Time-dependent response versus scan angle for MODIS reflective solar bands

Abstract

The MODIS instrument is currently operated onboard NASA's Terra and Aqua spacecrafts, launched in December 1999 and May 2002, respectively. MODIS has 36 spectral bands, among which 20 are the Reflective Solar Bands (RSB), covering a spectral range from 0.41 to 2.2 microns. The RSB are calibrated on-orbit using an onboard Solar Diffuser (SD), together with lunar observations and measurements from an onboard Spectroradiometric Calibration Assembly (SRCA). MODIS views the Earth's surface and the onboard calibrators via a two-sided scan mirror. Previous analysis of on-orbit observations from the SD, Moon, and SRCA has revealed that the Response Versus Scan angle (RVS) of the scan mirror is time, Angle of Incidence (AOI), and wavelength (band) dependent. Consequently, algorithms have been developed to track the on-orbit RVS change. In addition to the SD, Moon, and SRCA observations, the Earth View (EV) measurements at different AOI are trended and used to derive the time-dependent RVS look-up tables (LUT) for the RSB calibration in the Level 1B (L1B). On-orbit RVS algorithms were first applied to MODIS Version 4 and have, since then, been revised several times for both Terra and Aqua MODIS in order to adequately track on-orbit changes. In this paper, we present MODIS RSB RVS algorithm development history, focusing on the recent improvements for the upcoming Version 6. Results show that the RVS change is larger at shorter wavelengths and is different for the two mirror sides. For both Terra and Aqua MODIS, the degradation is also detector dependent for a few visible spectral bands.