Tracking GPS Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 182

first starting point is obtained from the acquisition method. If there is no shifting
of input data points, the difference between numbers of the same row should
be 50,000, which represents 10 ms of data. Sometimes the starting points are
slightly different from the values obtained from the tracking program.
In satellites 6, 10, and 26 there are no input data point shifts. However, the
first starting points of satellites 6 and 10 are off by 1 point from the rest of the
data. For satellite 17 the last data point has a shift. For satellite 23 the second
data point shifts 2 points then shifts back one point and for satellite 28 the data
points shift back and forth and these effects are caused by noise.
The values of the beginning of the C/A code keep increasing every 10 ms as
shown in Table 8.1. Each value is about 50,000 points more than the previous
one. In a long record of data these values can become extremely large. It is
inconvenient to store these values. However, these values need not be so large.
The reason for keeping these large values is easier to explain in the next chapter.
In actual programming these values are kept between 1 and 5,000. For example,
the beginning of the C/A code for satellite 6 will have the same value of 2,885
instead of the values listed in the table. The beginning of the C/A code is used
to find the beginning of the subframes, which can be located within 1 ms of
input data. This topic will be discussed in the next chapter. Since within 1 ms
the beginning of the C/A code is from 1 to 5,000, a value within this range is
sufficient to locate the beginning of the first subframe.
The time resolution in the above data is 200 ns, determined by the sampling
frequency of 5 MHz, thus, the beginning of the C/A code can be measured with
this time resolution. With each value of the beginning of the C/A code there is
a fine time x calculated by Equations (8.45) or (8.49), which is not included in
Table 8.1. These fine times can be used to improve the overall time resolution.
8.14 COMBINING RF AND C/A CODE
In Sections 8.8 to 8.12 the BASS tracking method is discussed. In order to track
the input signals for each satellite, two quantities are locally generated: a complex
RF frequency and the C/A code. Once the C/A is generated, it is used all the time.
The locally generated complex RF signal is updated at most every 10 ms because
the carrier frequency changes very slowly for a stationary receiver as discussed
in Chapter 3. The locally generated C/A code and RF signal can correlate with
the input signal simultaneously. One convenient approach is to combine the C/A
code and the Rf signal through point-by-point multiplication to generate a new
code. This new code can be used as the prompt code. This prompt code is shifted
two data points to the left and right to obtain the early and late codes. These
three codes consisting of the C/A code and RF signals are used to correlate with
the input signal. The phase of the RF in the prompt signal is important because it
is used to find the fine frequency and the phase transition in the navigation data.
The amplitudes of the early and late codes are used only to determine the fine
time resolution; therefore, the phase of RF signals in the early and late codes is