Abstract:
In many GPS applications, such as high dynamics vehicles or an appliance which uses sleep mode to save power consumption, a fast acquisition of signal is strongly required. In this paper, a new fast method to acquire GPS signal is proposed.The proposed methods find the candidates of Doppler shift using Squared-D algorithm, which is based upon a signal squaring method and dramatically reduces the search space of Doppler shift. This paper also proposes XMC (eXtended Multi-Correlator) to efficiently search C/A code. It is distinguished from the multi-correlator in that the combined replica of several codes is used. The expected performance of XMC method is equal to that of the multi-correlator while computational burden is reduced to 90%.
Keywords: fast acquisition, XMC, extended multi-correlator, Squared-D
1. Introduction
Most GPS receivers perform time-consuming two-dimensional search to acquire the signal from satellites because they have no code phase and Doppler shift information. It takes at least a few seconds even if the almanac is known. But in many applications, such as high dynamics vehicles or an appliance which uses sleep mode to save power consumption, a fast acquisition of signal is strongly required.
Correlators for fast acquiring the GPS signal are typically categorized into three types: multi-correlator, matched filter, and FFT[7]. However, these methods have limited applications because of computational burdens, power consumptions and their complexities.
A new algorithm to acquire GPS signal is proposed in this paper. It finds the candidates of Doppler shift using a new method called Squared-D, which is based upon a signal squaring method. It dramatically reduces the search space of Doppler shift because the Squared-D method gives several numbers of candidates of Doppler shift. Another new method called XMC (eXtended Multi-Correlator) is proposed to efficiently search C/A code. XMC is an extension of the multi-correlator method. It is distinguished from other method in that the combined replica of several codes is used instead of separate ones. The phase of code can be estimated with small numbers of correlator’s arms under this structure. Theoretically, 90% numbers of arms are used to get an equivalent performance of the multi-correlator. Thus the expected performance of XMC method is equal to that of the multi-correlator while the complexities and computational burdens are reduced to 90%. It is expected that the proposed method be easily adapted to many applications where the fast acquisition are required.

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