Intercomparison of Terra and Aqua MODIS using unscheduled lunar observations

MODIS is the key instrument for the NASA's EOS Terra and Aqua missions, launched in December, 1999 and May, 2002 respectively. The reflective solar bands (RSB) on Terra and Aqua MODIS are calibrated independently using an on-board solar diffuser and a solar diffuser stability monitor. Near-monthly lunar observations are also a major component of the on-orbit calibration strategy facilitating a response-versus-scan angle (RVS) characterization on-orbit. With a few exceptions, the regularly scheduled lunar observations have been performed with the same phase angles ranging from -55 degrees to -56 degrees for Aqua MODIS and 55 degrees to 56 degrees for Terra MODIS. Previous efforts have demonstrated that the observations of the Moon serve as an effective mechanism to perform an on-orbit cross-calibration of the two MODIS instruments. In addition to the regularly scheduled lunar observations that require a roll maneuver, both MODIS instruments also view the Moon via its space-view (SV) port for three to four months in a year that covers a wider range of phase angles. In this paper, we expand on the previous effort to provide an assessment of the RSB calibration difference between Terra and Aqua MODIS based on unscheduled lunar observations made over a range of phase angles. Also, discussed in this paper are strategies that could benefit other EOS sensors such as SNPP and NOAA 20 VIIRS.