Nonuniform azimuth image shift observed in the Radarsat images of ships in motion

This paper describes, for the first time to the authors' knowledge, the phenomenon of nonuniform azimuth image shift of a rigid body observed in the Radarsat synthetic aperture radar (SAR) images of cruising ships. The effect is caused by the different slant-range velocities of coherent scatterers across the hull associated with the ship motions. The slant-range velocity is estimated from the SAR image of an identified ship, and it is compared with that computed from a numerical model using the ship's specification and meteorological data. The result indicates that the dominant contribution to the nonuniform image shift is the pitching motion of the ship. Further comparison is made with the wave orbital velocity, and the results are shown to be in good agreement. Comparisons are also made between the SAR-derived slant-range velocities of two unknown ships and wave orbital velocities, and reasonable agreement is obtained. One of the ships' images exhibits not only nonuniform shift but also image skew. The latter skewing effect may be caused by rolling of the ship.