ISPRS Gulfport Calibration Workshop December 3, 2003 1

Results from On-Orbit Measurements and Results from On-Orbit Measurements and

Modeling Studies for the MODIS Solar Modeling Studies for the MODIS Solar Diffuser Attenuation ScreenDiffuser Attenuation Screen

B. Guenther1, J. Xiong2, E. Waluschka2, W. E. Barnes1 and V. V. Salomonson2

1. University of Maryland, Baltimore County, South Campus, Baltimore, MD 21250; and NASA’s Goddard Space Flight Center

2. 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

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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

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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

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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

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SD Screen Design