To Measure Earth’s Water, NASA Satellite Relies on a Ka-band Radar Interferometer

December 26, 2022 by Aaron Carman

Using radar technology onboard a satellite, NASA’s latest launch is mapping the planet’s water supply and helping combat climate change.

NASA has successfully launched the Surface Water and Ocean Topography (SWOT) mission, a low-earth orbit satellite built to measure the interactions of the Earth’s oceans, rivers, and lakes. NASA says this feat represents a “quantum leap” in progressing knowledge of the Earth’s water supply. Working among the subsystems and instruments onboard the payload is the Ka-band Radar Interferometer (KaRIn): a new instrument capable of measuring the water level with up to a centimeter of accuracy, all from space.


A Falcon 9 rocket transports the SWOT craft and onboard KaRIn instrument

A Falcon 9 rocket transports the SWOT craft and onboard KaRIn instrument into low-earth orbit, where it will undergo a calibration phase before collecting data. Image used courtesy of NASA JPL


The SWOT mission is not the first time NASA has shown interest in characterizing the Earth in an effort to combat climate change. Earlier this year, the EMIT mission was launched to determine the mineral content of dust clouds. The SWOT mission does, however, represent a major advancement in effective ranging from space and developing a planetary-level climate model. In this article, we'll discuss the technology behind the SWOT mission and KaRIn and how it gives climate scientists a new tool for understanding the planet.


New Data Every 21 Days

When it comes to developing a model for climate change, scientists need accurate data on a planetary scale. Several key indicators such as global tide levels are required, and oftentimes the lack of information can lead to an inaccurate model. This becomes especially true for systems involving freshwater, where fewer traditional resources are allocated to measurements.


An image of the KaRIn instrument’s working principle

The animation illustrates the method that the SWOT mission uses to measure water levels across the globe. The 21-day repeat orbit ensures that scientists always have the latest information. Animated image used courtesy of NASA JPL


To increase data availability, NASA launched a joint effort with the French space agency CNES to survey the planet with a considerably broader view. Following a successful launch, the spacecraft is currently in a six-month calibration and validation phase, after which it will begin sending over one terabyte of data per day. In addition to the massive amount of data, the SWOT mission’s 21-day repeat orbit ensures that new data can constantly be gathered.


Using Radio Waves to Measure Sea Levels

One of the greatest engineering feats onboard the SWOT spacecraft, the KaRIn uses Ka-band (26.5 GHz–40 GHz) radio waves to detect the distance from the craft to the surface with up to 1 cm of resolution. The instrument closely resembles synthetic aperture radar (SAR), which uses the motion of the transceiver to simulate a large antenna with a narrow beamwidth.


An image of the KaRIn instrument’s working principle

An image of the KaRIn instrument’s working principle shows two swaths being illuminated by the satellite. Topography for both oceans and freshwater systems can be characterized using the information produced by KaRIn and the satellite. Image used courtesy of NASA JPL


This narrow beamwidth, combined with the stereoscopic vision provided by two antennas, allows the instrument to accurately determine the distance to the surface, which in turn is used to determine the water level at one point. By repeating this measurement over time and using existing methods to determine the satellite’s orbiting altitude, an accurate model of the water’s motion may be drawn to identify currents, eddies, or changes in water level.


Learning More About the Blue Planet

As the SWOT mission continues and the spacecraft begins collecting and transmitting data, it will be interesting to see if any issues arise with the craft or the KaRIn instrument. The signal processing, for example, is quite cumbersome for SAR and could introduce points of error over time or prove to be a source of inefficiency. In addition, the high level of integration may introduce numerous points of failure if one subsystem begins to fail.

Despite the fact that it may be too early to call the entire mission a perfect success, the advances made by NASA and CNES are a remarkable feat of engineering, and the data produced by SWOT is expected to be invaluable for the future of climate science.