Acquisition of GPS C/A Code Signals
James Tsui · 2000
In order to track and decode the information in the GPS signal, an acquisition 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 program. From the tracking program information such as the navigation data can be obtained. As mentioned earlier, 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 is also discussed. The FFT method and the conventional method generate the same results. The FFT method can be considered 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 strong, the fast, low-sensitivity acquisition method can find it. If the signal is weak, the low-sensitivity acquisition will miss it but the conventional method will find it. If the signal is very weak, the long data length acquisition should be used. Once the signals are found, two important parameters must be measured. One is the beginning of the C/A code period and the other one is the carrier frequency of the input signal. In this and the following chapters, the data used are collected from the down-converted system. The data are considered real in contrast to complex data. The conventional approach to perform signal acquisition is through hardware in the time domain. The acquisition is performed on the input data in a continuous manner. Once the signal is found, the information will immediately pass to the tracking hardware. In some receivers the acquisition can be performed on many satellites in parallel. When a software receiver is used, the acquisition is usually performed on a block of data. When the desired signal is found, the information is passed on to the tracking program. If the receiver is working in real time, the tracking program will work on data currently collected by the receiver.