Study On Coherent Processing For Frequency Agility Radar

Frequency agility radar is born for electronic counter-countermeasures(ECCM).By modulating different carrier frequencies in a cognitive or random way to transmission pulses,it possesses low waveform interception performance,and can effectively against a variety of jamming modes,such as oppressive,deception and repeater jamming,etc.In particular,it is also capable of resisting mainlobe jamming.At the same time,frequency agility radar possesses the high-range-resolution performance and the ideal thumbtack ambiguity function.However,the price of its excellent anti-jamming performance is that there are many problems in the coherent processing for frequency agility waveforms,which limits its promotion in practical applications.In this paper,the waveform characteristics of frequency agility radar are theoretically analyzed,and the range-Doppler two-dimensional(2-D)coherent processing,adaptive clutter suppression and range sidelobe suppression under sparse frequency conditions are mainly studied.The corresponding method are also proposed to solve these issues.The details are as follows:1.Aiming at the problem of target crosstalk and heavy computational burden in multitarget environment,a 2-D coherent processing method is proposed for frequency agility radar.The possession of an ideal thumbtack ambiguity function implies the potential of realizing joint high-resolution range-Doppler estimation in frequency agility radar.However,achieving multi-target coherent detection and 2-D parameter estimation with high computational efficiency is still a difficult issue.In this paper,a novel algorithm,which estimates multiple targets’ velocities by using different frequencies pulses and then generates their high-range-resolution profiles(HRRPs),is proposed.In velocity estimation,the phase cancellation and generalized Doppler spectrum generation are adopted,and the cross-terms produced by phase cancellation is suppressed to guarantee the quality of the generalized Doppler spectrum.In HRRPs generation,a special dictionary matrix is structured by using the estimated velocities obtained in the previous step,and each target HRRP,without interference from other targets,is generated via sparse restoration technique.The algorithm avoids the computational burden brought by separately performing Doppler and range process,and guarantees the performance in a multi-target scenario via the cross-terms suppression and sparse restoration.2.To solve the incompatibility between frequency agility and moving target detection(MTD),an adaptive clutter suppression method is proposed for frequency agility radar.Due to the different pulse frequencies,the conventional MTD technique based on Doppler filtering is not compatible with frequency agility radars,which limits the application of frequency agility radar in clutter environment.In this paper,the effect of clutter on the coherent processing for frequency agility radar is first analyzed.The result shows that coherent processing can improve signal-to-clutter-ratio(SCR)on target Doppler channel by N(number of pulses in one CPI)times,which suppresses weak clutter scatterers to a large extent whilst strong clutter scatterers would still be left influential.Focusing on dominant clutter scatterers,the adaptive Range-Doppler clutter suppression(ARD-CS)algorithm,which based on designing clutter suppression filters with 2-D property to obtain the clear HRRP of moving target in clutter environment,is then proposed for RSF radar.The advanced point of the ARD-CS algorithm is that it can adaptively suppress clutter by using different frequency pulses,and the Doppler extend phenomenon of clutter is also taken into account.The former makes the algorithm possesses a higher degree of freedom in clutter suppression,and the latter enables it to be applied in actual clutter environment.3.Focusing on the range sidelobe problem caused by frequency sparse,a low-sidelobe HRRP generation method is proposed for frequency agility radar.In frequency agility radar,both the random and cognitive selection of the pulse carrier frequencies will lead to the phenomenon of frequency sparse(the frequency selection is the under-sampling of the available frequencies),which make the HRRP generated by matched filtering suffer from high noise-like sidelobes.In this paper,we take the sinc-envelopes formed by linearfrequency-modulation(LFM)pulses as well as frequency agility phases into account,and propose a novel range sidelobe suppression filters design method for frequency agility waveform based on a generalized minimum peak sidelobe level(PSL)principle and convex optimization technique.The characteristic of this method is that the target HRRP with much lower sidelobes can be obtained,and the performance can be traded with the instantaneous bandwidth of the receiver without reducing the frequency sparsity(without reducing the flexibility of waveform design),which provides a new idea for solving the sidelobe suppression problem of sparse frequency agility radar.