Weak Gps Signal Acquisition Algorithm Research

Under normal conditions GPS signal strength is about -130dBm, acquiring GPS signal can be achieved without difficulties when performing navigation and position function by GPS. However, when GPS receivers are operating in urban canyon, indoors, or in the woods where the S/N of GPS signal decrease harshly, it is hard to implement in acquiring GPS signal. The attenuation is caused by the obstacle of buildings and the high density leaves in the woods, which decreases the strength of signal, also by the multi-path effect generated in the signal transmittance process which bring about the interference between the similar codes.Therefore, in order to improve the technology of acquiring GPS signal to achieve the goal of locating in signal largely decreased environment, like indoors, in this thesis main work is focused on elevating the sensitivity of GPS receiver. A point should be mentioned is, in order to have accurate S/N, that simulated signals are used for the study in this thesis. Nowadays, research on acquiring GPS which signal strength decreasing largely, possesses huge application value in the ever expanding field of civil navigation and location and in the future wars which break out in the urban canyon and in the woods. The main contributions are as follows:The system model of GPS receiver based on software radio is built; two important factors are taken into consideration- selection of sampling time and that of sampling frequency.According to the structure of C/A code in GPS signal and considering the sampling time and sampling frequency, the GPS signal model is brought up, which take the noise in receiver, interference between codes, and the interference from ionosphere into account.The basic factors are also taken into consideration, possible range of Doppler shift, the length of processing data, and frequency step in the acquiring. Based on the discussion above, conventional GPS signal acquisition algorithms are simulated, which includes FFT method, circular coherent method, delay and integration method and long data processing method. Several key issues which determine the sensitivity of GPS receiver are particularly analyzed, afterwards block GPS signal acquisition algorithm is studied and corresponding simulation has been performed.Spectral correlation is taken to acquiring GPS signal, which ameliorate the sensitivity of receiver and lower the S/N threshold at which GPS signal can be acquired.