Azimuth Envelope Alignment for Focusing Maneuvering Ships in SAR Images

Synthetic aperture radar (SAR) is a crucial technology in marine surveillance. Moving ships are always defocused in SAR images, which affects the following identification process. Inverse SAR (ISAR) technology has been employed to focus moving targets in SAR images, typically involving two steps: translation motion compensation and rotation motion compensation. Range envelope alignment is generally considered a well-established method for translation motion compensation. In rotation motion compensation, current approaches, however, rely on signal parameter estimation and optimization algorithms, which lack robust methods. This article presents a rotational motion compensation framework, named “azimuth envelope alignment,” which extends the range envelope alignment to the azimuth direction. First, a novel concept of “azimuth envelope” is introduced, and the process of azimuth envelope alignment is discussed. Then, a specific method is presented: the use of the iterative adaptive approach (IAA) and resampling for implementation of “azimuth envelope alignment.” This method first estimates the alignment parameter (AP) using IAA, followed by achieving alignment through resampling. This method exploits the high-precision estimation capability of the IAA while avoiding its sparse estimation results, enabling the robust, nonsparse refocusing of moving targets. Finally, simulations and experimental data validate the effectiveness of the proposed algorithm. The azimuth envelope alignment framework introduced in this article can serve as a general approach for moving target imaging, similar to range envelope alignment.