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A new satellite mission called Surface Water and Ocean Topography (SWOT) is being developed jointly by the U.S. National Aeronautics and Space Administration and France’s Centre National d’Etudes Spatiales. Based on the success of conventional nadir-looking altimetry missions in the past, SWOT will utilize the technique of radar interferometry for making wide-swath altimetric measurements of the elevation of surface water on land and the ocean’s surface topography. The new measurements will provide information on the changing ocean currents that are key to the prediction of climate change, as well as the shifting fresh water resources resulting from the dynamic water cycle.
The noise level of conventional radar altimeters limits the along-track spatial resolution to 50-100 km over the oceans. The large spacing between the satellite ground tracks limits the resolution of two-dimensional gridded data to 200 km. Yet most of the kinetic energy of ocean circulation takes place at the scales unresolved by conventional altimetry. SWOT observations will provide the critical new information at these scales for developing and testing ocean models that are designed for predicting future climate change.
In contrast to ocean observations, land surface water measurements are limited mostly to in situ networks of gauges. While radar altimetry over surface waters has demonstrated the potential of this technique in land hydrology, a number of limitations exist. Raw radar altimetry echoes reflected from land surface are complex, with multiple peaks caused by multiple reflections from water, vegetation canopy and rough topography, resulting in much less valid data over land than over the ocean. Yet one of the most threatening consequences of a warming climate is the shifting water resources. Monitoring the global water on land is critical for assessing the storage and discharge of lakes and rivers.
The technology of SWOT is based on the heritage of the Shuttle Radar Topography Mission (SRTM) that successfully mapped the elevation of global land topography from a 10-day space shuttle mission. A higher frequency at Ka band (~35 GHz) is chosen for the radar to achieve high precision with a much shorter inteferometry baseline of 10 m. Small near-nadir look angles (~ 4 degrees), required for minimizing elevation errors, limit the swath width to 120 km. An orbit with inclination of 78 degrees and 22 day repeat period was chosen for gapless coverage and good tidal aliasing properties. With this configuration, SWOT is expected to achieve 1 cm precision at 1 km x 1 km pixels over the ocean and 10 cm precision over 50 m x 50 m pixels over land waters. Other payloads of the mission include a conventional dual-frequency altimeter for calibration to large-scale ocean topography, a water-vapor radiometer for correcting range delay caused by water vapor over the ocean, and precision orbit determination package (GPS, DORIS, and laser retroreflector). SWOT is currently being developed for a planned launch in 2021.
This presentation will describe the current SWOT mission status, including technical development challenges regarding the Payload Instrument, Spacecraft, ground data system and calibration/validation plans.
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