Study On STAP In Heterogeneous Environment For Airborne Phased Array Radar

Airborne phased array radar is easily affected by ground clutter when operating in the air-search or ground-search mode. To achieve a reliable detection of moving targets, the chief problem to be solved must be clutter suppression. Due to the movement of radar platform, land scatters in different directions will have different radial velocities, which makes the received clutter echo spread in the Doppler domain and with a strong space-time-coupling characteristic.Due to the spread of clutter power in the spatial domain and temporal domain, the traditional one-dimensional spatial or temporal filter can hardly obtain a well matched clutter notch, so it cannot suppress clutter effectively. By fully utilizing the two-dimensional radar system degree of freedom(DOF), space-time adaptive processing(STAP) technique can remove ground clutter effectively while maintaining a certain amount of coherent accumulation gain in the direction of target, which improves the moving target detection performance in airborne phased array radar, especially for low-speed targets or weak targets buried in sidelobe clutter. To achieve a desirable processing performance, STAP must employ independent and identically distributed(i.i.d.) training samples which need to be more than two times the adaptive DOF. However, in practice, the echo data received by airborne radar is always nonhomogeneous, leading to significantly degraded clutter suppression performance of STAP algorithms. The improvement of STAP performance in nonhomogeneous environments is considered in this dissertation, and the main contents of the dissertation can be summarized as follows:1. The performance improvement of STAP methods in training-limited scenarios is studied. When airborne radar operating in nonhomogeneous environments, the independent and identically distributed(i.i.d.) training samples obtained by STAP are always limited, which results in an inaccurate estimation of the clutter covariance matrix and a severely degraded STAP performance. Therefore, a novel EFA method based on the persymmetry property of the clutter covariance matrix is proposed. Via exploiting the persymmetry property in spatial domain, temporal domain and spatial-temporal domain respectively in the covariance matrix estimation, the proposed method can increase the training samples by three times theoretically, realizing a more efficient utilization of the limited i.i.d. samples, and dramatically improve the STAP performance in training-limited scenarios with the computational complexity and architectural complexity increase slightly.2. The suppression of nonhomogeneous clutter in airborne radar is studied. Non-sidelooking array configurations result in range-dependent short-range clutter, and nonhomogeneous radar scenes lead to large variation of clutter power among different range samples, which both can make the radar returns nonhomogeneous, destroy the hypothesis condition of i.i.d. samples, and then damage the clutter suppression performance of STAP. However, as long as radar parameters are given, the distribution of ground clutter in the angle-Doppler plane obeys a certain configuration, generally named as clutter ridge, and in practice due to the presence of all kinds of nonideal factors, the clutter ridge may have some deviation or extension. Therefore, a robust adaptive method for nonhomogeneous clutter suppression in airborne radar is proposed. At first, some a priori knowledge about radar system and platform movement is utilized to construct the clutter basis, and then an iterative least square fitting is conducted on the test data by taking into account a typical nonideal factor, namely array error. At last, the residual data after fitting is fed into conventional pulse-Doppler processor and CA-CFAR detector in sequence. In this way, nonhomogeneous clutter can be removed effectively and moving targets can be detected reliably.3. The clutter and deception jamming suppression in airborne radar is studied. By utilizing the sparsity of target or clutter in the angle-Doppler plane, the recently proposed SR-STAP conducts sparse representation for them in an oversampling DFT basis in order to realize target detection or clutter covariance matrix reconstruction. However, in reality no radar echo signal is strictly sparse in any a priori known basis, so the SR-STAP performance will suffer due to the unavoidable presence of basis mismatch. Aiming at finding a more appropriate clutter representation basis, here we propose a moving target detection method based on data fitting, which chooses the adjacent range samples as representation basis, conducts a least square fitting on the test data under the norm constraint of fitting coefficients, and then uses fitting errors to feed the CA-CFAR detector. The proposed method can not only suppress clutter signal effectively, but also handle dense deception jamming that conventional STAP methods can hardly deal with.4. The echo signal model and clutter suppression of rotating antenna system in airborne radar is studied. An analysis on the physical model construction of the echo signal received by airborne rotating antennas is first carried out, and a conclusion is obtained here that due to the presence of rotating motion, there will be a linear angle-dependent perturbation phase on the signal steering vector of a ground scatter in both spatial and temporal domains. Thus, in one hand, the coherent accumulation gain of a target will have a larger loss because of the mismatched space-time steering vector, in another hand, the clutter component presents a serious spectral diffusion as well, which damages its suppression performance of conventional STAP methods. For this, we propose a modified joint domain localized method. When a reduced-rank process in the beam domain is carried out, a corresponding phase compensation is constructed and conducted for data samples in different ranges and different beams at the same time, which can remove the clutter spectral diffusion and sample data nonhomogeneity. In addition, the modification to the constrained target steering vector also ensures the coherent accumulation gain. It is verified that the proposed method can significantly improve the clutter suppression and target detection performance of STAP in rotating antenna system.