i Detection of Smoke and Dust Aerosols Using Multi-sensor Satellite ...

development of future remote sensing systems. Fig. 7.2 is a layout of the SRCA (Xiong et
al., 2006). It consists of three subassemblies: a light source (VIS/NIR and infrared IR), a
monochromator/optical relay, and a collimator. The VIS/NIR source is a spherical
integration source (SIS) with four 10-W lamps and two 1-W lamps (one of the 10-W
lamps and one of the 1-W lamps are backups) to provide multiple levels of illumination
for the RSB characterization. A thermal source provides IR energy. The multi-lamp
configurations allow each band to be operated at a good SNR level. When the SRCA is in
spatial mode, a beam combiner on the filter wheel is used. The light coming out of the
SIS passes through it while the IR beam is reflected from its surface. The combined
beams provide illumination for all 36 bands. The light passes through the beam combiner
and is focused onto the monochromator’s entrance slit. After reflection by a collimating
mirror, the beam passes onto a mirror or grating (the grating is used for the spectral
characterizations). The beam is then refocused onto an exit slit (or various
interchangeable reticles) by the focusing mirror. The follow-up Cassegrain telescope
system expands and collimates the beam before it exits the SRCA and is viewed by the
MODIS scan mirror. The SRCA spatial mode was originally performed bi-monthly.
When the scan mirror is viewing the SRCA, the Earth scene illumination can pass
through the MODIS system and interfere with the measurement. In order to minimize this
effect, the SRCA is only operated in the night portion of the orbit.
When the SRCA is operated in spatial mode, an entrance slit equivalent to a 5 km
(scan direction) 12 km (track direction) nadir Instantaneous Field Of View (IFOV) (Fig.
7.3a) is used (Xiong et al., 2005). The mirror is set up in position so that the
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