Direct surface current field imaging from space by along-track InSAR and conventional SAR

Since the SEASAT mission in 1978, spaceborne synthetic aperture radar (SAR) images of ocean scenes have been used successfully for a variety of applications. However, despite the fact that a SAR is a Doppler radar, conventional SAR images do not provide direct information on target velocities, since all Doppler information is normally utilised to obtain the best possible spatial resolution under the assumption that targets are not moving. Recent technological progress has enabled us to overcome this shortcoming to some extent: The split-antenna mode of TerraSAR-X permits an acquisition of two SAR images with a short time lag for alongtrack interferometry, and advanced processing of conventional SAR raw data, demonstrated with ENVISAT ASAR data, permits velocity retrievals at a reduced spatial resolution even with a single antenna. We describe the methodologies and the theoretical advantages and limitations of both approaches, review experimental results from the last decade, and discuss possible and desirable future developments.