Research On Anti-jamming Techniques For Receiver Of Satellite Navigation System

Satellite Navigation System has been widely used in military and civilapplications. It has all-functional, all-weather, continuous and real-time characteristics,and can provide the precise position, velocity and timing information for the users.The environments where the receiver of satellite navigation system works becomemore and more complex, and the satellite signals are susceptible to intentional orunintentional interferences, so the anti-jamming ability of the satellite navigationreceiver has become an important issue, which is related to whether the receiver canwork or not. It is significant to research the anti-jamming techniques for the receiverof satellite navigation system, which can improve the work performance of satellitenavigation system under the complex electromagnetic environments, and enhance theadaptability for various conditions.In this dissertation, the anti-jamming techniques for the receiver of satellitenavigation system have been studied deeply. The global positioning system (GPS) ischose as the study objective, and three issues have been mainly researched, whichinclude direction-of-arrival (DOA) estimation for coherent GPS signals with stronginterferences, nonstationary interference suppression, and robust beamforming forinterference suppression. The main innovations of this dissertation are summarized asfollows:For the GPS complex propagation environments with strong interferences,coherent GPS signals and noise, a DOA estimation method for coherent GPS signalsis proposed based on symmetric uniform linear array. Exploiting the second-orderstatistical property, the interferences are first suppressed by orthogonal projection andthe noise is eliminated by the spatial difference matrix. Then, the coherent GPS signalsare decorrelated by the reconstructed Toeplitz matrices, and the DOA estimation isimproved by a new constructed matrix using all data. The proposed method can obtainaccurate DOA estimation for coherent GPS signals at high interference-to-signal ratio(JSR) and low signal-to-noise ratio (SNR). On this basis, in order to adapt the DOAestimation to uniform linear array, and further improve the accuracy of DOAestimation, a DOA estimation method for coherent GPS signals based on obliqueprojection is presented. By introducing oblique projection to DOA estimation, interferences and noise are suppressed simultaneously, and only the weak coherentGPS signals are retained without distortion. The coherent GPS signals are thenresolved by the improved forward/backward spatial smoothing decorrelation scheme.The simulation results illustrate that the proposed method can achieve more accurateDOA estimation for coherent GPS signals in GPS propagation environments,especially at high JSR and low SNR. The aforementioned methods realize the sourceparameter estimation under the GPS propagation environments, provide a prioriknowledge for adaptive beamforming algorithms to suppress interferences, and areuseful to improve the anti-jamming performance of GPS receiver.Aiming at the linear frequency modulation (LFM) interferences which are thetypical nonstationary interferences and widely encountered, a LFM interferencesuppression method is proposed based on fractional Fourier transform (FRFT) byusing the space-time subspace projection. The propose method introduces FRFT tononstationary interference suppression, extends the traditional single sensor subspaceprojection to multisensor space-time subspace projection, effectively suppressesstrong LFM interferences, and improves the output signal-to-interference-plus-noiseratio (SINR). Based on the above method, the complex scenario where LFMinterferences and coherent GPS signals coexist is further considered. By introducingoblique projection to nonstationary interference suppression and using the space-timesubspace oblique projection, a LFM interference suppression method based on FRFTis presented. The presented method not only can completely suppress LFMinterferences and coherent GPS signals, but also can reserve the desired GPS signalwithout distortion. Theoretical analyses and simulation results show that the presentedmethod significantly improves the anti-jamming performance, and is applicable to theGPS propagation environments.In the presence of the steering vector errors, the performance of the standardadaptive beamformer may decline. In order to solve this problem, a robustbeamforming method based on subspace is proposed for interference suppression. Byutilizing the orthogonal property of subspaces, and combining with the constraint ofsteering vector uncertainty set, the proposed method estimates the actual steeringvector, and solves the optimum weight vector of Capon beamformer. The computersimulations are used to assess the performance of the proposed method. Viasimulation results, it is demonstrated that the proposed method is robust to thesteering vector mismatches, and can improve the output SINR. On this basis, to further take account of the case where steering vector mismatches and coherent GPSsignals coexist, a robust beamforming method is presented for interferencesuppression under multipath environments. The presented method introduces thestandard orthogonal basis of null space, and pre-filters the coherent GPS signalswithout a priori knowledge of attenuation coefficients of coherent GPS signals. Andthen, the actual steering vector is estimated in the range of the steering vectoruncertainty set, and the optimum weight vector is solved by using the pre-filtered data.The simulation results indicate that the presented method can not only provide aimproved robustness against the steering vector mismatches, but also obtain thehigher SINR by suppressing interferences and coherent GPS signals.