Focusing very long dwell spaceborne SAR imagery of ship targets: coherent and non-coherent ISAR approaches

The advent of very-long dwell spotlight SAR modes marks a significant milestone in maritime surveillance applications. By observing the same area for extended periods, these modes are well suited for extracting detailed information from moving ships. This paper presents a processing chain specifically designed for exploiting very-long dwell, addressing the challenge of motion-induced blurring, a phenomenon that is exacerbated in these SAR images, where the target may undergo complex motion such as acceleration or rotation. The method utilizes sub-aperture processing to estimate target motion parameters (radial velocity and Doppler-rate) across consecutive CPIs. These estimates are then used to reconstruct the target's range history beyond the conventional second-order approximation over the entire dwell time, in turn enabling the generation of two long-dwell products: a non-coherent multiframe image, achieved by combining multiple sub-apertures, and a coherent long-dwell image, where the entire synthetic aperture is refocused. Both products significantly enhance structural details, providing high-resolution imagery of moving ships. The effectiveness of the method is validated using real ICEYE SLC data acquired over the Rotterdam port with an observation time of 25 s.