The SWOT (Surface Water and Ocean Topography) Mission: Spaceborne Radar Interferometry for Oceanographic and Hydrological Applications

Satellite altimetry has revolutionized the study of the global oceans for the past two decades. Precision altimetry missions like TOPEX/Poseidon, Jason-1, and OSTM/Jason­ 2, complemented by other missions like ERS 1 and 2, ENVISAT, and GFO, have provided unprecedented observations of the ocean surface topography at scales larger than about 200 km and made significant advances in our understanding of global ocean circulation and sea level change. However, the measurement errors have prevented resolving scales shorter than 100 km, the submesoscales that are important for understanding the dynamics of the ocean kinetic energy and the vertical transfer processes in the ocean. These processes are critical to the understanding of the role of the ocean in regulating global climate change. Altimetry measurements have also been applied to the study of the water levels of rivers and lakes, but the coarse resolution of the data has severely limited its ability in addressing key hydrological questions on the storage of water on land and its discharge. These are important questions on the distribution of fresh water on land that is being seriously affected by global climate change. A new space mission called Surface Water and Ocean Topography (SWOT) is being developed jointly by a collaborative effort of the international oceanographic and hydrological communities for making high-resolution measurement of the water elevation of both the ocean and land surface water to answer the questions about the oceanic submesoscale processes and the storage and discharge of land surface water. The key instrument payload is a Ka-band radar interferometer capable of making highresolution wide-swath altimetry measurement. The development of the mission has been proceeding as a joint effort of NASA and CNES, following the long-lasting tradition in collaboration in radar altimetry over the past 25 years. This paper describes the science objectives and requirements as well as the measurement approach of SWOT, which is anticipated to be launched in 2016. SWOT will demonstrate this new approach to advancing both oceanography and land hydrology and set a standard for future altimetry missions.