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NASA prepares mission to study depth, flow of seas, lakes, rivers

Surface Water and Ocean Topography (SWOT) will produce near-instantaneous images of sea surface height and measure the height of land water bodies, revealing changes in sea, lake and river levels down to centimeter resolution (less than half an inch).

Posted 2022-12-14T21:41:10+00:00 - Updated 2022-12-14T21:55:36+00:00

This illustration depicts the Surface Water and Ocean Topography (SWOT) satellite with solar arrays fully deployed. Measuring 48.8 feet (14.9 meters) in length with an area of 335 square feet (31 square meters), the two arrays will remain pointed at the Sun via small motors to provide 8 kilowatts of power; the spacecraft has a 1.5-kilowatt total power demand. The scientific heart of the SWOT satellite is the Ka-band Radar Interferometer (KaRIn) instrument, which will measure the height of water in Earth's lakes, rivers, reservoirs, and ocean. To do that, KaRIn will transmit radar pulses to Earth's surface and use two antennas – seen to the left and right of the spacecraft bus – to triangulate the return signals that bounce back. Mounted at the ends of a boom 33 feet (10 meters) long, the antennas will collect data over two swaths of Earth's surface at a time, each of them 30 miles (50 kilometers) wide and located on either side of the satellite. KaRIn will operate in two modes: A lower-resolution mode over the ocean will involve significant onboard processing of the data to reduce the volume of information sent during downlinks to Earth; a higher-resolution mode will be used mainly over land.

Tony Rice

, NASA Ambassador

NASA is targeting early Thursday for the launch the Surface Water and Ocean Topography (SWOT). The SUV-size satellite will help researchers understand how much water flows into and out of Earth’s freshwater bodies and will provide insight into the ocean’s role in how climate change unfolds.

SWOT is a joint mission developed by NASA and the French space agency Centre National d’Études Spatiales (CNES), with contributions from the Canadian Space Agency and the UK Space Agency. It follows on previous satellite altimeter missions used to map the oceans including TOPEX/Poseidon, and the Jason series with coverage and resolution never seen before.

This latest mission in NASA's ocean surface topography from space program is different from its predecessors which could only look straight down (the nadir point) with their altimeters. SWOT will produce near-instantaneous images of sea surface height and measure the height of land water bodies, revealing changes in sea, lake and river levels down to centimeter resolution (less than half an inch).

SWOT will provide NASA’s first global survey of Earth’s surface water. (Source: NASA/JPL-Caltech)

This will provide never-before-seen insight into small-scale ocean features like coastal currents that move nutrients vital to marine food webs. It will also help researchers understand how fresh water flows through lakes and rivers and improve forecasting drought and floods.

SWOT will monitor rivers at least 100 meters wide or about as wide as a football field is long. This includes rivers like the Neuse in North Carolina. The team has already made a goal to improve that resolution to measure rivers half that size.

The spacecraft will orbit at an inclination of 77º degrees, which means it will cover over everywhere on Earth between 77º north latitude and 77º south latitude, with an average revisit time of 11 days.

All that coverage and resolution will generate about 20 terabytes of data, the equivalent of 25 hours of HD streaming video, each day, delivered to the cloud where researchers. emergency managers and others around the world can freely access it.

Tracks of the KaRIn Interferometer (green shading) and altimeter (red lines) which are repeated every 3 weeks (Credit: NASA/CNES)

Radar interferometry

SWOT will use new instrument called the Ka-band Radar Interferometer (KaRIn), which bounces radar pulses off the water’s surface and receives the return signal via a pair two antennas which are spaced 33 feet (10 meters) apart on a boom (about the width of a tennis court). The differences in the return signals provide insight not only into the altitude but allow scientists to differentiate between land, water and ice.

The KaRIn instrument illuminates two parallel tracks of approximately 50 kilometres on either side of a nadir track from a traditional altimeter. The signals are received by two antennas 10 meters apart and are then processed to yield interferometry measurements. Image credit: NASA/JPL-Caltech

The mission will measure along a wider path than previous ones, about 75 miles (120 km) across.

Doing all this from a height of 554 miles (891 kilometers) presented a challenge to JPL engineers. To achieve the resolution demanded by the mission, that radar boom must remain stable within 2 microns – or about 3% of the thickness of a human hair.

Liftoff is targeted for 6:46 a.m. EST (3:46 a.m. PST) Thursday, Dec. 15, on a SpaceX Falcon 9 rocket from Space Launch Complex 4-East at Vandenberg Space Force Base in California. Live launch coverage begins on NASA TV at 6 a.m.