GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 147

are generated by different initial loads of the register. The register is reset every
10,230 chips for CM and every 767,250 chips for CL. The CM code repeats 75
times for each repetition of the CL code. At the 511.5-kchip/s rate, the period of the
CM code is 20 ms (one P(Y) code data bit period) and the period of the CL code is
1.5 seconds (one X1 epoch or Z-count).
The rate one-half constraint-length FEC scheme used to encode the 25-bps L2C
navigation data into a 50-baud bit stream is shown in Figure 4.23.
The minimum specified received L2C power level for signals broadcast from the
Block IIR-M and IIF satellites is − 160 dBW [10].

4.5.2 L5
The GPS L5 signal is generated as shown in Figure 4.24. QPSK is used to combine
an in-phase signal component (I5) and a quadraphase signal component (Q5). Dif-
ferent length-10,230 PRN codes are used for I5 and Q5. I5 is modulated by 50-bps
navigation data that, after the use of FEC using the same convolutional encoding as
L2C, results in an overall symbol rate of 100 baud. A 10.23-MHz chipping rate is
employed for both the I5 and Q5 PRN codes resulting in a 1-ms code repetition
period.

G2 (133 octal)

Output symbols
(50 sps )
(Alternating G1/G2)

Data input
(25 bps)

G1 (171 octal)

Symbol
clock

Figure 4.23 L2C data convolution encoder.

100-Hz Symbol Clock

10-symbol
Neuman-
Hofman
code

I5

100 sps 1 kbaud

L5 data
message

Add
CRC

Encode
with FEC

276 bits

300 bits

50-Hz data clock

QPSK
modulator

L5
Signal

XI(t)

1ms
epochs

Code
generator

XQ(t)

10.23-MHz
Code Clock

Q5

1 kbaud

20-symbol
Neuman-
Hofman
code

Carrier

Figure 4.24 L5 signal generation.