Global Positioning System Reference
In-Depth Information
FIGURE 5.2. Acquisition plot for PRN 21. Signals originating from PRN 21 are present in
the received signal. This is seen from the significant peak in the acquisition plot. The peak
location is related to a C/A code phase and a frequency of the signal.
After multiplication with the locally generated code, the signal must be mixed
with a locally generated carrier wave. This is done to remove the carrier wave
of the received signal. To remove the carrier wave from the signal, the frequency
of the locally generated signal must be close to the signal carrier frequency. As
mentioned earlier, the frequency can change up to
10 kHz from the nominal
frequency, so different frequencies within this area must be tested. To identify
whether or not a satellite is visible, it is sufficient to search the frequency in steps
of 500 Hz resulting in 41 different frequencies in case of a fast-moving receiver
and 21 in case of a static receiver; see Akos (1997), page 85.
After mixing with the locally generated carrier wave, all signal components are
squared and summed providing a numerical value.
The acquisition procedure works as a search procedure. For each of the dif-
ferent frequencies 1023 different code phases are tried. When all possibilities for
code phase and frequency are tried, a search for the maximum value is performed.
If the maximum value exceeds a determined threshold, the satellite is acquired
with the corresponding frequency and phase shift. Figure 5.2 shows a typical ac-
quisition plot performed for a visible satellite. The plot shows a significant peak,
which indicates high correlation.
Figure 5.3 shows a typical acquisition plot, performed for a satellite that is not
currently visible at the GPS receiver. In this plot, all values are nearly identical,
indicating low correlation.
±
5.1.2 Tracking
The main purpose of tracking is to refine the coarse values of code phase and
frequency and to keep track of these as the signal properties change over time.
Search WWH ::




Custom Search