Research And Improvement On The Technology For GPS P-Code Direct Acquisition

As it stands today, GPS adopts two types of pseudorandom noise code spreading sequences, coarse/acquisition code for Standard Positioning Service and precision code for Precise Positioning Serivice, respectively. Compared with coarse/acquisition code, precision code can not only provide high precise positioning, but also has high tolerance to jamming and spoofing. Conventional approaches of precision code acquisition use coarse/acquisition code to obtain the starting of next subframe in order to realize phase synchronization of precision code. When coarse/acquisition code is degraded during certain military operations, direct acquisition of P-code is the only option available to maintain navigation and positioning services. However, it is difficult to achieve direct acquisition of precision code because of its high chip rate and long period, which already becomes a technical nodus in the field of GPS. Consequently, it is very meaningful for the improvement on the satellite navigation and positioning system of our country to make research on precision code direct acquisition technology.Firstly, this thesis introduces the structure of the GPS signal and its modulation scheme. Then, various approaches of precision code direct acquisition is summarized, which includes acquisition methods based on FFT. Meanwhile, the influence of bit reversal and Doppler frequency shift on the acquisition process is expatiated and the maximum data length for precision code acquisition is evaluated. Then the principle and algorithm structure of various precision code acquisition schemes are described in detail, which includs the acquisition method of joint FFT block processing and frequency domain parallel search. Three block processing methods which can effectively reduce the amount of FFT operations are researched, and corresponding simulations are performed. The average acquisition time of these schemes are also listed and compared.Secondly, an improved algorithm is proposed aiming at the ubiquitous problems of ambiguous peak widely encountered in existing algorithms of precision code direct acquisition. Based on the acquisition model of conventional algorithms, the overall structure of the improved algorithm is designed. The improved algorithm is described in detail, which includes the approach of extracting the Q branch's spectrum and the hold-overlapping method. The curve of correlation peak value of each Doppler compensation branch versus carrier frequency shift is shown, which proves the improved performance of the extracting the Q branch's spectrum approach corresponding to the ambiguous peak problem. Meanwhile, the curve of correlation peak value versus code phase shift is given, which proves the effect of peak value enhancement of the hold-overlapping method. Simulations have been done in order to compare and analyze the detection probability and the error estimation probability of carrier frequency offset between the improved and conventiaonal algorithm. Comparisons between the improved and conventional algorithms in the aspect of detection probability and the error estimation probability of carrier frequency shift are made through simulations.Finally, the overall acquisition process and various acquisition parameters of the improved algorithm are simulated, and the acquisition results of code phase and Doppler frequency offset are shown respectively. Then the relationships between false alarm probability and estimation threshold, detection probability vs SNR, acquisition time vs code length are analyzed, and the adaptive threshold coefficients under different SNRs are given. The superiority of the improved algorithm is is proved by comparing the SNR gain curves between the improved and conventional algorithms.