Tracking Weak Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 275

1 st 1-ms data taken 2 nd 1-ms data taken

initial C/A

Input data

initial C/A

Local code

FIGURE 11.2 Take one ms of input data.

is less than 100 ns. Second, it requires only 1 ms of input data to contain an
entire C/A code; thus only 1 ms of input data is taken each time. This situa-
tion is shown in Figure 11.2. Any 1 ms of input data (5000 points obtained by
sampling at 5 MHz) contains all the satellites, but different satellites have dif-
ferent C/A codes with different initial points. In this figure only one satellite
is shown.
In the acquisition of weak signals three parameters are obtained: the rough
initial phase of the C/A code, the carrier frequency, and the beginning of the
navigation data. The initial phase of the C/A code and the carrier frequency
are used to generate the local signal. The beginning of the navigation data can
be used to determine the beginning of the tracking process. It is discussed in
Section 10.17 that 40 ms of data (20 ms before and 20 ms after the navigation
phase transition) are used to determine whether a navigation phase transition
exists. In every 20 ms there is one possible navigation data transition. The data
transition is also an initial C/A code point. The tracking should start with a

1 ms

initial navigation data of sat A

initial C/A of sat B

initial navigation data of sat B

initial C/A of sat A

FIGURE 11.3 Tracking starts from a navigation data point.