GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 126

tion of two 1-bit values yielding a 1-bit product using the convention that logical 0 is
plus and logical 1 is minus. There are 204,600 P(Y) code epochs between data
epochs and 20,460 C/A code epochs between data epochs, so the number of times
that the phase could change in the PRN code sequences due to data modulation is
relatively infrequent, but the spectrum changes due to this modulation are very
significant.
Figure 4.8 illustrates how the signal waveforms would appear before and after
the BPSK modulation of one P(Y) code data

transition. There are 154 carrier cycles per P(Y) code chip and 1,540 carrier cycles
per C/A code chip on L1, so the phase shifts on the L1 carrier are relatively infre-
quent. The L2 frequency (1,227.60 MHz) can be modulated by either the P(Y) code
⊕

⊕ data transition and one C/A code ⊕

data or by the P(Y) code alone as selected by the CS. The
P(Y) code and C/A codes are never present simultaneously on L2 prior to GPS mod-
ernization (see Section 4.5), unlike the case with L1. In general, the P(Y) code

data or the C/A code ⊕

data
is the one selected by the CS. There are 120 carrier cycles per P(Y) code chip on L2,
so the phase transitions on the L2 carrier are relatively infrequent. Table 4.2 summa-
rizes the GPS signal structure on L1 and L2.
The PPS user has the algorithms, the special Y-code hardware per channel, and
the key to gain access to the Y-code. PPS receivers formerly included a precise posi-
tioning service security module (PPSSM) to store and process the cryptographic keys
and an auxiliary output chip (AOC) to produce the Y-code. Current generation PPS
receivers are built around a security architecture referred to as the selective availabil-

⊕

(a)

(b)

(c)

(d)

(e)

(f)

(g)

0 180 360 540 720 900 1080

Phase (degrees)
Figure 4.8 GPS L1 carrier modulation: (a) L1 carrier (0º phase), (b) L1 carrier (90º phase), (c) P(Y)
code ⊕ data, (d) C/A code ⊕ data, (e) P(Y) code ⊕ data BPSK modulated on L1 carrier (0º phase)
with 3-dB attenuation, (f) C/A code ⊕ data BPSK modulated on L1 carrier (90º phase), and (g)
composite modulated L1 carrier signal.