From The Editor | November 10, 2022

SWOT: NASA's Plan To Map Earth's Waterways

John Oncea

By John Oncea, Editor


When Lewis and Clark led the Corps of Discovery Expedition in the first decade of the 1800s, they had no idea where they were going or how they were going to get there. If only they had some sort of satellite that would have enabled them to view every river at least 330 feet across from Camp Dubois, IL to Fort Clatsop, OR. Imagine how much simpler their expedition would have been.

While it came 220 years too late for Lewis and Clark, that technology exists today as announced by NASA, and it’s going to allow researchers to develop a “better understanding of Earth’s water cycle” which will in turn “both aid in better management of water resources and expand knowledge of how climate change affects lakes, rivers, and reservoirs.”

The satellite, officially known as the Surface Water and Ocean Topography (SWOT), is a “collaboration between NASA and the French space agency Centre National d ’Études Spatial (CNES), with contributions from the Canadian Space Agency and the United Kingdom Space Agency.”

SWOT is scheduled to launch on December 5 on a SpaceX Falcon 9 rocket from Space Launch Complex 4E at Vandenberg Air Force Base in California. NASA's Launch Services Program at Kennedy Space Center in Florida will manage the SpaceX launch service.


According to NASA, SWOT's primary payload is the Ka-band Radar Interferometer (KaRIn) that is being developed by JPL. The Canadian Space Agency will provide a high-power assembly component for KaRIn. CNES will build the Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) Antenna and Nadir Altimeter. JPL will also provide a Global Positioning System (GPS) science receiver, a Laser Retroreflector and a two-beam Microwave Radiometer.

SWOT's payload is comprised of the following instruments:

  • KaRIn

    • Using JPL-developed instrument technology, radar interferometry, KaRIn will measure ocean and surface water levels over a 120-km (75-mi) wide swath with a ~20 km (~12 mi) gap along nadir. It will operate in two modes:

      • Low-Resolution over the ocean with significant onboard processing to reduce data volume.

      • High-Resolution over broad, primarily continental, regions defined by the SWOT Science Team, focusing on hydrology studies.

  • Jason-class Altimeter will collect data in the gap between the KaRIn swaths. It will send and receive signals that travel straight up and down. Each pulse's round-trip travel time will be used to determine Sea Surface height.

  • DORIS Antenna will pick up signals from 50-60 ground-based radio beacons, equally distributed over Earth to ensure good coverage.

  • Microwave Radiometer will measure the amount of water vapor between SWOT and Earth's surface. More water vapor means slower radar signals.

  • X-band Antenna will be used for high-rate data downlink.

  • Laser Reflector Assembly is an array of mirrors that will provide a target for laser tracking measurements from the ground.

  • Global Positioning System (GPS) Receiver will pick up tracking signals from the constellation of GPS satellites.

Mounted below the Payload Module, the Spacecraft Bus is a multi-purpose platform housing many of the electronics required for the observatory to function:

  • S-band antenna will be used to communicate with Earth using a radio transmitter and receiver.

  • Command and data handler will manage the observatory's communication links and perform various tasks (e.g., data storage) using a centralized processor.

  • Electrical power subsystem is where all of the spacecraft’s power is generated, stored, and distributed.

  • Thermal control is a subsystem responsible for maintaining the temperatures of each component on the observatory within its allowable limits.

  • Solar arrays will be deployed from opposite sides of the Spacecraft Bus while SWOT is in orbit, using small drive motors to keep them pointed at the Sun.

  • Attitude control system will determine the observatory's orientation using star trackers. The satellite's attitude will be carried out by magnetic torquer bars and reaction wheels.

  • Propulsion will be used to adjust SWOT's orbit by firing a combination of its onboard thrusters. This subsystem also includes a propellant tank.

Once in orbit, it will make measurements of over 95% of Earth’s lakes, rivers, and reservoirs. “Current databases maybe have information on a couple thousand lakes around the world,” said Tamlin Pavelsky, the NASA freshwater science lead for SWOT, based at the University of North Carolina, Chapel Hill. “SWOT will push that number to between 2 million and 6 million.”

Among the mission’s scientific objectives are to characterize the ocean mesoscale and sub-mesoscale circulation at spatial resolutions of 10 km and larger. The hydrologic science objectives of the SWOT mission are:

  • To provide a global inventory of all terrestrial surface water bodies whose surface area exceeds 250 m2 (lakes, reservoirs, wetlands) and rivers whose width exceeds 100 m (requirement) (50 m goal).
  • To measure the global storage change in terrestrial surface water bodies (for man-made reservoirs, total storage) at sub-monthly, seasonal, and annual time scales.
  • To estimate the global change in river discharge at sub-monthly, seasonal, and annual time scales.

Among the anticipated benefits are better management of water-sharing issues; more accurate weather and climate forecasting; better management of freshwater for urban, industrial, and agricultural consumption; improved flood modeling; and environmental risk reduction.

The data NASA gleans from SWOT will benefit the environment and help combat global warming by providing a deeper understanding of every type of Earth’s waterways. That data also will benefit any entity that utilizes those waterways.

Consider the military, for example. It utilizes waterway transportation extensively to transport everything from petroleum, oils, and lubricants (POLs) to heavy machinery to troops because of the cost advantages waterway transportation provides. Routing on waterways can improve dependability in transit time, lower security risks, enhance training opportunities, and provide significant savings in both transit and up/downloading costs. Knowing which waterways are deep enough and wide enough to accommodate the boats navigating them will prove to be invaluable.

Think also about the military's role in responding to hurricanes. When Hurricane Ian made landfall in late September dozens of organizations jumped in to help including the Coast Guard and multiple National Guard units.

According to The U.S. Department of Defense “More than 5,200 National Guard soldiers and airmen from Florida and seven other states assisted with Hurricane Ian response efforts, ultimately saving the lives of more than 2,000 people and 50 pets. Their work in Florida included … conducting search and rescue operations. The team effort also included the Coast Guard and the Army Corps of Engineers.”

While those responding performed admirably one can theorize what more could have been accomplished if they had more detailed, accurate maps – such as those they will have after SWOT completes its mission – of the waterways in the Florida Keys, Gold Coast, and Southwest Florida to aid them in their search and rescue efforts.

Of course, eliminating the need for any search and rescue post-hurricane is the ultimate goal and the data SWOT is going to mine will allow for more accurate, detailed forecasts which, in turn, will aid in the evacuation of cities in any future hurricane’s path.

According to the National Oceanic and Atmospheric Administration (NOAA), “Forecasting where a hurricane will go and how strong it will be starts with data. Hurricane specialists at NOAA study satellite imagery and computer models to make forecast decisions for advisories that go to emergency managers, media, and the public for hurricanes, tropical storms, and tropical depressions.”

Combining SWOT’s data combined with the NOAA satellites constantly observing tropical cyclones (hurricanes, tropical storms, or tropical depressions) will allow for better modeling and predictions and, ultimately, save lives.

A NASA Jet Propulsion Laboratory video features Lee-Lueng Fu, Fellow and Senior Research Scientist at NASA Jet Propulsion Laboratory, saying, “Now we are facing a time that we need to be very precise. Therefore, we can accurately predict what will happen in our coastal cities 50 years from now.” It concludes with Eva Peral, Radar Digital Systems Group Supervisor at NASA Jet Propulsion Laboratory, saying, “Without really understanding the earth, we cannot protect it because we know that the missions that we work on are going to have an impact on our children and grandchildren.”