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MODIS Aqua SDSM
Gerhard Meister,
presentation to CalVal, 2006
MODIS SDSM
• SDSM (Solar Diffuser Stability Monitor) views SD, sun, and background
• SDSM sun screen installed at wrong angle
9 Detectors, from 412nm to 936 nmh
• 9 detectors from 412nm to 936nm
VIIRS SDSM
Scan Directions
SDSM sun screen: theory
• MODIS screen misalignment confirmed by MCST modeling for yaw maneuvers, but not for regular calibrations
MODIS SDSM: measured SD/sun ratios for detector 9 (936nm)
Slope: -0.2%
Effect of reducing number of data points on linear fit to detector 9 data:
MCST method: divide detectors by detector 9
Divide detector 7 (857nm) by detector 9
No
Divide detector 1 (412nm) by detector 9
Epsilon trend: divide detector 6 (747nm) by detector 7 (857nm)
MCST modeled slope of the ratio: -0.45%My estimate: -0.3 to -0.5%
Conclusions
• No evidence that assumption ‘SD is stable at 936nm’ is wrong, data is too noisy
• Independent of the above assumption, the SD reflectance ratio of 748nm to 870nm is modeled well by MCST (to within 0.1%)
• SD degradation may be slower than model prediction (0.2% at 412nm) (expected from modified calibration schedule, but this explanation not well supported by data)