Receiver Hardware Considerations - Fundamentals of Global Positioning System Receivers: A Software Approach - page 126

The first impact to be discussed is on the center frequency of the digitized sig-
nal. For the down-converted approach, the sampling frequency inaccuracy causes
a small error in the output frequency. For example, if the IF is at 21.25 MHz and
the sampling frequency is at 5 MHz, the digitized output should be at 1.25 MHz.
This value can be found from Equation (6.5) and the corresponding n 4. If

the true sampling frequency f s = 5 , 000 , 100 Hz, there is an error frequency of
100 Hz. The center frequency of the digitized signal is at 1,249,600 Hz. The
error frequency is 400 Hz, which is four times the error in the sampling fre-
quency because in this case n = 4. This frequency error will affect the search
range of the acquisition procedure.
For a direct digitization system, the error in the sampling frequency will create
a larger error in the output frequency. As discussed in Section 6.9, if the sampling
frequency f s = 5 , 161 , 000 Hz, with the input signal at 1575.42 MHz, the output
will be 1.315 MHz with n

=

= 305 from Equation (6.5). If f s = 5 , 161 , 100 Hz,
which is off by 100 Hz from the desired value, the input will be aliased to
1,284,500 Hz, which is off by 30,500 Hz because n

= 305. The frequency error
will have a severe impact on the acquisition procedure. Therefore, for the direct
digitization approach the accuracy of the sampling frequency is rather important.
The second impact of inaccurate sampling frequency is on the processing of
the signal. In a software receiver, both the acquisition and tracking programs
take the sampling frequency as input. If the actual sampling frequency is off
by too much the acquisition program might not cover the anticipated frequency
range and would not find the signal. For a small error in sampling frequency, it
will not have a significant effect on the acquisition and the tracking programs.
For example, the sampling frequency of 3.2 MHz used to collect the complex
data must be off slightly because all the Doppler frequencies calculated are of
one sign. If the correct sampling frequency is used in the program, the Doppler
frequency should have both positive and negative values because the receiving
antenna is stationary. From these experimental results, no obvious adverse effect
on the acquisition and tracking is discovered due to the slight inaccuracy of the
sampling frequency.
The most important effect of sampling frequency inaccuracy may be the pseu-
dorange measured. The differential pseudorange is measured by sampling time,
which will be discussed in Section 9.9. If the sampling frequency is not accurate,
the sampling time will be off. The inaccuracy in the pseudorange will affect the
accuracy of the user position measured.

6.16 SUMMARY

This chapter discusses the front end of a GPS receiver. The antenna should have
a broad beam to receive signals from the zenith to the horizon. It should be
right handed circularly polarized to reduce reflected signals. The overall gain
of the amplifier chain depends on the input voltage of the ADC. Usually the