Tracking GPS Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 183

not important. The fine frequency is calculated from Equation (8.42) and the new
frequency is used in generating the local RF signal for the next 10 ms of data.
The phase discontinuity in the Kernel function must be calculated and necessary
adjustments must be made as discussed in Section 8.9.
The outputs from the BASS method are the phase angles obtained from
the prompt code. The phase angles are calculated every millisecond. After the
phase adjustment discussed in Section 8.9, the phase should either be continuous
between two milliseconds of data or changing by π . The absolute values of the
phase angle are not very important because it depends on the initial sampling
point. The difference angle is defined as the phase difference between two adja-
cent milliseconds. If the difference angle is within ± π /2, it is considered as no
phase shift. If the difference is outside the range of ± π /2, there is a π phase
shift. The π phase shift should happen in multiples of 20 ms.
8.15 TRACKING OF LONGER DATA AND FIRST PHASE TRANSITION
The BASS tracking program discussed in this chapter is based on tracking 1 ms
of input data. If the signal is weak, it is possible to track more than 1 ms of data
to improve sensitivity. It seems that the maximum data length that can be tracked
coherently is 20 ms without very sophisticated processing because the navigation
data is 20 ms long. The input data must be properly selected and there should not
be a phase transition within the selected 20 ms of data. Under this condition, the
sensitivity is improved by 13 dB (10 log (20/1)) over tracking 1 ms of data. If
the data length is longer than 20 ms, it may contain a phase transition due to the
navigation data. The phase transition will disturb the operation of the tracking
program. If multiples of 20 ms of data are used, the tracking process must cover
all the possible combinations of phase transition. The tracking program can be
rather complicated and calculation intensive.
In order to track 20 ms of data without a navigation data phase transition,
one must find a phase transition in the data. This requirement puts an additional
restraint on the acquisition program. The acquisition is not only required to find
the beginning of the C/A code and the carrier frequency, it must also find a phase
transition. If the phase transition cannot be found, it is impractical to track 20 ms
of data. Therefore, finding the phase transition becomes an important requirement
to process weak signals. In Chapter 10, the first navigation data transition is
determined in the acquisition program rather than the tracking.
8.16 SUMMARY
In this chapter the concept of tracking a GPS signal is discussed. Two approaches
are presented: the conventional and the BASS methods. A general discussion
on the conventional phase-locked loop is presented and its application to GPS
receiver is discussed. In addition, a BASS method is presented in detail. This
method needs to generate the C/A code only once; thus it may save calculation