Imaging Algorithm of Missile-borne SAR in Diving and Squint Mode

Abstract

In this paper, we propose an imaging algorithm of m issile-borne SAR in diving and squint mode. The diving acceleration, vertical speed, and squint angle caused a large range cell migration (RCM). The algorithm divides the RCM correction (RCMC) into two steps. Firstly, correcting the main part of range walk in time domain. Secondly, correcting the remaining RCMC in range frequency-azimuth Doppler frequency domain, using an analytical form of the signal 2-D spectrum derived by the method of series reversion. Theoretical analysis and simulation results illustrate its validity, and satisfy the imaging quality of missile-borne SAR in high squint model. Index Terms - Missile-borne SAR, diving and squint mode, RCMC, series reversion. I. Introduction Compared with general airborne or space-borne SAR, missile-borne SAR has significantly different characteristics. In order to attack the target, missile-borne SAR cannot do the uniform line motion, and also it has a certain acceleration and vertical velocity in the diving flight. Moreover, it usually works in a squint mode in order to acquire a period of turning time. Therefore, the general SAR imaging algorithms are no longer suitable for missile-borne SAR (1-2). In (3), the 2-D spectrum of target echo was derived by the method of series reversion, and the RCMC, range and azimuth compression were accomplished in the 2-D frequency domain. But the algorithm was only suitable for the center single point target imaging of the scene in side-looking mode. An imaging algorithm for missile-borne SAR based on azimuth nonlinear chirp scaling (NLCS) was proposed in (4). After finished RCMC and range compression in the 2-D frequency domain by the method of series reversion, azimuth variation of Doppler FM rates for echo signal were compensated with the operation of azimuth NLCS, which could improve focusing depth and effect. However, the computation is too heavy, and it also only worked in side-looking mode. In (5), the spectral overlapping phenomenon in distance was solved by introducing a new CS factor, and the influence caused by velocity and acceleration of the missile were compensated. A deramp method was introduced to cope with the Doppler spectral overlapping phenomenon in azimuth, but this algorithm was too complex to realize. According to the characteristics of the missile-borne SAR platform motion, a space geometry relationship of the diving and squint movement is established in this paper. Then the instantaneous range between radar and target is calculated, and the point target echo signal model is derived. The Doppler frequency center is corrected in range frequency-azimuth time domain (6). The 2-D spectrum of the point target echo signal is deduced using the method of series reversion. After finishing the pulse compression processing, a focused point target image is got. At last, a missile-borne SAR simulation is used to illustrate its validity of the algorithm. II. Geometric Model and Instantaneous Range