Study On Target Detection In Airborne Passive Radar System

Airborne passive radar system uses existing illuminators of opportunity as transmitters,utilizes airborne radars as receivers to realize the target detection.Comparing with the traditional airborne radar,airborne passive radar system has benefits of low cost,strong survivability and no electromagnetic pollution.Comparing with the traditional passive radar,its elevated position of the receiver provides additional advantages for target detection including increased visible detection range.Thus,the study on the airborne passive radar system attracts universal attention home and abroad.However,owing to the motion of the passive radar platform,the ground clutter is spread over a region in Doppler frequency.This makes the conventional one-dimensional filter a failure.Though the displaced phase center antenna(DPCA)and space-time adaptive processing(STAP)can improve the clutter suppression ability for airborne radar,some challenges which exist in the airborne passive radar remains to be solved.First,for long-time coherent integration of signals,the parameters of the direct-path signal cause the channel phase deviation of DPCA algorithm,leading to a suppression ability loss.Second,the high-sidelobes property of the continuous wave signal significantly affects the background interference of STAP algorithm,thereby decreasing the target detection performance.Third,both the direct path signal and multipath signals involved in the reference channel are used to matched filtering in the presence of a contaminated reference signal,and the clutter snapshot caused by the multipath signals degrades the target detection ability.What's worse,the prior knowledge is expensive to acquire and it is hard to avoid array gain/phase errors in practice,which makes the above challenges more complex.To address these problems,the corresponding clutter suppression methods are proposed for airborne passive radar systems to achieve target detection.The detailed research work of the manuscript is arranged as follows.1.In airborne passive radar system,the long-time integration improves the weak-target detection performance but leads to the range cell migration and residual clutter for the DPCA algorithm.To address this problem,a weak-target detection method based on the prior knowledge for airborne passive radar is proposed.Firstly,the Keystone transform is used to correct the range cell migration,and then the phase deviation related to parameters of the direct-path signal is theoretically derived.Finally,the phase deviation is compensated based on the prior knowledge to achieve clutter suppression,thereby improving the weak-target detection capability.2.As the inaccurate prior knowledge degrades the weak-target detection performance of the proposed method,an improved detection method is proposed for airborne passive radar.The objective function related to parameters of the direct path signal is firstly derived based on the detection signal-to-noise ratio.Then,the phase deviation is compensated according to the estimated parameters to realize the clutter suppression.The simulation results show that the improved method can effectively solve the problem of residual clutter and detection ability degradation due to the inaccurate prior knowledge.3.The ambiguity function of the continuous wave signal has a high range sidelobes,which degrades the target detection performance for the STAP algorithm.Although the cascaded suppression method can overcome this problem,the high computational complexity of the existing cascaded method restricts its applicability.To address these problems,a cascaded suppression method based on the space-time sidelobes cancellation algorithm is proposed.Firstly,a sparse measurement model of sidelobes is constructed,and then the weight vector is updated iteratively to achieve the sidelobes suppression.Finally,the STAP algorithm is used to eliminate the clutter.The simulation results show that the proposed method can effectively eliminate the influence of interference sidelobes on the STAP algorithm,and provide a trade-off between target detection performance and computational complexity.4.As the presence of array gain/phase error degrades the suppression performance of proposed cascaded method,an improved cascaded suppression method based on the joint iterative algorithm is proposed.Firstly,the signal model of interference sidelobes and clutter in the presence of array error is established,and the sparse measurement model and cost function are modified.Then the joint iterative method is used to solve the weight vector and error vector to realize sidelobe cancellation.Finally,the clutter is suppressed by the STAP algorithm.The simulation results show that the improved method can achieve the satisfactory suppression performance,and provide a trade-off between target detection capability and computational complexity in the presence of array errors.5.In airborne passive radar system,the reference signal is inevitably contaminated by multipath signals in practice.In this case,the clutter caused by the multipath signals(MP clutter)degrades the target detection performance of the STAP algorithm.To address this problem,a cascaded suppression method is proposed.Based on the analysis of MP clutter,the sparse measurement model is firstly derived.Then,the weight vector is updated iteratively to achieve the MP clutter suppression.Finally,the STAP algorithm is used to eliminate the clutter caused by the direct path signal(DP clutter).The simulation results show that the proposed cascaded suppression method can effectively eliminate the influence of the contaminated reference signal on the STAP algorithm and avoid the target self-cancellation phenomenon.6.As the fixed sparse parameter is selected in the proposed cascaded method,which decreases its convergence speed,an improved cascaded method based on joint iteration of MP clutter cancellation weight vector and sparse parameter is proposed.The weight vector and the sparse parameter are alternately updated by joint iterative algorithm,and the influence of an impure reference signal on the STAP algorithm is eliminated.The simulation results show that the improved method can effectively improve the convergence speed and the suppression performance.