LUnar Spectral Irradiance (LUSI) is a program to radiometrically characterize the Moon for use as an on-orbit reference standard suitable for satellite instrumentation used to study and monitor the Earth's climate. Establishing the Moon as a low-uncertainty, SI-traceable radiometric standard will ensure the reliability of measurements for long-term climate studies.
The desire to monitor and understand changes in the Earth's climate has led to increasingly challenging radiometric accuracy requirements for space-based remote sensing instruments. Low-uncertainty measurements are needed both to refine climate models and to monitor changes over decadal periods of time that span the collective lifetimes of many instruments. While low-uncertainty laboratory calibration prior to launch is possible, changes incurred from the launch and operating in the harsh environment of space can be significant contributors to measurement uncertainty. Despite efforts to mitigate these uncertainties, on-orbit inter-comparison of remote sensing instruments show significant disagreements.
Without a reliable, common radiometric scale, detecting small, long-term climatic changes is limited by the construction of composite data sets from different sensors that overlap in time and view a common transient scene. This approach is subject to many problems: accumulating errors from the instrument cross-calibrations (e.g. mismatches in sensor characteristics and observation time), uncertainty due to radiometric drift, and the risk of a gap in the data set from either malfunctioning sensors, launch failure or program delays.
At present the uncertainty of the absolute lunar spectral irradiance is still too high to resolve differences in the calibration of satellite radiometer instruments, or to use the Moon as an absolute or a cross calibration reference at the level needed for climate monitoring.
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, a spectrally resolved model that allows trending at the 0.1 % level absent many years of observation over limited phase angles and assumptions about sensor degradation is currently unavailable.
NIST has developed a ground-based system to measure the lunar spectral irradiance that is scheduled to begin continuous operation from the NOAA Mauna Loa Observatory in the second half of 2020. An airborne system (Air-LUSI) for deployment on NASA’s high-altitude ER-2 aircraft was developed in collaboration with scientists at University of Maryland, Baltimore County, NASA Goddard, University of Guelph and USGS. Air-LUSI can make measurements from above >90% of the earth’s atmosphere. Both instrument can be periodically calibrated against NIST standards to maintain high accuracy measurements.
In combination with a lunar model that accounts for the effects of lunar libration and phase angle of the space-based remote sensing instruments, these low-uncertainty measurements will enable the characterization necessary for the Moon to serve as an absolute radiometric standard for remote-sensing instruments.
For information about the design of these instruments and their characterization, see LUSI at MLO and Air-LUSI.