Usually dedicated to ground imaging, Spot SAR processing can also be applied to moving targets, when proper autofocusing is used. In this case, the simultaneous motions of carrier and target both have an impact on the azimuth resolution. That is why these motions have to be estimated and their effects corrected. The process is then a joint SAR-Inverse SAR (ISAR) imaging. For ship targets, the typical roll, pitch and yaw motions (usually unknown for real, non co-operative targets) produce some residual uncorrected migrations of scatterers. The consequence of these migrations is a blurring of SAR/ISAR range-Doppler maps of the target.
A "snapshot" technique, based on a short imaging time, allows some robustness to these residual migrations during imaging time, but it has two main disadvantages. First, it makes it difficult to achieve high azimuth resolution. Second, it produces a series of range-Doppler maps, which include both useful and unsuitable images for extracting the outline of the ship target. The interest of a particular image depends on the moment in the unknown rotation of the target.
We developed a criterion enabling us to choose suitable snapshots in a series, and also a segmentation technique adapted to the typical shapes of ship targets. This criterion can be adapted to range undersampled data and the presumed Doppler spread of the target return. Its complexity and accuracy may therefore be adapted to the context. The criterion and the segmentation technique have both been tested on synthetic and real in-flight data. A highly effective way of producing easy recognizable height profiles of ship targets at sea has been achieved.