Abstract
Two MODIS instruments are currently onboard NASA's EOS Terra and Aqua spacecraft. MODIS has 20 reflective solar bands (RSB), which cover a spectral range from 0.41 to 2.2 micron. The RSB are calibrated on-orbit by an onboard solar diffuser (SD) whose degradation is tracked by a solar diffuser stability monitor (SDSM), an onboard spectroradiometric calibration assembly (SRCA) and monthly lunar observations through the instrument's space view (SV) port. MODIS views the Earth surface, SV, and the onboard calibrators via a two-sided scan mirror. The reflectance of the scan mirror depends on the angles of incidence (AOI) as well as the wavelength of the incident light. The dependence of the scan mirror's reflectance on the AOI is described by the response versus scan angle (RVS), which was measured prelaunch for both Terra and Aqua MODIS RSB. Analysis of the SD, lunar, and SRCA calibrations shows that the RVS was changing on-orbit and the changes are strongly AOI dependent for short wavelength bands. Algorithms have been developed to track the on-orbit RVS change using the SD, Moon, and SRCA observations as well as the Earth view (EV) response mirror side ratios and to derive the time-dependent RVS look-up tables (LUT) for the RSB calibration in the MODIS Level 1B (L1B) algorithm. Time-dependent RVS has been applied to MODIS RSB since MODIS L1B Version 4 and has been improved in Version 5 and Version 6 for both Terra and Aqua MODIS. This report reviews MODIS RSB RVS algorithms based on the most recent MODIS L1B Version (Version 6), derives the RVS uncertainties at the AOIs of the SD and SV, and estimates the uncertainties at other AOI. For most RSB, the reflectance calibration uncertainties are less than the 2% specification. For some of the shorter wavelength bands, especially in Terra MODIS, the reflectance calibration uncertainties is seen to be greater than 2% in recent years at some AOI between the SD and the SV.
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