A Numerical Model of ISAR Distortion and an

Distortion in the ISAR image of a target is known to cause by non-uniform rotational motion of the target during image formation. The conventional explanation describes the image distortion as a quadratic phase distortion effect. This quadratic phase distortion effect is significant only when the target image is integrated over an extended period of time; i.e., over a large angular rotation by the target. In many measured ISAR images such as those from in-flight aircraft, the distortion can be quite severe; but the image integration time is only a few seconds in duration and the target’s relative rotational displacement is only a few degrees at the most. Quadratic phase distortion is not adequate in explaining the severe blurring in many of these observations. Furthermore, time-frequency analysis of the distorted images often reveals that the Doppler frequency of the target is fluctuating randomly in nature and displays no quadratic phase behaviour temporally. A numerical model based on a time-varying target rotation rate has been developed to model the observed distortion. It has successfully modelled the severe distortion with all the observed characteristics. The model’s simulated results have been validated by experimental data. The severe distortion is attributed to the phase modulation effect where a time-varying Doppler frequency provides the smearing mechanism. It has been shown that the quadratic phase distortion can be considered as a special case of the phase modulation effect. The distortion in ISAR images can be refocused by signal processing techniques. Time-frequency methods, in particular, are quite effective in refocusing distorted (or blurred) ISAR images. In time-frequency processing, an ISAR image of a target is extracted at a particular instant of time; a focused image is thus obtained because the target’s motion can be considered as relatively uniform over a short duration. However, there are a large number of time instants to deal with in time-frequency processing. It is impractical to examine all refocused ISAR images to search for the best images. An efficient ISAR refocusing procedure has been developed. An optimum refocused image can be obtained quickly without having to process systematically a large number refocused ISAR images. Examples of refocused ISAR images are given using measured data. 1.0 INTRODUCTION Inverse Synthetic Aperture Radar (ISAR) imaging provides a 2-dimensional radar image of a target. A 2dimensional picture can potentially offer crucial distinctive information about the features of the target that can lead to more accurate target identification in Non-Cooperative Target Recognition application. An ISAR image is generated from the target’s rotational motion. This motion can sometimes be quite complex, such as the motion of a fast, maneuvering jet aircraft. As a result, severe distortion can occur in the ISAR image of the target [1]. An illustration is shown in Figure 1. Two ISAR images of an in-flight aircraft were taken in a consecutive sequence in time. The image on the left (a) is severely blurred; the image on the right (b) is well Paper presented at the RTO SET Symposium on “Target Identification and Recognition Using RF Systems”, held in Oslo, Norway, 11-13 October 2004, and published in RTO-MP-SET-080.