CHAPTER 11SATELLITE NAVIGATIONINTRODUCTION1100.EarlyDevelopmentsInSatelliteNavigationbetween the satellite and the navigator.Knowing the satel-lite orbit precisely,the navigator's absolute position can beaccuratelydeterminedfromthetimerate ofchange ofrangeThe idea that led to development of the satellite navi-to the satellite.gationsystemsdatesbackto1957andthefirst launchofanartificial satellite into orbit,Russia's Sputnik I.Dr.WilliamThe Johns Hopkins University Applied Physics Labora-H.GuierandDr.GeorgeC.WieffenbachattheAppliedtory developed NAVSAT for the U. S.Navy.The operationPhysics Laboratory of the Johns Hopkins University wereofthesystem isunderthecontroloftheU.S.NavyAstronau-monitoringthefamous“beeps"transmittedbythepassingtics Group with headquarters at Point Mugu, California.satellite. They plotted the received signals at precise inter-vals,and noticed that a characteristic Dopplercurve1102. System Configuration, Operation, Andemerged. Since celestial bodies followed fixed orbits, theyTerminationreasoned that this curve could be used to describe the satel-lite orbit. Later, they demonstrated that they couldTheNAVSAT consists of 10 orbiting satellites and3determineall oftheorbital parametersforapassingsatelliteorbiting spares;anetwork oftrackingstations continuouslyby doppler observation of a single pass from a single fixedmonitoring the satellites and updating the information theystation. The doppler shift apparent while receiving a trans-transmit; and the receivers and computers for processingmission from a passing satellite proved to be an effectivesignals.measuringdeviceforestablishingthesatelliteorbit.Eachsatelliteis inanominallycircularpolarorbitatanDr.Frank T.McClure,also of theApplied Physicsapproximatealtitudeof600nauticalmiles.Thereareusual-Laboratory,reasoned that if the satellite orbit wasknown,ly five satellites operating in the system.Five satellites indoppler shiftmeasurementscouldbeusedtodetermineorbitprovideredundancy;theminimum constellationforone's position on earth.His studies in support of this hy-system operation is four.This redundancy allows for an un-pothesis earned him the first National Aeronautics andexpected failure ofa satellite and the relatively longperiodSpace Administration award for important contributions tooftime requiredto schedule,prepare,andlaunch areplacespacedevelopment.ment satellite.This redundancy also provides for turningIn1958, the Applied Physics Laboratory proposed ex-offa satellitewhen (onrareoccasions)its orbital planepre-ploringthepossibilityof anoperational satellitedopplercesses near another satellite's plane, or when the timingnavigation system.TheChief of Naval Operations then set(phasing)of several satellites in theirorbits aretemporarilyforth requirements for such a system.The first successfulsuch thatmany satellitespass nearly simultaneouslynearlaunching of a prototype system satellite in April 1960one of the poles.demonstratedthedopplersystem'soperationalfeasibilityEach satellite contains: (1)receiver equipment to ac-cept injection data and operational commandsfromthe1101.NAVSAT,TheFirst SatelliteNavigationSystemground, (2)a decoder for digitizing the data, (3)switchinglogic and memorybanks for sorting and storing the digitalThe Navy Navigation Satellite System (NAVSATdata, (4) control circuits to cause the data to be read out atalsoknownasTRANSIT)wasthefirstoperational satellitespecifictimesintheproperformat,(5)anencodertotrans-navigation system.The system's accuracywas better thanlate the digital data to phasemodulation,(6)ultra stable50.1 nautical mile anywhere in the world.It was used prima-MHz oscillators,and (7)1.5-watt transmitters tobroadcastrilyforthenavigationofsurfaceshipsandsubmarines:butthe 150-and 400-MHz oscillator-regulated frequencies thatit also had some applications in air navigation. It was alsocarry thedatato earth.used in hydrographic surveying and geodetic positionThe transit launch program ended in 1988. Accordingdetermination.to the Federal Radionavigation Plan, the Navy will ceaseNAVSATusesthedopplershiftofradiosignalstrans-operationofNAVSATbythe end of1996,as thenewGlo-bal Positioning System (GPS)comes into operation.mitted from a satellite to measure the relative velocity179
179 CHAPTER 11 SATELLITE NAVIGATION INTRODUCTION 1100. Early Developments In Satellite Navigation The idea that led to development of the satellite navigation systems dates back to 1957 and the first launch of an artificial satellite into orbit, Russia’s Sputnik I. Dr. William H. Guier and Dr. George C. Wieffenbach at the Applied Physics Laboratory of the Johns Hopkins University were monitoring the famous “beeps” transmitted by the passing satellite. They plotted the received signals at precise intervals, and noticed that a characteristic Doppler curve emerged. Since celestial bodies followed fixed orbits, they reasoned that this curve could be used to describe the satellite orbit. Later, they demonstrated that they could determine all of the orbital parameters for a passing satellite by doppler observation of a single pass from a single fixed station. The doppler shift apparent while receiving a transmission from a passing satellite proved to be an effective measuring device for establishing the satellite orbit. Dr. Frank T. McClure, also of the Applied Physics Laboratory, reasoned that if the satellite orbit was known, doppler shift measurements could be used to determine one’s position on earth. His studies in support of this hypothesis earned him the first National Aeronautics and Space Administration award for important contributions to space development. In 1958, the Applied Physics Laboratory proposed exploring the possibility of an operational satellite doppler navigation system. The Chief of Naval Operations then set forth requirements for such a system. The first successful launching of a prototype system satellite in April 1960 demonstrated the doppler system’s operational feasibility. 1101. NAVSAT, The First Satellite Navigation System The Navy Navigation Satellite System (NAVSAT, also known as TRANSIT) was the first operational satellite navigation system. The system’s accuracy was better than 0.1 nautical mile anywhere in the world. It was used primarily for the navigation of surface ships and submarines; but it also had some applications in air navigation. It was also used in hydrographic surveying and geodetic position determination. NAVSAT uses the doppler shift of radio signals transmitted from a satellite to measure the relative velocity between the satellite and the navigator. Knowing the satellite orbit precisely, the navigator’s absolute position can be accurately determined from the time rate of change of range to the satellite. The Johns Hopkins University Applied Physics Laboratory developed NAVSAT for the U. S. Navy. The operation of the system is under the control of the U. S. Navy Astronautics Group with headquarters at Point Mugu, California. 1102. System Configuration, Operation, And Termination The NAVSAT consists of 10 orbiting satellites and 3 orbiting spares; a network of tracking stations continuously monitoring the satellites and updating the information they transmit; and the receivers and computers for processing signals. Each satellite is in a nominally circular polar orbit at an approximate altitude of 600 nautical miles. There are usually five satellites operating in the system. Five satellites in orbit provide redundancy; the minimum constellation for system operation is four. This redundancy allows for an unexpected failure of a satellite and the relatively long period of time required to schedule, prepare, and launch a replacement satellite. This redundancy also provides for turning off a satellite when (on rare occasions) its orbital plane precesses near another satellite’s plane, or when the timing (phasing) of several satellites in their orbits are temporarily such that many satellites pass nearly simultaneously near one of the poles. Each satellite contains: (1) receiver equipment to accept injection data and operational commands from the ground, (2) a decoder for digitizing the data, (3) switching logic and memory banks for sorting and storing the digital data, (4) control circuits to cause the data to be read out at specific times in the proper format, (5) an encoder to translate the digital data to phase modulation, (6) ultra stable 5 MHz oscillators, and (7) 1.5-watt transmitters to broadcast the 150- and 400-MHz oscillator-regulated frequencies that carry the data to earth. The transit launch program ended in 1988. According to the Federal Radionavigation Plan, the Navy will cease operation of NAVSAT by the end of 1996, as the new Global Positioning System (GPS) comes into operation
180SATELLITENAVIGATIONTHEGLOBALPOSITIONINGSYSTEM1103.BasicSystemDescriptioncated in Hawai, Colorado Springs,Kwajalein,DiegoGarcia, and Ascension Island, passively track the satel-TheFederalRadionavigationPlanhasdesignatedlites,accumulating ranging data from the satellitesthe Navigation System using Timing and Rangingsignals andrelaying themtotheMCS.TheMCSprocess-(NAVSTAR)GlobalPositioningSystem(GPS)asthees thisinformationtodeterminesatelliteposition andsignal data accuracy,updates the navigation message ofprimarynavigation systemoftheU.S.government.GPSeach satellite and relays this information to theground an-is a spaced-based radio positioning system which pro-tennas.Thegroundantennasthen transmitthisvides suitably equipped users with highly accurateposition,velocity,and time data.It consists of three ma-information tothesatellites.Thegroundantennas,locatedjor segments: a space segment, a control segment, andat Ascension Island, Diego Garcia, and Kwajalein, arealso usedfortransmitting and receiving satellite controla user segment.information.The space segment contains 24 satellites. PreciseThe user segment is designed for different require-spacing of the satellites in orbit is arranged such that aments of various users. These receivers can be used inminimum of four satellites are in view to a user at anytime on a worldwide basis.Each satellite transmits sig-high,medium, and lowdynamic applications.An exam-ple of a low dynamic application would be a fixednalsontworadiofrequencies,superimposedonwhichantenna or slowly drifting marine craft.An example ofaarenavigationand svstemdata.Included inthisdata ismediumdynamicapplicationwouldbeamarineorlandpredicted satelliteephemeris,atmosphericpropagationcorrection data,satellite clock error information,and sat-vehicletravelingat a constant controlled speed.Finally,ellite health data. This segment consists of 21an example of a high dynamic application would beahigh performance aircraft or a spacecraft. The useroperational satellites with three satellites orbiting as ac-tive spares. The satellites orbit in six separate orbitalequipment is designed to receive and process signalsfromfourormoreorbitingsatelliteseithersimultaneous-planes.The orbital planes have an inclination relative tothe equator of 55and an orbital height of 20,200 km.ly or sequentially.The processor in the receiverthenThesatellites complete an orbitapproximatelyonce ev-converts these signals to three-dimensional navigationinformationbased ontheWorldGeodeticSystem1984ery12hours.GPS satellitestransmitpseudorandom noise(PRN)reference ellipsoid. The user segment can consist ofstand-alone receivers or equipment that is integrated intosequence-modulated radio frequencies, designated L1another navigation system.Since GPS is used in a wide(1575.42MHz)andL2(1227.60MHz).Thesatellitetrans-mits botha Coarse Acquisition Code (C/A code)and avarietyofapplicationsfrommarinenavigationtolandsurveying, these receivers can vary greatly in functionPrecision Code (P code).Both the P and C/A codes areand design.transmittedontheL1carrier,onlythePcodeistransmittedontheL2carrier.Superimposed onboththeC/AandP1104. System Capabilitiescodes is the Navigation message.This message containssatellite ephemeris data, atmospheric propagation correc-tiondataandsatelliteclockbiasGPS provides multiple users withaccurate,continu-GPSassigns a unique C/Acode anda uniquePcodetoous,worldwide,all-weather,common-grid,three-dimensional positioningand navigation information.each satellite.Thispractice,knownas codedivision multi-ple access (CDMA), allows all satellites the use of aToobtainanavigation solution of position(latitudecommon carrier frequencywhile still allowing the receiverlongitude,and altitude)and time (fourunknowns),fourto determine which satellite is transmitting.CDMAalso al-satellitesmustbe selected.TheGPS usermeasures pseu-lows for easy user identification of eachGPS satellite.dorangeandpseudorangeratebysynchronizingandSince each satellite broadcasts using its own unique C/Atracking the navigation signal from each of the four se-andPcodecombination,it canbeassigned a uniquePRNlected satellites. Pseudorange is the true distancesequencenumber.Thisnumberishowasatelliteisidenti-between the satellite and the userplus an offset dueto thefiedwhentheGPScontrolsystemcommunicateswithusersuser'sclockbias.Pseudorangerateisthetrueslantrangeabout aparticularGPSsatellite.rateplusanoffset due tothefrequency errorof the user'sThe control segment includes a master control sta-clock.By decoding the ephemeris data and system tim-tion (MCS), a number of monitor stations, and grounding information on each satellite's signal, the user'santennaslocated throughout theworld.Themastercontrolreceiver/processorcan convertthe pseudorangeandstation.locatedinColoradoSprings,Colorado.consistsofpseudorange rate to three-dimensional position and ve-equipmentand facilities requiredfor satellitemonitoring.locity.Fourmeasurementsarenecessarytosolveforthetelemetry,tracking,commanding,control,uploading,andthreeunknowncomponentsofposition(orvelocity)andnavigation message generation.The monitor stations, lo-the unknown user time (or frequency)bias
180 SATELLITE NAVIGATION THE GLOBAL POSITIONING SYSTEM 1103. Basic System Description The Federal Radionavigation Plan has designated the Navigation System using Timing and Ranging (NAVSTAR) Global Positioning System (GPS) as the primary navigation system of the U.S. government. GPS is a spaced-based radio positioning system which provides suitably equipped users with highly accurate position, velocity, and time data. It consists of three major segments: a space segment, a control segment, and a user segment. The space segment contains 24 satellites. Precise spacing of the satellites in orbit is arranged such that a minimum of four satellites are in view to a user at any time on a worldwide basis. Each satellite transmits signals on two radio frequencies, superimposed on which are navigation and system data. Included in this data is predicted satellite ephemeris, atmospheric propagation correction data, satellite clock error information, and satellite health data. This segment consists of 21 operational satellites with three satellites orbiting as active spares. The satellites orbit in six separate orbital planes. The orbital planes have an inclination relative to the equator of 55° and an orbital height of 20,200 km. The satellites complete an orbit approximately once every 12 hours. GPS satellites transmit pseudorandom noise (PRN) sequence-modulated radio frequencies, designated L1 (1575.42 MHz) and L2 (1227.60 MHz). The satellite transmits both a Coarse Acquisition Code (C/A code) and a Precision Code (P code). Both the P and C/A codes are transmitted on the L1 carrier; only the P code is transmitted on the L2 carrier. Superimposed on both the C/A and P codes is the Navigation message. This message contains satellite ephemeris data, atmospheric propagation correction data, and satellite clock bias. GPS assigns a unique C/A code and a unique P code to each satellite. This practice, known as code division multiple access (CDMA), allows all satellites the use of a common carrier frequency while still allowing the receiver to determine which satellite is transmitting. CDMA also allows for easy user identification of each GPS satellite. Since each satellite broadcasts using its own unique C/A and P code combination, it can be assigned a unique PRN sequence number. This number is how a satellite is identified when the GPS control system communicates with users about a particular GPS satellite. The control segment includes a master control station (MCS), a number of monitor stations, and ground antennas located throughout the world. The master control station, located in Colorado Springs, Colorado, consists of equipment and facilities required for satellite monitoring, telemetry, tracking, commanding, control, uploading, and navigation message generation. The monitor stations, located in Hawaii, Colorado Springs, Kwajalein, Diego Garcia, and Ascension Island, passively track the satellites, accumulating ranging data from the satellites’ signals and relaying them to the MCS. The MCS processes this information to determine satellite position and signal data accuracy, updates the navigation message of each satellite and relays this information to the ground antennas. The ground antennas then transmit this information to the satellites. The ground antennas, located at Ascension Island, Diego Garcia, and Kwajalein, are also used for transmitting and receiving satellite control information. The user segment is designed for different requirements of various users. These receivers can be used in high, medium, and low dynamic applications. An example of a low dynamic application would be a fixed antenna or slowly drifting marine craft. An example of a medium dynamic application would be a marine or land vehicle traveling at a constant controlled speed. Finally, an example of a high dynamic application would be a high performance aircraft or a spacecraft. The user equipment is designed to receive and process signals from four or more orbiting satellites either simultaneously or sequentially. The processor in the receiver then converts these signals to three-dimensional navigation information based on the World Geodetic System 1984 reference ellipsoid. The user segment can consist of stand-alone receivers or equipment that is integrated into another navigation system. Since GPS is used in a wide variety of applications, from marine navigation to land surveying, these receivers can vary greatly in function and design. 1104. System Capabilities GPS provides multiple users with accurate, continuous, worldwide, all-weather, common-grid, threedimensional positioning and navigation information. To obtain a navigation solution of position (latitude, longitude, and altitude) and time (four unknowns), four satellites must be selected. The GPS user measures pseudorange and pseudorange rate by synchronizing and tracking the navigation signal from each of the four selected satellites. Pseudorange is the true distance between the satellite and the user plus an offset due to the user’s clock bias. Pseudorange rate is the true slant range rate plus an offset due to the frequency error of the user’s clock. By decoding the ephemeris data and system timing information on each satellite’s signal, the user’s receiver/processor can convert the pseudorange and pseudorange rate to three-dimensional position and velocity. Four measurements are necessary to solve for the three unknown components of position (or velocity) and the unknown user time (or frequency) bias
SATELLITENAVIGATION181The navigation accuracy that can be achieved by any(as it would were there no transmission error present);rath-user dependsprimarilyon thevariability of theerrors iner,it will plot as a triangle.Thenavigator can then applyaconstantbearing correction to eachLOPuntilthecorrectionmakingpseudorangemeasurements,theinstantaneousge-ometry of the satellites as seen fromthe user's location onapplied equals the bearing transmission error.When theEarth,and the presence of Selective Avaliability (SA).Se-correction applied equals the original transmission errorlectiveAvailability is discussedfurtherbelow.theresultant fix should plotas a pinpoint.The situation withGPS receiver timing inaccuracies is analogous; time mea-surement error simply replaces bearing measurement error1105.Global Positioning System Basic Conceptsin the analysis. Assuming that the satellite clocks are per-fectly synchronized and the receiver clock's error isAs discussed above,GPS measures distancesbetweenconstant.thesubtractionofthatconstanterrorfromtheresatellites in orbit and a receiver on or abovethe earth andsulting distancedeterminations will reduce thefix errorcomputes spheres of position from those distances.The in-until a“pinpoint"position is obtained. It is important totersections of those spheres of position then determine thenoteherethatthenumberoflinesofpositionreguiredtoreceiver'sposition.employthistechniqueis a functionof thenumberof linesThe distance measurements described above are done byof position required to obtain a fix.In the two dimensionalcomparingtiming signals generated simultaneouslybythesat-visual plotting scenario described above,only two LOP'sellitesand receiver's internal clocks.These signals,were required to constitute a fix.The bearing error intro-characterized by a special waveform known as the pseudo-duced another unknown into theprocess, resulting in threerandom code,aregenerated in phasewith eachother.Thesigtotalunknowns(thexcoordinateofposition.theycoordinal from the satellitearrives at the receiver following a timenate ofposition,and the bearing error).Becauseofthe threedelayproportional to itsdistancetraveled.This timedelay isunknowns, three LOP's were required to employ this cor-detected by the phase shift between the received pseudo-ran-rection technique. GPS determines position in threedom codeand the code generated bythe receiver.Knowing thedimensions; thepresenceofreceiverclock erroradds an ad-time required for the signal to reach the receiver from the sat-ditional unknown.Therefore, four timing measurementsellite allows the receiver to calculatethe distance from thearerequired to solve for the resulting four unknowns.satellite.The receiver,therefore,must be located on a spherecentered at thesatellite witharadius equal to thisdistance mea-1106.GPSSignalCodingsurement.The intersection of three spheres of position yieldstwo possiblepoints ofreceiverposition.Oneof thesepointsTwo separate carrier frequencies carry the signal trans-can be disregarded since it is hundreds of miles from the sur-face of the earth. Theoretically,then, only three timemitted by a GPS satellite. The first carrier frequency (LI)measurements arerequired to obtain a fixfromGPStransmitson1575.42MHz,thesecond(L2)transmitson1227.60MHz.TheGPSsignalconsistsofthreeseparateIn practice,however,afourthmeasurement is requiredmessages:theP-code,transmitted on bothL1 and L2;theC/to obtain an accurate position from GPS. This is due to re-Acode.transmittedonLlonlv,andanavigationdatamesceiver clock error.Timing signals travel from the satellitesage.The P code and C/A code messages are divided intotothereceiveratthespeed oflight:evenextremelyslighindividualbitsknownaschips.Thefrequencyatwhichbitstiming errors between the clocks on the satellite and in theare sent for each typeof signal is known as thechippingreceiver will lead to tremendous range errors.The satel-rate. The chipping rate for the P-code is 10.23 MHz (10.23lite's atomic clock is accurate to 10-9 seconds; installing ax 106 bits per second); for the C/A code, 1.023 MHz (1.023clock that accurate on a receiver would make the receiverx106bitspersecond),andforthedatamessage,50Hz(50prohibitively expensive.Therefore,receiverclock accuracybitsper second).ThePand C/A codes phasemodulatetheis sacrificed, and an additional satellite timing measure-carriers,the C/A code is transmitted at a phase angle of 900ment is made.The fix eror caused by the inaccuracies infrom thePcode.Theperiods ofrepetitionfortheC/AandPthe receiver clock is reduced by simultaneously subtractingcodes differ.The C/A code repeats once every millisecond;a constanttiming errorfrom four satellite timing measure-the P-code sequence repeats every seven days.ments until apinpointfixisreached.Thisprocess isanalogousto thenavigator's plottingof a visual fix whenAsstatedabovetheGPScarrierfrequenciesarephasemodulated. This is simply another way of saying that thebearing transmission error is present in his bearing repeatersystem.With that bearing error present, twovisual LOP'sdigital"1's"and"O's"contained in the P and C/A codes arewill not intersect at a vessel's true position, there will be anindicated along the carrier by a shift in the carrierphase.This is analogous to sending the same data along a carriererror introduced due to the fixed, constant error in thebear-ingtransmissionprocess.Therearetwowaystoovercomeby varying its amplitude (amplitude modulation, or AM) orsuch an error.The navigator can buy extremely accurateitsfrequency(frequencymodulation, orFM).See Figure(andexpensive)bearingtransmissionanddisplayequip-1i106a. Inphase modulation, thefrequency and the ampli-tude of the carrier are unchanged by the “informationment, or he can simply take a bearing to a third visualnavigation aid.Theresulting fix will not plot as a pinpointsignal,"and the digital information is transmitted by shiff-
SATELLITE NAVIGATION 181 The navigation accuracy that can be achieved by any user depends primarily on the variability of the errors in making pseudorange measurements, the instantaneous geometry of the satellites as seen from the user’s location on Earth, and the presence of Selective Avaliability (SA). Selective Availability is discussed further below. 1105. Global Positioning System Basic Concepts As discussed above, GPS measures distances between satellites in orbit and a receiver on or above the earth and computes spheres of position from those distances. The intersections of those spheres of position then determine the receiver’s position. The distance measurements described above are done by comparing timing signals generated simultaneously by the satellites’ and receiver’s internal clocks. These signals, characterized by a special wave form known as the pseudorandom code, are generated in phase with each other. The signal from the satellite arrives at the receiver following a time delay proportional to its distance traveled. This time delay is detected by the phase shift between the received pseudo-random code and the code generated by the receiver. Knowing the time required for the signal to reach the receiver from the satellite allows the receiver to calculate the distance from the satellite. The receiver, therefore, must be located on a sphere centered at the satellite with a radius equal to this distance measurement. The intersection of three spheres of position yields two possible points of receiver position. One of these points can be disregarded since it is hundreds of miles from the surface of the earth. Theoretically, then, only three time measurements are required to obtain a fix from GPS. In practice, however, a fourth measurement is required to obtain an accurate position from GPS. This is due to receiver clock error. Timing signals travel from the satellite to the receiver at the speed of light; even extremely slight timing errors between the clocks on the satellite and in the receiver will lead to tremendous range errors. The satellite’s atomic clock is accurate to 10-9 seconds; installing a clock that accurate on a receiver would make the receiver prohibitively expensive. Therefore, receiver clock accuracy is sacrificed, and an additional satellite timing measurement is made. The fix error caused by the inaccuracies in the receiver clock is reduced by simultaneously subtracting a constant timing error from four satellite timing measurements until a pinpoint fix is reached. This process is analogous to the navigator’s plotting of a visual fix when bearing transmission error is present in his bearing repeater system. With that bearing error present, two visual LOP’s will not intersect at a vessel’s true position; there will be an error introduced due to the fixed, constant error in the bearing transmission process. There are two ways to overcome such an error. The navigator can buy extremely accurate (and expensive) bearing transmission and display equipment, or he can simply take a bearing to a third visual navigation aid. The resulting fix will not plot as a pinpoint (as it would were there no transmission error present); rather, it will plot as a triangle. The navigator can then apply a constant bearing correction to each LOP until the correction applied equals the bearing transmission error. When the correction applied equals the original transmission error, the resultant fix should plot as a pinpoint. The situation with GPS receiver timing inaccuracies is analogous; time measurement error simply replaces bearing measurement error in the analysis. Assuming that the satellite clocks are perfectly synchronized and the receiver clock’s error is constant, the subtraction of that constant error from the resulting distance determinations will reduce the fix error until a “pinpoint” position is obtained. It is important to note here that the number of lines of position required to employ this technique is a function of the number of lines of position required to obtain a fix. In the two dimensional visual plotting scenario described above, only two LOP’s were required to constitute a fix. The bearing error introduced another unknown into the process, resulting in three total unknowns (the x coordinate of position, the y coordinate of position, and the bearing error). Because of the three unknowns, three LOP’s were required to employ this correction technique. GPS determines position in three dimensions; the presence of receiver clock error adds an additional unknown. Therefore, four timing measurements are required to solve for the resulting four unknowns. 1106. GPS Signal Coding Two separate carrier frequencies carry the signal transmitted by a GPS satellite. The first carrier frequency (L1) transmits on 1575.42 MHz; the second (L2) transmits on 1227.60 MHz. The GPS signal consists of three separate messages: the P-code, transmitted on both L1 and L2; the C/ A code, transmitted on L1 only; and a navigation data message. The P code and C/A code messages are divided into individual bits known as chips. The frequency at which bits are sent for each type of signal is known as the chipping rate. The chipping rate for the P-code is 10.23 MHz (10.23 × 106 bits per second); for the C/A code, 1.023 MHz (1.023 × 106 bits per second); and for the data message, 50 Hz (50 bits per second). The P and C/A codes phase modulate the carriers; the C/A code is transmitted at a phase angle of 90° from the P code. The periods of repetition for the C/A and P codes differ. The C/A code repeats once every millisecond; the P-code sequence repeats every seven days. As stated above the GPS carrier frequencies are phase modulated. This is simply another way of saying that the digital “1’s” and “0’s” contained in the P and C/A codes are indicated along the carrier by a shift in the carrier phase. This is analogous to sending the same data along a carrier by varying its amplitude (amplitude modulation, or AM) or its frequency (frequency modulation, or FM). See Figure 1106a. In phase modulation, the frequency and the amplitude of the carrier are unchanged by the “information signal,” and the digital information is transmitted by shift-
182SATELLITENAVIGATIONSTRINGOFONESANDZEROSTOBETRANSMITTED:sAMPLITUDEMODULATION(AM)FREQUENCYMODULATION (FM)smsPHASEMODULATION (PM)Figure1106a.Digital datatransmissionwithamplitude,frequencyandphasemodulationTHEL1SIGNALTHEL2SIGNALP(dBW)P(dBW)2.046 MHzC/A-CODE160-CODEP-CODE-163-1569F(Hz)DF(Hz)1227.6MHz1227.6MHz20.46Ml20.46MHzFigure1106b.Modulation of theLI and L2carrier frequencies with theC/Aand Pcode signals.POWERMSGNASVARRIERSPREADNOISEFigure1106c.GPSsignal spreadingandrecoveryfromsatellitetoreceiver
182 SATELLITE NAVIGATION Figure 1106a. Digital data transmission with amplitude, frequency and phase modulation. Figure 1106b. Modulation of the L1 and L2 carrier frequencies with the C/A and P code signals. Figure 1106c. GPS signal spreading and recovery from satellite to receiver
183SATELLITENAVIGATIONingthecarrier'sphase.Thephasemodulationemployed bysignal with the square wave function generated by the re-GPS isknown as bi-phase shiftkeying (BPSK)ceiver.The computer logic of the receiver recognizes thesquare wave signals as either a +1 or a 0 depending onDue to this BPSK, the carrier frequency is “spread”"whether the signal is “"on" or “"off" The signals are pro-aboutitscenterfrequencybyanamountequal totwicethecessed and matched byusing an autocorrelationfunction."chipping rate"of the modulating signal. In the case of theP code, this spreading is equal to (2 × 10.23 MHz)=20.46This process defines the necessity for a "pseudo-ran-dom code."The code must be repeatable (i.e., non-random)MHz.For the C/Acode,the spreading is equal to (2×1.023MHz)=2.046MHz.SeeFigure1106b.NotethattheL1because it is in comparing the two signals that the receivercarrier signal, modulated with both theP code and C/Amakes its distance calculations.At the same time,the codecode, is shaped differently from the L2 carrier, modulatedmust be random for the correlation process to work; the ran-with only the P code.This spreading of the carrier signaldomness of the signals must be such that the matchinglowers thetotal signal strength below the thermal noiseprocess excludes all possiblecombinations except the com-threshold present at thereceiver.This effect is demonstrat-bination that occurs when the generated signal is shifted aed inFigure 1106c.When the satellite signal is multiplieddistance proportional to the received signal's time delaywith the C/Aand Pcodesgenerated bythereceiver,thesat-These simultaneous requirements to be both repeatableellite signal will be collapsed into the original carrier(non-random)and random giveriseto thedescription offrequencyband.The signal power is then raised abovethe"pseudo-random";the signal has enough repeatability tothermalnoiselevelenable the receiver to make the required measurementwhile simultaneously retaining enough randomness to en-Thenavigation message is superimposed on both thePsure incorrect calculations are excluded.codeand C/Acodewithadatarateof50bitspersecond (50Hz.) The navigation message consists of 25 data frames,eachframeconsistingof1500bits.Eachframeisdivided1108.Precise Positioning Service And Standardintofive subframes of300bits each.It will,therefore,takePositioning Service30seconds toreceiveonedataframeand12.5minutestoreceive all 25 frames.The navigation message containsTwo levels of navigational accuracy areprovided byGPS system time of transmission;a hand over wordthe GPS:the Precise Positioning Service (PPS)and the(HOW),allowingthetransitionbetweentrackingtheC/AStandard Positioning Service (SPS).GPS was designed,codeto the P code;ephemeris and clock data for the satel-first and foremost, by the U.S.Department of Defense as alitebeingtracked,andalmanacdataforthesatellitesinUnited States military asset, its extremely accurate posi-orbit.It also contains coefficients for ionospheric delaytioning capability is an asset access to which the U.s.models used by C/Areceivers and coefficients used to cal-militarywould like to limit during time of war.Therefore,culateUniversal CoordinatedTime(UTC)thePPs is available only to authorized users, mainly theU.S.militaryand authorized allies.SPS, onthe other hand,1107.TheCorrelationProcessis available worldwidetoanyone possessingaGPS receiv-er.PPS,therefore,providesamoreaccuratepositionthandoes SPSThecorrelation process compares the signal receivedwith the signal generated internal to the receiver. It doesTwo cryptographicmethods areemployed todenythePPS accuracy to civilian users: selective availability (SA)this by comparingthe square wave function of the receivedSA/A-S ConfigurationSIS Interface ConditionsPPS UsersSPs UsersSA Set to ZeroFull accuracy,Full accuracy,*P-Code, no errorsA-S OffC/A-Code, no errorsspoofablespoofableSA at Non-Zero ValueP-Code, errorsFullaccuracy,Limited accuracy,A-S offC/A-Code, errorsspoofablespoofableSA Setto ZeroY-Code, no errorsFull accuracyFull accuracy,***Not spoofable**A-S OnC/A-Code, no errorsspoofableSA at Non-Zero ValueY-Code, errorsFull accuracyLimited accuracy,Not spoofable**A-S OnC/A-Code,errorsspoofable*Full accuracy" defined as equivelent to a PPS-capable UE operated in a similar manner.*Certain PPS-capable UE do not have P-or Y-code tracking abilities and remain spoofabledespite A-S protectionbeing applied***Assuming negligable accuracy degradation due to C/A-code operation (but moresusceptibletojamming)Figure 1108. Effect of SA and A-S on GPS accuracy
SATELLITE NAVIGATION 183 ing the carrier’s phase. The phase modulation employed by GPS is known as bi-phase shift keying (BPSK). Due to this BPSK, the carrier frequency is “spread” about its center frequency by an amount equal to twice the “chipping rate” of the modulating signal. In the case of the P code, this spreading is equal to (2 × 10.23 MHz) = 20.46 MHz. For the C/A code, the spreading is equal to (2 × 1.023 MHz) = 2.046 MHz. See Figure 1106b. Note that the L1 carrier signal, modulated with both the P code and C/A code, is shaped differently from the L2 carrier, modulated with only the P code. This spreading of the carrier signal lowers the total signal strength below the thermal noise threshold present at the receiver. This effect is demonstrated in Figure 1106c. When the satellite signal is multiplied with the C/A and P codes generated by the receiver, the satellite signal will be collapsed into the original carrier frequency band. The signal power is then raised above the thermal noise level. The navigation message is superimposed on both the P code and C/A code with a data rate of 50 bits per second (50 Hz.) The navigation message consists of 25 data frames, each frame consisting of 1500 bits. Each frame is divided into five subframes of 300 bits each. It will, therefore, take 30 seconds to receive one data frame and 12.5 minutes to receive all 25 frames. The navigation message contains GPS system time of transmission; a hand over word (HOW), allowing the transition between tracking the C/A code to the P code; ephemeris and clock data for the satellite being tracked; and almanac data for the satellites in orbit. It also contains coefficients for ionospheric delay models used by C/A receivers and coefficients used to calculate Universal Coordinated Time (UTC). 1107. The Correlation Process The correlation process compares the signal received with the signal generated internal to the receiver. It does this by comparing the square wave function of the received signal with the square wave function generated by the receiver. The computer logic of the receiver recognizes the square wave signals as either a +1 or a 0 depending on whether the signal is “on” or “off.” The signals are processed and matched by using an autocorrelation function. This process defines the necessity for a “pseudo-random code.” The code must be repeatable (i.e., non-random) because it is in comparing the two signals that the receiver makes its distance calculations. At the same time, the code must be random for the correlation process to work; the randomness of the signals must be such that the matching process excludes all possible combinations except the combination that occurs when the generated signal is shifted a distance proportional to the received signal’s time delay. These simultaneous requirements to be both repeatable (non-random) and random give rise to the description of “pseudo-random”; the signal has enough repeatability to enable the receiver to make the required measurement while simultaneously retaining enough randomness to ensure incorrect calculations are excluded. 1108. Precise Positioning Service And Standard Positioning Service Two levels of navigational accuracy are provided by the GPS: the Precise Positioning Service (PPS) and the Standard Positioning Service (SPS). GPS was designed, first and foremost, by the U.S. Department of Defense as a United States military asset; its extremely accurate positioning capability is an asset access to which the U.S. military would like to limit during time of war. Therefore, the PPS is available only to authorized users, mainly the U.S. military and authorized allies. SPS, on the other hand, is available worldwide to anyone possessing a GPS receiver. PPS, therefore, provides a more accurate position than does SPS. Two cryptographic methods are employed to deny the PPS accuracy to civilian users: selective availability (SA) SA/A-S Configuration SIS Interface Conditions PPS Users SPS Users SA Set to Zero A-S Off P-Code, no errors C/A-Code, no errors Full accuracy, spoofable Full accuracy,* spoofable SA at Non-Zero Value A-S Off P-Code, errors C/A-Code, errors Full accuracy, spoofable Limited accuracy, spoofable SA Set to Zero A-S On Y-Code, no errors C/A-Code, no errors Full accuracy, Not spoofable** Full accuracy,*** spoofable SA at Non-Zero Value A-S On Y-Code, errors C/A-Code, errors Full accuracy, Not spoofable** Limited accuracy, spoofable * ** *** “Full accuracy” defined as equivelent to a PPS-capable UE operated in a similar manner. Certain PPS-capable UE do not have P- or Y-code tracking abilities and remain spoofable despite A-S protection being applied Assuming negligable accuracy degradation due to C/A-code operation (but more susceptible to jamming). Figure 1108. Effect of SA and A-S on GPS accuracy