Precise relative calibration of Earth-orbiting remote sensors, using the Moon

An observation strategy is described that uses the Moon to monitor the radiometric stability of Earthorbiting remote-sensing instruments without requiring precise lunar-radiance measurements over the full range of lunar cycles. The Moon can be used as a constant irradiance reference for monitoring on-orbit shifts in instrument responsivity if (i) the Moon is observed at equivalent phase angles, (ii) the Moon is observed at equivalent sensorMoon libration angles, (iii) the Moon is observed at equivalent Sun-Moon libration angles, and (iv) the Moon is observed at equivalent scan rates and aspect angles. Irradiance changes due to changes in Sun-to-Moon and Moon-to-sensor distance can be precisely computed from ephemerides using inverse square scaling. A preliminary model of libration effects being developed by the United States Geological Survey from Clementine lunar-orbiter measurements was used to evaluate the effects of lunar libration in the 400 nm to 1000 nm spectral region. If the satellite platform can be rotated to view the Moon from a position in the orbit that exactly provides the required lunar phase angle, lunar-irradiance levels for 48 of the 75 lunar cycles in the 1994-1999 epoch examined can be matched to within 0.1 %. All lunar cycles in the epoch have at least one match to within 0.27 %. A pixel integration procedure that minimizes calibration uncertainties due to sensor pointing and optics contamination is presented and evaluated for the NASA Earth Observing System (EOS) Moderate Resolution Imaging Spectroradiometer (MODIS). The total combined relative uncertainties between suitable lunar reference pairs are expected to be less than 1 %.