Nadir radar altimetry has revolutionized oceanography by providing, for now 15 years, global measurements of ocean surface topography. Long-term measurements of large-scale circulation and heat storage of the global oceans have led to discoveries such as the effects of changes in ocean circulation on climate (e.g., El Niño and La Niña). Radar altimetry has also provided high-precision sea level measurements with global coverage. As sea level rise represents a major threat of on-going global warming, such observations are precious for separating natural climate variability effects from human-induced changes, identifying most vulnerable coastal areas and improving climate models used for sea level projections.
In contrast to ocean observations, land surface water measurements are limited mostly to in situ networks of gauges that record water surface elevations at fixed points along river channels. Globally, the spatial and temporal distribution of water stored on the land surface and moving through river channels is known only crudely. Furthermore, water movement in wetlands and across floodplains throughout the world is essentially unmeasured, significantly limiting our understanding of flood processes. Over rivers, in situ networks are declining worldwide due to economic and political reasons.
A critical limitation of the classical Nadir altimetry from Space, for both ocean dynamics and land hydrology, is the 200- to 300-kilometer spacing between satellite orbital tracks, which is unable to resolve small-scale features in oceanography (e.g., currents and oceanic mesoscale processes that contain 90% of the kinetic energy of the oceans) and miss a large number of surface water bodies on land (small lakes and reservoirs, most rivers, etc.)
The physical oceanography and surface freshwater hydrology communities have now joined together in recognizing the potential of high-resolution, space borne measurements of water surface elevations. They propose a new mission named SWOT (Surface Water Ocean Topography) based on the concept of wide-swath interferometric altimetry for mesoscale oceanography and land surface hydrology.
The SWOT mission has been recommended by the National Research Council decadal review “Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond” for implementation by NASA. The SWOT mission is a partnership between NASA and CNES agencies.
The primary SWOT instrument is an interferometric altimeter which has a rich heritage based on (1) the many highly successful ocean observing radar altimeters, (2) the Shuttle Radar Topography Mission, and (3) the development effort of the Wide Swath Ocean Altimeter. It is a near-nadir viewing, 120 km wide, swath based instrument that will use two Ka-band synthetic aperture radar (SAR) antennae at opposite ends of a 10 m boom to measure the highly reflective water surface. Interferometry SAR processing of the returned pulses yields a 5 m azimuth and 10 m to 70 m range resolution, with elevation precision of ± 50 cm. Polynomial based averaging over areas less than 1 km2 increases the height precision to less than ± 1 cm.