Envisat radar altimetry for coastal and inland waters: case-study of the Concordia ship to understand non-water targets using a tomographic technique

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 treated as contaminants of the pure radar signal reflected by the water surface. Despite the native low resolution of conventional altimetric 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 and eventually remove the associated electromagnetic artifacts. In this paper, we use a tomographic technique to retrieve useful information about the location and geometry of such particular targets. We show results from a case-study based on the Costa Concordia cruise ship, which smashed its hull on 13th of January 2012 against the coast of Giglio Island, a tiny piece of land in the Tuscan Archipelago (Italy) 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 tomographic reconstruction of the scene observed by the radar. The geometric characteristics of the ship and of the apparent electromagnetic target described in this paper are well compatible with the Concordia ship in its final position. 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.