1. ISPRS Gulfport Calibration Workshop December 3, 2003
Results from On-Orbit Measurements and Modeling Studies for the MODIS Solar Diffuser Attenuation Screen
B. Guenther1, J. Xiong2, E. Waluschka2, W. E. Barnes1 and V. V. Salomonson2
University of Maryland, Baltimore County, South Campus, Baltimore, MD 21250; and NASA’s Goddard Space Flight Center
NASA’s Goddard Space Flight Center, Greenbelt, Maryland 20771
ISPRS Gulfport Calibration Workshop December 3, 2003

2. Presentation Objective
Demonstrate measurement and model results for SD screen effects in MODIS data system
Screen design criteria was ~ hold constant the number of “pin-holes” seen by each detector during calibration observations
Reality:
Ocean color products, trended with high quality surface measurement system (MOBY), leads to stability as most stringent criteria (≤ 0.3% stability)
Criteria for precision may be more demanding than was used in screen design criteria
Show final steps before modeling results incorporated in calibration algorithm
ISPRS Gulfport Calibration Workshop December 3, 2003

3. ISPRS Gulfport Calibration Workshop December 3, 2003
MODIS Schematic
“Paddle-wheel” cross-track scanning imaging system
Scanning Mirror points to diffuser scatter plate each scan
ISPRS Gulfport Calibration Workshop December 3, 2003

4. ISPRS Gulfport Calibration Workshop December 3, 2003
MODIS Focal Planes
Multiple detectors require equalization (flat fielding)
Flat Fielding performed on RSB from solar diffuser observations
High quality images useful
for science products
10, 20 or 40 pixels in scan
direction
ISPRS Gulfport Calibration Workshop December 3, 2003

5. Flat Fielding On-orbit
Thermal Bands (TEB) use V-groove Blackbody source
Reflected Solar Bands (RSB) use solar-illuminated Spectralon® scatter plate
Broad range of surface solar illumination angles, and broad range of reflectance in scenes complicates requirements for scatter place
MODIS uses deployable screen for two radiance values off the diffuser scatter plate
Screen used on low reflectance bands
ISPRS Gulfport Calibration Workshop December 3, 2003

6. System Response w/o & w/ SD Screen
ISPRS Gulfport Calibration Workshop December 3, 2003

7. Projection of Sun via SD Screen onto SD (Pin-Hole Pattern)
Screen used for low reflectance bands
Rays from each SD screen pin hole onto SD
Fired in 0.5◦ cone
Ellipse size on scatter plate varies due to tilt between surfaces
ISPRS Gulfport Calibration Workshop December 3, 2003

8. Detector Projection onto Scan Mirror and Solar Diffuser
Screen
SD Scatter Plate
Scan Mirror
ISPRS Gulfport Calibration Workshop December 3, 2003

9. Modeling Approach, Only Vis Bands Shown
High, uniform density of rays sent backwards from detector
“Count” each ray that hits an ellipse on SD
Weight each hit by SD_screen to SD_scatter_plate distance
Assume scatter plate perfect Lambert scatter surface
ISPRS Gulfport Calibration Workshop December 3, 2003

10. Simulations for Sun Angles, for a single December Day
ISPRS Gulfport Calibration Workshop December 3, 2003

11. Simulation Single Scan Line Moving Mirror, Fixed Sun Angle
ISPRS Gulfport Calibration Workshop December 3, 2003

12. Single Detector, Single Orbit, One December Day
ISPRS Gulfport Calibration Workshop December 3, 2003

13. ISPRS Gulfport Calibration Workshop December 3, 2003
Conclusions
Simulations and measurements headed for convergence
Simulations extended to two other months
Sensor calibration coefficients corrected for scatter plate BRF, compared with simulations
May incorporate simulations directly into derived calibration coefficients
Fundamental design issue is relationship of scatter plate location to a sensor pupil location
ISPRS Gulfport Calibration Workshop December 3, 2003

14. ISPRS Gulfport Calibration Workshop December 3, 2003
Backup Charts
ISPRS Gulfport Calibration Workshop December 3, 2003

15. ISPRS Gulfport Calibration Workshop December 3, 2003
OBC-BB
ISPRS Gulfport Calibration Workshop December 3, 2003

16. ISPRS Gulfport Calibration Workshop December 3, 2003
SD Screen Design
ISPRS Gulfport Calibration Workshop December 3, 2003