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.
Technical Description
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.
