Research On Radar Radio Frequency Stealth Waveform Design

With the rapid improvement of the performance of passive detection system,the airborne radar faces the severe survival test in the complex battlefield environment.The use of stealth technology has become a trend of modern warfare patterns,radio frequency(RF)stealth,radar stealth and infrared stealth have been the main stealthy techniques.The radar waveform with good RF stealth performance can not only reduce the probability of airborne radar being intercepted by passive detection system,but also improve the anti-interception and anti-interference ability of airborne radar in complex electromagnetic environment,which has important military significance and application value.This dissertation conducts a deep study on how to design the radar RF stealth waveforms on the guarantee of higher range resolution and velocity resolution of airborne radar waveform.The research achievements and main contributions in the dissertation are as follows:1.The model and design methods of orthogonal frequency division multiplexing nonlinear frequency modulation(OFDM-NLFM)RF stealth compound signals have been proposed based on the idea of frequency diversity.Firstly,in view of the problem that the output sidelobes of the orthogonal frequency division linear frequency modulation(OFDM-LFM)signal are very high and easy to be recognized after matching filter,the linear frequency modulation features in each transmitting channel have been replaced with nonlinear frequency modulation features and OFDM-NLFM signal on the basis of a single window has been proposed in this paper.The simulation results have shown that with the number of transmit channels increases,OFDM-NLFM signal has the large frequency modulation bandwidth and low interception factor,and improves the low interception of airborne radar.With the increase of the frequency interval,the cross-correlations between the signals are reduced gradually,which is beneficial to the matching reception of each signal.Compared with OFDM-LFM signal,OFDM-NLFM signal has lower autocorrelation sidelobes and narrower autocorrelation mainlobe,and the target resolution is good.Then,in order to increase the diversity and complexity of the transmitting signal,OFDM-NLFM signal on basis of combination windows has been proposed by using the combination window technique and the advantages of different window functions.The proposed method optimizes the spectrum structure and autocorrelation parameters,which makes the combined window OFDM-NLFM signal more difficult to be detected and identified by the enemy while carrying the target detection task,which is more beneficial to radar RF stealth.2.The model and design methods of inter-pulse binary phase-coded and intra-pulse nonlinear frequency modulation(BPC-NLFM)RF stealth compound signals has been proposed.In view of the weak performances on radar target detection and RF stealth of inter-pulse binary phase-coded and intra-pulse linear frequency modulation(BPC-LFM)signal,the linear frequency modulation features in each subpulse have been replaced with nonlinear frequency modulation features by using the low autocorrelation sidelobe advantage of NLFM signal and BPC-NLFM compound signal has been proposed in this paper.The simulation results has shown that BPC-NLFM signal not only possesses inter-pulse phase agility but also has an irregular and complex time-frequency characteristic in the subpulse,which makes it more difficult for the passive interception receivers to sort and identify radar radiation sources.Under the same parameters,compared with BPC-LFM signal,BPC-NLFM signal has lower autocorrelation sidelobe level and higher mainlobe narrowing rate of the distance-velocity ambiguity functions which improves the target resolutions.3.The model and design methods of inter-pulse frequency coded and intra-pulse nonlinear frequency modulation(Costas-NLFM)RF stealth compound signals has been proposed.Firstly,in view of of high-distance gating lobes after the pulse compression of Costas signal at large frequency intervals,the inter-pulse frequency-encoded intra-pulse linear frequency modulation(Costas-LFM)compound signal has been obtained based on the principle of eliminating the gating lobes in this paper.The simulation results have shown that Costas-LFM signal is a RF stealth signal with the characteristics of complex and random Power Spectrum Density(PSD),large bandwidth,low interception factor and high range resolution.Then,in view of the high distance sidelobes caused by the linear frequency modulation,the linear frequency modulation in Costas-LFM has been replaced with nonlinear frequency modulation and Costas-NLFM signal has been proposed.The simulation results have shown that Costas-NLFM signal is a RF stealth signal with the characteristics of random PSD and low PSD amplitude.Finally,in order to further increase the intra-pulse frequency agility,the same or different Costas sub-sequence is used to modulate the frequency characteristics of the Costas main sequence in the subpulse to obtain the inter-pulse Costas and intra-pulse Costas(Costas-Costas)signal in this paper.The simulation results show that Costas-Costas signal is a RF stealth signal which has the characteristics of multiple transmitting frequencies and waveform styles,high speed frequency agility,idealized ambiguity function and low interception factor,which is suitable for the multi-user radars of sharing the spectrum resources.4.The model and design methods of inter-pulse Costas and NLFM hybrid frequency modulation and intra-pulse multiple phase-coded(CN-MPC)RF stealth compound signals has been proposed.Firstly,the inter-pulse NLFM and intra-pulse MPC(NLFM-MPC)compound signal has been obtained by using nonlinear frequency modulation between subpulses under the condition of keeping the multiple phase coding modulation within the subpulses.The simulation results have shown that the intraband PSD of NLFM-MPC signal is complicated,and its autocorrelation sidelobe levels decrease because of its multi-phase agility.However,the target resolutions of NLFM-MPC signal are low due to the influence of the energy distributions of NLFM signal and MPC signal.Then,in view of the low target resolutions of NLFM-MPC signal and increasing the inter-pulse frequency agility,the nonlinear frequency modulation has been replaced with the Costas frequency hopping feature between subpulses,and the inter-pulse Costas and intra-pulse MPC(Costas-MPC)compound signal has been obtained in this paper.The simulation results have shown that the energy of Costas-MPC signal is concentrated along the doppler axis and has strong low interception and anti-interference.Compared with Costas signal,the autocorrelation sidelobe levels of Costas-MPC signal are greatly reduced,and the mainlobe narrowing rate is obviously increased.The signal has the higher target resolutions and the lower probability of interception.Finally,in order to further increase the complexity of inter-pulse frequency modulation,the single frequency modulation mode of the nonlinear frequency or Costas hopping frequency modulation has been replaced with the hybrid frequency modulation mode of the nonlinear frequency and Costas hopping frequency modulation between subpulses,and the inter-pulse Costas and NLFM hybrid frequency modulation and intra-pulse MPC(CN-MPC)has been proposed in this paper.The simulation results have shown that CN-MPC signal has the most powerful anti-interception and anti-jamming performance of airborne radar because of its large time-frequency complexity,numerous parameters and low-peak zero-sidelobe ambiguity function,and CN-MPC signal has the extremely high distance-velocity resolutions of airborne radar because of its narrowest autocorrelation main lobe and the sharpest ambiguous main peak.