The Surface Water and Ocean Topography Mission

The Surface Water and Ocean Topography SWOT mission is a partnership between two communities, physical oceanography and hydrology, to share high vertical accuracy and high spatial resolution topography data produced by the science payload, whose principal instrument is a Ka-band radar Interferometer. The SWOT mission will provide large-scale data sets of ocean sea-surface height resolving scales of 15km (in ocean wavelength) and larger, allowing the characterization of ocean mesoscale and submesoscale circulation. Present altimeter constellations can only resolve the ocean circulation at ocean wavelengths larger than 200km. SWOT will address fundamental questions on the dynamics of ocean variability at wavelengths shorter than 200km, which encompasses mesoscale and submesoscale processes such as the formation, evolution, and dissipation of eddy variability (including narrow currents, fronts, and quasi-geostrophic turbulence) and their role in airsea interaction. The SWOT mission will also provide measurements of water storage changes in terrestrial surface water bodies and will provide estimates of discharge in large (wider than 100m) rivers, globally. The SWOT measurements will directly measure the surface water (lakes, reservoirs, rivers, and wetlands) component of the water cycle. The core of the SWOT payload consists of a Ka-band Radar Interferometer (KaRIn) - a dual-antenna synthetic aperture radar specifically designed to make high precision height and backscatter measurements enabling the key oceanographic and hydrology data sets. The SWOT payload also includes: a Nadir Altimeter (NA) system, a radiometer for tropospheric path delay corrections, and a precision orbit determination instrument suite consisting of Global Positioning System-Payload (GPSP), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) receivers, and a Laser Retroreflector Array (LRA). SWOT is a partnership mission between NASA, the French Space Agency (CNES), The United Kingdom Space Agency (UKSA), and The Canadian Space Agency (CSA). The spacecraft bus and command & data ground stations are provided by CNES, while the launch vehicle and payload module are provided by NASA/JPL. The SWOT payload module instrument suite consists of the NASA/JPL provided KaRIn instrument, cross-track radiometer, GPS, and laser retroreflector, as well as the CNES provided DORIS and nadir pointing radar altimeter. CNES also provides the Radio Frequency Unit (RFU) sub-system for which the UKSA contributes the Duplexer assembly. The RFU is a key subsystem of the KaRIn instrument. CSA contributes the Extended Interaction Klystron (EIK) which is one of the KaRIn subsystem. This paper describes the mission design, implementation of the Payload, and some of the interface and design challenges.