Research On Technique Of Acquiring Pseudo-random Noise Codes In GPS

GPS(Global Position System) is a new generation Navigation and Position System which can supply many users with exact timing and positioning service. The signal that GPS satellites transmit is formed via the course: firstly the spectrum of base band signal is extended by PN(Pseudo-random Noise) code, and then the spread spectrum signal modulates the carrier wave by way of BPSK(Binary Phase-Shift Keying). We mainly discuss rapid acquisition of PN codes in GPS in this dissertation.In each kind of spread spectrum communication systems, PN codes are playing very important role. Research on acquisition of PN codes began several ten years ago and a lot of methods came forth. Two kind of PN codes are used in GPS, they are C/A(coarse acqusition) code and P(Y)(precision) code respectively.C/A code has short period and low complexity. Because of the fluence of carrier frequence Doppler shifts, acqusition of C/A code is a two-dimensional search process which includes searching carrier frequence Doppler shifts and the phase of C/A code.A scheme called PMF-FFT(Patial Matched Flter-Fast Fourier Transform) is presented in this paper. C/A code can be captured in virtue of estimating. Doppler frequence shifts with FFT,then the estimation is used to adjust NCO(Numerically Controlled Oscillator) until C/A code is captured. The characteristic of FFT is fully utilized in this scheme and the former two-dimensional search process becomes a one-dimensional search processs. It reduces the acqusition time greatly and improves Real-time performance of C/A code receivers, beside that, application of the PMF in the front of FFT decreases the rate of received GPS signal and reduces the number of data which is used to implement FFT efficiently, so a mass of hardware resource is saved. However, it has a remarkable shortcoming that the correlation peak of FFT output will decrease along with the increment of Doppler frequence shifts and the degree of decrement is related with the value of X(the length of PMF). What the value of X is has direct effect on the size of FFT, so it should be setted according actual instance. FFT also has a shortcoming that it can bring Scalloping Loss. It was improved by adding Hanning Window in the dissertation and the results were satisfied.Compared to C/A code, P-Code and Y-Code which forms by encrypting P-Code in GPS have faster chipping rate, higher precision capability and better anti-jamming property. Traditionally, P(Y)-Code is acquired depending on handover from C/A code rather than direct acquisition. The most important characteristic of P(Y)-Code is it has even one week long period, so using traditional methods to acquire it will spend too much time, which can not be accepted for Real-time receivers. Adoption massive physical correlators in parallel can shorten the acqusition time, but it will result in costly expense. A new overlop average method presented in this dissertation is an improvement on direct average method. Firstly, it introduces data average method to achieve fast direct P(Y)-Code acquisition and the small size FFT decreases hardware resource enormously, which can be realized easily. On the other hand, compared to direct average method, the most important advantage of overlop average method is it's correlation peak has no relation with code phase shifts, which is also same even though noise is available. The author compared overlop average method with other classical methods which were used to acquire P(Y)-Code and found it was superior to these classical methods in some aspects of acquisition perfomance, e.g. it had better anti-jamming than direct average method and shorter acquisition time than direct method.We labor their respective characteristic of the two different PN codes in GPS and describe several common methods for acquiring PN codes, and then expound the principle of slippage correlation method which is usually utilized in old receivers. For the scheme of PMF-FFT which is used to acquire C/A code, we analyse it's work principle and disscusse it's acquisition performance penetratingly, especially seriously analyses the improved technique of PMF and FFT respectively. Simulation of the results of PMF-FFT scheme with MATLAB and comparison with old methods in acquisition performance are presented. At last, actualization framework of PMF-FFT with DSP(Digital Signal Processing) is given. For the overlop averaged method which is used to acquire P(Y)-Code, we mainly introduce it's arithmetic process and explain why it is superior to direct averaged method in this dissertation, moreover, we compare it with other methods in some aspects of acquisition performance seriously. To effectively demonstrate the advantage of overlop average method, both it's arithmetic principle and simulation with MATLAB are included, simultaneously some concernful data is tabulated in order to compare their acquisition performance among different methods.