GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 138

1 R()

G

τ

− 1 / 1,023

− 101 1,022 1,024

1,023

τ (chips)

Figure 4.14 Normalized and simplified autocorrelation function of a typical C/A code with in

chips.

R()
P

τ

1

− 1/p

− 10 1

p1 p1 − +

12

p 6.1871 10 chips = ×

p

τ (chips)

Figure 4.15 Normalized and simplified autocorrelation function of a typical P(Y) code with τ in

chips.

Table 4.7 Comparisons Between C/A Code and P(Y) Code Autocorrelation

C/A Code P(Y) Code

Maximum autocorrelation amplitude 1 1

1
1 023

1
61871

Typical autocorrelation amplitude
outside the correlation interval

−

−

, . × 10 12

Typical autocorrelation in decibels with
respect to maximum correlation

− 30.1 − 127.9

Autocorrelation period 1 ms 1 week

Autocorrelation interval (chips) 2 2

Autocorrelation time interval (ns) 1,955.0 195.5

Autocorrelation range interval (m) 586.1 58.6

R c = chipping rate (Mchip/s) 1.023 10.23

T c = chipping period (ns) 977.5 97.8

Range of one chip (m) 293.0 29.3

cant differences in values. Table 4.7 compares these characteristics. From Table 4.7,
it can be observed that P(Y) code can be considered essentially uncorrelated with
itself (typically

127.9 dB) for all intervals outside the correlation interval, whereas,
the C/A code is adequately uncorrelated with itself (typically

−

30.1 dB) outside its
correlation interval. However, the C/A codes can be as poorly uncorrelated with
themselves as

−

21.1 dB outside the correlation interval—fortunately this occurs only
a small percentage of the time.
When the GPS codes are combined with the 50-Hz navigation message data,
there is essentially an imperceptible effect on the resulting autocorrelation functions

−