Global Positioning System Reference
In-Depth Information
Input data 50,000 points
1
2
3
4
5
6
7
8
9
10
Down converted input signal using the known carrier frequency
1
2
3
4
5
6
7
8
9
10
1
+
2
Divided into 10 sections
and summed into 5000 pts
+
10
Circular correlation
Produce 2500-pt output
Local C/A code 5000 pts
This operation performs 1 ms acquisition
FIGURE 12.2
Folding10msofdatainto1ms.
this operation, 10 ms of input data will generate output. This operation is much
simpler than the method described in Section 10.10 because less bandwidth is
covered.
Having less output data brings two advantages. One is that less searching time
is involved in finding the maximum of the correlation outputs. Another is that
the number of false alarm generated can be reduced. Although the probability
of false alarm does not change, a smaller number of outputs can produce fewer
false alarms.
In this operation when the input signal is
±
50 Hz from the desired frequency
f
0
, the output will be about 3.92 dB down. One can use the method of recov-
ering loss between frequencies by operating on two adjacent outputs, separated
by 100 Hz, and produce a 5 by 2500 matrix output. The undesirable loss, as
discussed in Section 10.11, can then be reduced.
12.4 CIRCULAR CORRELATION BY PARTITION
(
2
)
This approach can be applied to a periodic or a nonperiodic signal. The C/A code
can be considered as periodic and the P code or P(Y) code as nonperiodic. The
basic principle of this acquisition method will be explained first; then an example
of actual use will be presented. The input signal must be down-converted into the
baseband first. In order to discuss this method, the output sampling rate described
in Section 10.9 will be used. From the output sampling rate, one can determine
the overall processing bandwidth. Normally, it is desirable to cover a 10 kHz
(
±
5 kHz) bandwidth. When this bandwidth is used, the signal with Doppler
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