GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 146

Navigation
message
(25 bps)

Rate 1/2 FEC

10,230 chip-code
generator

Chip-by-chip
multiplexer

CM code

L2C signal

511.5 kHz clock

767,250 chip-code
generator

CL code

1/2

1.023-MHz clock

Figure 4.21 Baseband L2C signal generator.

L2C signal is formed by the chip-by-chip multiplexing of the CM (with data) and CL
codes. The fact that L2C devotes one-half its power to a component without data
(CL) is an important design feature shared by the other modernized GPS signals.
This feature enables very robust tracking of the signal by a GPS receiver (see Section
5.3.1).
The L2C signal has an overall chip rate of 2 1.023

Mchip/s, which accounts for its similar power spectrum to the C/A code. There are
important differences between the L2C and C/A code signal power spectra, how-
ever. Since both CM and CL are much longer than the length-1,023 C/A code, the
maximum lines in the L2C power spectrum are far lower than the maximum lines in
the C/A code power spectrum. As will be discussed in Chapter 6, the lower lines in
the L2C power spectrum lead to greatly increased robustness in the presence of
narrowband interference.
The CM and CL codes are generated using the same 27-stage linear feedback
shift register shown in Figure 4.22. A shorthand notation is used in the diagram. The
number that appears in each block in the figure represents the number of stages
(each holding 1 bit) between feedback taps. CM and CL codes for different satellites

× 511.5-kchip/s rate =

3 3 2 3 3 2 2 3 1 1 1 3

Output

Initial conditions

Shift direction

Figure 4.22 CM and CL PRN code generation.