A basic difficulty in processing SAR imagery of ships at sea is that oftentimes the ship motion is great enough within a radar dwell to cause not only blurring of the image but distortion as well. One can readily see this effect on small boats even in moderate seas. As the sea state worsens, larger ships become affected to the point where all ships behave like small boats. Processing SAR imagery in the conventional manner could easily yield ship lengths that are too large by a factor of three to five.
The wave induced ship motion is such that the imagery must be processed in the manner of Inverse SAR (ISAR). EMSI has developed a technique that exploits the combination of rolling, pitching and yawing to generate three-dimensional positions of the major scatterers on the ship. We have tested our technique on small boats and ships and have found we can produce accurate length measurements – a key requirement for accurate classification.