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Abstract:
This term paper provides necessary material for the Carrier phase tracking and Code phase tracking for GPS receiver. We started with the general discussion, How GPS signal is correlated the PRN code that is transmitted by the satellite. ... It covers carrier phase tracking in detail giving necessary block diagrams of this module. And in the end it covers code phase tracking in detail. This paper helps in the understanding of GPS signal tracking.
GPS SIGNAL ACQUISITION AND TRACKING
In practice, a GPS receiver must replicate the PRN code that is transmitted by the SV that is being acquired by the receiver, then it must shift the phase of the replica code until it correlates with the SV PRN code. The same correlation properties occur when cross-correlating the transmitted PRN code with a replica code as occurs for the mathematical autocorrelation process for a given PRN code. When the phase of the GPS receiver replica code matches the phase of the incoming SV code, there is maximum correlation. When the phase of the replica code is offset by more than one chip on either side of the incoming SV code, there is minimum correlation. This is indeed the manner in which a GPS receiver detects the SV signal when acquiring or tracking the SV signal in the code-phase dimension. It is important to understand that the GPS receiver must also detect the SV in the carrier-phase dimension by replicating the carrier frequency plus Doppler (and usually eventually obtains carrier phase lock with the SV signal by this means). Thus, the GPS signal acquisition and racking process is a two-dimensional (code and carrier) signal replication process.
In the code or range dimension, the GPS receiver accomplishes the cross-correlation process by first searching for the phase of the desired SV and then tacking the SV code state by adjusting the nominal chipping rate of its replica code generator to compensate for the Doppler-induced effect on the SV PRN code due to line-of-sight relative dynamics between the receiver and the SV. There is also an apparent Doppler effect on the code tracking loop caused by the frequency offset in the receivers reference oscillator with respect to its specified frequency. This error effect, which is the time bias rate determined by the navigation solution, is quite small for the code tracking loop and is usually neglected. 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 integration 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.
However, if the receiver has not simultaneously adjusted (tuned) its replica carrier 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 frequency response roll-off characteristic of the GPS receiver. ... If the signal was successfully acquired because the SV code and frequency were successfully replicated during the search process, but then the receiver subsequently loses track of the SV frequency, i then the receiver subsequently loses code track as well. Thus, in the carrier Doppler il frequency dimension, the GPS receiver accomplishes the carrier matching (wipeoff) I 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 carrier frequency of its replica carrier generator to compensate for the Doppler- induced effect on the SV carrier signal due to line-of-sight 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 receivers reference oscillator with respect to its specified frequency. ...
The two-dimensional acquisition and tracking process can best be explained and understood in progressive steps. ... The two- dimensional search and acquisition process is easier to understand if the two-dimensional steady-state tracking process is explained first. The two-dimensional code and carrier tracking process is easier to understand if the carrier tracking process is explained first. ... This high-level overview will then be followed by more detailed explanations of the carrier and code tracking loops, including the most useful equations.
GPS Receiver Code and Carrier Tracking
Most modern GPS receiver designs are digital receivers. ... Only the carrier frequency is lowered. ... Not shown in the block diagram are the baseband timing signals that are provided to the digital receiver channels by the frequency synthesizer phase-Iocked to the reference oscillators stable frequency. The IF must be high enough to provide a single-sided bandwidth that will support the PRN code chipping frequency. ... The name "digital receiver channel" is somewhat misleading since it is not the ASIC but the receiver processing function that usually implements numerous baseband functions such as the loop discriminators and filters, data demodulation, meters, phase-Iock indicators, and so forth. ...
Carrier Tracking Loops
Figure 2 illustrates the block diagram of the GPS receiver carrier-tracking loop. The programmable designs of the carrier predetection integrators, the carrier loop discriminators, and the carrier loop filters characterize the receiver carrier-tracking loop. These three functions determine the two most important performance characteristics of the receiver carrier loop design: the carrier loop thermal noise error and the maximum line-of-sight dynamic stress threshold.
Approximate Word count = 4455
Approximate Pages = 17.8
(250 words per page double spaced)
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