Efficient Code Manipulation for Direct Acquisition of The Long CL Code of The GPS L2C Signal

摘要

The CL code of the GPS L2 civil signal is not modulated by a navigation message, and so there is no restrictions on the integration length due to data bits or bit edges. This advantage is hindered by the long length of the CL code, which is 1500 ms, as the processing power and memory needed for acquisition is very high. The correlation process must cover code delays that span 1500 ms, and Doppler shifts that not only span the Doppler range, but also have a resolution proportional to the inverse of the integration time. This paper introduces a novel algorithm that efficiently manipulates the CL code to reduce the number of code delays and eliminate the Doppler shift from received signal. The replica CL local code is fragmented into an even number of shorter codes. Those shorter codes are multiplied to generate another shorter code that represents the whole CL code. A method is developed to guarantee the generation of an identical shorter code from the received signal, which is complicated because of the unknown code delay and the unknown change in the received CL code length due to the Doppler effect. In addition, a denoising filter is designed to deal with the increased noise level due to the multiplication of different parts of the noisy received signal. The probabilities of detection and false alarm are analyzed and used to demonstrated the acquisition of signals with different carrier to noise ratios.