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)