MODIS Instrument Characteristics, Performance, and Data for Climate Studies - Satellite-based Applications on Climate Change - page 35

Fig. 3.2 Calibration and characterization components of the Aqua and Terra MODIS instruments
including the frequency for their application
Additionally, the solar diffuser (SD) showed some degradation that was particu-
larly significant at the shorter wavelengths (e.g., bands 3, 8, 9, and 10). Figure 3.4
shows the extent of the changes in the solar diffuser properties for both instruments.
These changes were observed by the solar diffuser stability monitor (SDSM). The
wavelengths of SDSM detectors (1-8) are as follows: 0.41, 0.47, 0.53, 0.55, 0.65,
0.75, 0.86, and 0.90 μ m. The largest changes occurred at the shortest wavelengths
and become progressively smaller as the wavelengths become larger. In the case of
the Terra SD, the door covering the opening to the solar diffuse experienced
difficulties and was left open to insure that proper measurements could be provided.
Leaving the door caused much larger exposure of the solar diffuser to the sun and
thus greater degradation in wavelength response over time. In the case of the Aqua
SD, the SD/SDM calibration frequency has been gradually reduced at about 2,200
days into the mission to lengthen the lifetime of the SD door opening and closing
mechanism and is now done every 6 weeks as compared to weekly at the beginning
of the Aqua mission.
Most of the other MODIS components have been relatively stable and generally
meet requirements. For instance, most of the detectors (475 out of 490) continue to
meet sensor design signal-to-noise (SNR) requirements. The on-orbit spectral and
spatial performance (e.g., band shifts or spatial registration of bands to each other)
have largely remained quite stable and within specifications (0.1 km) over the