GPS C/A Code Signal Structure - Fundamentals of Global Positioning System Receivers: A Software Approach - page 76

FIGURE 5.5 C/A code generator.
Figure 5.5 shows the C/A code generator. Another modulo-2 adder is used
to generate the C/A code, which uses the outputs from G1 and G2 as inputs.
The initial values of the two shift registers G1 and G2 are all 1's and they must
be loaded in the registers first. The satellite identification is determined by the
two output positions of the G2 generator. There are 37 unique output positions.
Among these 37 outputs, 32 are utilized for the C/A codes of 32 satellites,
but only 24 satellites are in orbit. The other five outputs are reserved for other
applications such as ground transmission.
Table 5.3 lists the code phase assignments. In this table there are five columns
and the first column gives the satellite ID number, which is from 1 to 32.
The second column gives the PRN signal number; and it is from 1 to 37.
It should be noted that the C/A codes of PRN signal numbers 34 and 37 are
the same. The third column provides the code phase selections that are used to
form the output of the G2 generator. The fourth column provides the code delay
measured in chips. This delay is the difference between the MLS output and the
G2 output. This is redundant information of column 3, because once the code
phase selections are chosen this delay is determined. The last column provides
the first 10 bits of the C/A code generated for each satellite. These values can
be used to check whether the generated code is wrong. This number is in an
octal format.
The following example will illustrate the use of the information listed in
Table 5.3. For example, in order to generate the C/A code of satellite 19, the 3
and 6 tabs must be selected for the G2 generator. With this selection, the G2