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

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Early tomorrow morning, a new Earth-monitoring instrument, the Surface Water and Ocean Topography mission, or SWOT, will launch into space on a SpaceX rocket. The satellite will conduct the first global survey of Earth’s freshwater systems from space, observing not only the oceans, but also lakes, rivers and coastal areas. It will be able to measure the height of the 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 adviser at NASA, said at a news conference.

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

“It will help us understand where water is, where it comes from and where it goes”

“At the moment we can see fairly well with satellite images where rivers and lakes are located. We can see their area quite well,” said Tamlin Pavelsky, chief of SWOT hydrology science at the University of North Carolina. “But we’re not doing nearly as well in terms of our ability to see the height of the water in it.”

Once SWOT begins its observations, researchers can collect data on water height and area, giving them a more complete picture of the volumes of water found in a given location. By measuring that volume over time, they can see the dynamic nature of water systems. “We will be able to see how the volume of lakes and reservoirs increases and decreases over time. And for rivers, we can track the amount of water flowing through rivers from space,” Pavelsky said.

SWOT will focus on larger bodies of water. It will be able to monitor lakes larger than 15 hectares and rivers wider than 100 meters. That includes millions of lakes and millions of miles of rivers around the world, the data of which will be publicly available. Currently, most rivers are monitored using ground-based meters, which can provide more regular measurements, but have the disadvantage of being expensive to install and not available in all locations.

Environmental scientists already use satellite data to monitor the water in lakes and rivers, but they’re working with satellites that aren’t designed for that. SWOT is specifically designed to measure water height using instruments such as the Ka-band Radar Interferometer or KaRIn.

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

An important feature of KaRIn is its size. The antennas are located at the ends of two 5-meter booms on either side of the SWOT spacecraft. Because they are widely spaced, they can be used to view larger areas of the Earth’s surface, allowing the instrument to take measurements of large areas of the world more quickly. The resolution of the SWOT tools will be 10 times higher than current technologies, providing more detailed data and covering more targets.

“This matters a lot whether you think of a really ecologically sensitive lake, or if you think of a lake in a rural part of India where people depend on that water to irrigate their crops,” Pavelsky said. “SWOT is going to provide the free and open data everyone needs to track these really important resources.”

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