Global Positioning System Reference
In-Depth Information
CHAPTERSEVEN
Acquisition of GPS C/A
Code Signals
7.1 INTRODUCTION
In order to track and decode the information in the GPS signal, an acquisi-
tion method must be used to detect the presence of the signal. Once the signal is
detected, the necessary parameters must be obtained and passed to a tracking pro-
gram. From the tracking program information such as the navigation data can be
obtained. As mentioned in Section 3.5, the acquisition method must search over
a frequency range of
±
10 kHz to cover all of the expected Doppler frequency
range for high-speed aircraft. In order to accomplish the search in a short time,
the bandwidth of the searching program cannot be very narrow. Using a narrow
bandwidth for searching means taking many steps to cover the desired frequency
range and it is time consuming. Searching through with a wide bandwidth filter
will provide relatively poor sensitivity. On the other hand, the tracking method
has a very narrow bandwidth; thus high sensitivity can be achieved.
In this chapter three acquisition methods will be discussed: conventional, fast
Fourier transform (FFT), and delay and multiplication. The concept of acquiring a
weak signal using a relatively long record will also be discussed. In this edition,
two chapters (10 and 11) are devoted to weak signals. The FFT method and the
conventional method generate the same results. The FFT method can be consid-
ered as a reduced computational version of the conventional method. The delay
and multiplication method can operate faster than the FFT method with inferior
performance, that is, lower signal-to-noise ratio. In other words, there is a trade-
off between these two methods that is speed versus sensitivity. If the signal is
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