Tracking Weak Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 281

Input signal

A B C

1 ms 1 ms 1 ms

Multiply point by point

Local signal

beginning C/A code

Results from input multiplying local

a

b a b a b

A

B C

Regrouped segments

a

b a b a b

1 2

One compressed output

One compressed output

FIGURE 11.8 Peak correlation outputs from each ms.

sets of 5000 input data points by combining a and b portions, as shown in
Figure 11.8.
One second of data only generates 1000 compressed outputs. Since one com-
pressed data point is the result of processing 1 ms input data, the corresponding
bandwidth is 1000 Hz. The operation speed requirement on compressed data is
not stringent because of the low output data rate.

11.7 OBTAINING NAVIGATION DATA AND FINDING CARRIER
FREQUENCY

Once 1 ms of data is compressed into one data point, the actual navigation data
transition can be found. Since the location of the navigation data transition has
been determined in the acquisition program and the tracking program starts from
the beginning of a navigation data, as discussed in Section 11.4, the navigation
data transition can be determined every 20 ms. The approach uses 40 ms of
compressed data and two reference signals: one with phase transition at 20 ms
and one without phase transition. The procedure has been discussed in detail in
step 1 of Section 10.17. Once the navigation data transition is determined, the
navigation data of 1 sec (1000 points) can be generated. These navigation data
are multiplied with the compressed data, and this operation will strip off the
navigation data from one second of input (1000 points). The result is a complex
continuous wave (cw). An FFT is performed on these data putting the results
in the frequency domain, which covers a bandwidth of 1 kHz with a resolution
of 1 Hz. The amplitude of a typical frequency domain is shown in Figure 11.9.