Moon-seeking sensor helps earth sensors

Moon-seeking sensor helps earth sensors
Air-LUSI takes off aboard an ER2 out of NASA’s Armstrong Flight Research Center in Palmdale, California, for an airborne campaign to measure the Moon. (Ken Ulbrich, NASA)

An instrument aboard a high-altitude NASA plane is using measurements of the Moon’s brightness to help Earth observing sensors make more accurate measurements, according to an article on by Elizabeth Goldbaum of NASA’s Goddard Space Flight Center.

The airborne Lunar Spectral Irradiance Instrument (air-LUSI) is flying aboard NASA's ER-2 airplane at altitudes of 70,000 feet above ground. The flights are occurring at night to avoid scattered light from the Sun, between November 13 through 17 from NASA's Armstrong Flight Research Center in Palmdale, California.

The NASA-funded instrument is "measuring how much sunlight is reflected by the Moon at various phases in order to accurately characterize it and expand how the Moon is used to calibrate Earth observing sensors," said Kevin Turpie, a professor at the University of Maryland, who is leading the air-LUSI effort. The project is funded by NASA's Earth Science Division and the National Institute of Standards and Technology (NIST).

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Earth-observing sensors, like the Visible Infrared Imaging Radiometric Suite (VIIRS) aboard the NASA/NOAA/DOD Suomi National Polar-orbiting Partnership satellite and the NOAA-20 meteorological satellite, collect images of cloud cover, land surface cover and ocean color. These sensors have to withstand high-energy particles and ultraviolet light, which can degrade their performance over time.

To account for any changes in sensitivity, VIIRS and other satellite instruments calibrate their sensors by looking at a known reference and comparing how the most recent look compares to previous ones. If the sensor sees the reference differently than before, it knows it needs to recalibrate or adjust its sensitivity.

Currently, many instruments carry an opaque or white material, called a diffuser, that reflects sunlight and acts as a reference for sensor calibration. The Moon is considered an ideal diffuser since its reflectance of sunlight is stable and more similar to Earth's in brightness.

Scientists have long known about the Moon's potential. "Not long after the Apollo program, a group at the U.S. Geological Survey (USGS) developed a way of characterizing the Moon so that Earth observing satellites could use it for calibration," Turpie said.

Air-LUSI's instruments are able to obtain highly accurate lunar spectral irradiance measurements that will have the lowest ever uncertainty (less than 1%), Turpie said, which establishes the Moon as an absolute calibration reference and helps remote sensing scientists determine if Earth observing sensors, like VIIRS, are recording actual changes on Earth or changes in their instruments.

Although Earth observing missions can look at the Moon at the same time and phase every month as a way to notice trends in their instruments' sensitivity, they haven't yet been able to use the Moon as an absolute calibration reference, said Kurt Thome, a project scientist for Earth observing missions at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

The instrument has three subsystems. The first component is called IRIS, short for Irradiance Instrument Subsystem, and was designed by NIST. It includes an instrument able to take precise measurements of the Moon while sitting in a temperature and pressure-controlled enclosure.

The second component is a robotic telescope mount called ARTEMIS (Autonomous, Robotic Telescope Mount Instrument Subsystem) designed and built by the University of Guelph. ARTEMIS has a camera that scans the sky until it finds the Moon and directs the telescope to point at it and keep it locked in place, regardless of aircraft motion.

The final component is the High-altitude ER-2 Adaptation, or HERA. HERA includes all the connective tissue, like cables and mounting equipment, which holds the instrument together and to the plane, as well as the thermal stabilizing components. Air-LUSI is able to record data during flight and download the data from the plane to the ground.

In the near future, an operational weather satellite would benefit from being able to look to the Moon as an absolute calibration reference, Thome said. This includes the currently-flying Suomi National Polar-orbiting Partnership (Suomi NPP) and Joint Polar Satellite System-20 (JPSS) satellites, as well as those to come in the future from both NOAA and their international partners, according to Thome.

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