Receiver Hardware Considerations - Fundamentals of Global Positioning System Receivers: A Software Approach - page 120

Actual complex data with zero center frequency have been collected. Since
the acquisition and tracking programs used in this topic can process only real
data, the complex data are converted into real data through software. The details
will be presented in Section 6.14.

6.11 ALIASING TWO OR MORE INPUT BANDS INTO A BASEBAND ( 8,9 )

If one desires to receive signals from two separate bands, the straightforward
way is to use two mixers and two local oscillators to convert the two input bands
into desired IF ranges such as adjacent bands, combine, and digitize them. If
direct digitization is used and the correct sampling frequency is selected, two
input bands can be aliased into a desired output band. Figure 6.10 shows the
arrangement of aliasing two input bands into the baseband for a real data collec-
tion system.
The aliased signals in the baseband can be either overlapped or separated.
In Figure 6.10 the two signals in the baseband are separated. Separated bands
have better signal-to-noise ratio because the noise in the two bands is separated.
Separated spectra occupy a wider frequency range and require a higher sampling
rate. The overlapped bands have lower signal-to-noise ratio because the noise of
two bands is added together. Overlapped spectra occupy a narrower bandwidth
and require a lower sampling rate. The aliasing scheme can be used to fold more
than two input bands together and it also applies to complex data collection.
Before the input bands can be folded together, analog filters must be used to
properly filter the desired input bands.
Let us use three examples to illustrate the band aliasing idea. In the first
example the two P code channels from both L1 and L2 frequencies are aliased
into two separated bands in the baseband. Since the P code has a bandwidth of
approximately 20 MHz, two P code bands will occupy 40 MHz. The minimum
sampling frequency to cover these bands is about 100 MHz (40

×

2 . 5), if 2.5
rather than the Nyquist sampling rate of 2 is used as the minimum required
sampling rate. The two input frequency ranges are 1565.42 - 1585.42 MHz and

FIGURE 6.10 Aliasing two input bands to baseband.