Acquisition of GPS C/A Code Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 153

FIGURE 7.13 Frequency component of the despread signal of satellite 6.

component occurs at k = 7. From Figures 7.12 and 7.13, one can see that the

initial point of the C/A code and the frequency are clearly shown. Since the data
are actually collected, the accuracy of the fine frequency is difficult to determine
because the Doppler frequency is unknown. The fine frequency also depends on
the frequency accuracy of the local oscillator used in the down conversion and
the accuracy of the sampling frequency.
One way to get a feeling of the calculated fine frequency accuracy is to
use different portions of the data. Six fine frequencies are calculated from dif-
ferent portions of the input data. The data used are 1 - 25,000, 5001 - 30,001,
10,001 - 35,001, 15,001 - 40,001, 20,001 - 45,001, 25,001 - 50,001. These data are
five milliseconds long and the starting points are shifted by 1 ms. Between two
adjacent data sets four out of the five milliseconds of data are the same. There-
fore, the calculated fine frequency should be close. The fine frequency differences
between these six sets are − 2.4, 9.0, − 8.2, 5.4, and 2.3 Hz. These data are col-
lected at a stationary set. The frequency change per millisecond should be very
small as discussed in Chapter 3. Thus, the frequency difference can be considered
as the inaccuracy of the acquisition method. When the signal strength changes,
the difference of the fine frequency also changes. For a weak signal the frequency
difference can be in tens of Hertz, if the same calculation method is used.
The acquisition performed on a weak signal (satellite 24) is shown in
Figures 7.14 and 7.15. From these figures, it is difficult to assess whether the