Results from On-Orbit Measurements and Modeling Studies for the MODIS Solar Diffuser Attenuation Screen

1. 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. 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. 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 LineMoving 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. 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. Backup Charts ISPRS Gulfport Calibration Workshop December 3, 2003

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

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