Understanding target-like signals in coastal altimetry: Experimentation of a tomographic imaging technique

Usage and availability of Sea Surface Height (SSH) information from satellite radar altimeters undergo known limitations in the coastal zone, where such data are of great importance and usefulness. In fact, coastal regions are a crucial zone to be investigated and monitored, due to the high impact that sea level and circulation changes have on the environmental security and the related economic and societal issues. It is known that radar returns from the sea surface sometimes present target-like echoes ("bright targets"), especially in correspondence of particular features of the coastal zone, thus entailing a potential interference with the measurement of SSH. Such spiky echoes generate hyperbolic patterns in the radargram domain, which the recent literature has tentatively explained as resulting from flat water areas in the proximity of the coastline, but the physical mechanism that underlies their occurrence still remains unclear. To probe further into this aspect, this work describes a novel application of a microwave tomographic reconstruction approach, applied to the Envisat RA-2 signals, tested on selected passes over the Pianosa Island (a 10 km2 island in the NW Mediterranean). The aim of this study is the analytical identification of the signal contamination sources in terms of location and extension of their associated electromagnetic anomaly. The obtained results confirm the idea that the origin of such signatures is connected with particular conditions of the sea surface, which are easier to be found in the proximity of coastal closed areas such as gulfs, but presumably not limited to such circumstances.