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Summary
LUnar Spectral Irradiance (LUSI) is a project to measure the light from the Moon for use as an on-orbit reference standard suitable for radiometric calibration of Earth-observing satellite sensors.
Description
Earth observing satellite sensors were once the exclusive domain of large satellite platforms used by government agencies for the purpose of national security, defense, weather prediction and disaster mitigation. These satellites typically carry onboard radiometric calibrators that along with pre-launch testing, improve the quality of the radiometric information gathered by the instruments, but add size, mass and power requirements. However, both the government and private sector are launching a new generation of small satellites and cubesats that lack the payload capacity and budget to carry onboard calibrators, but would still benefit from on-orbit radiometric calibration. In addition to the benefit of calibrating each sensor individually, calibration also allows the direct comparison of imagery from different sensors now and into the future.
The Moon has a highly stable reflectance that allows its irradiance to be predicted at any time from a model, given knowledge of the position of the Sun, the Moon, and the observer. (The solar irradiance that illuminates the Moon is sufficiently stable and is part of the model). To date, satellite sensors have successfully used the Moon as a stability reference for monitoring changes in instrument performance, usually degradation, with time. The Sea-viewing Wide Field-of-view Sensor (SeaWiFS), for example, has modeled slow degradation of its sensor bands at the 0.1 % level using many observations of the Moon acquired over a period of more than 10 years. However, existing lunar irradiance models do not have an absolute, SI-traceable radiometric scale. That is the problem this project is solving. Making a series of low-uncertainty, fiducial reference measurements of the lunar irradiance will enable the characterization necessary for the Moon to serve as an absolute radiometric standard for remote-sensing instruments.
NIST has developed an airborne system (Air-LUSI) for deployment on the NASA Airborne Science Program’s high-altitude ER-2 aircraft in collaboration with scientists at University of Maryland, Baltimore County, NASA Goddard, McMaster University, and U. S. Geological Survey. Air-LUSI can make measurements from above >90% of the earth’s atmosphere. The instrument can be calibrated against NIST standards before each flight to maintain high-accuracy measurements.
For information about the design of these instruments and their characterization, see Airborne Lunar Spectral Irradiance (air-LUSI) Instrument.