How NASA’s newest satellite will monitor lakes and rivers from space

Early tomorrow morning, a new Earth monitoring instrument called the Surface Water and Ocean Topography mission, or SWOT, will launch into space on a SpaceX rocket. The satellite will take the first global survey of Earth’s freshwater systems from space, observing not only the oceans but also lakes, rivers, and coastal regions. It will be able to measure the height of water in these systems for the first time.

“It will help us understand where water is, where it’s coming from, and where it’s going,” Katherine Calvin, chief scientist and senior climate advisor at NASA, explained in a press conference. 

Understanding Earth’s water and how it moves is crucial for both modeling climate change over the long term and dealing with urgent events like floods and droughts. As these events are driven by global ocean currents and weather events, a global view is required to be able to understand them. Even the satellite itself is international. NASA collaborated with France’s space agency (CNES) on SWOT, and Canada and the UK also worked on the satellite. 

“Right now with satellite imagery, we can see pretty well where rivers and lakes are located. We can see their area pretty well,” Tamlin Pavelsky, SWOT hydrology science lead at the University of North Carolina, said. “But we don’t do nearly so well in terms of our ability to see the height of the water in them.”

Once SWOT begins its observations, researchers will be able to gather data on the height of water as well as its area, giving them a more complete picture of the volumes of water found in a particular location. Measuring that volume over time lets them see the dynamic nature of water systems. “We’ll be able to see how the volume of lakes and reservoirs increases and decreases over time. And for rivers, we’ll be able to track the volume of water flowing through rivers from space,” Pavelsky said.

SWOT will focus on larger bodies of water. It will be able to monitor lakes that are larger than 15 acres and rivers wider than 330 feet. That will include millions of lakes and millions of miles of rivers around the globe, data on all of which will be publicly available. Currently, most rivers are monitored using ground-based gauges, which can take more regular readings but have disadvantages in that they are expensive to install and are not available in all locations. 

Environmental scientists already use satellite data to monitor water in lakes and rivers, but they are working with satellites not designed for this purpose. SWOT is specifically designed to measure water elevation using instruments like its Ka-band Radar Interferometer, or KaRIn. 

Radar interferometry works by sending out two sets of radar signals, which bounce off the Earth and back to the transmitter, then analyzing the interference of these two signals. One of these signals is sent on a slightly longer path, so once the two pings arrive back at the detector, researchers can look at the difference between the two and use that information to precisely calculate distance. That allows researchers to see details about the depth of a body of water.

One important feature of KaRIn is its size. Its antennae are located at the end of two 5-meter-long booms on either side of the SWOT spacecraft. As they are spread widely apart, they can be used to view larger areas of the Earth’s surface, letting the instrument take measurements from large parts of the globe more quickly. The resolution of SWOT’s instruments will be 10 times higher than current technologies, giving more detailed data as well as covering more targets.

“This matters a lot whether you’re thinking about a really ecological vulnerable lake, or if you’re thinking about a lake in a rural part of India where people depend on that water for irrigating their crops,” Pavelsky said. “SWOT is going to provide the free and open data that everyone needs in order to be able to track these really important resources.”