| The airborne warning and control system faces new challenges because of deployment of stealth fighters.Using P waveband with longer wavelength can increase radar cross-section of targets,so it can be used to improve the ability of anti-stealth.There is lots of ground clutter in the echoes of the airborne early warning radar.The fast moving targets with lower echo power are obscured easily by the strong clutter which comes from the high-level antenna side lobe of P waveband.In order to suppress clutter effectively and improve the detection performance of airborne early warning radar,space-time adaptive processing(STAP)technology which makes full use of temporal and spatial information is introduced.However,the STAP performance is influenced greatly by the non-stationarity of clutter.For most cases,the direction of the velocity of the aircraft is misaligned with the array axis due to antenna rotation.In the non-side looking array,the clutter spectrum changes with range,especially for the near-range clutter,leading to non-stationarity of clutter.At the same time,the diffusion of clutter because of the antenna rotation will also induce non-stationarity.of clutter.The fact that it is difficult to obtain enouth independent and identical distributede samples from the non-stationary clutter makes the non-stationary clutter suppression necessary.In order to improve the detection performance,the thesis focuses on clutter suppression methods for airborne rotating antenna radar in P waveband.To weaken the diffusion of clutter spectrum caused by antenna movement,a rotating phase compensation method is studied.The method obtains compensation coefficient of the first range ambiguity by matching function.To deal with the clutter non-stationarity of non-side looking array,two methods of near-range clutter cancellation are studied.For the first one,the near-range clutter component is extracted based on projection process to realize cancellation with near-range clutter component in orignal data.For the second method,the adaptive weight of the auxiliary antenna is obtained by using first filler pulse as training samples,then the near-range clutter cancellation of the main antenna and the auxiliary antenna can be realized.These two methods can suppress near-range clutter and improve clutter stationarity.For the difficulty in estimating the covariance matrix of heterogeneous clutter,a method combining the covariance of original data with reconstructed data is studied.The method can suppress clutter effectively when samples are not enough and the cell under test is singular.The main contents of the thesis are summarized as follows:1.The clutter suppression method of rotating antenna is studied.The established signal model has an amplitude modulation and its antenna can rotate around any axis.Due to the antenna rotation movement,there is a linear angle-dependent phase on the signal steering vector of a ground scatter in both spatial and temporal domains.Based on the characteristic,a rotating phase compensation method in both spatial and temporal domains is studied.The method obtains compensation coefficient of the first range ambiguity by matching function and can weaken the difussion of clutter spectrum.It can improve clutter stationarity and be used as a preprocessing of STAP.2.The elevation filtering method of near-range clutter received by non-side looking array is studied.The range dependence of non-side looking array is mainly caused by the near-range clutter.Considering the characteristic that the near-range clutter and far-range clutter are distinguishable in elevation space,two methods for near-range clutter suppression are studied.For the first method,the near-range clutter component is extracted based on projection process to realize clutter cancellation with near-range clutter component in orignal data.For the second method,the adaptive weight of the auxiliary antenna is obtained by using first filler pulse as training sample,then the near-range clutter cancellation of the main antenna and the auxiliary antenna can be realized.These two methods can suppress the near-range clutter and improve the clutter stationarity,and they can be used as a preprocessing of STAP.3.The estimation method of covariance matrix in heterogeneous environment is studied.Training samples in heterogeneous environment usually are not independent and identical distributed,which will result in the degradation of STAP performance.In order to solve the problem that the covariance matrix is difficult to estimate accurately in heterogeneous environment,a covariance estimation method is studied,which combines the covariance of original data with the covariance of reconstructed data.The method can suppress clutter effectively when samples are not enough and the cell under test is singular. |
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