Use of Lunar Irradiance for Earth Climate Observations
Kurt Thome [kurt.thome@nasa.gov], NASA Goddard Space Flight Center, Greenbelt, MD
Lunar irradiance measurements have been used as a means for on-orbit calibration of
earth imaging sensors since the early 1990’s because of the invariant nature of the
moon’s surface. Characterization of lunar irradiance includes corrections for libration and
phase angle that improve the precision of lunar-based calibrations to levels of uncertainty
<0.1%. Absolute knowledge of the lunar irradiance is not as well known. Examples of
past use of the moon and current status of lunar calibration accuracy and precision are
described. The role of lunar irradiance in the development of climate-quality data sets is
covered using the Climate Absolute Radiance and Refractivity Observatory (CLARREO)
as an example. CLARREO is a climate-focused mission designed to observe climate
change on decadal time scales. Instrument approaches to meet CLARREO requirements
require factors of 3 to 10 improvements in benchmark accuracy over current
observations. The reflected solar instrument of CLARREO achieves the needed
improvements through coordinated measurements of solar and lunar irradiances. The sun
provides the ultimate reference for reflectance retrieval while the lunar irradiance
measurements act as a verification of the solar irradiance measurements. Lessons learned
in characterizing solar irradiance provide opportunities to improve the understanding of
absolute lunar irradiance.