Science

1. Provide a global inventory of water resources.
2. Understand where the water is, where it’s coming from, and where it’s going.
3. Observe the fine details of the ocean’s surface topography.
4. Improve understanding of the ocean’s role in climate change.
5. Measure ocean conditions near coastlines.

SWOT will measure sea surface height, or the ocean’s surface topography. This will enable scientists to study currents and eddies less than 13 miles (20 kilometers) across – up to 10 times smaller than has been previously detectable with other sea level satellites.

SWOT’s ability to “see” smaller areas of Earth’s surface will also enable it to fill in observational gaps along coastlines. The spacecraft will collect more precise data than other satellites can along coastlines, where sea level rise can directly impact communities and coastal ecosystems. For example, SWOT will help researchers better understand how rising seas will affect things like storm surges and coastal flooding.

Observing the ocean at these smaller scales will also help researchers assess its role in moderating climate change. Earth’s ocean has absorbed more than 90% of the excess heat trapped by human-caused greenhouse gas emissions. Most of the uptake of that heat is thought to occur via currents and eddies less than 60 miles (100 kilometers) across. Gaining greater insight into this process could be key to figuring out whether there’s a limit to the ocean’s ability to absorb the heat trapped by the carbon and methane emissions from human activities.

SWOT will produce the world’s first comprehensive global survey of Earth’s freshwater from space, providing data on more than 95% of the world’s lakes larger than 15 acres (62,500 square meters) and rivers wider than 330 feet (100 meters) across. As of now, researchers have reliable measurements of water levels for only a few thousand lakes worldwide and little to no data on some important river systems like that of the Koyukuk River in Alaska. Its network of smaller rivers and tributaries drains an area roughly the size of Kansas.

SWOT will measure the height of water in lakes, rivers, and reservoirs, as well as the surface area, or extent, of that water. Using this information, scientists will be able to calculate how much water moves through those freshwater bodies as climate change accelerates Earth’s water cycle. Warmer temperatures mean that the atmosphere can hold more water (in the form of water vapor), which can result in things like volatile precipitation patterns – torrential downpours rather than a steady, gentle rain, for example. Such variability can, in turn, wreak havoc on a community’s ability to manage its water resources effectively.

SWOT will also help with tracking changes in water volume over time. Satellites already in orbit can measure the height of water – in the ocean, very large lakes, or very wide rivers, for example – or the surface area of a water body. But to calculate changes in volume over time, scientists need to match up extent and height measurements that may have been collected by different instruments on different days and at different times. This makes it difficult to know basic information, such as how much water flows through the world’s rivers and how much that volume varies. SWOT will eliminate the need to cobble together the extent and height information from different sources and at the same time give researchers a global view of Earth’s surface water.

By advancing research into Earth’s ocean and freshwater bodies, SWOT will aid communities around the world. Scientists, policymakers, and resource managers will be able to apply SWOT data to challenges, including flood projections, drought monitoring, and assessing how climate change may impact coastal communities and ecosystems.

For example, currently, water levels during a flood are measured after the fact using what are called wrack marks – debris indicating what is often considered the high-water mark during a flood. But wrack marks don’t convey how water levels varied during a flood. If SWOT is overhead during such an event, the satellite could capture water levels at particular locations and times, and could contribute to improved flood-forecasting systems in flood-prone river basins.

The satellite will also provide much more detailed information than other satellites about the coastal ocean. Basic data, such as average sea levels near the coast, has gaps, depending on where instruments that measure sea surface height are located. SWOT’s improved resolution and global coverage will deliver sea surface height data for many more coastal locations. Knowing where the water is will not only lead to better models of ocean circulation close to the coast, it will help with severe-storm forecasting and storm-surge projections.

SWOT’s data and the tools to support researchers in analyzing the information will be free and accessible. For more information on how SWOT data will be applied to local communities, visit the SWOT early adopters website.

The scientific heart of the SWOT satellite, the Ka-band Radar Interferometer (KaRIn) instrument, 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 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: The higher-resolution mode will be used mainly over land.

This science instrument was provided by NASA’s Jet Propulsion Laboratory. The French space agency Centre National d’Études Spatiales (CNES) and Thales Alenia Space built the radio-frequency subsystem, a key part of this instrument. The development of the Duplexer, a high-power switching system that is part of the radio-frequency unit, was funded by the UK Space Agency at Honeywell UK. The Canadian Space Agency contributed the extended interaction klystrons, which are at the heart of the instrument’s high-power transmitter assembly.

The nadir (Earth-facing) altimeter instrument measures the height of water on Earth’s surface in a narrow strip directly below the satellite, between the swaths the KaRIn instrument observes. Provided by CNES and used on previous ocean-observing satellites, including the Jason series of spacecraft, the altimeter beams radio signals to the water’s surface and measures how long the reflected signal takes to return to a receiver. This instrument builds upon the dual-frequency Poseidon family of altimeters that trace their roots to the TOPEX/Poseidon mission launched in 1992. The Poseidon family of altimeters was developed and built by Thales Alenia Space.

Water vapor affects the propagation of radar signals from the KaRIn and nadir altimeter instruments, causing the water level they’re measuring to appear higher or lower than it actually is. The microwave radiometer instrument provides water vapor measurements used to correct this effect. It uses heritage technology employed on the Jason-3 mission, pointing two measurement beams in between the two KaRIn swaths. NASA JPL provided this instrument.

Precisely determining the position of the SWOT spacecraft in orbit is critical when collecting measurements. To do this, SWOT carries a state-of-the-art precise orbit determination instrument package. Each of the package’s three science instruments uses a different approach to provide tracking measurements of SWOT’s position. The measurements are then used to determine the 3D position and speed of the satellite. The instruments include:

• Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS): The DORIS instrument measures the radio signals from 50 to 60 global ground stations that compose the International DORIS Service. Each ground station acts as a beacon to broadcast two stable radio frequencies at 2036.25 megahertz (S-band) and 401.25 megahertz (VHF). SWOT’s DORIS instrument measures the Doppler shift of the radio beacons’ frequencies to ultimately determine the satellite’s position. CNES provided this instrument and it was built by Thales Alenia Space.
• Laser retroreflector array (LRA): LRA consists of nine precisely shaped mirrors located on the spacecraft’s Earth-facing side. Ground-based laser-ranging stations send laser beams to the satellite’s LRA mirrors, which reflect them back to the stations. By measuring how long it takes the laser beams to return to Earth’s surface, engineers can determine the distance between the spacecraft and the station. NASA JPL provided this instrument.
• Global positioning system (GPS) receiver: The GPS receives tracking signals transmitted by GPS satellites orbiting Earth. The instrument uses these signals to determine the distance between the spacecraft and the GPS satellites. NASA JPL provided this instrument.