Research On Cross-Range Scaling In ISAR Imaging

Inverse synthetic aperture radar (ISAR),which accomplishes two-dimensional high resolution by transmitting broadband signal and using Doppler frequency discrimination, has been used to generate high resolution two dimensional images of moving targets, such as satellites, missiles, aircraft and ships, in all-time, all-weather and long range conditions. It plays significant roles in both military and civilian areas. The range resolution of ISAR image will be decided by the radar signal bandwidth, which is easy to identify; and the cross-range resolution is decided by the wavelength and the total rotation angle of the target relative to radar line of sight (RLOS) during the imaging time. The total rotation angle is unknown due to the non-cooperative characteristics of the ISAR target, so its cross-range resolution is unknown, as a result, the ISAR image can not reflect the real size of the target, which is not conducive to target recognition. Therefore, the determination of cross-range resolution, i.e., the problem of cross-range scaling is studied in this paper.After analyzing data of the broadband and narrowband echo, three effective cross-range scaling algorithms are mainly studied. The main contents are listed as follows:1.The fundamental principles of ISAR imaging are introduced, and the cross-range scaling solutions are given targeting both the broadband and narrowband echo data, which lay theoretical foundation for further detailed analysis.2.For ISAR broadband echo data, on one hand, the cross-range scaling is studied based on FrFT: after range compression, the slow time ISAR echo sequence in each range bin are linear frequency-modulated (LFM) signals, the chirp rate of which contains the information of the target angular velocity. FrFT is used to detect the LFM signal to estimate the chirp rate and obtain the rotation angular velocity during imaging, then the total rotation angle can be calculated, and cross-range scaling can be carried out. On the other hand, the cross-range scaling is studied based on image entropy: applying the matching relationship between the final generated ISAR image and rotation angle, searching in the possible range of the rotation angle, an image will be generated using polar reformatting and the image entropy is calculated at the same time. When the searched angle is closest to the real rotation angle, the image entropy reaches the minimum, and the total angle is obtained, then the cross-range scaling is accomplished.3.For narrowband echo data, the cross-range scaling is studied based on the target track: according to the track information provided by the narrowband echo signals, including slant range, azimuth and pitch angle, the target trajectory is filtered using converted measurement Kalman filter (CMKF) algorithm, and more accurate trajectory information will be obtained after data fitting. Then the aspect angle of radar can be calculated during the process of target motion, and then the total rotation angle relative to RLOS within imaging time is obtained, which enables cross-range scaling.