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Fundamentals of gps receivers


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  • 1. Chapter 2Introduction to the Global Positioning SystemIn Chap. 1 we introduced some of the working principles of the GPS by analogywith a simple linear model. Now its time to move into more detailed discussion ofthe GPS satellite system and how it works. In this chapter we will cover the basicsof how the satellites are configured, making the path delay measurements, a quicklook at the user position solution equations, and a high-level block diagram of GPSreceiver will be presented.2.1 The Satellite SystemGPS is comprised of 24 satellites orbiting the earth at a distance of 20,000 km asmeasured from mean sea level. See Fig. 2.1. Figure 2.2 shows a more detailedpicture of a user getting position information from four satellites. Each satellitecontains a very precise clock. All the clocks and hence all the timing signalsassociated with each satellite are in near-perfect synchronism. The basic principleof operation is similar to the Roadway Distance Measurement example of Chap. 1.A rough model is to imagine a system of satellites emitting a “pulse” of light atexactly the same instant as all the others (Important: GPS is NOT a pulsed system).If you were at the exact center of the earth and the satellites were in perfect circularorbits (GPS orbits are nearly circular) these pulses would all arrive at your receiverat the exactly same instant. For all other points the pulses would (typically) arrive atdifferent times. If one has knowledge of the positions of the satellites and canmeasure the path delay (distance) from at least four satellites to the user receiver,the user position X, Y, Z in ECEF (Earth Centered Earth Fixed) coordinates and userclock bias can be solved for.D. Doberstein, Fundamentals of GPS Receivers: A Hardware Approach, 23DOI 10.1007/978-1-4614-0409-5_2, # Springer Science+Business Media, LLC 2012
  • 2. 24 2 Introduction to the Global Positioning System c DKD INSTRUMENTS EARTHc DKD INSTRUMENTSFig. 2.1 The earth and its constellation of GPS satellites2.2 Physical Constants of a GPS Satellite Orbit That Passes Directly OverheadFigure 2.3 shows a simple model of one satellite circling earth directly overhead atthe user zenith. We will use this simple model to compute some constantsassociated with a user at or near the earth’s surface. We will assume that the usercan be at a maximum altitude above sea level of 10 km. The minimum altitude willbe assumed to be sea level. With this information and the known mean diameter of aGPS orbit we can calculate the minimum and maximum distance to the satellite as itpasses overhead. Figure 2.3 shows the elements used in this calculation and otherresults. From this information we find that the maximum distance a GPS satellite is25,593 km. The minimum distance is found to be 20,000 km. These distancescorrespond to path delays of 66 and 85 ms, respectively. As we have seen inChap. 1, we can use this knowledge of maximum and minimum distance to setthe receiver’s reference clock to approximate GPS time.
  • 3. 2.3 A Model for the GPS SV Clock System 25 ZK R3 R1 EARTH R4 c DKD INSTRUMENTS USER EQU ATOR IA L PL R2 ANE YKXK c DKD INSTRUMENTSFig. 2.2 User at earths surface using four SV’s for position determination2.3 A Model for the GPS SV Clock SystemEach GPS SV has its own clock that “free runs” with respect to the other clocks in thesystem. GPS uses a “master clock” method in which all the clocks are “referenced” tothe master clock by the use of error terms for each SV clock. We discussed thisalready in Chap. 1. In this chapter we will use the clock model of Fig. 1.7 minus theoutside second-counter dial. In addition to the omitted second-counter dial, we willnot show the dial that indicates the week of the year, year dial, etc. We have notdiscussed these new dials but they are present in the time-keeping method employedby GPS. For the purpose of discussion and understanding GPS we will often use theclock model of Fig. 1.7 minus the dials above the 0–1 s time increment. If we show ordiscuss a SV or Receiver replica clock without all the dials, the reader will assume themissing dials are “present” but not shown for reasons of clarity. The smallest time increment dial of our clock model will always be the0–0.977 ms dial. As mentioned in Chap. 1 this dial has a very fine “effective”resolution as used in the SV. But this statement is an approximation. In reality the0–0.977 ms dial is the one dial of all the clock dials in our model that does not have adirect physical counter part in the “true” SV clock. In other words, our clock modelat the sub microsecond level is not a completely accurate model of the GPS clock.
  • 4. 26 2 Introduction to the Global Positioning System SATELLITE ORBIT RMIN RMAX USER USER HORIZON LINE Rsv EARTH RADIUS OF EARTH ~ 6,368 KM c DKD INSTRUMENTSUSER TO SATELLITE DATA GPS SATELITE DATA R ~ 20,000KM MIN MEAN ALTITUDE ABOVE SEA LEVEL ~ 20,000KM ~RMAX 25,593 KM Rsv = RADIUS OF ORBIT WRT CENTER OF THE EARTH ~ 26,550 KM ORBITAL RATE : 2 ORBITS IN 24 HOURSMin Time Delay ~ 66msec ORBITAL SPEED ~ 3874M/SECMax Time Delay ~ 86msecFig. 2.3 The range from the receiver to a GPS satellite and physical constants (for a GPS satelliteorbit that passes directly overhead)When we use the clock model to describe the receiver’s reference clock (or SVreplica clocks) we will see that the 0–0.977 ms dial does have a direct physicalcounter part in the receiver. The impact of having the smallest time increment dialnot an exact model for the SV clock is not an important issue for this text as our goalis position accuracy of Æ100 m.2.4 Calculating Tbias Using One SV, User Position KnownPerhaps the simplest application of GPS is synchronizing a receiver’s clock when thereceiver’s position is known. In this example we will form an first-order estimate ofthe Tbias term associated with receiver clock. Figure 2.4 shows a model of ourexample system. The SV has its own clock and sends its clock timing information tothe earth-based GPS receiver using an encoded radio wave. The receiver has twoclocks, a reference clock and a replica of the SV clock reconstructed from thereceived radio wave. Figure 2.4 shows the receiver clock corrected with the calcu-lated Tbias term, i.e., it is in synchronism with the SV clock.
  • 5. 2.4 Calculating Tbias Using One SV, User Position Known 27 100MSEC 0 EC 20 0M SE MS C 20 C SE 0 1 0.9 77 uSE C GPS SATELLITE 300 MSEC MSEC900 2MSEC 3MSEC 0 4M 0 SE C C = SPEED OF LIGHT EC L) C R = t*C C 5M SE uS SE SE 77 OTA C .9 T M 1M =0 S 20 SEC 400MS C TIC TI 1 023 6M EC 17MSEC 18MS (1 EC EC 7MSEC 800MS 8MSEC t = Trec - Tsent - Tbias SEC 16M C SE 9M C SE M 15 C SE 14 M MS 10 EC SEC C SE 13MS 11M EC 12MSEC C ME S 00 M C 0M SE 50 5 0 70 600MSEC R= (Xu - Xsv)2 + (Yu - Ysv)2 + (Zu - Zsv)2 Tbias = Trec - Tsent - R C SV POSITION (X, Y, Z) t= R Pa th De lay c DKD INSTRUMENTS USER REC. 1 SEC 0 1 SEC 0 20MSEC 20MSEC C 10 C 10 SE 0M SE 0M 0M SE 0M SE 90 C 90 C 1MSEC 0 1MSEC 0 C C 0.977uSEC 0 SE 200M SE 200M 0.977uSEC 0 800M 800M SEC 1 TIC =0.977uSEC SEC (1023 TICS TOTAL) 1 TIC =0.977uSEC (1023 TICS TOTAL) 20MSEC 0 20MSEC 0 C 2M C 2M MSE SE C MSE SE 18 18 C C C MSE 3M MSE 3M SEC SEC 300M SE 300M SE 17 17 C C 700M SEC 700M SEC SEC 4MS SEC 4MS 15MSEC 16M 15MSEC 16M EC EC 5MSEC 5MSEC EC 14M EC 14M 6MS 6MS SEC SEC C C 13 SE 13 SE MSE 7M MSE 7M C 40 40 C C SE C 12 0M MSE C SE 12 C 0M 0M SE SE MSE SE 60 C 8M C 0M SE 11M 60 C 8M C SEC EC 11M EC 9MS SEC 9MS 10MSEC 10MSEC 500MSEC 500MSEC 500MSEC REPLICA OF RECEIVER CLOCK FROM SV REFERENCE CLOCK USER POSITION (Xu, Yu, Zu) EARTHFig. 2.4 Calculation of Tbias using a single SV, user and SV position known In order to solve for Tbias we need to calculate the distance R from the SV to theUser receiver and measure Trec and Tsent as shown in Fig. 2.4. The calculation of Ruses the distance equation which requires the user and SV positions in X, Y, Z. We haveassumed the user position is known. In addition to the SV clock information the SV
  • 6. 28 2 Introduction to the Global Positioning Systemposition information is encoded onto the radio wave that is transmitted to the receiver.We will assume for now that this information is provided in X, Y, Z coordinates.We will see later that getting SV position information in X, Y, Z format is nontrivial. The Tsent and Trec information are obtained as before by just taking a “snapshot” of the receiver reference clock and the receiver’s replica of the SV clock. Ifwe record the SV position at the same instant that we record Trec and Tsent, we willhave all the information needed to solve for Tbias. It is important to realize that dueto SV motion we must “capture” the SV position data at the same moment wecapture the state of the receiver’s clocks. If we do not properly capture SV position,Tsent, and Trec, then the computed distance, R, would be incorrect for the measuredpath delay. Now that we have all the information needed we can calculate R, Dt, and Tbias.If we continually update the measurements and calculations, the receiver referenceclock will “track” the SV clock. This allows the GPS time receiver to replicate thestability of the SV clock. SV clocks are atomic based and so the stability is veryhigh. If we modify the receiver’s reference clock to output a “pulse” every time the1 s dial passes the 0 tic mark and a 1 pps signal will be generated. This is a commonsignal many GPS receivers provide.2.5 GPS Time Receiver Using Master Clock and the Delay term TatmIn the previous example we computed Tbias when the user position was known. Inthis example we will include the effects of SV clock error with respect to the GPSmaster clock and the additional delay caused by diffraction of the radio beam as itpasses through the earth’s atmosphere. Figure 2.5 shows the details of our new model. The receiver’s reference clock isshown corrected to the master clock time. The SV clock has a small error withrespect to master clock. The error is less than a millisecond. As before we capturethe state of the receiver’s reference clock and the replica clock. This information isused in conjunction with the computed path length R to form our estimate of Tbias.There is a difference from our first example and that is in the path delay. Theexpression for the path delay now has two additional terms. One, of course, is theSV clock error with respect to the master clock, Terr_sv. The other is the termTatm. The term Tatm is the extra delay experienced by the radio beam as it travelsthrough the earth’s atmosphere. To a first approximation the atmosphere acts like alens and “bends” the radio beam from the SV to receiver. This bending of the radiobeam as it passes through the atmosphere causes the extra delay. Normally the delayTatm is broken into two parts, one for the Ionosphere and one for the Tropospherelayers of the earth’s atmosphere. Here we have combined them into one term withthe sign convention following most of all GPS literature. How big is the added
  • 7. 2.5 GPS Time Receiver Using Master Clock and the Delay term Tatm 29 100MSEC 0 EC 20 0M SE C MS 20 C 0 SE E C GPS SATELLITE1 0.9 77 uS 300M SEC EC900MS 2MSEC 3MSE C 4M 0 0 SE C C = SPEED OF LIGHT 5M EC L) C R = t*C SE C u S TA SE C SE 400M 77 O C M SE .9 T 1M =0 S 20 6M C TIC SEC EC TI 3 1 02 18MS 7MSEC (1 EC 17MSEC 800MS C 8MSE t = Trecsv - Tsentsv - Tbias + Terrsv - Tatmsv C SE C 16M SE 9M C SE C M 15 SE M 14 10 MS C EC SE 11M 13MS C SE EC 12MSEC C 0M 50 SE (Xu - Xsv)2 + (Yu - Ysv)2 + (Zu - Zsv)2 700M 600MSEC R= R Tbias = Trecsv - Tsentsv + Terrsv - Tatmsv - C SV POSITION (X, Y, Z) SV CLOCK ERROR= Terr_sv c DKD INSTRUMENTS t= R Pa th De lay MASTER CLOCK 1 SEC 20MSEC 0 C 10 SE 0M 0M SE 90 C USER REC. 1MSEC 0 0.977uSEC 0 1 SEC 0 1 SEC 0 200M C SE 1 TIC =0.977uSEC 20MSEC 20MSEC SEC 800M (1023 TICS TOTAL) C 10 C 10 SE 0M SE 0M 0M SE 0M SE 90 C 90 C 20MSEC 0 C 2M MSE SE C 18 C MSE 3M 0 SEC 1MSEC 300M SE 17 0 C 1MSEC 700M SEC C C SEC 4MS 0.977uSEC 0 SE 200M SE 200M 0.977uSEC 0 15MSEC 16M 800M EC 800M SEC 1 TIC =0.977uSEC SEC 5MSEC (1023 TICS TOTAL) 1 TIC =0.977uSEC (1023 TICS TOTAL) EC 14M 6MS SEC C 13 SE MSE 7M C 40 20MSEC 0 20MSEC 0 C SE 12 C 0M MSE SE 0M SE C 2M 60 C 8M C C 2M 11M SEC EC MSE SE C MSE SE 9MS 18 18 C 10MSEC C C MSE 3M MSE 3M SEC SEC SE 300M SE 300M 17 17 C C 700M SEC 700M SEC 4MS SE 4MS SEC 15MSEC 16M 15MSEC 16M C EC 500MSEC EC 5MSEC 5MSEC EC 14M EC 14M 6MS 6MS SEC SEC C C 13 SE 13 SE MSE 7M MSE 7M C 40 C C 40 SE C 12 0M MSE C SE SE 12 C 0M 0M SE 0M MSE SE SE 60 C 8M C C 8M 11M EC 60 11M C SEC 9MS SEC EC 10MSEC 9MS 10MSEC 500MSEC 500MSEC 500MSEC c DKD INSTRUMENTS REPLICA OF CLOCK FROM SV RECEIVER REFERENCE CLOCK USER POSITION (Xu,Yu, Zu) EARTHFig. 2.5 Calculation of Tbias using a single SV, user and SV position known with master clockand atmospheric delay termdelay? Expressed as an increase to the R term this “extra” distance is in 50 m range.The maximum value occurs when the SV is low in sky and decreases as it rises. Thisdelay is relatively large and consequently places the error introduced by Tatm at thetop of a long list of error sources. Estimating the value of Tatm requires knowledgeof the SV position in relation to the receiver, exact time of day, sun spot activity,
  • 8. 30 2 Introduction to the Global Positioning Systemday/night, etc. Needless to say, the effects of the Troposphere and Ionosphere on aradio beam are extremely complex and beyond the scope of this text. The reader isdirected to the bibliography for suitable references. By including the master clock correction and a term for atmospheric delay weform a model that achieves greater accuracy than our previous model. We also putinto place the primary components needed to do the full solution to the user positionproblem. If we were to look at a 1 pps signal generated from the receiver’s referenceclock it “track” the GPS master clock with greater precision. This example also illustrates that by refining the path delay estimate we canachieve greater accuracy in the computed value of Tbias. This will also be true forcomputed user position. It is hoped the reader sees the pattern emerging of succes-sive refinements to the path delay, which results in greater accuracy for bothposition and time measurements.2.6 Solving For User Position Using Four SatellitesWith the previous examples and models we have laid the groundwork for the task ofdetermining user position, Xu, Yu, Zu in ECEF coordinates and the user clock error,Tbias. Figure 2.6 shows the system using four SV’s and four SV clock replicas in theuser receiver. We again assume the user receiver is at or near the surface of the earth(i.e., each path delay is 66–85 ms). The four SV clocks are referenced to a masterclock. The receiver’s reference clock is shown with its error removed. In other wordsthe receiver reference clock is displaying master clock time. For each SV to userdistance a path delay is measured/computed. Additionally a distance Ri is assigned tothe equivalent distance corresponding to path delay multiplied by the speed of light. Figure 2.7 shows the set of equations. Each distance, Ri, can be computed fromtwo different methods. The first method uses the path delay, which containsmeasured, computed, and SV-supplied terms. Each path delay has its own uniqueTatmsv_i term derived from estimates of user position. As user position estimatesare refined, this term will also be refined in accuracy. The clock error for each SV issent to the receiver and is identified by Terrsv_i. Normally the Trec time would be the same for all the path delays. It is possible tohave separate Trec times if the user receiver does not move appreciably from oneTrec time to the next. As before we take a “snapshot” of the receiver’s referenceclock and the replica clock or clocks to record Trec and Tsentsv_i. If we choose toinclude all the clocks in our picture, then the value of Trec could be the same for allSV path delay equations. This last point needs a bit more clarification. If the userreceiver is stationary (or moving slow compared to the time to measure each pathdelay) then we can measure the path delay to each SV separately. That is, we couldfirst do SV1, then SV2, etc. In other words, a sequential measurement using a singlechannel receiver. As long as the drift rate of the receiver’s reference clock is lowenough over the time all four measurements are made, this method will work fine.Many early GPS receivers used this method as they were single channel receivers.
  • 9. 1 SEC 0 1 SEC 0 1 SEC 0 1 SEC 0 20MSEC 20MSEC 20MSEC 20MSEC C 10 C 10 SE SE 10 C 0M 0M 0M 0M C 0M SE 10 90 SE 90 SE SE SE 0M 0M C C 0M C 90 SE 90 C 1MSEC 0 1MSEC 0 C C 0 1MSEC 0 C 1MSEC SE SE C 0.977uSEC 0 SE 0.977uSEC 0.977uSEC 0.977uSEC 200 0 0 0 0M 0M 200 200 SE 0M 80 80 200 MS 80 EC 1 TIC =0.977u SEC 1 TIC =0.977u SEC 800M MSEC MSEC (1023 TICS TOTAL) (1023 TICS TOTAL) 1 TIC =0.977u SEC 1 TIC =0.977u SEC MSEC (1023 TICS TOTAL) (1023 TICS TOTAL) 20MSEC 0 20MSEC 0 20MSEC 0 20MSEC 0 EC 2M EC 2M 2M MS SE MS SE EC SE EC 2M 18 C 18 C MS C MS SE 18 18 C EC EC EC 3M 3M MS MS EC 3M EC EC 17 17 SE SE 3M MS SE MS EC C C C C 300 17 SE MS MS 17 C SE MS 300M 300M MS SEC SEC 700 700 EC 300M SEC 4MS 4MS SE SE 4MS 700 EC C C 4MS SE EC EC 700M EC C EC 15MSEC 16M 15MSEC 16M 5MSEC 5MSEC 15MSEC 16M 5MSEC 15MSEC 16MS 5MSEC EC EC EC 14MS 14MS EC 14M 6MS 6MS 14MS EC EC 6MS 6MS EC SEC C C C 13 13 SE SE C SE 13 13 MS MS 7M 7M SE 7M MS EC EC MS C C 7M 40 12 12 40 40 EC EC SE SE C MS C MS C 0M 0M C 0M C 12 40 0M EC SE 0M EC SE 12 SE MS SE MS C SE SE SE SE 0M 60 11M 8M C 60 11M 8M C EC 8M C 0M EC SE SE EC EC 0M SEC SEC 8M 10MSEC 9MS 10MSEC 9MS 60 11MS EC 10MSEC 9MS EC 60 11M SEC 10MSEC 9MS EC C Terr_sv1 (Xsv_1, Ysv_1, Zsv_1) (Xsv_3, Ysv_3, Zsv_3) (Xsv_2, Ysv_2, Zsv_2) Terr_sv2 (Xsv_4, Ysv_4, Zsv_4) Terr_sv3 t MASTER CLOCK Terr_sv4 3 = lay 1 SEC 0 De 20MSEC R3 elay Pa th a th C 10 SE 0M 0M SE 90 C =P th D De R4 t1 a la R2 t = y R1 1MSEC 0 0.977uSEC 0 4 Pa th 2 t =P 200 EC 1 TIC =0.977uSEC MS MS Dela (1023 TICS TOTAL) EC 800 y 20MSEC 0 EC 2M SE MS C 18 EC 3M MS SE EC 17 C 300 MS EC MS 700 4MS EC EC 15MSEC 16MS EC 5MSEC 14MS 6MS EC C 13 SE MS 7M EC C 40 12 C 0M MS SE 0M EC SE SE 8M C 11MS 60 EC EC 9MS 10MSEC 2.6 Solving For User Position Using Four Satellites 500MSEC USER POSITION (Xu, Yu, Zu) 1 SEC 0 1 SEC 0 1 SEC 0 1 SEC 0 USER REC. 1 SEC 0 20MSEC 20MSEC 20MSEC 20MSEC 20MSEC C 10 SE 0M C 10 C 10 C 10 C 10 0M SE SE 0M SE 0M SE 0M SE 0M 90 C 0M SE 0M SE 0M SE 0M SE 90 C 90 C 90 C 90 C 1MSEC 0 C 0 1MSEC 0 1MSEC 0 1MSEC 0 C C C 1MSEC C 0.977uSEC 0 SE 200 0.977uSEC 0 0.977uSEC 0 0.977uSEC 0 SE SE SE SE 200 200 200 0.977uSEC 0 200 MS 800M MS MS MS MS 800M 800M 800M EC 1 TIC =0.977uSEC 800M EC EC EC 1 TIC =0.977uSEC EC 1 TIC =0.977uSEC (1023 TICS TOTAL) 1 TIC =0.977uSEC 1 TIC =0.977uSEC (1023 TICS TOTAL) (1023 TICS TOTAL) (1023 TICS TOTAL) (1023 TICS TOTAL) 20MSEC 0 20MSEC 20MSEC 20MSEC 2M 0 0 0 20MSEC 0 EC SE MS EC 2M EC 2M EC 2M 2M 18 C SE SE SE EC MS C MS C MS C MS SE 18 18 18 18 C EC 3M EC EC EC MS EC 3M 3M 3M SE EC 3M 17 MS MS C MS 300 MS SE SE SE EC EC EC SE 17 17 17 EC C C C 17 300 300 MS C 300 300 MS MS MS MS SEC MS MS MS MS 4MS 700 MS SEC SEC SEC EC SEC 4MS 4MS 4MS 700 700 700 EC 4MS 700 EC EC EC EC EC EC EC EC 15MSEC 16M 5MSEC 15MSEC 16M 15MSEC 16M 15MSEC 16M 15MSEC 16M 5MSEC 5MSEC 5MSEC 5MSEC EC 14M EC EC EC EC 14M 14M 14M 6MS 14M SEC 6MS 6MS 6MS 6MS SEC SEC SEC C SEC 13 C C SE C C 13 13 13 MS SE SE SE 7M 13 SE MS MS MS EC 7M 7M 7M 40 MS C 7M 12 EC EC EC 0M C C 40 C 40 C 40 EC SE MS C 40 EC SE SE SE 12 C 0M 12 C 0M SE 12 C 0M 12 MS SE 0M MS MS C 0M 8M SE C EC SE SE SE SE EC SE SE MS SE 11M EC 60 0M 0M SE EC 0M SEC 8M C 8M C 8M C 0M EC 8M 9MS 60 11M EC 60 11M EC 60 11M EC C 10MSEC 11M 60 SEC 9MS SEC 9MS SEC 9MS SEC EC 10MSEC 10MSEC 10MSEC 9MS 10MSEC 500MSEC 500MSEC 500MSEC 500MSEC 500MSEC 500MSEC REPLICA OF REPLICA OF REPLICA OF REPLICA OF RECEIVER CLOCK FROM SV_4 CLOCK FROM SV_3 CLOCK FROM SV_2 CLOCK FROM SV_1 REFERENCE CLOCK EARTH 31Fig. 2.6 Calculation of user position and Tbias using four SV’s
  • 10. 32 2 Introduction to the Global Positioning SystemFig. 2.7 Equation set forsolving for user position andTbias using four SV’sThe drawback is a better reference clock is needed as the method is relying on Tbiasbeing approximately constant over the entire measurement sequence of the fourSV’s path delays. Being able to “measure” four Trec and four Tsent times simulta-neously requires a four-channel receiver. This is the assumption made when writingthe equations of Fig. 2.7. The distance Ri can also be computed by using the distance formula as we did inthe single SV example for obtaining Tbias. Each distance is computed from theuser’s position Xu, Yu, Zu and the position of the SV, which is assumed to be sentfrom SV as Xsv_i, Ysv_i, Zsv_i coordinates. By using the distance equations combinedwith the path delay equations we have enough information to determine thereceiver’s position and the receiver clock error. Unlike our simple solution forTbias using one SV, the equation set of Fig. 2.7 cannot be solved in closed form forTbias and Xu, Yu, Zu. The reason is that the equation for distance contains thesquare root function. This introduces a nonlinearity which precludes a “closedform” solution. To solve the equations a iterative method is employed. An iterativesolution for user position and Tbias is presented in Chap. 5.
  • 11. 2.8 A Simplified Model of the GPS Receiver 332.7 The Pseudo-RangeWhen we subtract the Tsent reading of SV from the Trec reading we are computing ameasured path delay. But our measured path delay has errors. Tbias is rarely, if ever,exactly zero. Because of this error the distance obtained by computing the range fromknown user position and known SV position plus the distance equivalent of the userclock error, Tbias, is called the “Pseudo Range.” This statement reflects the fact thatthis range is not exact because it has the error of receiver clock bias error expressed init. In the terms used in this text and following conventional polarities of error terms,the Pseudo-Range for any given SVi would be: Pseudo - Rangei ¼ ½ðxu À xsv i Þ2 þ ðyu À ysv i Þ2 þ ðzu À zsv i Þ2 Š1=2 þ Tbias  C (2.1)A related measured path delay version/estimation of the Pseudo-Range is: C  Dti ! C  ½Trecsv i À Tsentsv i þ Terrsv i À Tatmsv i Š (2.2)2.8 A Simplified Model of the GPS ReceiverFigure 2.8 shows a high-level block diagram of a GPS receiver. The receiver shownhas four channels. Only one of the four channels is fully detailed. The other threeare shown as repeated blocks. Each channel has an identical SV clock replicationblock. This block replicates the SV clock from the received SV RF signal. Only onereceiver reference clock is needed. The RF carrier is typically converted to a lowerfrequency and then sent to each channel. Dividing down Receiver Reference Oscillator (RRO) forms the replica clock andthe receiver reference clock. The receiver reference clock as shown can be identicalin construction to the receiver’s replica clocks or it could be of different construc-tion. Later in Chap. 6 we will introduce an alternate reference clock based on thetagged SNAP_SHOT method. This approach can result in a simpler reference clockimplementation. The reference clock reset function shown is used to start the clockat predetermined time or to “subtract out” Tbias from the reference clock so that itdisplays Master Clock time (within the errors of receiver). The receiver’s computercan force Tbias to minimum by dynamically loading N0, N1, N2 and N3 values intothe receiver reference clock registers. In some receivers this is optional and theerror is held in software. It is important to understand that the SV Replica Clock is never truly recovered.We can only form better and better estimates of the true SV clock as the receiverdesign is advanced. This same comment holds for the receiver’s reference clock.We can never get to the point when Tbias is exactly zero, the goal is to drive it to aminimum.
  • 13. 2.9 The Receiver Reference Oscillator 35 The SV replica clock must be properly reset in order to be a “faithful” delayedreproduction of the received SV clock. In the reception process the first two dialsare correctly set by the correlation process used on the SV signal. In practice, the1 ms dial is set by a quasi-dynamic process while the 0.977 ms dial is dynamicallycontrolled to maintain “lock” on the received SV clock. The Preset inputs on thesetwo dials is optional as some receivers do not “preload” values here. Since theoutput of the 0.977 dial drives all the other dials locking this dial to the received SVclock “locks” all the subsequent dials. The 0.977 dial may be a fixed divider incombination with a modulator or a pure analog method, which would use a separatereference oscillator from the receiver’s reference clock (not shown). The 20 ms and 1 s dials must be set by examination of the received data from theSV for that channel. Once the dials are initially set to the correct initial value they“free run” except for the 0.977 dial, which is under a servo loop control to keep lockon the received SV clock. Each channel must independently set its dials as perthe method just described. After all channel clock dials are properly set then aSNAP_SHOT can be taken which records the state of all replica and reference dials(details of capture method not shown). The data captured from replica and reference dials is now sent to the computer.In addition, each channel decodes the SV position information for the time thesignal left the SV (i.e., Tsent). The computer now has SV position, Trec, and Tsentfor all four channels. Now the computer can solve the equations of Fig. 2.7(by iteration methods) for user position and receiver clock bias. In the hardware section of the text we will get into the details of exactly how allthis is accomplished. In Chap. 3 we will see how the transmitter, or SV, clock isconstructed.2.9 The Receiver Reference OscillatorThe reference oscillator used in the GPS SV is an atomic-based system or “atomicclock.” Atomic clocks are extremely accurate and stable over long periods oftime. Due to reasons of cost and size the RRO used in “lower end” GPS receiverscannot be an atomic clock. Typically, lower-cost GPS receivers use oscillatorsaccurate to Æ0.5 ppm, while the SV atomic clocks will exceed Æ0.00001 ppmaccuracy. To investigate the effect of RRO error suppose we reset the receiver’s referenceclock such that it reads exactly 0 at the same instant as the GPS Master clock. Wethen let the reference clock “free run.” After a period of time (an interval dependenton the PPM rating of the RRO) our reference clock would typically drift from theGPS Master clock resulting in a difference of displayed time on the dials of the twoclocks. This occurs because our reference oscillator has a static frequency offset anddrift rate. The result is the receiver reference clock will lose synchronism with respectto the Master clock as time advances. Once the error grows to approximately 1 ms we
  • 14. 36 2 Introduction to the Global Positioning Systemwould see an equivalent distance error of 300 m in our path delay measurements.In general, higher-quality GPS receivers use higher-quality reference oscillators inorder to mitigate the problems associated with RRO errors.2.10 Satellite Position InformationIn our model we assumed that the XYZ data was sent down to receiver on anencoded RF signal. The format was assumed to be the satellites position expressedin ECEF coordinates, X, Y, Z. Unfortunately it is not that simple. Rather thansending its position in X, Y, Z coordinates each SV sends a very complex set ofOrbital Parameters for its particular orbit. This set of orbital data for GPS is calledthe Ephemeris data. Orbital parameters can be used to describe the precise orbit characteristics of anyorbiting body about the earth. The parameters contain the diameter of the orbit, itsdeviation from circular, its angle w.r.t. the polar axis, etc. If you know the orbitalparameters of an orbiting body precisely, you can predict where it is and where itsgoing, provided you know what time it is. The SV orbits are fairly stable and repeatable. So once you have the orbitalparameters for all the satellites at a particular day and time you can compute theirpositions at a later date (not too much later) with a fair degree of accuracy, using thesame set of orbital data. In fact, as we will find out later each SV sends down theorbital parameters for not only itself but for all the other SV’s of GPS but withreduced precision (Almanac Data). This allows the receiver that has a successfulcontact with just one SV to obtain position information for all the other satellites.Together with the initial estimate of present GPS time this information can assistthe receiver in obtaining its first estimate of user position. Once the receiver gets the orbital data it now has the task of converting this datato X,Y,Z in ECEF. In order to do this accurately it must know what “time” it is. Wecan get this information from the Tsent data present on the receiver SV replicaclocks. Once we have the Tsent time we can use the orbital parameters to work outthe X,Y,Z position of the SV at the Tsent instant of time.2.11 SummaryIts time to take a step back and examine where we are in our quest to understand theGPS receiver and to get our first-order solution of the user’s position. The first twochapters have laid the groundwork for understanding the basic principles of theusing synchronized clocks to measure distance by measuring path delay. In thischapter we have discussed some of the details of the satellite system. It is hoped thatthe reader can start to see the real complexity involved and now has a grasp on thebasic principles and working mechanisms behind GPS.
  • 15. 2.11 Summary 37 The first two chapters were written on a lay basis so as to insure that as manyreaders as possible can get the basics of GPS. From here on some readers may nothave the background to proceed. Those readers may need to consult other texts toget the needed background knowledge to understand the discussions ahead. TheGPS signal structure will be the next topic.
  • 16.