GPS Software Receivers - Fundamentals of Global Positioning System Receivers: A Software Approach - page 207

A computer program ( p9 7 ) is listed at the end of the chapter to illustrate the
calculation of the user position. This program integrates the user position and the
satellite position correction together.
9.15 TRANSITION FROM ACQUISITION TO TRACKING PROGRAM
The above discussion is based on data digitized and stored in memory. However,
in building a real-time receiver, the information obtained from the acquisition
must be passed to the tracking program in a timely manner. In a software
approach, the acquisition performs on a set of data already collected and the
tracking will process the incoming data. For ordinary signal strength it takes
the acquisition program slightly less than 1 second to process 1 ms of digitized
data to find the signal using a 400 MHz pentium computer. Therefore, the track-
ing program will process data collected about 1 second later than that used for
acquisition. Figure 9.7 shows this operation.
The question to be answered is whether the maximum acquisition time is
short enough such that the tracking program still can process the new data. This
section will present the maximum allowed time separation between the data for
acquisition and the data for tracking. These results are obtained experimentally.
Two parameters, the carrier frequency and the beginning of the C/A code
obtained from the acquisition program, will pass to the tracking program. If
these two parameters are known, one can start to track the data. The carrier
obtained from the acquisition will be used in the tracking program but a different
beginning of the C/A code from the acquisition must be used in the incoming data.
Theoretically, the beginning of the C/A code can be predicted from the carrier
frequency. However, as mentioned in Section 6.15, the sampling frequency may
be off from the desired value. Therefore, a set of digitized data must first be
experimentally tested. The testing procedure is to process the collected data and
find the relation between the carrier frequency and the beginning of the C/A
code. This result can be obtained by tracking several satellites in the data. Once
this result is obtained, it can be used for all satellites.
In this data collection system, the oscillator used for down conversion and
the clock used for digitization are not phase locked. As a result, the following
discussion may be different, if the down conversion oscillator and the digitization
clock are phase locked.
The data used for this illustration are obtained from an I-Q channel digi-
tizer. The nominal sampling frequency is 3.2 MHz as discussed in Section 6.13.
FIGURE 9.7 Transition time required from acquisition to tracking.