Page 19 - NGTU_paper_withoutVideo

Modern Geomatics Technologies and Applications

The C/A codes are generated at the rate of 1023 chips/ms. The shift registers, which are used for code generation, can
be described by polynomials. The initial states of the polynomials and the phase between them define the code for a specific
satellite. The initial states for both polynomials are set to the same value, i.e. 1:
(0) = {1, 1, 1, 1, 1, 1 ,1 ,1, 1,1 }
G (0) = G
Figure 6 show Gold polynomial (G 2) shift register generator configuration.

Figure 6. Gold polynomial (G 2) shift register generator configuration [11, 16].

The G 1 and G 2 registers are clocked at a 1.023 MHz rate. Thirty-six of the possible combinations over GPS satellites
can be selected. Figure 7 show the combinations of GPS satellites for C/A code generation.

Figure 7. Combinations of GPS satellites for C/A code generation

As Figure 7 shows, there is a unique combination for each satellite to generate the C/A code. For example, satellite
number 23 uses the combination of power 1 and power 3 of the second polynomial (G ), and also for satellite number 16,
combinations 9 and 10 are used. A Boolean operator working on two variables that has the value of one if one but not both of
the variables has a value of one. To make different C/A codes for the satellites, the output of the two shift registers (G and G )

6