Study On Moving Target Detection With Frequency Diverse Array Radar

Moving target detection is one of the foremost tasks in radar applications. In airborne/spaceborne radar system, moving target compete with strong clutter of the ground/sea, which induces great difficulty. Space-time adaptive processing (STAP) combines the spatial and temporal information to distinguish target from clutter, which improves the slow-moving target detection performance significantly. Thus, the STAP radar plays an important role in moving target reconnaissance, surveillance and tracking and it is widely used in military and civil areas. In application of the high moving radar platform, the Doppler spectrum of clutter spread severely, which should be handled with high pulse repeat frequency (PRF) radar. However, high PRF involves serious range ambiguity problem. In this case, range ambiguity clutter suppression becomes the key issue in moving target detection. Besides, the precise target information is usually inaccessible in practical applications, which induces a mismatched target constraint of the target steering vector. Moreover, it commonly incurs small sample support problem and non-stationary/non-homogeneous clutter scenarios in practical applications, which dramatically reduces the target detection performance of traditional method. Study on robust beamforming method to alleviate the performance loss due to errors is of great importance in clutter suppression and target detection. Additionally, electronic countermeasure usually occurs in real scenarios, inducing great threaten to radar system in detecting moving target. Thus, it calls for effective strategies and schemes to mitigate the jamming signals, especially for the deceptive jamming signals.In this thesis, focusing on the problems, i.e., electronic countermeasure, range ambiguity of the clutter, mismatch target constraint and ambiguity in target parameter estimation, in moving target detection with high-speed radar systems, I have proposed novel signal processing methods addressing the aforementioned problems using frequency diverse array (FDA) framework. Degrees-of-freedom (DoFs) in space, time and frequency domains are sufficiently exploited. The proposed methods can considerably improve the performance of the radar systems in jamming suppression, clutter suppression, agility of the processor and robustness of signal processing method. Under the support of national nature science funds, i.e., "research on moving target detection approach for high-speed radar platform", "research on method for space target detection, imaging and recognition using spaceborne radar systems" and "research on cooperationally detection and identification of weak and time-variant target based on spaceborne radar systems", I have conducted a throughout study on moving target detection with airborne/spaceborne radar systems, including deceptive jamming and clutter suppression, robust adaptive beamforming, range ambiguous clutter suppression and target parameter estimation. The following provides a brief summary:1、Focused on the jamming supperession in modern electromagnetic scenarios, especially for the deceptive jamming suppression issue, a novel adaptive signal processing approach using multiple-input multiple-output (MIMO) radar with FDA framework. The proposed method effectively suppresses the deceptive jamming and performs actual target detection. In Chapter 2, the general signal model of FDA radar is presented and the distribution of target in transmit-receive spatial domains is studied. Besides, the scheme generating the deceptive jamming and the distribution of the deceptive jamming are analyzed. Since FDA provides additional DoFs in range dimension, the deceptive jamming and actual target are distinguishable from each other in the transmit-receive spatial domains. Thus, a robust adaptive beamforming approach is devised to realize deceptive jamming suppression. Basically, the proposed method explores the difference of target and deceptive jamming in range dimension and thus it can suppress the deceptive jamming signals even from the mainlobe direction.2、Focused on the clutter and jamming suppression problem, a novel space-time-range adaptive processing (STRAP) method is proposed within the FDA framework. By using the MIMO technique which can utilize the transmit DoFs by using the virtual aperture technique, the proposed STRAP method incorporates information in space, time and range dimensions and performs clutter and jamming suppression simultaneously. Thanks to the DoFs in range dimension which is provided by the FDA, target and deceptive jamming can be discriminated effectively. Actually, the distributions f clutter and deceptive jamming behave differently. The proposed method consists of two steps:firstly, performing deceptive jamming suppression using the subspace projection approach using the received data before compensation, and secondly, realizing clutter suppression with the data after compensation.3、Focused on the range ambiguous clutter suppression problem in high-speed radar systems, a novel range ambiguous clutter suppression approach is proposed by using FDA in vertical dimension. In Chapter 6, the signal model of airborne radar with vertical-FDA is presented and the distribution of the clutter in vertical spatial frequency domain is studied. Compared with the distribution of clutter in vertical spatial frequency domain for the traditional phased array radar, the clutters from different ambiguous range regions are fully separated. By using the proposed compensation approach in vertical dimension followed by a pre-STAP scheme, clutters from different ambiguous range regions can be extracted independently. Thus, range ambiguous clutter can be suppressed and target detection can be performed in each separated range region. In this case, the non-homogeneity of the clutter is mitigated in some extend. Moreover, the parallel processor scheme in target detection can be performed since the range ambiguous clutter is separated.4、Focused on the performance degradation due to mismatched target steering vector and target contamination problems, a joint magnitude and phase constrained STAP method is proposed. In this approach, the phase response characteristic is studied. Similar to the linearly distortionless response characteristic of the finite impulse response filter, conjugate symmetric weight constraint is introduced. In the sequel, the form of the phase response is devised. The proposed approach improves the performance considerably in terms of output signal-to-clutter-plus-noise ratio (SCNR), robustness as well as well-maintained mainlobe of the beampattern. With radar application in forward-looking geometry, the target detection performance can be significantly enhanced by using the joint magnitude and phase constrained STAP method. The performance degradation due to mismatched target steering vector and the target contamination problems can be alleviated.5、Focused on the performance degradation due to uncertainty of target steering vector, a novel robust adaptive beamforming approach is proposed based on response vector optimization. As the traditional linearly constrained minimum variance (LCMV) beamformer only adopts the magnitude response, i.e., the all one response vector, it cannot suppression the sidelobe of the beampattern. In the proposed approach, the magnitude and phase constraints are studied. The objective function is formulated in order to further improve the output SCNR while the constraint is obtained to meet the requirement of mainlobe gain. Since the induced problem is non-convex, the problem is further expressed in higher dimensions and semidefinite programming is applied to solve the problem. Note that the computational complexity is negligible as the length of the response vector is usually very small. Additionally, the existence principle of the rank-one solution of the semidefinite programming problem is theoretically proved and the effectiveness of the proposed approach in target contamination environment is justified. Simulation examples demonstrate that the proposed approach outperforms the state-of-the-art techniques in terms of output SCNR and side lobe suppression.6、Focused on the ambiguity problem in target parameter estimation for high-speed radar systems, a novel approach is proposed to jointly estimate the range and angle of the target using MIMO radar with FDA. By exploiting the DoFs of FDA in range dimension and combining the DoFs in angle dimension, the proposed method can effectively realize range resolution of target. To further improve the range estimation accuracy, a prior knowledge based compensation method is proposed. With firmly analysis, the distribution of target in the transmit-receive spatial domains after compensation is studied, which indicates the separation of targets from different ambiguous range regions. The proposed range and angle estiamtion method consists of three steps:firstly, estimate the angle of target in receive spatial domain since the receive spatial frequency is only dependent on the impinging angle of target, secondly, perform range ambiguity resolution in transmit-receive spatial domains, and thirdly, estimate the accurate range of target using superresolution technique. Simulation results demonstrate that the proposed approach can not only realize range resolution but also improves the range estimation peroformance significantly.