Abstract
The Moon is a very useful calibration target for Earth-observing sensors in orbit because its surface is radiometrically stable and it has a radiant flux comparable to Earth scenes. To predict the lunar irradiance given an illumination and viewing geometry, the United States Geological Survey (USGS) has developed the Robotic Lunar Observatory (ROLO) Model of exo-atmospheric lunar spectral irradiance. The USGS ROLO model represents the current most precise knowledge of lunar spectral irradiance and is used frequently as a relative calibration standard by space-borne Earth-observing sensors. However, instrument calibration teams have expressed the need for an absolute lunar reference with higher accuracy.
The objective of the airborne LUnar Spectral Irradiance (air-LUSI) mission is to make highly accurate, SI-traceable measurements of lunar spectral irradiance in the VNIR spectral region from NASA’s high-altitude ER-2 aircraft. To that end, the air-LUSI system employs an autonomous, robotic telescope system that tracks the Moon in flight, and a stable spectrometer housed in an enclosure providing a robustly controlled environment. These instrument subsystems are situated in a wing pod of the ER-2 aircraft with a small dorsal view port. Through this port, the telescope can observe the Moon from above 95% of the Earth’s atmosphere.
air-LUSI successfully conducted a Demonstration Flight Campaign on five consecutive nights from 12 to 17 November 2019. Each night, the air-LUSI system observed the Moon at about 68,000 feet altitude. Each observation period lasted 30 to 40 minutes and measured the lunar spectral irradiance at wavelengths from about 380 to 1000 nm. The five flights corresponded to lunar phase angles of 10°, 21°, 34°, 46° and 59°. The measurement uncertainty is currently estimated to be about 0.8% or less through the mid-visible range. With this new capability, the air-LUSI team plans to acquire additional lunar spectral irradiance measurements and apply this state-of-the-art data set to improve the accuracy of ROLO predictions. This paper will summarize the air-LUSI objectives and provide an overview of the Demonstration Flight Campaign and lessons learned that could further improve air-LUSI accuracy in future flights.
Airborne Lunar Spectral Irradiance (air-LUSI) Missioni Capability Demonstration
The Moon is a very useful calibration target for Earth-observing sensors in orbit because its surface is radiometrically stable and it has a radiant flux comparable to Earth scenes. To predict the lunar irradiance given an illumination and viewing geometry, the United States Geological Survey (USGS) has developed the Robotic Lunar Observatory (ROLO) Model of exo-atmospheric lunar spectral irradiance. The USGS ROLO model represents the current most precise knowledge of lunar spectral irradiance and is used frequently as a relative calibration standard by space-borne Earth-observing sensors. However, instrument calibration teams have expressed the need for an absolute lunar reference with higher accuracy.
The objective of the airborne LUnar Spectral Irradiance (air-LUSI) mission is to make highly accurate, SI-traceable measurements of lunar spectral irradiance in the VNIR spectral region from NASA’s high-altitude ER-2 aircraft. To that end, the air-LUSI system employs an autonomous, robotic telescope system that tracks the Moon in flight, and a stable spectrometer housed in an enclosure providing a robustly controlled environment. These instrument subsystems are situated in a wing pod of the ER-2 aircraft with a small dorsal view port. Through this port, the telescope can observe the Moon from above 95% of the Earth’s atmosphere.
air-LUSI successfully conducted a Demonstration Flight Campaign on five consecutive nights from 12 to 17 November 2019. Each night, the air-LUSI system observed the Moon at about 68,000 feet altitude. Each observation period lasted 30 to 40 minutes and measured the lunar spectral irradiance at wavelengths from about 380 to 1000 nm. The five flights corresponded to lunar phase angles of 10°, 21°, 34°, 46° and 59°. The measurement uncertainty is currently estimated to be about 0.8% or less through the mid-visible range. With this new capability, the air-LUSI team plans to acquire additional lunar spectral irradiance measurements and apply this state-of-the-art data set to improve the accuracy of ROLO predictions. This paper will summarize the air-LUSI objectives and provide an overview of the Demonstration Flight Campaign and lessons learned that could further improve air-LUSI accuracy in future flights.