The Imaging Techniques Of Missile-borne Synthetic Aperture Radar

Synthetic aperture radar (SAR) can achieve long range, high resolution imagery in all weather conditions, day and night, which contributes to an improvement of the orientation performance on precision guided weapons in strike. However, there are three problems must be resolved in missile-borne SAR applications: 1) the cross range resolution of SAR is degraded near the direction of flight due to the decrease in the Doppler bandwidth; 2) Serious motion errors are induced by the trajectory deviations from the straight flight track and forward velocity variations under maneuvers; 3) the large computational and memory requirements in real time SAR imaging are difficult to fulfilled on missile platforms. All the problems are researched on in this dissertation and the main achievements are as follows:1. The high resolution imaging algorithms of highly squinted missile-borne SAR are investigated. A modified azimuth nonlinear CS (ANCS) algorithm is proposed to resolve the limitation of the focus depth, which caused by linear range walk correction (RWC) in time domain. Simulation results are presented showing that it can be used toprocess data with up to approximately 50°angle with 1m resolution. Based on the squinted SAR geometry, the RD and CS algorithms are studied. A detail study is preformed on the Taylor extension of the range model and the decoupling errors of the signal expression in 2D frequency domain. It is concluded that the cubic range cell migration correction (RCMC) and secondary range compression (SRC) are significant in highly squinted SAR imaging. The investigation on RWC demonstrates that: the decoupling phase errors can be decreased after RWC; however, the focus depth of map is limited due to the azimuth variations of the target's Doppler rate. So the modified ANCS algorithm is developed to resolve this problem. After the cubic RCMC, an azimuth nonlinear phase term is introduced to equalize the Doppler rate in the same range cell. The modified algorithm can process highly squinted data and extend the focus depth without deterioration of the image quality.2. The SAR image formation and motion compensation (MOCO) algorithms for constant accelerations platform are investigated intensively. A refined RD algorithm is proposed to compensate the constant accelerations in 2D frequency domain, which can improve the cross range resolution and PSLR greatly. In combination with the contrast optimization auto-focus (COA) approach, a generalized block diagram is given for data processing. Firstly, according to the range model and the return signal expression, it is concluded that the velocity error along the flight path and the displacement along the line of sight (LOS) must be compensated in data processing. Secondly, two algorithms including refined RD algorithm and spectral analysis (SPECAN) algorithm are derived to compensate the motion errors effectively. Geometric correction is implemented simply in the SPECAN algorithm too. In a different way, all the displacement along the LOS can also be compensated in range frequency domain. Using the COA approach to correct the residual phase errors, a generalized procedure for imaging and MOCO of the constant accelerations platform is provided. Finally, this procedure is tested by using data from an airborne flight experiment.3. The forward looking imaging techniques are investigated. A multi-channel de-convolution imaging method applied to mono-pulse radar is proposed to sharper the angular resolution in the direction of flight. Simulation results show that: about 10 times of improvement in angular resolution over real aperture image can be achieved. To enhance cross range resolution around the boresight, the Doppler beam sharpen (DBS) technique is used. As an unfocused SAR technique, the main specification and processing parameters of DBS in diving phase are deeply analyzed and the characteristics of cross range resolution are tested by simulation. To eliminate the DBS blind sector in the boresight, the return signals of the sum beam and the difference beam are deconvolved to sharpen the angular resolution. With the consideration of the truncation effect, the de-convolution operators are refined to restrain the noise caused by de-convolution effectively.4. The techniques to implement real time DSP systems in missile-borne SAR are investigated. Using multiple generalized DSPs as the processing units, the structures of real time missile-borne SAR processor are given. The mapping of SAR imaging procedure to a multi-DSPs system is presented and optimized. At first, according to the characteristics of the generalized DSPs, three SAR processors are proposed to adapt different missile-borne SAR applications. Subsequently, the mapping method of the SAR imaging procedure of constant accelerations platform to a processor formed by multiple TS201s is analyzed. Meanwhile, the key steps in the imaging algorithm are optimized in details. Finally, based on the master-slave structure processor, a missile-borne DBS imaging processor is implemented.