Research On Anti-jamming Acquisition Of GPS Signal

As the GPS signal is very weak and transmitted at a fixed frequency,it leads to the satellite signals are susceptible to intentional or unintentional interference.Although GPS receivers have an inherent anti-jamming ability due to their spreading spectrum characteristics.However,when the interference power exceeds the system's anti-jamming ability,the GPS receiver performace degrades severely.At present,GPS receiver for civil use has been widely applied.The civilian C/A code receiver is vulnerable to the influence of the narrow-band interference,especially the continuous wave interference have been shown to have severe impacts on the acquisition performance of the GPS receiver.Therefore,aiming at this type of interference,the research of interference cancellation technology is of great significance.Firstly,the internal working principle of GPS has been studied and the intermediate frequency signal model of GPS is conducted.On the basis of that,two classical algorithms are analysed which including sequence acquisition algorithm in time domain and acquisition based on circular correlation.Matlab is used to carry on the simulation for that.Secondly,aiming at continuous wave interference,frequency domain interference suppression algorithm——N-Sigma algorithm is studied in the first place.In this algorithm,the received signal is needed to be transformed to frequency domain in order to eliminate the contaminated frequency bins.By selecting a proper threshold,if the magnitude of the frequency bin is greater than the threshold,the value is set to zero.As the jamming frequency bins are eliminated,the original signal is inevitable to be eliminated at the same time.Hence,the application of add-windows and 50% overlapping technique are lucubrated.After the excision process,inverse FFT blocks translate the frequency domain signals back to the time domain so that accomplish the subsequent acquisition.Even though the N-Sigmal algorithm has the ability to suppress the narrow interference,it requires caculation of the statistical value of the frequency-domain data to determine the threshold.To obtain a more accurate threshold,a higher frequency resolution is preferred,which means more FFT points are required.Besides the application of the forward and inverse FFT leads to a higher computational complexity.Aiming at these disadvantages,an adaptive short-time Fourier transform(STFT)-based jamming mitigation system is proposed.Firstly,a time-frequency(t-f)analysis of the received GPS signal is made to detect the contaminated frequency bins and a second-order IIR adaptive notch filter with a lattice structure is used to null the estimated contaminated frequencies.Finally,acquisition algorithm based on circular correlation is adopted to capture the signal that has been eliminated the interference.The algorithm adopts the t-f filtering technique to analyse the jamming,it mitigates the jamming signal immediately without the technique of the forward and inverse FFT.Compared with the N-Sigma algorithm,the proposed algorithm has more advantages such as its simple arithmetic,easy realization and lower computational complexity.Simulation results show that the proposed algorithm perform better on jamming mitigation,besides,it leads to a more accurate acquisition result.Finally,the effects of the time-varying single-tone jamming on GPS is studied,and the BER expression is derived.Then,these two jamming mitigation algorithms are simulated in the presence of time-varying single-tone interference,the experimental results suggest the proposed method also has a better performance on the time-varying single-tone jamming.