Envisat radar altimetry as a tool for coastal and inland waters: the use of a microwave tomographic approach to achieve information about location and geometry of non-water targets

Satellite altimetry systems use a nadir-looking radar to sense the water surface, in order to estimate water heights. Non-water targets (e.g. land, ships) are normally considered as contaminants of the pure radar signal reflected by the water surface. Despite the native low resolution of conventional altimetry platforms, there's still the possibility to extract information about eventual land or artificial scatterers inside the antenna footprint, giving the opportunity to better understand how to detect, characterize and eventually remove the associated electromagnetic artefacts. In this paper, we use a tomographic technique to retrieve useful information about the location and geometry of these particular targets. In this work, we will show results from a case-study based on the Costa Concordia cruise ship, which smashed its hull against the coast of Giglio Island, a tiny piece of land in the Tuscan Archipelago of the Northwestern Mediterranean. The ship is a strong artificial reflector located off-nadir with respect to the closest Envisat track (orbit 274) to the island. A signature in the Envisat waveforms due to the presence of the Concordia ship is revealed by a change-detection analysis applied to the images achieved by tomographic approach for different passes of the Envisat altimeter before and after the accident. The geometric characteristics of the ship and of the apparent electromagnetic target achieved by the approach are compatible with the Concordia ship in its final position. Therefore, the tomographic technique is therefore a promising tool to make a mapping of targets in coastal and inland waters and to enhance the possibilities to mitigate such effects when dealing with water height measurements in the presence of such targets.