A brand new instrument with its eye on the Moon is taking off aboard a high-altitude NASA aircraft to measure the Moon’s brightness and finally assist Earth observing sensors make extra correct measurements.
The airborne Lunar Spectral Irradiance Instrument (air-LUSI) is flying aboard NASA’s ER-2 airplane. The ER-2 is in a position to soar above clouds, about 70,000 toes above floor. The flights, which happen at night time to keep away from scattered gentle from the Solar, started November 13 and can wrap up November 17 from NASA’s Armstrong Flight Analysis Heart 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,” mentioned Kevin Turpie, a professor on the College of Maryland, Baltimore County, main the air-LUSI effort. Turpie and his group are funded by NASA’s Earth Science Division and the Nationwide Institute of Requirements and Expertise (NIST).
How the Moon helps Earth sensors
Earth-observing sensors, just like the Seen Infrared Imaging Radiometric Suite (VIIRS) aboard the NASA/NOAA/DOD Suomi Nationwide Polar-orbiting Partnership satellite tv for pc and the NOAA-20 meteorological satellite tv for pc, acquire photographs of cloud cowl, land floor cowl, and ocean colour. Whereas these sensors are diligently doing their jobs, in addition they have to brace in opposition to high-energy particles and stand up to ultraviolet gentle, which degrade their sensors over time.
To account for any adjustments in sensitivity, VIIRS and different satellite tv for pc devices calibrate their sensors by taking a look at a identified reference and evaluating how the newest look compares to earlier ones. If the sensor sees the reference otherwise than earlier than, it is aware of it wants to recalibrate or alter its sensitivity.
At present, many devices carry an opaque or white materials, referred to as a diffuser, that displays daylight and acts as a reference for sensor calibration. Nonetheless, though the Solar offers a gradual output, its harsh rays degrade the diffuser over time. The Moon, alternatively, is a perfect diffuser since its reflectance of daylight is steady and extra related to Earth’s in brightness.
Scientists have lengthy identified in regards to 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 mentioned.
The USGS Robotic Lunar Observatory (ROLO) in Flagstaff, Arizona, measured the Moon on a month-to-month foundation from 1995 to 2003. Tom Stone, a scientist at USGS, together with Hugh Kieffer, a former scientist with USGS, developed a ROLO-based mannequin that has and continues to be used to assist calibrate Earth observing sensors in devices, like SeaWiFS, which operated from 1997 to 2010 and measured ocean colour to monitor phytoplankton. SeaWiFS regarded on the Moon frequently to observe any adjustments in its instrument’s sensitivity.
Though a variety of Earth observing mission calibration groups use ROLO, there will be giant discrepancies of their lunar calibration information, Stone mentioned. The hope is that air-LUSI’s extremely correct measurements will characterize these discrepancies and decide in the event that they’re brought on by inside biases within the ROLO mannequin or one thing else. “We can’t validate ROLO calibrations to any better than 5%,” Stone mentioned. “Air-LUSI can improve ROLO or determine what needs to be improved.”
Air-LUSI’s novel devices are in a position to acquire extremely correct lunar spectral irradiance measurements that can have the bottom ever uncertainty (lower than 1%), Turpie mentioned, which establishes the Moon as an absolute calibration reference and helps distant sensing scientists decide if Earth observing sensors, like VIIRS, are recording precise adjustments on Earth or adjustments of their devices.
Though Earth observing missions can take a look at the Moon on the similar time and part each month as a method to discover traits of their devices’ sensitivity, they haven’t but been in a position to use the Moon as an absolute calibration reference, Kurt Thome, a undertaking scientist for Earth observing missions at NASA’s Goddard House Flight Heart in Greenbelt, Maryland, mentioned.
What does it imply to be an absolute calibration reference? In case you evaluate two individuals standing subsequent to one another, it’s simple to see which particular person is taller. Nonetheless, if these two individuals are at reverse ends of the world, the one method to evaluate their heights can be with an absolute reference, like a ruler. Air-LUSI is aiming to make the Moon an absolute calibration reference, which suggests an instrument would solely want to take a look at the Moon as soon as to decide the instrument’s absolute sensitivity, whereas evaluating appears over time to see if the instrument is altering, Thome mentioned.
A collaborative effort
To assemble details about the Moon, air-LUSI contains three subsystems, which require experience from a number of organizations, mentioned Turpie. His group contains individuals from NIST, the USGS, the College of Guelph in Ontario, Canada and NASA.
The primary element is known as IRIS, quick for Irradiance Instrument Subsystem, and was designed by NIST. It contains an instrument in a position to take exact measurements of the Moon whereas sitting in a temperature and pressure-controlled enclosure.
The second element is a robotic telescope mount referred to as ARTEMIS (Autonomous, Robotic Telescope Mount Instrument Subsystem) designed and constructed by the College of Guelph. ARTEMIS has a digicam that scans the sky till it finds the Moon and directs the telescope to level at it and preserve it locked in place, no matter plane movement.
The ultimate element is the Excessive-altitude ER-2 Adaptation, or HERA. HERA contains all of the connective tissue, like cables and mounting gear, which holds the instrument collectively and to the aircraft, in addition to the thermal stabilizing elements. Air-LUSI is in a position to document information throughout flight and obtain the info from the aircraft to the bottom.
One small step for air-LUSI, one large leap for Earth science
Within the close to future, an operational climate satellite tv for pc would profit from having the ability to look to the Moon as an absolute calibration reference, Thome mentioned. This contains the currently-flying Suomi Nationwide Polar-orbiting Partnership (Suomi NPP) and Joint Polar Satellite tv for pc System-20 (JPSS) satellites, in addition to these to come sooner or later from each NOAA and their worldwide companions. Every satellite tv for pc might calibrate its devices by the Moon to evaluate how its sensors are holding up to the opposite satellites’ sensors, Thome mentioned.
NASA’s upcoming Ocean Coloration Imager, aboard the Phytoplankton Aerosols Clouds and ocean Ecology (PACE) satellite tv for pc, additionally intends to use the Moon for calibration, Turpie mentioned.
“Air-LUSI’s Moon measurements make it easier for people to justify using the Moon to calibrate their instruments,” Thome mentioned.