ais.

Because t)is assu#tion is not correct9 t)e #lat-or :ill tilt o-- level as t)eeart) rotates9 in t)e sae anner as in t)e gyroco#ass ea#le.

T)e tilt is detected &y t)e acceleroeters as &e-ore9 &ut rat)er t)an sending all

tor;uing to align to nort)9 t)e signal is s#lit &et:een t)e : and y gyros. ventually the right combination o! earth rotation rate compensation to the two

gyros is &etermine& !or the particular wan&er angle.

◦ The ratio o! earth rotation rate compensation is use& to compute the initial

wan&er angle.

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?ightweight &igital computers permit the system to eliminate the gimbals which

create strap*&own systems. These sensors are strappe& &own to the vehicle an& computers are use& to

compute lateral an& vertical velocities.

There are a !ew sensors that may be use& in strap*&own inertial systems.

The most common in aviation use is the ring laser gyro (26-).

=ptical yroscopes,

=ptical gyroscopes were universally use& in strap&own inertial navigators in

+44/.

These gyros o!!er e0tremely high &ynamic range" linearity" ban&wi&th"

rugge&ness" an& reliability.

Strap&own ? systems have become the pre&ominant inertial navigators !or

commercial an& military aircra!t.

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The laser gyro wor#s on a physical principle &iscovere& by (rench physicist .

Sagnac in the +4--s. Sagnac !oun& that the &i!!erence in time that two beams" each traveling in

opposite &irections" ta#e to travel aroun& a close& path mounte& on a rotating

plat!orm is &irectly proportional to the spee& at which the plat!orm is rotating.

Alt)oug) Sagnac and ot)ers deonstrated t)e conce#t in t)e la&oratory9 it :as notuntil t)e 1/!0s9 :it) t)e advent o- t)e laser &ea and its uni;ue #ro#erties9 t)at t)e

#rinci#le could &e used in a gyro. The #ey properties o! the laser that ma#e the laser gyro possible are,

◦ the laserMs coherent light beam"

◦ its single !re'uency"

◦ its small amount o! &i!!usion"

◦ an&" its ability to be easily !ocuse&" split" an& &e!lecte&.

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The ? is compose& o! segments o! transmission paths con!igure& as either

a s'uare or a triangle an& connecte& with mirrors. (igure )*%E shows a &iagram o! a triangular ?.

T)e irrors in t)e diagra are located at 59!9 and $.

(ne o- t)e irrors *!+ is #artially silvered9 allo:ing lig)t t)roug) to t)e detector */+.

A laser *,+ is launc)ed into t)e transission #at) *4+ in &ot) directions9 esta&lis)ing astanding :ave resonant :it) t)e lengt) o- t)e #at).

T)e &eas are reco&ined and sent to t)e out#ut detector */+.

In the absence o! rotation" the path lengths will be the same an& the output will

be the total constructive inter!erence o! the two beams.

I! the apparatus rotates" there will be a &i!!erence in the path lengths travele&

by the two beams resulting in a net phase &i!!erence an& &estructive

inter!erence. The net signal will vary in amplitu&e &epen&ing on the phase shi!tB the resulting

amplitu&e is a measurement o! the phase shi!t" an& conse'uently" the

rotation rate about the bo&yMs a0is 576.

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Two*mo&e ?s 5(igure


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)igure 7.6 &o*+ode ring laser gyro ,courtesy o- Litton uidance and /ontrol 0yste+s.

0ource Aionics Naigation 0yste+s 0econd Edition ayton )ried.

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To provi&e a practical north*see#ing gyrocompass" three ?s are combine&

with three accelerometers to !orm a complete navigation" gui&ance" an& controlsystem.

(igure )*%4 &epicts a common ? INS system.

(ne o- t)e signi-icant attri&utes o- t)e laser gyro is its use o- very -e: oving #arts.

It is t)eoretically #ossi&le to &uild laser gyros :it)out any oving co#onents.

T)e s#inning ass gyros use gi&als9 &earings9 and tor;ue otors9 :)ereas t)eR% uses a ring o- laser lig)t9 rigid irrors9 and electronic devices.

T)ese di--erences ae t)e R% ore rugged t)an t)e s#inning ass gyros9 :it)greater relia&ility9 and a longer ean tie &et:een -ailures *TB+ :)ic) translatesto lo:er li-ecycle costs.

;ecause the laser gyro uses soli&*state components an& massless light" it is

insensitive to variations in the earthMs magnetic an& gravitational !iel&s.

S)oc and vi&ration )ave little i#act.

Hnli#e a conventional gyro that re'uires some time to warm up an& bring the

gyro up to spee&" the laser gyro is essentially rea&y as soon as it is turne& on.

T)is is an advantage -or ilitary aircra-t :it) alert re;uireents.

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There are some problems with the ?. The most severe problem encountere&

in the ? is that o! Qloc#*inR. In the +4/-Js" it was observe& that the ? was insensitive to low angular

rates" as illustrate& in (igure


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At )ig) rates9 t)e E-rictionF is overcoe &ecause t)e -re;uencies se#arate and t)egyro is ca#a&le o- easuring t)e rate.

8)en t)e laser gyro is rotated very slo:ly9 t)e -re;uencies o- t)e counterrotatinglasers &ecoe very close to eac) ot)er.

At t)is lo: rotation9 t)e nulls in t)e standing :ave get stuc on t)e irrors9 locingt)e -re;uency o- eac) &ea to t)e sae value9 and t)e inter-erence -ringes no longerove relative to t)e detector9 :)ic) causes t)e device to no longer trac angular#osition.

This @loc#*in@ e!!ect is compensate& !or by a&&ing &ithering. In a two*mo&e ?" mechanical biasing is employe& to overcome loc#*in.

T)e usual eans -or acco#lis)ing t)is is ec)anical Edit)er9 :)ic) is a largea#litude sinusoidal otion a##lied to t)e gyro &ody.

Ty#ically9 #ea dit)er rates are 100 degsec.

With &ithering" the entire apparatus is twiste& an& untwiste& about an a0is at a

rate convenient to the mechanical resonance o! the system" thus ensuring the

angular velocity o! the system is usually !ar !rom the loc#*in threshol&.

The out put o! the gyro must then be compensate& !or the &ither motion so

that the true rotation o! the vehicle can be &etermine&.

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irror

*tilted do:n

Intensitydiode assy

irror

*tilted do:n Anode

igure $.10 ultioscillator R%*courtesy o- itton %iuidance and

=ontrol Systes+.Source6 Avionics Navigation SystesSecond 7dition9 >ayton9 ried.

(ara&ay

rotator

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There are other gyros that may be use& in strap*&own systems.

A &evice relate& to the ? is the !iber*optic gyro 5(=6. The (= wor#s

e0actly as the ?" however" it utili3es transmission paths with a coile& !iber*

optic cable rather than a laser.

A vi&rating gyro taes =oriolis e--ects into account t)at is9 a vi&rating eleent*resonator+9 :)en rotated :ill cause a secondary vi&ration :)ic) is ort)ogonal to t)eoriginal vi&rating direction.

By sensing t)e secondary vi&ration9 t)e rate o- turn can &e detected. or vi&ration ecitation and detection9 t)e #ieCoelectric e--ect is o-ten used t)ere-ore

vi&rating gyros are o-ten called #ieCo9 ceraic9 or ;uartC gyros9 even t)oug) vi&rationand detection do not necessarily use t)e #ieCo e--ect.

T)is ty#e o- gyro is suita&le -or ass #roduction and is alost aintenance -ree.

T)is device )as a dra:&ac in t)at :)en it is used under eternal vi&ration9 it cannot

distinguis) &et:een secondary vi&ration and eternal vi&ration.

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In ost a##lications t)e syste )as t:o ass&alanced tuning -ors integrated on asilicon c)i# arranged J)andle to )andleJ so -orces cancel.

As t)e -ors are t:isted a&out t)e ais o- t)e )andle9 t)e vi&ration o- t)e tines tendsto continue in t)e sae #lane o- otion.

T)is otion )as to &e resisted &y electrostatic -orces -ro t)e electrodes under t)etines.

By easuring t)e di--erence in ca#acitance &et:een t:o tines o- a -or t)e systecan deterine t)e rate o- angular otion.

There are also accelerometer*only systems.

These systems use !our pen&ular accelerometers to measure all the possible

movements an& rotations.

Lsually t)ese are ounted :it) t)e :eig)ts in t)e corners o- a tetra)edron9 t)ere-oret)ey are called tetrahedral inertial platforms *TI's+.

8)en t)e ve)icle rolls9 t)e asses at o##osite ends :ill &e accelerated in o##osite

directions. 8)en t)e ve)icle )as linear acceleration9 t)e asses are accelerated in t)e sae

direction. These &evices are ine0pensive" but at the current time they su!!er !rom inaccuracy.

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%REA!

En+ &f "irt Sei&n

13

%ench Tetin, &f Inerti-l


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The e'uipment use& to test plat!orms or strap&own systems inclu&e the rate

table 5two*or three*a0is6 which applies angular rates about &i!!erent a0es an&the vibration table which supplies oscillatory motion to the system.

Rate ta&les are soeties used in Scors;y mode :)ere&y a sine otion on one aisis siultaneously a##lied :it) a cosine on t)e ot)er. T)is test induces a coning ratein t)e instruent #acage.

ate table an& vibration tests are also per!orme& over temperature using

thermal chambers.

N-0i,-ti&n S2te1

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lig)t test ites o- #articular interest to t)e evaluator include

◦ Tie to align

◦ Align ;uality

◦ Alignent ty#es

◦ Navigation circular error #ro&a&le *=7'+9 or dri-t rate

◦


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Tie to align is t)e iniu aount o- tie t)at t)e INS s)ould tae to #rovide ano#era&le navigation syste.

T)ese ties are eit)er called out in t)e anu-acturerDs s#eci-ications or &y t)e user int)e syste re;uireents.

It is i#ortant to note t)at t)e tie to align is associated :it) a navigation#er-orance nu&er.

In general9 t)e longer t)e syste is le-t in t)e alignent ode t)e greater t)e#er-orance9 or accuracy9 o- a syste.

or ea#le9 t)e s#eci-ication ay state t)at Jt)e AN/ASN


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I )ave revie:ed test re#orts o- INS evaluations :)ere t)e evaluator touted a dri-t rate

o- 0.5 n)r9 J-ar su#erior to t)e s#eci-ication.J L#on investigation :e -ind t)at t)e aircra-t :as le-t in t)e align ode -or

a tie longer t)an t)e 4 in called out in t)e s#eci-ication.

T)is constitutes a Jno testJ as -ar as s#eci-ication validation is concerned.

A typical &ata car& !or this test is shown in Table )*)

14"

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Note t)e in-oration t)at is i#ortant to t)is test.

T)e type o! alignment is eit)er noral9 stored )eading9 carrier9 interru#t9 or in-lig)t. T)ese ty#es o- alignents :ill &e addressed in ore detail s)ortly.

The latitu&e an& longitu&e are data entered &y t)e #ilot or o#erator.

T)e start time is t)e tie t)e syste is #laced into t)e align ode.

T)e rea&y light is the time t)at t)e o#erator is a##rised o- an o#era&le navigator*:)ic) ust eet t)e a##lica&le s#eci-ications+.

Navigate selecte& is t)e tie t)at t)e o#erator #laces t)e syste into navigate *t)istie ay not eceed t)e s#eci-ication tie -or navigation accuracy -lig)ts+.

T)e latitu&e an& longitu&e are again note& a!ter t)e o#erator #laces t)esyste to navigate *it s)ould &e t)e sae as entered &y t)e o#erator+.

T)e groun&spee& is also note& at the selection o! navigate and needs to &ee;ual to 3ero or t)e INS :ill start to dri-t a:ay ra#idly.

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The INS will also provi&e a 'uality or @X@ !actor &uring operation.

T)e U -actor is t)e INSDs &est guess at )o: )ealt)y it is. T)e U -actor is very siilar to t)e -igure o- erit *(+ :)ic) is #roduced in t)e

%'S.

T)e U -actor norally #rovides a nuerical value o- 1 t)roug) /9 :it) / &eing t)e&est.

In any integrated %'SINS systes9 t)e U -actor is used to deterine :)et)er INS

in-oration is good enoug) to &e used in t)e navigation solution.

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There are !ive general alignment types !or the INS.

It is important !or the tester to note that navigational accuracy is also

&epen&ent on the type o! alignment use&.

T)e general alignent ty#es are

1. Noral alignent

2. Stored )eading alignent

3. =arrier *=


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The normal alignment is the most common INS alignment type an& is use&

most o!ten operationally.

T)e o#erator oves t)e INS control -ro o-- to stand&y9 t)us #o:ering u# t)esyste.

"e#ending on t)e ty#e o- installation9 t)e o#erator ay go directly to t)e align odeor #ause in stand&y until t)e re;uired entries are ade.

The operator will be re'uire& to enter own*ship latitu&e an& longitu&eB this

allows the INS to per!orm its transport calculations. 5See ne0t sli&e6

I- t)e INS is a s#inning ass syste9 it :ill re;uire a :aru# #eriod9 :)ic) isde#endent on te#erature.

The speci!ication will call out how much time is allowe& !or the warm*up

perio&.

"uring t)is tie t)e gyros :ill coe u# to s#eed and t)e o#erator :ill &e given anindication :)en t)e :aru# #eriod )as ela#sed and t)e syste )as entered t)e -irst

#)ase o- t)e alignent. An ? system &oes not have a warm*up time an& will enter the alignment

phase imme&iately.

◦ The ne0t in&ication the operator will receive is completion o! the attitu&e

phase.

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Ali,n1ent T2e


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emember that this is the point where the system has levele& the plat!orm an&

the z accelerometer is aligne& with the gravity vector. 8e could go to navigate at t)is #oint9 &ut :e :ould )ave #lat-or re-erence data only

*)oriCon line+ t)e INS :ill not &e a&le to navigate.

The last phase is gyrocompassing" which allows the INS to properly align the

north*south>east*west a0es.

(nce acco#lis)ed9 t)e o#erator :ill receive a ready indication and t)e syste ay

&e #laced to navigate.

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*Si#+

A stored heading alignment is ost o-ten used in alert situations :)ere an aircra-t

needs to scra&le *tae o-- as soon as #ossi&le+. irst9 a noral alignent is #er-ored as #reviously descri&ed.

At t)e co#letion o- t)e alignent9 t)e o#erator re;uests t)at t)e )eading &e storedin t)e INS and t)en s)uts do:n t)e syste.

8)en t)e o#erator returns to t)e aircra-t in a ra#id reaction scenario9 )e :ill #o:er u#t)e INS as noral and t)e syste :ill advise )i t)at a stored )eading is availa&le.

T)e o#erator )as t)e o#tion o- acce#ting t)e stored )eading or entering into a noralalignent se;uence.

Since t)e #resent #osition and true )eading *and )ence true nort)+ are alreadyno:n &y t)e syste9 only #lat-or leveling is re;uired.

(nce t)e #lat-or is leveled9 an o#era&le navigator is availa&le.

T)e advantage is tie9 since #lat-or lateral re-erencing does not need to &eacco#lis)ed.

A stored )eading alignent usually taes a&out )al- t)e tie to #er-or as anoral alignent.

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*Si#+

T)ere are soe disadvantages to t)is alignent9 )o:ever6 t)e accuracyis not as good as a noral alignent and t)e aircra-t cannot &e oved in t)e interitie &et:een t)e -irst alignent and t)e second alignent.

T)e reason -or t)e oveent restriction s)ould &e o&vious t)e aircra-t a##lies itseart) rotation rate co#ensation to t)e gyros &ased on a no:n true )eading.

oving t)e #lat-or :ill cause t)e gyros to tu&le as soon as navigate is selected &y

t)e o#erator.

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Aircra!t carriers provi&e some uni'ue problems !or INS alignments.

As t)e aircra-t sits on t)e dec o- t)e carrier it is su&ected to t)e -or:ard otion o-t)e s)i# as :ell as t)e #itc)ing and rolling oveents o- t)e ocean.

A noral alignent :ould &e i#ossi&le to #er-or in suc) situations.

The carrier alignment mo&e 5sometimes calle& the CV align mo&e6 allows the

INS to per!orm an alignment in these con&itions.

*A$76 A ty#ical SINS alignent re;uired 1112 inutes as o##osed to a 10inute

s)ore&ased alignent.+ In #re%'S days9 t)e s)i#Ds INS9 or SINS9 in-oration :as -ed to t)e aircra-t &y eit)er

a )ard:ire or icro:ave lin.

T)e aircra-t INS :as said to &e aligned :)en it :as in sync :it) t)e s)i#Ds INS.

T)is et)od is still used today9 )o:ever9 %'S aes it #ossi&le -or t)e aircra-t to#er-or a =< alignent very ;uicly9 #roviding siilar accuracies.

15"


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To&ay" PS can provi&e the INS with present position" groun&spee&" an& trac#

at up&ates once per secon&.*Si#+

T)e alignent can tae as little as 30 see and #rovides accuracies co#ara&le to t)enoral alignent.

T)ere are t:o ty#es o- interrupted alignments6 po!er interrupt and ta:i interrupt .

T)e INS s)ould &e a&le to :it)stand #o:er interru#tions on t)e ground during t)e

alignent se;uence. T)e INS )as a &attery :)ic) s)ould #rovide eergency #o:er during transient

o#erations.

T)is alone is a good reason -or -lig)t testers to c)ec t)e &attery -or #ro#er o#eration&e-ore #er-oring any INS test.

T)is evaluation is easily acco#lis)ed &y starting t)e alignent on eternal #o:er

and trans-erring to internal #o:er id:ay t)roug) t)e se;uence. T)e transient s)ould &e trans#arent to t)e INS.

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The secon& type o! interruption is the ta0i interrupt" which is controlle& by the

position o! the han&bra#e. I! the han&bra#e is release& &uring an alignment" the alignment will be

suspen&e& until the han&bra#e is reset.

The INS uses all the in!ormation gathere& be!ore the suspension to continue

the alignment a!ter the han&bra#e is reset.

This works well for a short ta+i in the forward direction only .

I- t)e aircra-t is oved in )eading or -or any distance9 t)e alignent ust&e reinitiated9

T)e ode is designed to allo: t)e aircra-t to &e oved sl

5_Navigation Systems-Final (5)

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