Research On ISAR Motion Compensation

Inverse Synthetic Aperture Radar (ISAR) is a kind of high-resolution imaging radar,which can provide an all-weather, day/night capability to generate a high-resolution two-dimensional radar images of the targets over a long range. ISAR has received intensive attention because of its potential applications in both civilian and military applications. The objects of ISAR imaging are usually non-cooperative targets, the motion of which is unable to be forecasted, and then the motion compensation is a key technique in ISAR.The motion of the target relative to the radar can be decomposed into two parts: the translational motion of a reference point in the target and the rotational motion of others about the reference point. Only the latter is useful for the imaging. The former does not contribute to the imaging. It influences ISAR imaging in two aspects: First, it causes the range misalignment of the range profiles. Second, it introduces an additive phase component which induces defocus in cross-range. Therefore, the task of the motion compensation is to remove out the translational motion before imaging. It is usually divided into two steps: range alignment and phase compensation.The thesis has a comprehensive study on the some effective ISAR phase compensation algorithms, and then discusses the ISAR motion compensation algorithms of high-speed targets.Firstly, ISAR imaging principle is introduced, and motion compensation principle is described in detail. Then the main range alignment methods at the present time are introduced briefly, while the cross-correlation method of range alignment used in simulation of the later chapters is introduced particularly. Secondly, some effective ISAR phase compensation algorithms are researched.First, the Doppler centroid tracking method is studied, which is based on simple principle of phase compensation. And then some other phase compensations are researched deeply, including Phase Gradient Autofocus algorithm, the Rank One Phase Estimation algorithm and its improved algorithm, Fast Minimum Entropy Phase Compensation and Maximum-Contrast Phase Adjustment. In order to compare and analyze the performances of these algorithms, the thesis applies these algorithms to the simulated data and the ISAR real data. And combining with objective criterion for these algorithms, the thesis proves that FMEPC algorithm is an effective method for phase compensation.Finally, the thesis discusses the motion compensation of high-speed targets. Through calculating the echo signal of high-speed targets, it is discovered that the echo signal of high-speed targets is LFM signal. Therefore, the thesis must compensate the frequency modulation slope before motion compensation with usual method. In order to evaluate the frequency modulation slope, the thesis puts forward a motion compensation algorithm that is based on Fractional Fourier Transform. The result of simulation proves the validity of this algorithm.