GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 140

MHz. The combined power spectra of C/A code and P(Y) codes are centered at the
L1 carrier frequency. The first nulls of the C/A code power spectrum are at 1.023

MHz from the center frequency and the first nulls of the P(Y) code power spectrum
are at

±

10.23 MHz from the center frequency.
Figure 4.17 is a plot of the power spectrum of the GPS P(Y) code (plus 50-Hz
data) BPSK modulated onto the L2 carrier. The plot is virtually identical to Figure
4.15, except the center frequency is at the L2 carrier, 1,227.60 MHz, and the C/A
code modulation is removed. The first null of the P(Y) code is at

±

10.23 MHz.
Figure 4.18 is a plot of the power spectrum of the GPS C/A code (plus 50-Hz
data) BPSK modulated onto the L1 carrier with the P(Y) code turned off. The fre-
quency scale has been adjusted to be narrower than Figure 4.16 by a factor of ten in
order to inspect the C/A code power spectrum more closely. The resolution band-
width of the spectrum analyzer has been reduced to 3 kHz so that the line spectrum
of the C/A code is just beginning to be visible in the plot. The strong lines of the C/A
code [those above the nominal sinc 2 ( x ) envelope] are also somewhat observable. It
would be impossible to observe the line spectrum of the P(Y) code with a spectrum
analyzer because the resolution bandwidth corresponding to its extremely fine line
spacing would be unreasonably narrow.

±

4.3.4 Cross-Correlation Functions and CDMA Performance
The GPS modulation/demodulation concept is based on the use of a different PRN
code in each SV, but with the same code chipping rates and carrier frequencies on
each SV. This modulation/demodulation technique is called CDMA, as discussed in
Section 4.2.1. The CDMA technique requires the user GPS receiver to synthesize a

MKR 1227.60 MHz
− 73.40 dBm Ref 55.0 dBm Atten 10 dB −

hp
5 dB/

Marker
1227.60 MHz
73.40 dBm −

Center 1227.60 MHz
Res BW 300 kHz

Span 50.00 MHz
SWP 10 sec

VBW 30 Hz

Figure 4.17 Power spectrum of L2 P(Y) code from a GPS signal generator.