Tracking Weak Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 274

2. Initial C/A code phase (or initial phase of a C/A code): Although this term
has been used previously, it is restated here again to emphasize the rela-
tionship to the initial C/A code point. The initial C/A phase is a data point,
which is closest to an initial C/A code point. Most likely in a data set there
is no data point that matches the initial C/A code point. One can consider
that the initial code point is between two input data points. For example, if
there are 5000 data points in one ms and the initial C/A point is between
the 1000 and 1001 data points but closer to 1000, the initial C/A code
phase will be 1000. Sometimes the initial C/A code phase is also referred
to as the offset time.

In both the tracking methods discussed in Chapter 8 and in this chapter, the input
data are processed in 1 ms blocks. Since the data are sampled at 5 MHz, 5000
input data points will be grabbed and processed each time. There are several
differences, however, between these two methods. The method in Chapter 8 will
be briefly repeated here. The local C/A code is generated only once, and its initial
phase must match a C/A code in the input signal. If the initial phases of the two
C/A codes differ by more than 100 ns, the input data must shift by one point, as
was illustrated in Section 9.2. In order to make sure that the input data contain
1 ms of data of every satellite with all possible C/A code initial phase, 2 ms of
input data are taken every time. Each time the input data move only 1 ms, as
shown in Figure 11.1. In that approach the locally generated C/A code always
starts from the initial point.
In the present tracking method the locally generated C/A code does not start
from the initial of the code. It is generated to match the first data point in the
input signal. The initial point of the local C/A code can be anywhere in the
code, and it changes every 20 ms because of the code regeneration. Under this
condition two situations are different from the previous method. First, there is
no need to shift the input data to match the local C/A code phase, as discussed
in Section 10.2, because the worst mismatch after 20 ms is about 63.5 ns, which

initial C/A

Input data

1 st 2-ms data taken

2 nd 2-ms data taken

initial C/A

Local code

FIGURE 11.1 Input data taken for method discussed in Chapter 8.