Satellite Signal Acquisition, Tracking, and Data Demodulation - Understanding GPS: Principles and Applications - page 154

code tracking loop and is usually neglected for code tracking and measurement pur-
poses. The code correlation process is implemented as a real-time multiplication of
the 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 prompt
phase of its replica code generator at maximum correlation with the desired SV code
phase. Typically, three correlators are used for tracking purposes, one at the prompt
or 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. Modern
receivers use multiple (even massively multiple) correlators to speed up the search
process 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 signal
correlation process in the range dimension is severely attenuated by the resulting fre-
quency response roll-off characteristic of the GPS receiver. This has the consequence
that the receiver never acquires the SV. If the signal was successfully acquired
because the SV code and frequency were successfully replicated during the search
process, but the receiver subsequently loses track of the SV frequency, then the
receiver 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 and
then 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-induced
effect on the SV carrier signal due to LOS relative dynamics between the receiver and
the SV. There is also an apparent Doppler error effect on the carrier loop caused by
the frequency offset in the receiver's reference oscillator with respect to its specified
frequency. This error, which is common to all satellites being tracked by the
receiver, 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 important
to the search process (if known) and is an essential correction to the carrier Doppler
phase measurement process.
The two-dimensional search and tracking process can best be explained and
understood in progressive steps. The clearest explanation is in reverse sequence from
the events that actually take place in a typical real world GPS receiver, namely signal
search and acquisition followed by steady state tracking. The two-dimensional search
and acquisition process is easier to understand if the two-dimensional steady state
tracking process is explained first (Section 5.2). Once in steady state tracking, the
two-dimensional code and carrier tracking process is easier to understand if the car-
rier tracking process is explained first even though both the code and carrier tracking
processes are taking place simultaneously. This is the explanation sequence that will
be used. The explanation will first be given in the context of a generic GPS receiver
architecture with minimum use of equations. This high-level overview will then be fol-
lowed by more detailed explanations of the carrier (Section 5.3) and code tracking
loops (Section 5.4), including the most useful equations. Sections 5.5 through 5.7
cover additional topics regarding the tracking loops. Section 5.5 addresses the design
of loop filters. Section 5.6 discusses measurement errors and tracking thresholds. Sec-
tion 5.7 describes how the pseudorange, delta pseudorange, and integrated Doppler
measurements are formed from the natural measurements of a GPS receiver.