Because the Solar Diffuser is the primary calibration target for the reflective bands, it is essential to monitor its reflectance properties throughout the life of the mission. Keeping MODIS data accurate depends on the accuracy of its calibration instruments, which is why the Solar Diffuser has a monitoring component called the Solar Diffuser Stability Monitor (SDSM). When the Solar Diffuser’s reflectance properties change, the SDSM observes those changes, and MODIS data can be adjusted accordingly. Changes are monitored by alternately viewing the Sun and the solar-illuminated diffuser surface. These data from these observations are then fed back into the calibration algorithm to maintain the calibration of the instrument over the duration of the mission.
The SDSM consists of a spherical integrating source (SIS) with a single input aperture and nine filtered detectors. Each filter has a narrow spectral bandpass so that the change in reflectance is effectively monitored at nine discrete wavelengths between 0.4 mm and 1.0 mm. A three-position fold mirror enables the detectors to view sequentially a dark scene, direct sunlight, and illumination from the Solar Diffuser. The direct sunlight is attenuated via a two-percent transmitting screen to keep the radiance within the dynamic range of the SDSM’s detector/amplifier combination.