GPS Receiver-Related Subjects - Fundamentals of Global Positioning System Receivers: A Software Approach - page 316

Input data down converted to baseband

500,000 pts

Multiply

10-ms P code 500,000 pts Shift 2 samples each time by moving

2 samples along the input data

2500

500,000 pts

sum

200 pts Perform FFT
FIGURE 12.8 Acquisition on 10 ms P code signal.

1 pt

100 Hz bins, the insertion loss is also 3.92 dB. In order to recover this loss, the
outputs from two adjacent 100 Hz filters are combined, using the method similar
to the one discussed in Section 10.11. Because there are two hundred 100 Hz
filters, the overall coverage is ± 10 kHz, as discussed in Figure 12.3. However,
only the middle portion of the spectrum ( ± 5 kHz) can be used. Therefore this
method is not as efficient as the method discussed in Section 10.10.
The P code is nonperiodic and has been discussed in Section 5.4. If this
method is applied to P code, the corresponding sampling frequency should be
around 50 MHz to accommodate the 20-MHz bandwidth ( 2 ) . There are 500,000
digitized points in 10 ms of input data. In order to cover ± 5 kHz, the filter
bandwidth will still have to be 20 kHz. Under this condition 2500 input points
are summed into one point, as shown in Figure 12.8. The operation is similar
to the operation shown in Figure 12.4, but with two basic differences. First, the
input and the reference both contain 500,000 points in 10 ms, and 2,500 points
are summed into one output point in the time domain, which still contains 200
points. Second, to correlate with the input signal, the reference is shifted against
the input signal instead of making itself wrap around, because the P code is non-
periodic. The searching time window can be any value and is not limited to 1 ms.
In general, the requirement on the searching window is several ms. The actual
acquisition will use circular correlation by the partition method, as presented in
Figures 12.5, 12.6, and 12.7. The difference is that in Figures 12.5 and 12.6 the
total number of data points increased by 10 times. In Figure 12.7 the reference
is moving along the input signal rather than wrapping around it.

12.5 SAMPLING FREQUENCY CORRECTION THROUGH WIDE AREA
AUGMENTATION SYSTEM (WAAS) SIGNAL ( 3 )

The pseudorange calculated for a GPS receiver depends on the accuracy of the
sampling frequency. If the sampling frequency is inaccurate, the pseudorange
will have an error that reflects on the calculated user position. While an accurate
oscillator is expensive, the accuracy of a low-cost oscillator is usually poor and
the output frequency can be off from the specified value. Thus it is important