Increase the Coherent Processing Interval for SAR Focusing of Maneuvering Ships by Data Resampling

Synthetic aperture radar (SAR) is important for marine surveillance. The image resolution of SAR is obtained via coherent integration. However, with increasing coherent processing intervals (CPIs), the focusing quality of maneuvering ships in images will worsen, impacting target recognition. Defocusing is caused mainly by the change in the image projection plane (IPP) of the maneuvering ship and the change in the effective rotation vector (ERV). In this article, an improved inverse SAR (ISAR) method is used to focus the ships in SAR images. First, the factors influencing the instantaneous imaging position are analyzed. Then, a new method based on data resampling for maneuvering ship imaging with a long CPI is proposed. This method consists of two main steps: ERV estimation and data resampling. The change in the ERV is estimated by using the instantaneous imaging position of all the scatterers, and the ship echo is resampled according to the estimation of the time-varying ERV. After resampling, we can obtain the echo from a constant rotating ship and can ensure a well-focused ship by using the traditional range-Doppler algorithm. This new method overcomes the restriction of ERV variation. Thus, a longer CPI can be used for imaging, while the IPP remains stable. Finally, simulation and measured data are used to verify the effectiveness of the proposed algorithm.