Many current studies are attempting to enhance the quality of altimetry data close to the coasts. New processing methods and applications can then be developed for littoral and shallow-water regions, some of the most fragile and important areas of the oceans.
The shortage of altimetry data near the coasts (or their inferior quality) is due to several factors:
– the technique itself, since the radar echoes reflected off water, and off a combination of water and land are not identical, and basically only the former undergo processing by the ground segments. Other altimetry satellite measurements also suffer from the same problem, such as those from the radiometer (at a distance of about 50 km from the coast)
– – the fact that the basic distributed data (GDR) are mainly average over one second, thus covering about 7 km on the ground (data averaged over 1/20 s do exist, however).
– the computation of some corrections. Tides, in particular, are much more complex near the shores than in the open sea, and require a highly precise knowledge of the coastal geography to be accurately computed. Moreover, rapid variations (“high frequency”) must be taken into account in those areas (for the tides as well as for the atmospheric pressure). Wet tropospheric corrections, computed from radiometer measurements are also less precise, or even missing, near the coasts.
For Jason-1 or Topex/Poseidon, areas where radiometer measurements are typically edited out (ie disregarded) by standard processing (in red) and ones where they are retained (in green). The Aegean Sea, in particular, is completely overlooked.
Such studies are leading to advances that will soon make it possible to use altimetry data close to the coasts. With new possibilities such as the use of individual altimetry echoes (as opposed to averaged ones today), we can at last hope for real coastal data.