GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 141

MKR 1575.420 MHz
75.75 dBm −

Ref

−

65.0 dBm Atten 10 dB

hp
5 dB/

Marker
1575.420 MHz
75.75 dBm −

Center 1275.420 MHz
Res BW 3 kHz

VBW 30 Hz Span 5.00 MHz

SWP 100 sec

Figure 4.18 Power spectrum of L1 C/A code from a GPS signal generator showing the line spec-
trum of the C/A code.

replica of the SV-transmitted PRN code and to shift the phase of the replica PRN
code so that it correlates with a unique PRN code for each SV tracked. Each SV
PRN code used in the CDMA system must be minimally cross-correlated with
another SV's PRN code for any phase or Doppler shift combination within the
entire code period. The autocorrelation characteristics of the GPS codes have
already been discussed. The ideal cross-correlation functions of the GPS codes are
defined by the following equation:

∞
∫

() () ( )

R τ = PN t PN t + τ d τ = 0 (4.25)

ij i j

−∞

where:

PN i ( t ) = PRN waveform for satellite i

PN j ( t ) = PRN waveform for all other satellites j where j ≠ i

Equation (4.25) states that the PRN waveform of satellite i does not correlate
with the PRN waveform of any other satellite for any phase shift . In practice this is

impossible, just as it is impossible for a satellite to have the desirable characteristic
of zero autocorrelation outside its correlation interval. In order for the CDMA dis-
crimination technique to work, a certain level of cross-correlation signal rejection

τ