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New Measurements of the Lunar Spectral Irradiance for Satellite Calibration

S. Maxwell and J.T. Woodward

National Institute of Standards and Technology (NIST), Gaithersburg, MD 20880; 301-975-4950, E-mail: stephen.maxwell@nist.gov

Precise radiometric calibration of optical earth observing satellites enables improved understanding of the earth system, better long-term climate data records, and mitigation against gaps in coverage. While there are many techniques to calibrate earth observing satellites, both pre-launch and on-orbit, there are none currently in use that can deliver calibration uncertainties under 1% in the visible and near infrared (VNIR). 

To enable on-orbit calibrations at this level, the National Institute of Standards and Technology (NIST) is working on obtaining new, high-precision measurements of the spectral irradiance of the moon in the VNIR and short-wave infrared ranges. The moon’s brightness approximates that of an earth scene, can be easily observed by many satellites (indeed it is incidentally observed by many geostationary instruments), and has a stable reflectance that has been estimated to change by less than 10-8 per year. Further, it is already in use by dozens of satellite programs for relative calibrations both in the U.S. and internationally, using the highly successful U.S. Geological Survey (USGS) Robotic Lunar Observatory Model and its derivatives. 

The new lunar measurements are being taken through two programs. One NIST program has taken measurements from Mauna Loa Observatory (MLO) in 2017 and 2019, and intends to begin a multi-year measurement campaign later in 2021 at MLO. The program will run until enough of the 18.6 year Sun-Moon-Earth configuration cycle has been sampled to constrain a redeveloped lunar model. The other program operates jointly with NASA, the United States Geological Survey (USGS), the University of Maryland Baltimore County, and the University of Guelph to put a similar lunar-observing instrument on NASA’s ER-2 to make a smaller number of measurements above 95% of the atmosphere. These measurements, and possibly others, will then be used both to develop a new model of the lunar spectral irradiance and to update existing models.

This talk will describe the measurement programs, what the measurement goals are, and how the programs fit into the context of concurrent global efforts.

Figure 1

Figure 1. The NIST Lunar Spectral Irradiance Instrument at Mauna Loa Observatory, Hawaii (MLO-LUSI) observing moonrise in December 2017. 

Figure 2

Figure 2. The Airborne Lunar Spectral Irradiance Instrument (air-LUSI) being loaded into the wing pod of NASA's ER-w in November 2019.