Tracking Weak Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 291

TABLE 11.1 Fine Time Measured by Simulated GPS
Signals (One C/A Code used 20 ms)

Mean
Error(ns)

Standard
Deviation(ns) C/N 0

25 1.3 20

24 1.3 15

23 1.5 23

22 4 23

21 12 28

−

20 4 30

19 − 7 43

18 4 40

can also be found in Equation (11.8). The approach in Figure 11.13 makes y esa
approximately equal y lsa ; thus the error calculated will be small.
With this arrangement the fine time obtained from 1 second of data should
be zero (or x/ 2 = 0). The difference between the two C/A codes in the first ms

x /2. If the fine time obtained from 1 second of data is not equal to
zero, the difference between the two C/A codes in the input and the local signal
should be adjusted by the fine time measured.
Simulated data are used to measure the fine time through this approach. Two
sets of 8-second simulated GPS signals with Gaussian noise are used as input
signals ( 2 ) . The results are listed in Table 11.1. Note that under the weak signal
condition, the mean error is rather small, only a few ns, but the standard deviation
increases. The tracking program usually can track a signal less than C/N 0 =
24 dB, as indicated in the table.

should be −

11.12 TRACKING PROGRAM WITH REGENERATION OF C/A CODE
EVERY SECOND ( 3 )

24 dB, the program
must have enough sensitivity; therefore, the locally generated C/A code and C/A
code of input data would not have to match exactly. For this reason a tracking
program without regenerating C/A code is studied. This type of tracking saves
the generation of C/A code every 20 ms, but the tracking of the fine time of
every ms can be cumbersome. In addition the locally generated C/A code often
needs to be shifted a point. After every second the local C/A code and the carrier
frequency are generated again.
Let us use a numerical example to help explain this approach. If the Doppler
is 5 kHz, as discussed in Section 11.10, every ms the C/A code has a time shift
of − 3.174 ns. In one second the total time shift is − 3174 ns. This time shift
must be compensated; otherwise, the local C/A code and the C/A code in the
input signal can no longer be aligned, and the input signal can no longer be
tracked. It is desirable to make a data shift whenever the local and input C/A

Since the tracking program can track signals below C/N 0 =