NASA Uses Moonlight to Improve Satellite Accuracy – Watts Up With That?

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NASA’s Airborne Lunar Spectral Irradiation, or air-LUSI, flew aboard NASA’s ER-2 aircraft from March 12-16 to precisely measure the amount of light reflected off the Moon. Reflected moonlight is a constant source of light that researchers are harnessing to improve the accuracy and consistency of measurements between Earth-observing satellites.

“The Moon is extremely stable and is not greatly influenced by factors on Earth such as weather. It becomes a very good calibration reference, an independent reference point, by which we can set up our instruments and see what happens to our planet,” said air-LUSI principal investigator Kevin Turpie, a research professor at the University of of Maryland, College Park.

The air-LUSI flights are part of NASA’s comprehensive satellite calibration and validation efforts. The results will complement ground-based sites like Railroad Valley Playa in Nevada, and together provide orbiting satellites with a robust set of calibration data.

NASA has more than 20 Earth-observing satellites that give researchers a global perspective of the interconnected Earth system. Many of them measure light waves reflected, scattered, absorbed, or emitted by the Earth’s surface, water, and atmosphere. This light includes visible light, which humans see, as well as invisible ultraviolet and infrared wavelengths, and everything in between. Like musical instruments in an orchestra, individual satellite instruments must be “tuned” to each other so that researchers can get the most out of their data. By using the Moon as a “tuning fork,” scientists can more easily compare data from different satellites to look at global changes over long periods of time.

This electromagnetic spectrum shows how energy travels in waves; Humans can only see visible light, but NASA instruments use the full spectrum to observe Earth and more. Credits: NASA

That’s where air-LUSI comes in. Developed in collaboration with the National Institute of Standards and Technology (NIST), the US Geological Survey, and McMaster University, air-LUSI is a telescope that measures how much light is reflected from the lunar surface to assess the amount of energy that Earth observation satellites receive from the light of the moon. It was mounted aboard the managed ER-2 aircraft and was flying from NASA’s Armstrong Flight Research Center in Palmdale, California. The ER-2 is a high-altitude aircraft flown at 70,000 feet, above 95% of the atmosphere, that can scatter or absorb reflected sunlight. This allowed air-LUSI to collect very precise NIST-traceable measurements that are analogous to what a satellite would do from orbit. To improve the accuracy of lunar reflectance models, air-LUSI measurements are accurate to less than 1% uncertainty. During the March flights, air-LUSI measured the Moon for four nights just before full Moon.

This airborne approach has the advantage of studying the moonlight during the different phases of the Moon, while also being able to bring the instrument back between flights for evaluation, maintenance and, if necessary, repair.

The air-LUSI cylindrical telescope is positioned to measure a simulated Moon at the other end of a laboratory for testing and calibration before and after the flight campaign.
The air-LUSI telescope is shown positioned to measure a simulated Moon in a laboratory for testing and calibration before and after the flight campaign. Credits: Kevin Turpie

Making improvements for better accuracy

The air-LUSI spectrometer is hermetically sealed within an enclosure that keeps the instrument constantly at sea level temperature and pressure. The light collected by a telescope enters an integrating sphere that directs the light to the spectrometer, which is an instrument that measures variations in light waves. The air-LUSI first flew similar flights in November 2019. Since then, the air-LUSI team has continued to improve the accuracy of the instrument.

The team improved the internal monitor to better verify the instrument’s accuracy over a larger range of wavelengths, from ultraviolet to near-infrared. They were also able to redesign the integrating sphere to remove the small effects of temperature changes.

“This will help the instrument make measurements at the greater than 99% accuracy levels that we are looking for,” Turpie said.

Making these changes was a challenge. Delays from the COVID-19 pandemic prompted the chief engineer, who was working on instrument upgrades and repairs, to develop a new remote work plan. Both he and the principal investigator received special permission to have parts delivered directly to their homes so they could work on the instrument and be prepared for the 2022 flights.

Using the Moon as a common standard

The 2019 and 2022 data together have the potential to help scientists make data from Earth-observing satellites in the ultraviolet to near-infrared range more consistent. In addition, the common Moon standard would make it easier to compare and fine-tune current and future satellite observations. NASA’s upcoming Plankton, Aerosol, Cloud, Ocean Ecosystem (PACE) mission plans to use the Moon as a common reference point to make its observations more precise and interconsistent with other satellite measurements of Earth. Over the next decade, PACE and future orbiting sensors at NASA’s Earth System Observatory will help create a more cohesive picture of our planet.

“Having a common calibration source outside of Earth will help us achieve this goal,” Turpie said. “Once Air-LUSI measurements are used to improve the accuracy of the total amount of light coming from the Moon, we will be able to take much more accurate measurements of Earth using current and future space observatories.”

Banner image: NASA’s ER-2 aircraft is shown ready to refuel and prepare for flight. Photo Credit: Ken Ulrich

By Abby Graff
NASA Earth Science News Team

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