GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 142

performance must be achieved among all of the used PRN codes. Because the code
length is 6.1871 10 12 chips and the chipping rate is 10.23 Mchip/s (1-week period),

the cross-correlation level of the GPS P(Y) codes with any other GPS P(Y) code
approaches

×

127 dB with respect to maximum autocorrelation. Hence, the
cross-correlation of the P(Y) code of any GPS SV can be treated as uncorrelated with
any other GPS SV signals for any phase shift. Because of this excellent P(Y) code
cross-correlation performance, no further discussion is warranted.
Because the GPS C/A code length was a compromise at 1,023 chips with a chip-
ping rate of 1.023 Mchip/s (1-ms period), the cross-correlation properties can be
poor under certain circumstances. As shown in Table 4.8, the C/A code cross-corre-
lation functions have peak levels that can be as poor as

−

24 dB with respect to its
maximum autocorrelation for a zero Doppler difference between any two codes.
Table 4.9 shows that for higher Doppler difference levels at the worst-case intervals
of 1 kHz, the cross-correlation levels can be as poor as

−

− 21.1 dB.

4.4 Navigation Message Format

As described in Section 4.3, both the C/A code and P(Y) code signals are modulated
with 50-bps data. This data provides the user with the information necessary to
compute the precise locations of each visible satellite and time of transmission for
each navigation signal. The data also includes a significant set of auxiliary informa-
tion that may be used, for example, to assist the equipment in acquiring new satel-
lites, to translate from GPS system time to UTC (see Section 2.6), and to correct for a
number of errors that affect the range measurements. This section outlines the main
features of the GPS navigation message format. For a more complete description, the
interested reader is referred to [10].

Table 4.8 C/A Code Maximum Cross-Correlation
Power (Zero Doppler Differences)
Cumulative Probability
of Occurrence

Cross-Correlation
for Any Two Codes (dB)

0.23 -23.9

0.50 -24.2

1.00 -60.2

Table 4.9 C/A Code Maximum Cross-Correlation Power Summed for All 32 Codes (Increments
of 1-kHz Doppler Differences)
Cumulative
Probability of
Occurrence

Cross-
Correlation at
1 kHz (dB)

Cross-
Correlation at
2 kHz (dB)

Cross-
Correlation at
3 kHz (dB)

Cross-
Correlation at
4 kHz (dB)

Cross-
Correlation at
5 kHz (dB)

0.001 − 21.1 − 21.1 − 21.6 − 21.1 − 21.9

0.02

− 24.2 − 24.2 − 24.2 − 24.2 − 24.2

0.1

− 26.4 − 26.4 − 26.4 − 26.4 − 26.4

0.4 − 30.4 − 30.4 − 30.4 − 30.4 − 30.4