Latest International Water Satellite Packs an Engineering Punch
Meet the scientific heart of the Surface Water and Ocean Topography mission, which will see Earth’s water in higher definition than ever before.
Set for a Thursday, Dec. 15 launch, the Surface Water and Ocean Topography (SWOT) satellite promises to provide an extraordinary accounting of water over much of Earth’s surface. Its measurements of fresh water and the ocean will help researchers address some of the most pressing climate questions of our time and help communities prepare for a warming world. Making this possible is a scientific instrument called the Ka-band Radar Interferometer (KaRIn).
Years in development, the instrument has been designed to capture very precise measurements of the height of water in Earth’s freshwater bodies and the ocean. KaRIn will measure the height of water in the ocean, “seeing” features like currents and eddies that are less than 13 miles (20 kilometers) across – up to 10 times smaller than those detectable with other sea level satellites. It will also collect data on lakes and reservoirs larger than 15 acres (62,500 square meters) and rivers wider than 330 feet (100 meters) across.
“For freshwater, this will be a quantum leap in terms of our knowledge,” said Daniel Esteban-Fernandez, KaRIn instrument manager at NASA’s Jet Propulsion Laboratory in Southern California. For example, researchers currently have good data on only a few thousand lakes around the world; SWOT will increase that number to at least a million.
The cutting-edge KaRIn instrument lies at the heart of this international mission, the latest in a longstanding collaboration between NASA and the French space agency Centre National d’Études Spatiales (CNES), with contributions from the Canadian Space Agency (CSA) and the UK Space Agency.
A Bigger Picture
Until now, researchers looking to study a body of water relied on instruments that measure at specific locations – like gauges in rivers or the ocean – or that are space-based, gathering data along narrow “tracks” of Earth they can see from orbit. Researchers then have to extrapolate if they want a broader idea of what’s happening in a water body.
KaRIn is different. The radar instrument uses the Ka-band frequency at the microwave end of the electromagnetic spectrum to penetrate cloud cover and the dark of night. As a result, it can take measurements regardless of weather or time of day. The instrument configuration consists of one antenna at each end of a boom that’s 33 feet (10 meters) long. By bouncing radar pulses off the water’s surface and receiving the return signal with both antennas, KaRIn will collect data along a swath 30 miles (50 kilometers) wide on either side of the satellite. “With KaRIn data, we’ll be able to actually see what’s happening, rather than relying on these extrapolations,” said Tamlin Pavelsky, the NASA freshwater science lead for SWOT, based at the University of North Carolina, Chapel Hill.
The two KaRIn antennas will see the same spot on Earth from 553 miles (890 kilometers) above. Since the antennas look at a given point on Earth from two directions, the return signals reflected back to the satellite arrive at each antenna slightly out of step, or phase, with one another. Using this phase difference, the distance between the two antennas, and the radar wavelength, researchers can calculate the height of the water that KaRIn is looking at.
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