Tracking Weak Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 301

code length is more than 5000 points of input data. In one set of 5000 input data,
it may not contain an initial C/A code point. This data set should be skipped.
This phenomenon also occurs when the sampling frequency is not an integer and
a similar operation can be applied.
It was found that without this operation of processing one set of 5000 data
points twice or skipping it, every so often the fine time obtained was off by
1 ms. This affected the pseudorange, and the calculated user location was found
to have large errors. After modifications, the pseudorange was corrected and the
user position also corrected. This method is also used in the tracking program
discussed in Chapter 8, extended to 2 ms of input data taken each time.

11.15 EXPERIMENTAL RESULTS OF THE WEAK SIGNAL TRACKING

Experimental data collected using a 5 MHz sampling rate were tracked by two
methods: the original BASS method discussed in Chapter 8 and the weak signal
method presented in this chapter. The data were processed off line. The final
result was the user position. The user position was calculated and displayed every
second. Figure 11.19 shows the user position calculated from the weak signal
method. Figure 11.19a shows the user position in xy plane, and Figure 11.19b
shows the altitude variation versus time. Note that the maximum variation in xy
plane is about 5 meters.

Figure 11.20 shows the location calculated from the BASS tracking method.
Figure 11.20a shows the location in the xy plane, and Figure 11.20b shows alti-
tude variations plotted against time. Compared with the results in Figure 11.19,
the improvements from the weak signal method are obvious. These improve-
ments are expected because in the weak signal tracking one second of data is
used to calculate the fine time, whereas in the BASS method, the fine time is
calculated from 10 ms of data. The noise is reduced, which affects the calculated
pseudorange.

± 3 meters and the maximum altitude variation is about ±

REFERENCES

1. Tsui, J. Digital Techniques for Wideband Receivers , 2nd ed, p. 122, Artech House,
Norwood, MA, 2001.
2. Lin, D., Tsui, J., “A weak signal tracking technique for a stand-alone software GPS
receiver,” ION GPS 2002, pp. 2534 - 2538, Portland, OR, September 24 - 27, 2002.
3. Lin, D., AFRL/SNRP engineer, private communication.
4. Lin, D., Tsui, J., “Sensitivity improvement of a low cost commercial GPS receiver
using software approach,” ION 2004 National Technical Meeting, San Diego, CA,
January 26 - 28, 2004.