GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 149

replacing the GPS constellation with modernized SVs, the military user equipment
will combine P(Y) code, M code, and C/A code operation in the so-called YMCA
receiver. The primary military benefits that M code provides are improved security
plus spectral isolation from the civil signals to permit noninterfering higher power
M code modes that support antijam resistance. Other benefits include enhanced
tracking and data demodulation performance, robust acquisition, and compatibil-
ity with C/A code and P(Y) code. It accomplishes these objectives within the existing
GPS L1 (1,575.42 MHz) and L2 (1,227.60 MHz) frequency bands.
To accomplish the spectral separation shown in Figure 4.20, the new M code
employs BOC modulation [3]. Specifically, M code is a BOC s (10,5) signal. The first
parameter denotes the frequency of an underlying squarewave subcarrier, which is
10 1.023 MHz, and the second parameter denotes the underlying M code genera-

tor code chipping rate, which is 5

×

1.023 Mchip/s. Figure 4.26 depicts a very high
level block diagram of the M code generator. It illustrates the BOC square wave
modulation of the underlying M code generator that results in the split spectrum
signals of Figure 4.20.

×

M code
BPSK-R(5)
generator

BOC (10,5) M-code s

Square wave

f CO =
5.115 MHz

2 f CO = 10.23 MHz

Figure 4.26 M code signal generation.

Table 4.10 Summary of GPS Signal Characteristics

Center
Frequency
(MHz)

Null-to-Null
Bandwidth
(MHz)*

Modulation
Type

Data Rate
(bps) Signal PRN Code Length

L1 C/A code 1,575.42 BPSK-R(1) 50 2.046 1023

P: 6187104000000
Y: cryptographically generated

L1 P(Y) code 1,575.42 BPSK-R(10) 50 20.46

P: 6187104000000
Y: cryptographically generated

L2 P(Y) code 1,227.6 BPSK-R(10) 50 20.46

CM: 10,230
CL: 767,250
(2 PRN sequences are
chip-by-chip multiplexed)

L2C 1,227.6 BPSK-R(1) 25 2.046

I5: 10,230
Q5: 10,230
(two components are in phase
quadrature)

L5 1,176.45 BPSK-R(10) 50 20.46

L1 M code 1,575.42 BOC(10,5) N/A 30.69* Cryptographically generated

L2 M code 1,227.6 BOC(10,5) N/A 30.69* Cryptographically generated

L1C 1,575.42 BOC(1,1) N/A 4.092* N/A

* For binary offset carrier modulations, null-to-null bandwidth is defined here as bandwidth between the outer nulls of the largest spectral
* lobes.