New instrument uses radar to measure what the eye can't see

Researchers at the University of Illinois Urbana-Champaign developed a new instrument to measure the density of the dust that kicks up when spacecraft thrusters interact with planetary surfaces as it lands. Because cameras and other optical equipment are blinded by dense dust clouds, the new instrument uses millimeter-wave radar in a new way to accurately measure the dust and debris.

“Other measurement techniques exist, but our instrument addresses a sort of ‘missing middle.’ It is applicable to particle clouds which are too dense for optical measurements but too thin for state-of-the-art opaque multiphase techniques like X-rays or MRI. It is also capable of several thousands of measurements per second,” said Nicolas Rasmont, Ph.D. student in the Department of Aerospace Engineering. “Our instrument uses a radar to generate waves with a wavelength of 3.8 mm, just over an eighth of an inch. The wave travels through a cloud of particles, then is reflected, then captured back by our instrument to detect the presence of the particles.”

Rasmont said it starts with knowing that the speed of light is fastest in a vacuum and moves more slowly when it interacts with matter, like glass, water, or a cloud of dust. He credits German physicist Gustav Mie for developing the equation to calculate how light behaves when it encounters a particle.

“Because millimeter-waves are much larger than dust grains, they travel through the cloud unaffected, making it transparent to the radar,” Rasmont said. “Visible light, with a wavelength much smaller than dust grains, is rapidly scattered and absorbed, which makes the cloud opaque.”

“With the radar emitter and receiver, we can time the light traveling between two points with high accuracy. The particles slow the millimeter-waves slightly, which means that there is a small difference in time-of-flight between a wave traveling through a cloud of particles against a wave traveling in vacuum. That small difference in time-of-flight can be measured by comparing the two waves together, using a technique called interferometry.”

The instrument is called Radar Interferometry For Landing Ejecta. Rasmont said he first began developing RIFLE in 2020. “We had the idea of using millimeter waves and radar but had to work out the kinks,” he said. “Our first idea was to measure the absorption of the cloud: the more particles there are on the wave path, the more opaque the cloud is, and the weaker our radar signal should be.”

This initial concept had some problems. “During testing, we found that the cloud not only absorbs the waves, but it can also act as a lens, focusing the waves toward the radar. Counter-intuitively, a denser cloud can lead to a stronger signal, which makes absorption measurements quite inaccurate. This is when we got the idea of using interferometry for measuring dust instead of absorption.”

He said although some variants of radar interferometry are used in remote sensing to detect small terrain movements, his research group is likely the first to apply it to dense clouds of particles for fluid dynamics.

“We found that it was a very accurate way of measuring particles concentrations. From there, it was a rush to develop the first and second round of prototypes and then the final iteration, which is what we have right now and is being applied for a patent.”

To be certain their instrument worked, rather than relying purely on the formula to calculate the wave time-of-flight, they conducted an experiment to measure the time of flight between the emitter and the receiver for a known obstruction using an optical technique.

“We used a slot funnel to generate a thin curtain of known concentrations of dust, a camera, and a light source,” Rasmont said. “The camera can see the shadows of the particles. And if you have high enough magnification, you can see individual particles and count their shadows. Knowing the concentration for a single curtain of dust, we could add more curtains, and have a cloud of known size and known concentration to calibrate the radar.

“While optical techniques for measuring particles rely on knowing the size of particles, ours does not,” Rasmont said. “And optical techniques only measure thousands to millions of particles per cubic meter. In real life, you need to measure billions to trillions—like the very dense plumes of dust in a lunar landing.”

Rasmont said this work was supported by NASA through a Future Investigator in Earth and Space Science and Technology (FINESST) fellowship. For a lunar application, the instrument could be mounted between the legs of the spacecraft, or deployed in the descent process so it could gather the plume surface interaction data before the spacecraft lands.

Rasmont said RIFLE has a wide range of applications outside of lunar landings. “Many industrial processes involve mixing or carrying powders, like flour, grain, catalysts, coal, or other chemicals, with a fluid. To control and understand those processes, you want to measure the particle concentration of the mixture, which is exactly what our instrument provides.”

The study, “Millimeter-Wave Interferometry for Opaque Particle-Laden Flows,” by Nicolas Rasmont, Hussein Tatal Al-Rashdan, and aerospace faculty Gregory Elliott, Joshua Lucas Rovey, and Laura Villafañe Roca, is published in IEEE Transactions on Microwave Theory and Techniques. DOI: 10.1109/TMTT.2023.3277527