Research On Anti-jamming Method For GPS Based On Improved Spread Spectrum Technology

In the global positioning system(GPS),the navigation signal is very susceptible to interference,which will affect its navigation and positioning perfor mance in military and civil areas and make it impossible to provide accurate navigation and positioning information.If the system is disturbed by the spoofer,it even makes the receiver receive false positioning information,causing serious consequences.So the study of anti-jamming method for GPS system is very necessary.It is difficult to balance the relationship between spectrum utilization and anti-interference ability in the existing method,and it needs a strict synchronization mechanism,so we propose a hybrid spread spectrum method based on direct sequence spread spectrum/message-driven frequency hopping to resist the blocking jamming.It is difficult to solve the contradiction between the low signal-to-noise ratio and the system error rate,so we propose a method based on level by level spread spectrum to resist the forwarding spoofing.The proposed method is analyzed theoretically and simulated.Through the comparation with existing methods,it is proved that the method proposed in this thesis can effectively solve the interference problem in GPS.The main work of this thesis is as follows:First,a hybrid spread spectrum method based on DS/MDFH is proposed to resist blocking jamming.The GPS satellite-launched navigation signals are first processed by the direct sequence spread spectrum and the information achieved are grouped.Frequency-hopping operation of multiple sets of information is carried out simultaneously to increase the spectrum efficiency of the system.Then transfer a long pseudo information that has the same length with the spread spectrum information,which is used for carrier BPSK modulation to prevent eavesdropping.The simulation results indicate that this method can effectively resist single frequency interference,partial band interference and narrowband interference.Under certain signal noise ratio,DS/MDFH can effectively reduce the bit error ratio of the system and no strict synchronization mechanism is required by the receiver.A method based on level by level spread spectrum to counter the forwarding spoofing is proposed.Level by level spread spectrum and transmit the GPS satellite information,the spread spectrum sequences with different spread spectrum factors are randomly arranged in the spread spectrum set.The index of the spread spectrum sequence used in this level spread spectrum is randomly inserted into the current spread spectrum information,and the position of index insertion is used as the new information for spread spectrum operation.Because the role of the spoofing attacker is different from the legitimate receiver,if the spoofer wants to implement an effective attack,it must reconstruct and forward the information before the legitimate receiver receives the information,this method increases difficulties in spoofing attacks.By comparing this method with the existing methods,it is proved that this method can effectively resist the forwarding spoofing and can obtain better bit error ratio performance when the signal noise ratio is low.