Principles of GPS

8/10/2019 Principles of GPS

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GPS is a Satellite NavigationSystem

GPS is funded by and controlled by the U. S. Department of Defense (DOD).While there are many thousands of civil users of GPS worldwide! thesystem was desi"ned for and is operated by the U. S. military.

GPS provides specially coded satellite si"nals that can be processed in a

GPS receiver! enablin" the receiver to compute position! velocity and time.

#our GPS satellite si"nals are used to compute positions in threedimensions and the time o$set in the receiver cloc%.

Space Segment &he Space Se"ment of the system consists of the GPS satellites. &hese

space vehicles (S's) send radio si"nals from space.

&he nominal GPS Operational onstellation consists of * satellites thatorbit the earth in + hours. &here are often more than * operationalsatellites as new ones are launched to replace older satellites. &he satelliteorbits repeat almost the same "round trac% (as the earth turns beneaththem) once each day. &he orbit altitude is such that the satellites repeat

the same trac% and con,"uration over any point appro-imately each *hours (* minutes earlier each day). &here are si- orbital planes (withnominally four S's in each)! eually spaced (/0 de"rees apart)! and

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inclined at about ,fty,ve de"rees with respect to the euatorial plane.&his constellation provides the user with between ,ve and ei"ht S's visible

from any point on the earth.

User Segment &he GPS User Se"ment consists of the GPS receivers and the user

community. GPS receivers convert S' si"nals into position! velocity! andtime estimates. #our satellites are reuired to compute the four dimensionsof 1! 2! 3 (position) and &ime. GPS receivers are used for navi"ation!positionin"! time dissemination! and other research.

4avi"ation in three dimensions is the primary function of GPS. 4avi"ationreceivers are made for aircraft! ships! "round vehicles! and for handcarryin" by individuals.

Precise positionin" is possible usin" GPS receivers at reference locations

providin" corrections and relative positionin" data for remote receivers.Surveyin"! "eodetic control! and plate tectonic studies are e-amples.

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&ime and freuency dissemination! based on the precise cloc%s on board

the S's and controlled by the monitor stations! is another use for GPS.5stronomical observatories! telecommunications facilities! and laboratorystandards can be set to precise time si"nals or controlled to accuratefreuencies by special purpose GPS receivers.

6esearch pro7ects have used GPS si"nals to measure atmospheric

parameters.

GPS Satellite Signals

&he S's transmit two microwave carrier si"nals. &he 8+ freuency (+9:9.*;


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GPS Data

&he GPS 4avi"ation ;essa"e consists of timeta""ed data bits mar%in" thetime of transmission of each subframe at the time they are transmitted bythe S'. 5 data bit frame consists of +900 bits divided into ,ve @00bit

subframes. 5 data frame is transmitted every thirty seconds. &hree si-second subframes contain orbital and cloc% data. S' loc% corrections aresent in subframe one and precise S' orbital data sets (ephemeris dataparameters) for the transmittin" S' are sent in subframes two and three.Subframes four and ,ve are used to transmit di$erent pa"es of systemdata. 5n entire set of twenty,ve frames (+9 subframes) ma%es up thecomplete 4avi"ation ;essa"e that is sent over a +.9 minute period.

Data frames (+900 bits) are sent every thirty seconds. Bach frame consistsof ,ve subframes.

Data bit subframes (@00 bits transmitted over si- seconds) contain parity

bits that allow for data chec%in" and limited error correction.

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Position, and Time from GPS Code Phase Tracking (Navigation) &he GPS receiver produces replicas of the >5 and>or P (2)ode. Bach P64

code is a noiseli%e! but predetermined! uniue series of bits.

&he receiver produces the >5 code seuence for a speci,c S' with some

form of a >5 code "enerator. ;odern receivers usually store a completeset of precomputed >5 code chips in memory! but a hardware! shift

re"ister! implementation can also be used.

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&he >5 code "enerator produces a di$erent +0@ chip seuence for eachphase tap settin". An a shift re"ister implementation the code chips areshifted in time by slewin" the cloc% that controls the shift re"isters. An amemory loo%up scheme the reuired code chips are retrieved from

memory.

5 GPS receiver uses the detected si"nal power in the correlated si"nal to

ali"n the >5 code in the receiver with the code in the S' si"nal. Usually alate version of the code is compared with an early version to insure that the

correlation pea% is trac%ed.

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5 phase loc%ed loop that can loc% to either a positive or ne"ative halfcycle

(a biphase loc% loop) is used to demodulate the 90


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4oise errors are the combined e$ect of P64 code noise (around +

meter) and noise within the receiver noise (around + meter).

Cias errors result from Selective 5vailability and other factors

Selective 5vailability (S5)

S5 is the intentional de"radation of the SPS si"nals by a timevaryin" bias. S5 is controlled by the DOD to limit accuracy for

nonU. S. military and "overnment users. &he potentialaccuracy of the >5 code of around @0 meters is reduced to+00 meters (two standard deviations).

&he S5 bias on each satellite si"nal is di$erent! and so theresultin" position solution is a function of the combined S5 biasfrom each S' used in the navi"ation solution. Cecause S5 is achan"in" bias with low freuency terms in e-cess of a fewhours! position solutions or individual S' pseudoran"es cannotbe e$ectively avera"ed over periods shorter than a few hours.Di$erential corrections must be updated at a rate less than thecorrelation time of S5 (and other bias errors).

Other Cias Brror sources

S' cloc% errors uncorrected by ontrol Se"ment can result inone meter errors.

Bphemeris data errorsE + meter

&ropospheric delaysE + meter. &he troposphere is the lower part("round level to from F to +@ %m) of the atmosphere thate-periences the chan"es in temperature! pressure! andhumidity associated with weather chan"es. omple- models oftropospheric delay reuire estimates or measurements of theseparameters.

Unmodeled ionosphere delaysE +0 meters. &he ionosphere isthe layer of the atmosphere from 90 to 900 %m that consists ofioni=ed air. &he transmitted model can only remove about halfof the possible :0 ns of delay leavin" a ten meter unmodeledresidual.

;ultipathE 0.9 meters. ;ultipath is caused by reected si"nals

from surfaces near the receiver that can either interfere with orbe mista%en for the si"nal that follows the strai"ht line pathfrom the satellite. ;ultipath is diHcult to detect and sometime

hard to avoid. Clunders can result in errors of hundred of %ilometers.

ontrol se"ment mista%es due to computer or human error can cause

errors from one meter to hundreds of %ilometers.

User mista%es! includin" incorrect "eodetic datum selection! cancause errors from + to hundreds of meters.

6eceiver errors from software or hardware failures can cause blundererrors of any si=e.

4oise and bias errors combine! resultin" in typical ran"in" errors of around,fteen meters for each satellite used in the position solution.

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Di"erential GPS (DGPS) Techni#!es

&he idea behind all di$erential positionin" is to correct bias errors at onelocation with measured bias errors at a %nown position. 5 referencereceiver! or base station! computes corrections for each satellite si"nal.

Cecause individual pseudoran"es must be corrected prior to the formationof a navi"ation solution! DGPS implementations reuire software in thereference receiver that can trac% all S's in view and form individualpseudoran"e corrections for each S'. &hese corrections are passed to theremote! or rover! receiver which must be capable of applyin" theseindividual pseudoran"e corrections to each S' used in the navi"ationsolution. 5pplyin" a simple position correction from the reference receiverto the remote receiver has limited e$ect at useful ran"es because bothreceivers would have to be usin" the same set of S's in their navi"ationsolutions and have identical GDOP terms (not possible at di$erentlocations) to be identically a$ected by bias errors.

Di"erential Code GPS (Navigation)

Di$erential corrections may be used in realtime or later! with postprocessin" techniues.

6ealtime corrections can be transmitted by radio lin%. &he U.S. oast Guard maintains a networ% of di$erential monitors andtransmits DGPS corrections over radiobeacons coverin" muchof the U. S. coastline. DGPS corrections are often transmitted ina standard format speci,ed by the 6adio &echnical ommission;arine (6&;).

orrections can be recorded for post processin". ;any public

and private a"encies record DGPS corrections for distributionby electronic means.

Private DGPS services use leased #; subcarrier broadcasts!satellite lin%s! or private radiobeacons for realtimeapplications.

&o remove Selective 5vailability (and other bias errors)!

di$erential corrections should be computed at the referencestation and applied at the remote receiver at an update ratethat is less than the correlation time of S5. Su""ested DGPSupdate rates are usually less than twenty seconds.

DGPS removes commonmode errors! those errors common to boththe reference and remote receivers (not multipath or receiver noise).Brrors are more often common when receivers are close to"ether(less than +00 %m). Di$erential position accuracies of ++0 metersare possible with DGPS based on >5 code SPS si"nals.

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Geometric Dilution of Precision (GDOP) andVisibility

GPS ran"in" errors are ma"ni,ed by the ran"e vector di$erences betweenthe receiver and the S's. &he volume of the shape described by the unitvectors from the receiver to the S's used in a position ,- is inversely

proportional to GDOP. Poor GDOP! a lar"e value representin" a small unit vectorvolume!

results when an"les from receiver to the set of S's used are similar.

Good GDOP! a small value representin" a lar"e unitvectorvolume!

results when an"les from receiver to S's are di$erent.

GDOP is computed from the "eometric relationships between the receiverposition and the positions of the satellites the receiver is usin" for navi"ation.#or plannin" purposes GDOP is often computed from 5lmanacs and anestimated position. Bstimated GDOP does not ta%e into account obstacles thatbloc% the lineofsi"ht from the position to the satellites. Bstimated GDOP maynot be reali=able in the ,eld.

GDOP terms are usually computed usin" parameters from the navi"ation

solution process.

GDOP is computed from the "eometric relationships between the receiverposition and the positions of the satellites the receiver is usin" for navi"ation.

#or plannin" purposes GDOP is often computed from 5lmanacs and anestimated position. Bstimated GDOP does not ta%e into account obstacles thatbloc% the lineofsi"ht from the position to the satellites. Bstimated GDOP maynot be reali=able in the ,eld.

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Principles of GPS