“The moon is extremely stable and is not affected by things like climate on Earth to any significant extent. It becomes a very good calibration standard, an independent benchmark against which we can tune our instruments and see what’s happening to our planet,” said air-LUSI principal investigator Kevin Turpy, research professor at the University of Maryland at College Park.
Air-LUSI flights are part of NASA’s comprehensive calibration and verification effort. The results will complement ground sites such as Nevada’s Railroad Valley Playa and together provide orbiting satellites with a robust calibration data set.
NASA has more than 20 Earth observation satellites that give researchers a global perspective on 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 the visible light that humans see, as well as the invisible ultraviolet and infrared waves and everything in between. Like musical instruments in an orchestra, individual satellite instruments must be in tune with each other so that researchers can get the most out of their data.
By using the Moon as a fork, scientists can more easily compare data from different satellites to see global changes over long periods of time.
This is precisely the mission of Air-LUSI. Developed in partnership with the National Institute of Standards and Technology (NIST), the United States Geological Survey, and McMaster University, air-LUSI is a telescope that measures how much light is reflected off the lunar surface to estimate how much energy Earth-observing satellites receive from moonlight.
It was installed aboard an ER-2 aircraft operated by NASA’s Armstrong Flight Research Center in Palmdale, California. The ER-2 is a high-altitude aircraft that flew at an altitude of 21 km, above 95% of the atmosphere, which can scatter or absorb reflected sunlight.
This allowed Air-LUSI to collect very accurate, NIST-tracked measurements that are similar to those taken by a satellite from orbit.
To improve the accuracy of lunar reflection models, Air-LUSI measurements are accurate with less than 1% error. During the March flights, Air-LUSI took lunar measurements during the four nights immediately before the full moon.
The advantage of this airborne approach is that it is possible to study moonlight during the various phases of the moon and still return the instrument between flights for evaluation, maintenance and, if necessary, repairs.
The Air-LUSI Spectrometer is hermetically sealed in a housing that maintains the instrument at sea level temperature and pressure at all times.
The light collected by the telescope hits an integrating sphere, which directs the light to a spectrometer, a device that measures the fluctuations of light waves. air-LUSI first made such a flight in November 2019. Since then, the air-LUSI team has continued to improve the accuracy of the instrument.
The team has upgraded the internal monitor to be able to better check the instrument’s accuracy over a wider range of wavelengths, from ultraviolet to near infrared. They were also able to change the design of the integrating sphere to eliminate small effects of temperature change.
“This will help the instrument achieve the more than 99% accuracy we are aiming for,” Turpi said.
Making these changes was challenging. Delays due to the COVID-19 pandemic forced the chief engineer who was updating and repairing the instrument to develop a new plan for remote work.
He and the Principal Investigator received special permission to have the parts delivered directly to their homes so they can work on the instrument and be ready for flights in 2022.
Using the Moon as a Common Standard
Data from 2019 and 2022 together could help scientists make ultraviolet and near-infrared data from Earth observation satellites more consistent. In addition, a common lunar standard will make it easier to compare and fine-tune current and future satellite observations.
NASA’s upcoming Plankton, Aerosols, Clouds, and Ocean Ecosystems (PACE) mission plans to use the Moon as a common reference to make its observations more accurate and consistent with other Earth satellite measurements.
Over the next decade, PACE and NASA’s Earth System Observatory’s future orbital sensors will help create a more holistic picture of our planet.
“Having a common off-Earth calibration source will help us achieve this goal,” Turpi said. “Once Air-LUSI measurements are used to improve the accuracy of total light coming from the Moon, we will be able to make much more accurate measurements of the Earth with current and future space observatories.”
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