GPS Software Receivers - Fundamentals of Global Positioning System Receivers: A Software Approach - page 208

The data are converted into real form with an equivalent sampling frequency of
6.4 MHz and a center frequency of 1.6 MHz.
Figure 9.8 shows the accumulative beginning point shift of the C/A code
versus time. The results are straight lines; let us refer to them as the accumulative
shift lines. Each line contains 12,400 input points of data and is for a certain
satellite. If the sampling frequency is accurate, the lines should have both positive
and negative slopes to represent the positive and negative Doppler frequency shift.
The results in Figure 9.8 indicate that the sampling frequency is not at 1.6 MHz,
because the slopes of the lines are all positive.
The slopes of the lines are plotted against the measured Doppler frequency
shift and the result is shown in Figure 9.9. The measured Doppler frequency is
the difference between the measured frequency through the tracking program and
the assumed center frequency of 1.6 MHz. The results are close to a straight line.
From these two figures one can determine the desired shift of the beginning of
the C/A code. For example, if a certain value of Doppler frequency is measured,
from Figure 9.9 the slope of the accumulative line can be obtained. This slope
can be used to draw a line as shown in Figure 9.8. The accumulative beginning
point of the C/A code can be found as a function of time.
This method is used to find the maximum time from the data used for acquisi-
tion to the data to be tracked. For all of the satellites the minimum time obtained
is slightly over 30 seconds. It takes only 1 second to perform the acquisition;
therefore, there is plenty of time to pass the necessary information to the tracking
FIGURE 9.8 Accumulative beginning point shift of C/A code versus time.