Global Positioning System Reference
In-Depth Information
(a)
(b)
FIGURE 6.3. Output from serial search acquisition. (a) PRN 19 is not visible so no peak
is present. (b) PRN 21 is visible so a significant peak is present. The peak occurs at C/A
code phase = 359 chips and frequency = 9.5475 MHz.
As the name parallel frequency space search acquisition implies, this second
method of acquisition parallelizes the search for the one parameter. This method
utilizes the Fourier transform to perform a transformation from the time domain
into the frequency domain. Figure 6.4 is a block diagram of the parallel frequency
space search algorithm.
The incoming signal is multiplied by a locally generated PRN sequence, with
a code corresponding to a specific satellite and a code phase between 0 and 1022
chips. The resulting signal is transformed into the frequency domain by a Fourier
transform. The Fourier transform could be implemented as a discrete Fourier
transform (DFT) or a fast Fourier transform (FFT). The FFT is the faster of the
two; but it requires an input sequence with a radix-2 length, that is, 2 n ,where n
takes positive integer value.
Figure 6.5 illustrates the result of multiplying the incoming signal with a per-
fectly aligned locally generated PRN sequence. The result is a continuous wave
signal. Of course, this only happens when the locally generated PRN code is per-
fectly aligned with the code in the incoming signal. If the incoming signal contains
signal components from other satellites, these components will be minimized as
a result of the cross-correlation properties of the PRN sequences.
In parallel frequency space search acquisition, the upper signal in Figure 6.5 is
the input to the Fourier transform function. With a perfectly aligned PRN code,
the output of the Fourier transform will show a distinct peak in magnitude. The
Incoming
signal
Fourier
transform
Output
| | 2
PRN code
generator
FIGURE 6.4. Block diagram of the parallel frequency space search algorithm.
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