Global Positiong System

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ABSTRACT

The GPS (Global Positioning System) is a constellation of 24 well-spaced satellites that orbit

the arth and ma!e it possible for people with gro"nd recei#ers to pinpoint their geographic

location$ The location acc"racy is anywhere from %&& to %& meters for most e'"ipment$

cc"racy can be pinpointed to within one (%) meter with special military-appro#ed e'"ipment$

GPS e'"ipment is widely "sed in science and has now become s"fficiently low-cost so that

almost anyone can own a GPS recei#er$

GPS recei#ers are becoming cons"mer prod"cts$ n addition to their o"tdoor "se (hi!ing* cross-

co"ntry s!iing* ballooning* flying* and sailing)* recei#ers can be "sed in cars to relate the dri#er+s

location with traffic and weather information$ ,ere are some eb locations that describe GPS

recei#er prod"cts$The GPS is owned and operated by the .$S$ /epartment of /efense b"t is

a#ailable for general "se aro"nd the world$ 0riefly* here+s how it wor!s1

2% GPS satellites and three spare satellites are in orbit at %&*&& miles abo#e the arth$ The

satellites are spaced so that from any point on arth* fo"r satellites will be abo#e the hori3on$

ach satellite contains a comp"ter* an atomic cloc!* and a radio$ ith an "nderstanding of its

own orbit and the cloc!* it contin"ally broadcasts its changing position and time$ (nce a day*

each satellite chec!s its own sense of time and position with a gro"nd station and ma!es any

minor correction$) n the gro"nd* any GPS recei#er contains a comp"ter that triang"lates its

own position by getting bearings from three of the fo"r satellites$ The res"lt is pro#ided in the

form of a geographic position - longit"de and latit"de - to* for most recei#ers* within %&& meters$

f the recei#er is also e'"ipped with a display screen that shows a map* the position can be

shown on the map$ f a fo"rth satellite can be recei#ed* the recei#er5comp"ter can fig"re o"t the

altit"de as well as the geographic position$

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Contents Page No

Chapter 1 %$& ,istory of man and na#igation7777777777$$77777$$74

Chapter 2

2$& GPS classification7777777777777777$$77777$$ 2$% GPS as a satellite 7777777777777777$$77$$$$$$$$$$$$$$$82$2 Space segment$777777777777777777$$$777$$$$$$82$9 :ontrol segment777777777777777777$$$7777$82$4 .ser segment$77777777777777777777$$$777$;

Chapter 39$% 0ig idea geometrically$77$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$77777777777ten"ating circ"mstance7$$77777777777777777%;$? here GPS obtained77777$$$$$$777777777777777% yo"r position with any real degree of acc"racy when the s"n

or stars are #isible$ /"ring clo"dy weather* no reliable means of locating one+s position e>isted$

C"rthermore* altho"gh a compass can tell yo" in which direction yo" are headed* it can not tell

yo" in what direction yo" are act"ally tra#eling$ These two directions co"ld be somewhat

different* depending on conditions of wind* wa#es* and c"rrents$ Th"s* dead rec!oning is often

not a #ery satisfactory method of na#igating for airplanes and ships$

n the early %


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n a similar way* the introd"ction of radar in the %es as good as abo"t 9&

to %&& meters$ This offers good precision in non-dynamic sit"ations* b"t position fi>es are only

a#ailable when the satellites are in #iew* which* at best* is perhaps once e#ery ho"r$

GPS* "nli!e any of these other systems* offers a real-time position fi> with an acc"racy of 9 to

%&& meters on a 24 ho"r per day basis$ t also offers a way to determine time to a precision of

better than a few h"ndred nanoseconds almost anywhere on 5 aro"nd the s"rface of the earth$

These are capabilities that will completely re#ol"tioni3e many areas of life in the near f"t"re$

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Chapter 2

20 The 'PS C(ass"f"cat"on

The GPS (Global Positioning System) is a constellation of 24 well-spaced satellites that orbitthe arth and ma!e it possible for people with gro"nd recei#ers to pinpoint their geographic

location$ The location acc"racy is anywhere from %&& to %& meters for most e'"ipment$

cc"racy can be pinpointed to within one (%) meter with special military-appro#ed e'"ipment$

GPS e'"ipment is widely "sed in science and has now become s"fficiently low-cost so that

almost anyone can own a GPS recei#er$

GPS recei#ers are becoming cons"mer prod"cts$ n addition to their o"tdoor "se (hi!ing* cross-

co"ntry s!iing* ballooning* flying* and sailing)* recei#ers can be "sed in cars to relate the dri#er+s

location with traffic and weather information$ ,ere are some eb locations that describe GPS

recei#er prod"ctsThe GPS is owned and operated by the .$S$ /epartment of /efense b"t is

a#ailable for general "se aro"nd the world$ 0riefly* here+s how it wor!s1

2% GPS satellites and three spare satellites are in orbit at %&*&& miles abo#e the arth$ The

satellites are spaced so that from any point on arth* fo"r satellites will be abo#e the hori3on$

ach satellite contains a comp"ter* an atomic cloc!* and a radio$ ith an "nderstanding of its

own orbit and the cloc!* it contin"ally broadcasts its changing position and time$ (nce a day*each satellite chec!s its own sense of time and position with a gro"nd station and ma!es any

minor correction$) n the gro"nd* any GPS recei#er contains a comp"ter that triang"lates its

own position by getting bearings from three of the fo"r satellites$ The res"lt is pro#ided in the

form of a geographic position - longit"de and latit"de - to* for most recei#ers* within %&& meters$

f the recei#er is also e'"ipped with a display screen that shows a map* the position can be

shown on the map$ f a fo"rth satellite can be recei#ed* the recei#er5comp"ter can fig"re o"t the

altit"de as well as the geographic position$

f yo" are mo#ing* yo"r recei#er may also be able to calc"late yo"r speed and direction of tra#el

and gi#e yo" estimated times of arri#al to specified destinations$

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The GPS is being "sed in science to pro#ide data that has ne#er been a#ailable before in the

'"antity and degree of acc"racy that the GPS ma!es possible$ Scientists are "sing the GPS to

meas"re the mo#ement of the arctic ice sheets* the arth+s tectonic plates* and #olcanic acti#ity1

21 'PS as a sate(("te Na&"gat"on

GPS is f"nded by and controlled by the .$ S$ /epartment of /efense (//)$ hile there are

many tho"sands of ci#il "sers of GPS world-wide* the system was designed for and is operated

by the .$ S$ military$

GPS pro#ides specially coded satellite signals that can be processed in a GPS recei#er* enabling

the recei#er to comp"te position* #elocity and time$

Co"r GPS satellite signals are "sed to comp"te positions in three dimensions and the time offset

in the recei#er cloc!$

22 Space Seg)ent

The Space Segment of the system consists of the GPS satellites$ These space #ehicles (SEs) send

radio signals from space$

The nominal GPS perational :onstellation consists of 24 satellites that orbit the earth in %2

ho"rs$ There are often more than 24 operational satellites as new ones are la"nched to replace

older satellites$ The satellite orbits repeat almost the same gro"nd trac! (as the earth t"rns

beneath them) once each day$ The orbit altit"de is s"ch that the satellites repeat the same trac!

and config"ration o#er any point appro>imately each 24 ho"rs (4 min"tes earlier each day)$

There are si> orbital planes (with nominally fo"r SEs in each)* e'"ally spaced (& degrees apart)*

and inclined at abo"t fifty-fi#e degrees with respect to the e'"atorial plane$

23 Contro( seg)ent

The =aster :ontrol facility is located at Schrie#er ir Corce 0ase (formerly Calcon C0) in

:olorado$ These monitor stations meas"re signals from the SEs which are incorporated into

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orbital models for each satellites$ The models comp"te precise orbital data (ephemeris) and SE

cloc! corrections for each satellite$ The =aster :ontrol station "ploads ephemeris and cloc! data

to the SEs$ The SEs then send s"bsets of the orbital ephemeris data to GPS recei#ers o#er radio

signals$

24 *ser Seg)ent

The GPS .ser Segment consists of the GPS recei#ers and the "ser comm"nity$ GPS recei#ers

con#ert SE signals into position* #elocity* and time estimates$ Co"r satellites are re'"ired to

comp"te the fo"r dimensions of F* * H (position) and Time$ GPS recei#ers are "sed for

na#igation* positioning* time dissemination* and other research$ Aa#igation in three dimensions

is the primary f"nction of GPS$ Aa#igation recei#ers are made for aircraft* ships* gro"nd

#ehicles* and for hand carrying by indi#id"als$

!ere+s ho, 'PS ,or-s "n f"&e (og"ca( steps.

%$ The basis of GPS is Itriang"lationI from satellites$

2$ To Itriang"late*I a GPS recei#er meas"res distance "sing the tra#el time of radio signals$

9$ To meas"re tra#el time* GPS needs #ery acc"rate timing which it achie#es with some

tric!s$

4$ long with distance* yo" need to !now e>actly where the satellites are in space$ ,igh

orbits and caref"l monitoring are the secret$

?$ Cinally yo" m"st correct for any delays the signal e>periences as it tra#els thro"gh the

atmosphere$ mprobable as it may seem* the whole idea behind GPS is to "se satellites in

space as reference points for locations here on earth$

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Chapter 3

31 The B"g /ea 'eo)etr"ca((#

S"ppose we meas"re o"r distance from a satellite and find it to be %%*&&& miles$ Jnowing that

weKre %%*&&& miles from a partic"lar satellite narrows down all the possible locations we co"ld be

in the whole "ni#erse to the s"rface of a sphere that is centered on this satellite and has a radi"s

of %%*&&& miles$ Ae>t* say we meas"re o"r distance to a second satellite and find o"t that itKs

%2*&&& miles away$ That tells "s that weKre not only on the first sphere b"t weKre also on a sphere

thatKs %2*&&& miles from the second satellite$ r in other words* weKre somewhere on the circle

where these two spheres intersect* f we then ma!e a meas"rement from a third satellite and find

that weKre %9*&&& miles from that one* that narrows o"r position down e#en f"rther* to the two

points where the %9*&&& mile sphere c"ts thro"gh the circle thatKs the intersection of the first two

spheres$

32 Tr"ang(at"ng

Position is calc"lated from distance meas"rements (ranges) to satellites$

=athematically we need fo"r satellite ranges to determine e>act position$

Three ranges are eno"gh if we reLect ridic"lo"s answers or "se other tric!s$

nother range is re'"ired for technical reasons to be disc"ssed later$

33 $easr"ng "stance

e saw in the last section that a position is calc"lated from distance meas"rements to at least

three satellites$ 0"t how can yo" meas"re the distance to something thatKs floating aro"nd in

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spaceM e do it by timing how long it ta!es for a signal sent from the satellite to arri#e at o"r

recei#er$

34 The B"g /ea $athe)at"ca((#

n a sense* the whole thing boils down to those I#elocity times tra#el timeI math problems we

did in high school$ Bemember the old1 If a car goes & miles per ho"r for two ho"rs* how far

does it tra#elMI

Eelocity (& mph) > Time (2 ho"rs) N /istance (%2& miles)

n the case of GPS weKre meas"ring a radio signal so the #elocity is going to be the speed of light

or ro"ghly %;*&&& miles per second$ The problem is meas"ring the tra#el time$ something li!e

&$& seconds$ So weKre going to need some really precise cloc!s$ eKll tal! abo"t those soon$0"t

ass"ming we ha#e precise cloc!s* how do we meas"re tra#el timeM To e>plain it letKs "se a goofy

analogy1 S"ppose there was a way to get both the satellite and the recei#er to start playing IThe

Star Spangled 0annerI at precisely %2 noon$ These two #ersions wo"ld be o"t of sync$ The

#ersion coming from the satellite wo"ld be a little delayed beca"se it had to tra#el more than

%%*&&& miles$ f we wanted to see L"st how delayed the satelliteKs #ersion was* we co"ld start

delaying the recei#erKs #ersion "ntil they fell into perfect sync$ The amo"nt we ha#e to shift bac!

the recei#erKs #ersion is e'"al to the tra#el time of the satelliteKs #ersion$ So we L"st m"ltiply that

time the speed of light and bingo$ e ha#e got o"r distance to the satellite$

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Chapter 4

40 !o, 'PS ,or-s

nly instead of the Star Spangled 0anner the satellites and recei#ers "se something called a

IPse"do Bandom :odeI - which is probably easier to sing than the Star Spangled 0anner$

41$easr"ng "stance

%$ /istance to a satellite is determined by meas"ring how long a radio signal ta!es to reach

"s from that satellite$

2$ To ma!e the meas"rement we ass"me that both the satellite and o"r recei#er are

generating the same pse"do-random codes at e>actly the same time$

9$ 0y comparing how late the satelliteKs pse"do-random code appears compared to o"r

recei#erKs code* we determine how long it too! to reach "s$

4$ ="ltiply that tra#el time by the speed of light and yo"K#e got distance

42 T")"ng

f meas"ring the tra#el time of a radio signal is the !ey to GPS* then o"r stop watches had better

be darn good* beca"se if their timing is off by L"st a tho"sandth of a second* at the speed of light*

that translates into almost 2&& miles of errorOn the satellite side* timing is almost perfect

beca"se they ha#e incredibly precise atomic cloc!son board$0"t what abo"t o"r recei#ers here

on the gro"ndM Bemember that both the satellite and the recei#er need to be able to precisely

synchroni3e their pse"do-random codes to ma!e the system wor!f o"r recei#ers needed atomic

cloc!s (which cost "pwards of ?&J to %&&J) GPS wo"ld be a lame d"c! technology$ Aobody

co"ld afford it$

@"c!ily the designers of GPS came "p with a brilliant little tric! that lets "s get by with m"ch

less acc"rate cloc!s in o"r recei#ers$ This tric! is one of the !ey elements of GPS and as an

added side benefit it means that e#ery GPS recei#er is essentially an atomic-acc"racy cloc!$

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The secret to perfect timing is to ma!e an extrasatellite meas"rement$ ThatKs right* if three

perfect meas"rements can locate a point in 9-dimensional space* then fo"r imperfect

meas"rements can do the same thing$

43 tra $easre)ent Cres T")"ng ffset

f o"r recei#erKs cloc!s were perfect* then all o"r satellite ranges wo"ld intersect at a single point

(which is o"r position)$ 0"t with imperfect cloc!s* a fo"rth meas"rement* done as a cross-chec!*

will AT intersect with the first three$So the recei#erKs comp"ter says I.h-ohO there is a

discrepancy in my meas"rements$ m"st not be perfectly synced with "ni#ersal time$ISince any

offset from "ni#ersal time will affect all of o"r meas"rements* the recei#er loo!s for a single

correction factor that it can s"btract from all its timing meas"rements that wo"ld ca"se them all

to intersect at a single point$That correction brings the recei#erKs cloc! bac! into sync with

"ni#ersal time* and bingoO - yo"K#e got atomic acc"racy time right in the palm of yo"r hand$nce

it has that correction it applies to all the rest of its meas"rements and now weK#e got precise

positioning$ne conse'"ence of this principle is that any decent GPS recei#er will need to ha#e

at least fo"r channels so that it can ma!e the fo"r meas"rements sim"ltaneo"sly$ ith the

pse"do-random code as a roc! solid timing sync p"lse* and this e>tra meas"rement tric! to get "s

perfectly synced to "ni#ersal time* we ha#e got e#erything we need to meas"re o"r distance to a

satellite in space$

0"t for the triang"lation to wor! we not only need to !now distance* we also need to !now

e>actly where the satellites are$

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44 'ett"ng Perfect T")"ng

%$ cc"rate timing is the !ey to meas"ring distance to satellites$

2$ Satellites are acc"rate beca"se they ha#e atomic cloc!s on board$

9$ Becei#er cloc!s donKt ha#e to be too acc"rate beca"se an e>tra satellite range

meas"rement can remo#e errors$

45 Sate(("te Pos"t"ons

n this t"torial weK#e been ass"ming that we !now where the GPS satellites are so we can "se

them as reference points$0"t how do we !now exactlywhere they areM fter all theyKre floating

aro"nd %%*&&& miles "p in space$

46 A h"gh sate(("te gathers no )oss

That %%*&&& mile altit"de is act"ally a benefit in this case* beca"se something that high is well

clear of the atmosphere$ nd that means it will orbit according to #ery simple mathematics$The

ir Corce has inLected each GPS satellite into a #ery precise orbit* according to the GPS master

plan$n the gro"nd all GPS recei#ers ha#e an almanac programmed into their comp"ters that

tells them where in the s!y each satellite is* moment by momentThe basic orbits are '"ite e>act

b"t L"st to ma!e things perfect the GPS satellites are constantly monitored by the /epartment of

/efense$They "se #ery precise radar to chec! each satelliteKs e>act altit"de* position and speed$

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The errors theyKre chec!ing for are called Iephemeris errorsI beca"se they affect the satelliteKs

orbit or Iephemeris$I These errors are ca"sed by gra#itational p"lls from the moon and s"n and

by the press"re of solar radiation on the satellites$The errors are "s"ally #ery slight b"t if yo"

want great acc"racy they m"st be ta!en into acco"nt

47 'ett"ng the )essage ot

nce the /o/ has meas"red a satelliteKs e>act position* they relay that information bac! "p to the

satellite itself$ The satellite then incl"des this new corrected position information in the timing

signals itKs broadcasting$ So a GPS signal is more than L"st pse"do-random code for timing

p"rposes$ t also contains a na#igation message with ephemeris information as well$ith perfect

timing and the satelliteKs e>act position yo"Kd thin! weKd be ready to ma!e perfect position

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Chapter 5

50 rror Correct"on

Cirst* one of the basic ass"mptions weK#e been "sing thro"gho"t this t"torial is not e>actly tr"e$

eK#e been saying that yo" calc"late distance to a satellite by m"ltiplying a signalKs tra#el time

by the speed of light$ 0"t the speed of light is only constant in a #ac""m$ s a GPS signal passes

thro"gh the charged particles of the ionosphere and then thro"gh the water #apor in the

troposphereit gets slowed down a bit* and this creates the same !ind of error as bad cloc!s$

There are a co"ple of ways to minimi3e this !ind of error$ Cor one thing we can predict what atypical delay might be on a typical day$ This is called modelingand it helps b"t* of co"rse*

atmospheric conditions are rarely e>actly typical$ nother way to get a handle on these

atmosphere-ind"ced errors is to compare the relati#e speeds of two different signals$ This I d"al

fre'"encyI meas"rement is #ery sophisticated and is only possible with ad#anced recei#ers$

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51 Pro(e)s at the sate(("te

#en tho"gh the satellites are #ery sophisticated they do acco"nt for some tiny errors in the

system$ The atomic cloc!s they "se are #ery* #ery precise b"t theyKre not perfect$ =in"te

discrepancies can occ"r* and these translate into tra#el time meas"rement errors$ nd e#en

tho"gh the satellites positions are constantly monitored* they canKt be watched e#ery second$ So

slight position or IephemerisI errors can snea! in between monitoring times$ Tro"ble for the

GPS signal doesnKt end when it gets down to the gro"nd$ The signal may bo"nce off #ario"s local

obstr"ctions before it gets to o"r recei#er$

This is called m"ltipath error and is similar to the ghosting yo" might see on a TE$ Good

recei#ers "se sophisticated signal reLection techni'"es to minimi3e this problem$ 0asic geometry

itself can magnify these other errors with a principle called IGeometric /il"tion of PrecisionI or

G/P$ t so"nds complicated b"t the principle is '"ite simple$ f it pic!s satellites that are

widely separated the circles intersect at almost right angles and that minimi3es the error region$

Good recei#ers determine which satellites will gi#e the lowest G/P$

52 Correct"ng rrors

%$ The earthKs ionosphere and atmosphere ca"se delays in the GPS signal that translate into

position errors$ See a s"mmary of error so"rces$

2$ Some errors can be factored o"t "sing mathematics and modeling$

9$ The config"ration of the satellites in the s!y can magnify other errors$

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Chapter 6

60 "fferent"a( 'PS

hy we need /ifferential GPS

,ow /ifferential GPS wor!s

here to get /ifferential :orrections

ther ways to wor! with /ifferential GPS

d#anced :oncepts

61 ! N 'PS

0asic GPS is the most acc"rate radio-based na#igation system e#er de#eloped$ nd for many

applications itKs plenty acc"rate$ 0"t itKs h"man nat"re to want =BO So some crafty engineers

came "p with I/ifferential GPS*I a way to correct the #ario"s inacc"racies in the GPS system*

p"shing its acc"racy e#en farther$ /ifferential GPS or I/GPSI can yield meas"rements good to a

co"ple of meters in mo#ing applications and e#en better in stationary sit"ations$ That impro#ed

acc"racy has a profo"nd effect on the importance of GPS as a reso"rce$ ith it* GPS becomes

more than L"st a system for na#igating boats and planes aro"nd the world$ t becomes a "ni#ersal

meas"rement system capable of positioning things on a #ery precise scale$

62 !o, 'PS ,or-

/ifferential GPS in#ol#es the cooperation of two recei#ers* one thatKs stationary and another

thatKs ro#ing aro"nd ma!ing position meas"rements$ The stationary recei#er is the !ey$ t ties all

the satellite meas"rements into a solid local reference it wor!s$

63 The pro(e)

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Bemember that GPS recei#ers "se timing signals from at least fo"r satellites to establish a

position$ ach of those timing signals is going to ha#e some error or delay depending on what

sort of perils ha#e befallen it on its trip down to "s$ Since each of the timing signals that go into a

position calc"lation has some error* that calc"lation is going to be a compo"nding of those errors$

64 An etenat"ng c"rc)stance

@"c!ily the sheer scale of the GPS system comes to o"r resc"e$ The satellites are so far o"t in

space that the little distances we tra#el here on earth are insignificant$ So if two recei#ers are

fairly close to each other* say within a few h"ndred !ilometers* the signals that reach both of

them will ha#e tra#eled thro"gh #irt"ally the same slice of atmosphere* and so will ha#e #irt"ally

the same errors$ ThatKs the idea behind differential GPS1 e ha#e one recei#er meas"re the

timing errors and then pro#ide correction information to the other recei#ers that are ro#ing

aro"nd$ That way #irt"ally all errors can be eliminated from the system* e#en the pes!y Selecti#e

#ailability error that the /o/ p"ts in on p"rpose$ The idea is simple$ P"t the reference recei#er

on a point thatKs been #ery acc"rately s"r#eyed and !eep it there$ This reference station recei#es

the same GPS signals as the ro#ing recei#er b"t instead of wor!ing li!e a normal GPS recei#er it

attac!s the e'"ations backwards$ nstead of "sing timing signals to calc"late its position* it "ses

its !nown position to calc"late timing$ t fig"res o"t what the tra#el time of the GPS signals

sho"ld be* and compares it with what they act"ally are$ The difference is an Ierror correctionI

factor$ The recei#er then transmits this error information to the ro#ing recei#er so it can "se it to

correct its meas"rements Since the reference recei#er has no way of !nowing which of the many

a#ailable satellites a ro#ing recei#er might be "sing to calc"late its position* the reference

recei#er '"ic!ly r"ns thro"gh all the #isible satellites and comp"tes each of their errors$

tKs as if the reference recei#er is saying1 IJ e#erybody* right now the signal from satellite Q% is

ten nanoseconds delayed* satellite Q2 is three nanoseconds delayed* satellite Q9 is si>teen

nanoseconds delayed$$$I and so on$

65 here 'PS ta"ne

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n the early days of GPS* reference stations were established by pri#ate companies who had big

proLects demanding high acc"racy - gro"ps li!e s"r#eyors or oil drilling operations$ nd that is

still a #ery common approach$ o" b"y a reference recei#er and set "p a comm"nication lin!

with yo"r ro#ing recei#ers$ 0"t now there are eno"gh p"blic agencies transmitting corrections

that yo" might be able to get them for freeO The .nited States :oast G"ard and other

international agencies are establishing reference stations all o#er the place* especially aro"nd

pop"lar harbors and waterways$ These stations often transmit on the radio beacons that are

already in place for radio direction finding ("s"ally in the 9&&!,3 range)$nyone in the area can

recei#e these corrections and radically impro#e the acc"racy of their GPS meas"rements$ =ost

ships already ha#e radios capable of t"ning the direction finding beacons* so adding /GPS will

be '"ite easy$=any new GPS recei#ers are being designed to accept corrections* and some are

e#en e'"ipped with b"ilt-in radio recei#ers$

66 Post Process"ng 'PS

Aot all /GPS applications are created e'"al$ Some donKt need the radio lin! beca"se they donKt

need precise positioning immediately$ tKs one thing if yo"Kre trying to position a drill bit o#er a

partic"lar spot on the ocean floor from a pitching boat* b"t '"ite another if yo" L"st want to

record the trac! of a new road for incl"sion on a map$ f yo" donKt ha#e a reference recei#er there

may be alternati#e so"rce for corrections in yo"r area$ Some academic instit"tions are

e>perimenting with the nternet as a way of distrib"ting corrections$ In#erted /GPS*I can sa#e

money in certain trac!ing applications$ nyway* yo"Kd li!e this acc"racy b"t yo" donKt want to

b"y e>pensi#e Idifferential-readyI recei#ers for e#ery b"s$ ith an in#erted /GPS system the

b"ses wo"ld be e'"ipped with standard GPS recei#ers and a transmitter and wo"ld transmit their

standard GPS positions bac! to the trac!ing office$ t re'"ires a comp"ter to do the calc"lations*

a transmitter to transmit the data b"t it gi#es yo" a fleet of #ery acc"rate positions for the cost of

one reference station* a comp"ter and a lot of standard GPS recei#ers$ S"ch a dealO

Chapter 7

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70 A:ANC CNCPTS

f yo" want to !now where /GPS might be headed* ta!e a loo! at yo"r hand* beca"se soon

/GPS may be able to resol#e positions that are no farther apart than the width of yo"r littlefinger$ magine the possibilities$ "tomatic constr"ction e'"ipment co"ld translate :/

drawings into finished roads witho"t any man"al meas"rements$ Self-g"ided cars co"ld ta!e yo"

across town while yo" '"ietly read in the bac! seat$ To "nderstand how this !ind of GPS is being

de#eloped yo" need to "nderstand a little abo"t GPS signals$ f two recei#ers are fairly close to

each other* say within a few h"ndred !ilometers* the signals that reach both of them will ha#e

tra#eled thro"gh #irt"ally the same slice of atmosphere* and so will ha#e #irt"ally the same line$

The words I:ode-PhaseI and I:arrier-PhaseI refer to the partic"lar signal that we "se for timing

meas"rements$ .sing the GPS carrier fre'"ency can significantly impro#e the acc"racy of GPS$

Bemember that a GPS recei#er determines the tra#el time of a signal from a satellite by

comparing the Ipse"do random codeI itKs generating* with an identical code in the signal from

the satellite$ The amo"nt it has to slide the code is e'"al to the signalKs tra#el time$ The problem

is that the bits (or cycles) of the pse"do random code are so wide that e#en if yo" do get synced

"p thereKs still plenty of slop$

f yo" compared them logically yo"Kd say they matched$ hen signal is a one* signal 0 is aone$ hen signal is a 3ero* signal 0 is a 3ero$ 0"t yo" can see that while they match theyKre a

little o"t of phase$ Aotice that* e#en tho"gh they are the same most of the time* signal may

change state a little before signal 0$ This is the so"rce of positioning error$ ThatKs the problem

with code-phase GPS$ S"r#ey recei#ers beat the system by starting with the pse"do random code

and then mo#e on to meas"rements based on the carrier fre'"ency for that code$ This carrier

fre'"ency is m"ch higher so its p"lses are m"ch closer together and therefore more acc"rate$ f

yo"Kre r"sty on the s"bLect of carrier fre'"encies considers yo"r car radio$ The pse"do random

code has a bit rate of abo"t % =,3 b"t its carrier fre'"ency has a cycle rate of o#er a G,3

(which is %&&& times fasterO)t the speed of light the %$?8 G,3 GPS signal has a wa#elength of

ro"ghly twenty centimeters* so the carrier signal can act as a m"ch more acc"rate reference than

the pse"do random code by itself$ nd if we can get to within one percent of perfect phase li!e

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we do with code-phase recei#ers weKd ha#e 9 or 4 millimeter acc"racyO n essence this method is

co"nting the e>act n"mber of carrier cycles between the satellite and the recei#er$

The problem is that the carrier fre'"ency is hard to co"nt beca"se itKs so "niform$ #ery cycle

loo!s li!e e#ery other$ The pse"do random code on the other hand is intentionally comple> to

ma!e it easier to !now which cycle yo"Kre loo!ing at$ So the tric! with Icarrier-phase GPSI is to

"se code-phase techni'"es to get close$ f the code meas"rement can be made acc"rate to say* a

meter* then we only has a few wa#elengths of carrier to consider as we try to determine which

cycle really mar!s the edge of o"r timing p"lse$ Besol#ing this Icarrier phase ambig"ityI for L"st

a few cycles is a m"ch more tractable problem and as the comp"ters inside the recei#ers get

smarter and smarter itKs becoming possible to ma!e this !ind of meas"rement witho"t all the

rit"al that s"r#eyors go thro"gh They reali3ed the great benefits GPS co"ld bring to a#iation* b"tthey wanted more$ They wanted the acc"racy of /ifferential GPS and they wanted it across the

whole continent$ Their plan is called the Iide rea "gmentation SystemI or IS*I and itKs

basically a continental /GPS system$ The idea grew o"t of some #ery specific re'"irements that

basic GPS L"st co"ldnKt handle by itself$ t began with Isystem integrity$I GPS is #ery reliable b"t

e#ery once in a while a GPS satellite malf"nctions and gi#es inacc"rate data$

The GPS monitoring stations detect this sort of thing and transmit a system stat"s message that

tells recei#ers to disregard the bro!en satellite "ntil f"rther notice$ .nfort"nately this process can

ta!e many min"tes which co"ld be too late for an airplane in the middle of a landing$ So the C

got the idea that they co"ld set "p their own monitoring system that wo"ld respond m"ch

'"ic!er$ n fact* they fig"red they co"ld par! a geosynchrono"s satellite somewhere o#er the .$S$

that wo"ld instantly alert aircraft when there was a problem$ Then they reasoned that they co"ld

transmit this information right on a GPS channel so aircraft co"ld recei#e it on their GPS

recei#ers and wo"ldnKt need any additional radios$ The C fig"red that with abo"t 24 reference

recei#ers scattered across the .$S$ they co"ld gather pretty good correction data for most of the

co"ntry$ The ramifications of this go well beyond a#iation* beca"se the system g"arantees that

/GPS corrections will be raining o"t of the s!y for e#eryone to "se$ To complete the system the

C wants to e#ent"ally establish I@ocal rea "gmentation SystemsI near r"nways$ These

wo"ld wor! li!e the S b"t on a smaller scale$ The reference recei#ers wo"ld be near the

r"nways and so wo"ld be able to gi#e m"ch more acc"rate correction data to the incoming

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planes$ ith a @S aircraft wo"ld be able to "se GPS to ma!e :ategory 9 landings (3ero

#isibility)$

71 P*TT/N' 'PS T R;

GPS technology has mat"red into a reso"rce that goes far beyond its original design goals$ These

days scientists* sportsmen* farmers* soldiers* pilots* s"r#eyors* hi!ers* deli#ery dri#ers* sailors*

dispatchers* l"mberLac!s* fire-fighters* and people from many other wal!s of life are "sing GPS

in ways that ma!e their wor! more prod"cti#e* safer* and sometimes e#en easier$ These

applications fall into fi#e broad categories$

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23/31

The first and most ob#io"s application of GPS is the simple determination of a IpositionI or

location$ GPS is the first positioning system to offer highly precise location data for any point on

the planet* in any weather$ That alone wo"ld be eno"gh to '"alify it as a maLor "tility* b"t the

acc"racy of GPS and the creati#ity of its "sers is p"shing it into some s"rprising realms$

Sometimes an e>act reference locator is needed for e>tremely precise scientific wor!$ R"st

getting to the worldKs tallest mo"ntain was tric!y* b"t GPS made meas"ring the growth of =t$

#eresteasy$ The data collected strengthened past wor!* b"t also re#ealed that as the Jh"mb"

glacier mo#es toward #erestKs 0ase :amp* the mo"ntain itself is getting tall$

712 NA:/'AT/N

GPS helps yo" determine e>actly where yo" are* b"t sometimes important to !now how to get

somewhere else$ GPS was originally designed to pro#ide na#igation information for ships and

planes$ So itKs no s"rprise that while this technology is appropriate for na#igating on water* itKs

also #ery "sef"l in the air and on the land$

713 n the ater

tKs interesting that the sea* one of o"r oldest channels of transportation* has been re#ol"tioni3ed

by GPS* the newest na#igation technology$ Trimble introd"ced the worldKs first GPS recei#er for

marine na#igation in %pect* na#igating the worldKs oceans and

waterways is more precise than e#er$Today yo" will find Trimble recei#ers on #essels the world

o#er* from hardwor!ing fishing boats and long-ha"l container ships* to elegant l">"ry cr"ise

ships and recreational boaters$

714 TRAC;/N'.

f na#igation is the process of getting something from one location to another* then trac!ing is

the process of monitoring it as it mo#es along$ GPS "sed in conL"nction with comm"nication

lin!s and comp"ters can pro#ide the bac!bone for systems tailored to applications in agric"lt"re*

mass transit* "rban deli#ery* p"blic safety* and #essel and #ehicle trac!ing$ So itKs no s"rprise that

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police* amb"lance* and fire departments are adopting systems li!e TrimbleKs GPS-based E@

("tomatic Eehicle @ocation) =anager to pinpoint both the location of the emergency and the

location of the nearest response #ehicle on a comp"ter map$

:arrier-phase trac!ing of GPS signals has res"lted in a re#ol"tion in land s"r#eying$ line of sight

along the gro"nd is no longer necessary for precise positioning$ Positions can be meas"red "p to 9&

!m from reference point witho"t intermediate points$ This "se of GPS re'"ires specially e'"ipped

carrier trac!ing recei#ers$The @% and5or @2 carrier signals are "sed in carrier phase s"r#eying$ @%

carrier cycles ha#e a wa#elength of %< centimeters$ f trac!ed and meas"red these carrier signals

can pro#ide ranging meas"rements with relati#e acc"racies of millimeters "nder special

circ"mstances$

Trac!ing carrier phase signals pro#ides no time of transmission information$ The carrier signals*while mod"lated with time tagged binary codes* carry no time-tags that disting"ish one cycle from

another$ The meas"rements "sed in carrier phase trac!ing are differences in carrier phase cycles

and fractions of cycles o#er time$ t least two recei#ers trac! carrier signals at the same time$

onospheric delay differences at the two recei#ers m"st be small eno"gh to ins"re that carrier phase

cycles are properly acco"nted for$ This "s"ally re'"ires that the two recei#ers be within abo"t 9&

!m of each other$

715 T/$/N'

tKs a big world o"t there* and "sing GPS to s"r#ey and map it precisely sa#es time and money in

this most stringent of all applications$ Today* Trimble GPS ma!es it possible for a single

s"r#eyor to accomplish in a day what "sed to ta!e wee!s with an entire team$ nd they can do

their wor! with a higher le#el of acc"racy than e#er before$ Trimble pioneered the technology

which is now the method of choice for performing control s"r#eys* o"K#e seen how GPS

pinpoints a position* a ro"te* and a fleet of #ehicles$ =apping is the art and science of "sing GPS

to locate items* and then create maps and models of e#erything in the world$ nd we do mean

e#erything$ GPS is mapping the world$

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Chapter 8

'PS Sate(("te S"gna(s

The SEs transmit two microwa#e carrier signals$ The @% fre'"ency (%?8?$42 =,3) carries the

na#igation message and the SPS code signals$ The @2 fre'"ency (%228$& =,3) is "sed to

meas"re the ionospheric delay by PPS e'"ipped recei#ers$

Three binary codes shift the @% and5or @2 carrier phase$The :5 :ode (:oarse c'"isition)

mod"lates the @% carrier phase$ The :5 code is a repeating % =,3 Pse"do Bandom Aoise

(PBA) :ode$ This noise-li!e code mod"lates the @% carrier signal* spreading the spectr"m o#er

a % =,3 bandwidth$ The :5 code repeats e#ery %&29 bits (one millisecond)$ There is a different

:5 code PBA for each SE$ GPS satellites are often identified by their PBA n"mber* the "ni'"e

identifier for each pse"do-random-noise code$ The :5 code that mod"lates the @% carrier is the

basis for the ci#il SPS$The P-:ode (Precise) mod"lates both the @% and @2 carrier phases$ The P-

:ode is a #ery long (se#en days) %& =,3 PBA code$ n the nti-Spoofing (S) mode of

operation* the P-:ode is encrypted into the -:ode$ The encrypted -:ode re'"ires a classified

S =od"le for each recei#er channel and is for "se only by a"thori3ed "sers with cryptographic

!eys$ The P ()-:ode is the basis for the PPS$The Aa#igation =essage also mod"lates the @%-:5 code signal$ The Aa#igation =essage is a ?& ,3 signal consisting of data bits that describe

the GPS satellite orbits* cloc! corrections* and other system parameters$

81 'PS ata

The GPS Aa#igation =essage consists of time-tagged data bits mar!ing the time of

transmission of each s"bframe at the time they are transmitted by the SE$ data bit frame

consists of %?&& bits di#ided into fi#e 9&&-bit s"bframes$ data frame is transmitted e#ery

thirty seconds$ Three si>-second s"bframes contain orbital and cloc! data$ SE :loc!

corrections are sent in s"bframe one and precise SE orbital data sets (ephemeris data

parameters) for the transmitting SE are sent in s"bframes two and three$ S"bframes fo"r and

fi#e are "sed to transmit different pages of system data$ n entire set of twenty-fi#e frames

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(%2? s"bframes) ma!es "p the complete Aa#igation =essage that is sent o#er a %2$? min"te

period$

/ata frames (%?&& bits) are sent e#ery thirty seconds$ ach frame consists of fi#e s"bframes$

/ata bit s"bframes (9&& bits transmitted o#er si> seconds) contain parity bits that allow for

data chec!ing and limited error correction$

:loc! data parameters describe the SE cloc! and its relationship to GPS time$

phemeris data parameters describe SE orbits for short sections of the satellite orbits$

Aormally* a recei#er gathers new ephemeris data each ho"r* b"t can "se old data for "p to

fo"r ho"rs witho"t m"ch error$ The ephemeris parameters are "sed with an algorithm that

comp"tes the SE position for any time within the period of the orbit described by the

ephemeris parameter set$

lmanacs are appro>imate orbital data parameters for all SEs$ The ten-parameter almanacs

describe SE orbits o#er e>tended periods of time ("sef"l for months in some cases) and a set

for all SEs is sent by each SE o#er a period of %2$? min"tes (at least)$ Signal ac'"isition time

on recei#er start-"p can be significantly aided by the a#ailability of c"rrent almanacs$ The

appro>imate orbital data is "sed to preset the recei#er with the appro>imate position and

carrier /oppler fre'"ency (the fre'"ency shift ca"sed by the rate of change in range to the

mo#ing SE) of each SE in the constellation$

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ach complete SE data set incl"des an ionospheric model that is "sed in the recei#er to

appro>imates the phase delay thro"gh the ionosphere at any location and time$

ach SE sends the amo"nt to which GPS Time is offset from .ni#ersal :oordinated Time$

This correction can be "sed by the recei#er to set .T: to within %&& ns$

82 'PS Rece"&er B(oc- "agra)

83 'PS rror Sorces

GPS errors are a combination of noise* bias* bl"nders$ Aoise errors are the combined effect of

PBA code noise (aro"nd % meter) and noise within the recei#er noise (aro"nd % meter)$0ias errors

res"lt from Selecti#e #ailability and other factors $

84 Se(ect"&e A&a"(a"("t#

S is the intentional degradation of the SPS signals by a time #arying bias$ S is controlled by the

// to limit acc"racy for non-.$ S$ military and go#ernment "sers$

The potential acc"racy of the :5 code of aro"nd 9& meters is red"ced to %&& meters (two

standard de#iations)$

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The S bias on each satellite signal is different* and so the res"lting position sol"tion is a

f"nction of the combined S bias from each SE "sed in the na#igation sol"tion$ 0eca"se S is a

changing bias with low fre'"ency terms in e>cess of a few ho"rs* position sol"tions or indi#id"al

SE pse"do-ranges cannot be effecti#ely a#eraged o#er periods shorter than a few ho"rs

85 ther B"as rror sorces

Tropospheric delays1 % meter$ The troposphere is the lower part (gro"nd le#el to from ; to %9

!m) of the atmosphere that e>periences the changes in temperat"re* press"re* and h"midity

associated with weather changes$ :omple> models of tropospheric delay re'"ire estimates or

meas"rements of these parameters$ .n modeled ionosphere delays1 %& meters$ The ionosphere is

the layer of the atmosphere from ?& to ?&& !m that consists of ioni3ed air$ The transmitted model

can only remo#e abo"t half of the possible 8& ns of delay lea#ing a ten meter "n-modeled

resid"al$

="ltipath1 &$? meters$ ="ltipath is ca"sed by reflected signals from s"rfaces near the recei#er

that can either interfere with or be mista!en for the signal that follows the straight line path from

the satellite$ ="ltipath is diffic"lt to detect and sometime hard to a#oid$ 0l"nders can res"lt in

errors of h"ndred of !ilometers$

:ontrol segment mista!es d"e to comp"ter or h"man error can ca"se errors from one meter to

h"ndreds of !ilometers$ .ser mista!es* incl"ding incorrect geodetic dat"m selection* can ca"se

errors from % to h"ndreds of meters$ Becei#er errors from software or hardware fail"res can ca"se

bl"nder errors of any si3e$ Aoise and bias errors combine* res"lting in typical ranging errors of

aro"nd fifteen meters for each satellite "sed in the position sol"tion$

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Chapter 9

90 So)e


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Chapter10

S))ar#

Cor 0asic GPS "se* yo" only need to "nderstand the most basic of the recei#er+s f"nctions$ Set-

"p and initiali3ation f"nctions* Sa#ing a aypoint* and "sing the GT feat"re$ Cor this le#el of

"se* yo" don+t need to "nderstand the coordinate grid systems* dat"m or how to "se maps$

Therefore* a new GPS "ser can ma!e good "se of the "nit with the minimal amo"nt of

e>perience reading the man"al and wor!ing the men"s and !eypad$

Those who need to refer to maps and "se them to plot and read position coordinates m"st

"nderstand at least one of the coordinate grid systems$ nce the grid system is "nderstood and itscoordinate references can be identified on maps* it is a simple matter to learn to "se the tools and

techni'"es to properly identify coordinates$

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Reference

www$wi!ipedia$org

www$howst"ffwor!s$com

www$ieee$org
http://www.wikipedia.org/http://www.howstuffworks.com/http://www.ieee.org/http://www.wikipedia.org/http://www.howstuffworks.com/http://www.ieee.org/

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