Tracking GPS Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 181

respect to x and setting the result to zero. The result is

dy
dx = 2 ax + b = 0 r
x = −

b
2 a

(8.49)

Thus, x is the desired result. If x> 100 ns ( d /2) or x< d /2), it is

required to shift the input data with respect to the locally generated C/A code as
discussed in the previous section.
In actual application, every millisecond five y values are generated. Every 10
milliseconds the y values are averaged to generate five averaged y values. The
highest averaged y value and its two adjacent neighbors are used to find the
desired x value. This x value is considered as the fine time resolution and used
to determine whether a shift in input data is needed.
This method and the method discussed in Section 8.11 are both used in
the BASS tracking program. The differences between these two programs are
insignificant. The results from the method discussed in Section 8.11 are used in
Chapter 9.

− 100 ns ( −

8.13 OUTPUTS FROM THE BASS TRACKING PROGRAM

Besides the phase angles measured through the BASS program, two additional
outputs are important for calculating user position. One is the beginning of the
C/A code and the other is the fine time resolution. Since the update is performed
every 10 ms, the shift of the beginning of the C/A code is checked at this rate. If
the misalignment between the locally generated C/A code and the input data is
more than 100 ns, the input data must be shifted either to the right or to the left
one data point to better match the local generated code. The actual starting points
(reference to the input data points) of the C/A code for every 10 ms must be kept.
They will be used in the next chapter to find the beginning of the subframes.
The C/A code beginning points of 6 satellites for 81 ms of data are listed
in Table 8.1. These values are the input data points that represent the beginning
of a C/A code. For each satellite the data points are separated by 10 ms. The

TABLE 8.1 C/A Code Beginning Points

SV # Starting Point Beginning of C/A Code

6 2884 52885 102885 152885 202885 252885 302885 352885 402885

10 3814 53815 103815 153815 203815 253815 303815 353815 403815

17 0469 50470 100470 150470 200470 250470 300470 350470 400469

23 2200 52202 102201 152201 202201 252201 302201 352201 402201

26 2664 52664 102664 152664 202664 252664 302664 352664 402664

28 3269 53269 103270 153270 203270 253269 303269 353269 403269