Satellite Signal Acquisition, Tracking, and Data Demodulation

CHAPTER 5Satellite Signal Acquisition, Tracking, andData DemodulationPhillip W. WardNAVWARD GPS ConsultingJohn W. Betz and Christopher J. HegartyThe MITRE Corporation5.1 OverviewIn practice, a GPS receiver must first replicate the PRN code that is transmitted bythe SV being acquired by the receiver; then it must shift the phase of the replica codeuntil it correlates with the SV PRN code. When cross-correlating the transmittedPRN code with a replica code, the same correlation properties occur that occurs forthe mathematical autocorrelation process for a given PRN code. As will be seen inthis chapter, the mechanics of the receiver correlation process are very differentfrom the autocorrelation process because only selected points of the correlationenvelope are found and examined by the receiver. When the phase of the GPSreceiver replica code matches the phase of the incoming SV code, there is maximumcorrelation. When the phase of the replica code is offset by more than 1 chip oneither side of the incoming SV code, there is minimum correlation. This is indeed themanner in which a GPS receiver detects the SV signal when acquiring or tracking theSV signal in the code phase dimension. It is important to understand that the GPSreceiver must also detect the SV in the carrier phase dimension by replicating thecarrier frequency plus Doppler (and usually eventually obtains carrier phase lockwith the SV signal by this means). Thus, the GPS signal acquisition and trackingprocess is a two-dimensional (code and carrier) signal replication process.In the code or range dimension, the GPS receiver accomplishes the cross-corre-lation process by first searching for the phase of the desired SV and then tracking theSV code state. This is done by adjusting the nominal spreading code chip rate of itsreplica code generator to compensate for the Doppler-induced effect on the SV PRNcode due to LOS relative dynamics between the antenna phase centers of thereceiver and the SV. There is also an apparent Doppler effect on the code trackingloop caused by the frequency offset in the receivers reference oscillator with respectto its specified frequency. This common mode error effect, which is the time biasrate that is ultimately determined by the navigation solution, is quite small for the153code tracking loop and is usually neglected for code tracking and measurement pur-poses. The code correlation process is implemented as a real-time multiplication ofthe phase-shifted replica code with the incoming SV code, followed by an integra-tion and dump process. The objective of the GPS receiver is to keep the promptphase of its replica code generator at maximum correlation with the desired SV codephase. Typically, three correlators are used for tracking purposes, one at the promptor on-time correlation position for carrier tracking and the other two located sym-metrically early and late with respect to the prompt phase for code tracking. Modernreceivers use multiple (even massively multiple) correlators to speed up the searchprocess and some use multiple correlators for robust code tracking.However, if the receiver has not simultaneously adjusted (tuned) its replica car-rier signal so that it matches the frequency of the desired SV carrier, then the signalcorrelation process in the range dimension is severely attenuated by the resulting fre-quency response roll-off characteristic of the GPS receiver. This has the consequencethat the receiver never acquires the SV. If the signal was successfully acquiredbecause the SV code and frequency were successfully replicated during the searchprocess, but the receiver subsequently loses track of the SV frequency, then thereceiver subsequently loses code track as well. Thus, in the carrier Doppler fre-quency dimension, the GPS receiver accomplishes the carrier matching (wipeoff)process by first searching for the carrier Doppler frequency of the desired SV andthen tracking the SV carrier Doppler state. It does this by adjusting the nominal car-rier frequency of its replica carrier generator to compensate for the Doppler-inducedeffect on the SV carrier signal due to LOS relative dynamics between the receiver andthe SV. There is also an apparent Doppler error effect on the carrier loop caused bythe frequency offset in the receivers reference oscillator with respect to its specifiedfrequency. This error, which is common to all satellites being tracked by thereceiver, is determined by the navigation filter as the time bias rate in units of sec-onds per second. This error in the carrier Doppler phase measurement is importantto the search process (if known) and is an essential correction to the carrier Dopplerphase measurement process.The two-dimensional search and tracking process can best be explained andunderstood in progressive steps. The clearest explanation is in reverse sequence fromthe events that actually take place in a typical real world GPS receiver, namely signalsearch and acquisition followed by