GPS Receiver-Related Subjects - Fundamentals of Global Positioning System Receivers: A Software Approach - page 312

0

− 5

− 10

− 15

− 20

− 25

− 30
−

20 − 15 − 10 − 5 0

Frequency (KHz)

5 10 15 20

FIGURE 12.3 Insertion loss of a sinc function filter with 20 kHz resolution.

frequency at ± 5 kHz will suffer a 3.92 dB processing loss, as discussed in
Section 10.11. In order to reduce this loss, the processing bandwidth is increased
to 20 kHz, as shown in Figure 12.3. With this bandwidth the signal at ± 5kHz
has an acceptable loss of 0.91 dB, which is caused by the filter shape. If 1 ms
of data is summed into one output point, it provides 1 kHz resolution. In order
to have 20 kHz resolution, 50 µ s (1 ms/20) of data can be summed into one
output point.
Suppose that the data are collected by the system discussed in Section 6.8.
The intermediate frequency (IF) is at 21.25 MHz and the sampling rate is 5 MHz.
There is a total of 5000 input data points per ms, and 250 (5000/20) input data
points can be summed into one output point.
First, a periodic signal will be used to illustrate the basic idea because this
method can be applied to both periodic and nonperiodic signals. In this example
10 ms of input data with C/A code modulation are used for coherent integration.
There are a total 50,000 points of input data in 10 ms. The signal is processed
as follows:

1. The input signal is down-converted to baseband by multiplying it with a
complex continuous wave (cw) signal. The frequency of the cw signal is
at 21.25 MHz.
2. The locally generated 10-ms C/A code, containing 50,000 points, is mul-
tiplied with the input signal point by point, which results in a 50,000
point output.