Satellites have already revolutionized oceanography, and tomorrow they will do the same for hydrology. The French-U.S. SWOT mission (Surface Water and Ocean Topography) will be at the forefront, carrying a wide-swath Ka-band radar interferometer dubbed KaRIn that marks a break with today’s technologies.

With its two radar antennas perched at the end of a 10-metre boom, KaRIn will afford continuous coverage of a 120-kilometre swath where current radar altimeters are restricted to a strip of a few kilometres directly below the satellite. Thanks to this wide ground track, KaRIn will be able to acquire measurements of surface water height in more than 100-metre-wide rivers as well as lakes and flood zones with a surface area of  250 m x 250 m, with a 10-metre accuracy, and to quantify slopes with a 1.7cm/km accuracy (after averaging on a >1 km2 water surface area).

Combined with high-precision geoid models from the GOCE satellite and precise digital terrain models, SWOT data will radically improve hydrodynamic models used to estimate river discharges. They will also help to determine temporal variations in surface water stocks (lakes, reservoirs and wetlands) covering more than 250 square metres and in flow dynamics. To give an idea, it is estimated there are more than 30 million lakes larger than 1 hectare in the world.

Oceanographers are also eagerly awaiting SWOT, as KaRIn will be able to see mesoscale and sub-mesoscale circulation patterns covering several hundred to several tens of km, like eddies and filaments, to characterize their very dynamic vertical transport, and to study coastal circulation and refine current ocean and climate prediction models, all with centimetre accuracy.

The vital technical, scientific and application innovations that SWOT will bring draw on NASA and CNES’s joint altimetry legacy going back more than 20 years.