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Sun beta angle residuals in solar diffuser measurements of the MODIS ocean bands

Gerhard Meistera, Junqiang Sunb, Robert E. Eplee Jr.a, Frederick Patta, Jack Xiongb, and Charles McClaina

a: OBPG (Ocean Biology Processing Group; GSFC) b:MCST (MODIS Characterization Support Team; GSFC)

August 11, 2008

Earth Observing Systems XIII, SPIE Optonics + Photonics 2008, San Diego

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Background

• Ocean color remote sensing imposes very stringent requirements on radiometric precision (goal is 0.2% relative accuracy)

• Seasonal oscillations of calibration coefficients could lead to misinterpretations of ocean data

• Accuracy goal is especially demanding for prediction of calibration coefficients (necessary for operational sensors)

• This talk: β angle dependence for MODIS Aqua (update) and Terra (new)

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MODIS SD Measurement Setup

From: Waluschka et al, 2004.

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Simplified Solar Diffuser Geometry

Slide (modified) from R. Wolfe’s presentation at the MODIS Science Team meeting, Baltimore, March 2005

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Sun beta angle

Range for Terra and Aqua: β = 140 – 280

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Calibration coefficients: m1

• Measured m1, provided by MCST (approx. bimonthly)

• Not used in calibration LUTs• Calculated with:

m1 = BRF * cos(θSD) * Δ * VF / (dn * d2ES)

θSD = solar zenith angle on the SD VF = vignetting function from SD screen Δ = SD degradation measured by SDSM dn* = measured counts from SD (temperature corrected) minus dark current dES = distance Earth-Sun

Detector Ratio 10/1

For Band 12

Primaryand secondary

peaks arewell

correlatedwith

β angle

Detector Ratio 10/1

For Band 12

DetectorRatio

PlottedVersusβ angle

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MODIS Focal Planes

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Det.10/1 versus β for MODIS Aqua

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Det.10/1 versus β for MODIS Terra

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From detector ratios to absolute correction

• Detector ratios show effect, but correction must be applied absolute

• Old method: assume degradation (e.g. exponential) for m1, calculate correction using residuals (based on 3 years of data)

• New method: calculate rolling 1-year average, calculate correction using residuals (6 years of data)

• Residual ri = m1(ti)/<m1(ti)>, then fit 3rd order polynomial to ri(β)

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ri(β) for all MODIS Aqua bands for detector 10(solid=new fit, dashed=old correction)

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ri(β) for MODIS Aqua band 8, det.10

Scatter in band 8 is not random, it is correlated with time =>1-year averaging not accurate enough for band 8

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ri(β) for 4 MODIS Terra bands, detectors 1 and 10

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ri(β) for MODIS Terra may change with time

E.g. Band 16, detector 10: 0.4% m1 drop at beta angle maxima(dashed line) after SD anomaly (dotted line), but not before

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Operational prediction of m1 for MODIS Aqua

Top two plots: old correction

Bottom two plot: new correction

Notice the spread of the residuals is reduced with

the new correction =>More reliable prediction

into the future

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Relative detector trending: MODIS Aqua

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Relative detector trending: MODIS Terra

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SD screen on VIIRS

• VIIRS is follow-on sensor to MODIS• VIIRS SD screen not movable, no on-orbit

characterization of VF possible• VF will only be measured on component level,

not system level• Aqua beta angle correlation unexplained by

raytrace modeling• How can we be sure that component level VF

same as system level VF ?

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Summary

• Maximum variation in MODIS Aqua m1 measurements due to β angle: 0.8%, significant for ocean color

• Correction useful for predicting future trends• Variation in MODIS Terra: less than half,

possibly changing with time • Both sensors show similar detector degradation

relative to central detector• VIIRS sensor will only have component level

SD screen characterization