Abstract
The Moon is a very useful calibration target for orbiting Earth-observing sensors because its surface is radiometrically stable and it has a flux output comparable to Earth scenes.To predict the spectral lunar irradiance given 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, ROLO predictions are not traceable to the International System of Units (SI) .Consequently, the Moon is not currently used as an absolute standard. An SI-traceable, exo-atmospheric, lunar spectral irradiance with uncertainties less than 1% would meet many sensor calibration uncertainty requirements. Such measurements could also be used to quantify biases in land and ocean-based vicarious calibration approaches. For ocean color sensors for example, this would help quantify biases stemming from atmospheric correction in vicarious calibrations using NOAA’s Marine Optical Buoy. The objective of the airborne LUnar Spectral Irradiance (air-LUSI) mission is to make highly accurate (sub-0.5 % uncertainty), SI-traceable measurements of lunar spectral irradiance in the VNIR region from an aircraft at elevations nearing 70,000 feet.To that end, the air-LUSI system employs an autonomous, robotic telescope system and a stable spectrometer housed in a enclosure providing a robustly controlled environment. It is expected that measurements by the air-LUSI, corrected for residual atmospheric attenuation, can be used. The air-LUSI system was successfully integrated into a wing pod of a NASA ER-2 research aircraft in preparation for its maiden flight campaign, which was conducted in August 2018. These engineering flights confirmed the expected high level of performance of the air-LUSI subsystems. Lessons learned from the calibration methodology offer a path forward toward obtaining low uncertainty spectro-radiometric measurements, which will be demonstrated in upcoming flights. We present an overview of the mission, the performance of key subsystems during air-LUSI’s first flights, and offer an early look at what was learned from air-LUSI’s first measurements.
air-LUSI Measuring Lunar Spectral Irradiance from a High-Altitude Aircraft
The Moon is a very useful calibration target for orbiting Earth-observing sensors because its surface is radiometrically stable and it has a flux output comparable to Earth scenes.To predict the spectral lunar irradiance given 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, ROLO predictions are not traceable to the International System of Units (SI) .Consequently, the Moon is not currently used as an absolute standard. An SI-traceable, exo-atmospheric, lunar spectral irradiance with uncertainties less than 1% would meet many sensor calibration uncertainty requirements. Such measurements could also be used to quantify biases in land and ocean-based vicarious calibration approaches. For ocean color sensors for example, this would help quantify biases stemming from atmospheric correction in vicarious calibrations using NOAA’s Marine Optical Buoy. The objective of the airborne LUnar Spectral Irradiance (air-LUSI) mission is to make highly accurate (sub-0.5 % uncertainty), SI-traceable measurements of lunar spectral irradiance in the VNIR region from an aircraft at elevations nearing 70,000 feet.To that end, the air-LUSI system employs an autonomous, robotic telescope system and a stable spectrometer housed in a enclosure providing a robustly controlled environment. It is expected that measurements by the air-LUSI, corrected for residual atmospheric attenuation, can be used. The air-LUSI system was successfully integrated into a wing pod of a NASA ER-2 research aircraft in preparation for its maiden flight campaign, which was conducted in August 2018. These engineering flights confirmed the expected high level of performance of the air-LUSI subsystems. Lessons learned from the calibration methodology offer a path forward toward obtaining low uncertainty spectro-radiometric measurements, which will be demonstrated in upcoming flights. We present an overview of the mission, the performance of key subsystems during air-LUSI’s first flights, and offer an early look at what was learned from air-LUSI’s first measurements.