GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 139

and the power spectrum. When these are modulated onto the L-band carrier, there
is a translation to L-band of the power spectrum from the baseband frequencies that
have been described so far. Assuming that the PRN waveform is BPSK modulated
onto the carrier and that the carrier frequency and the code are not coherent, the
resulting power spectrum is given by [9]:

1
2

[ ]

() ( ) ( )

S ω = P S ω + ω + P S ω − ω (4.24)

L c PN c c PN c

where:

P c = unmodulated carrier power

ω c = carrier frequency (radians)

S PN ( ω c ) = power spectrum of the PRN code(s) (plus data) at baseband

As can be observed from (4.24), the baseband spectra are shifted up to the car-
rier frequency (and down to the negative carrier frequency). In the following GPS
L-band power spectrum illustrations, only the (upper) single-sided frequency is con-
sidered. The GPS signals were synthesized by a GPS signal generator and measured
by a Hewlett-Packard spectrum analyzer.
Figure 4.16 is a plot of the power spectrum of the GPS P(Y) code and C/A code
(plus 50-Hz data) BPSK modulated onto the L1 carrier. The spectrum analyzer per-
formed the plot using a 300-kHz resolution bandwidth, so it is impossible to
observe the line spectrum characteristics of either code. Therefore, the power spec-
trums appear to be continuous. The center frequency is at the L1 carrier, 1,575.42

MKR 1575.42 MHz
−

Ref 55.0 dBm Atten 10 dB −

61.85 dBm

hp
5 dB/

Marker
1575.42 MHz
61.85 dBm −

Center 1575.42 MHz
Res BW 300 kHz

Span 50.00 MHz
SWP 100 sec VBW3Hz

Figure 4.16 Power spectrum of L1 P(Y) code and C/A code from a GPS signal generator.