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NASA Publications National Aeronautics and Space Administration
1-1-1997
Atmospheric correction of visible to middle-infrared EOS-MODIS data over land surfaces:
Background, operational algorithm and validationE. F. VermoteUniversity of Maryland
N. El SaleousUniversity of Maryland
C. O. JusticeUniversity of Maryland
Y. J. Kaufman
Goddard Space Flight Center
J. L. PriveeGoddard Space Flight Center
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Vermote, E. F.; El Saleous, N.; Justice, C. O.; Kaufman, Y. J.; Privee, J. L.; Remer, L.; Roger, J. C.; and Tanre, D., "Atmosphericcorrection of visible to middle-infrared EOS-MODIS data over land surfaces: Background, operational algorithm and validation"(1997).NASA Publications. Paper 31.hp://digitalcommons.unl.edu/nasapub/31
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Authors
E. F. Vermote, N. El Saleous, C. O. Justice, Y. J. Kaufman, J. L. Privee, L. Remer, J. C. Roger, and D. Tanre
is article is available at DigitalCommons@University of Nebraska - Lincoln: hp://digitalcommons.unl.edu/nasapub/31
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. D14, PAGES 17,131-17,141, JULY 27, 1997
Atmospheric correction of visible to middle-infrared
EOS-MODIS data over land surfaces:
Background, operational algorithm and validation
E. F. Vermote, N. E1 Saleous, C. O. Justice, Y. J. Kaufman, 2 J. L. Privette,2
L. Remer,3 J. C. Roger, and D. Tanr6s
Abstract. The NASA moderate esolution magingspectroradiometerMODIS)
instrumentwill provide a global and improved sourceof information for the studyof land
surfaceswith a spatial esolutionof up to 250 m. Prior to the derivationof various
biophysical arametersbasedon surface eflectances,he top of the atmosphere ignals
need to be radiometrically alibratedand corrected or atmospheric ffects.The present
paper describesn detail the state of the art techniques hat will be used for atmospheric
correctionof MODIS bands 1 through 7, centered at 648, 858, 470, 555, 1240, 1640, and
2130 nm, respectively. reviousoperationalcorrectionschemes ave assumed standard
atmospherewith zero or constantaerosol oading and a uniform, Lambertian surface.The
MODIS operationalatmospheric orrectionalgorithm,reported here, usesaerosoland
water vapor information derived from the MODIS data, corrects or adjacency ffects and
takes nto account he directionalpropertiesof the observed urface.This paper also
describeshe operational mplementationof these echniques nd its optimization.The
techniques re applied o remote sensing ata from the LandsatThematic Mapper (TM),
the NOAA advanced ery high resolution adiometer AVHRR), and the MODIS
airbornesimulator MAS) and validatedagainstground-based easurementsrom the
Aerosol Robotic Network (AERONET).
1. Introduction
The use of MODIS data for retrieval of land parameters,
such as the bidirectional reflectance distribution function
(BRDF), albedo, egetationndices VIs), fractionof absorbed
photosyntheticallyctive adiation FPAR), and leaf area in-
dex LAI), requires hat the top of the atmosphereadiancebe
converted o surface reflectance.The processnecessary or
that conversions called atmospheric orrection.By applying
the proposed lgorithms nd associated rocessing ode,mod-
erate esolutionmaging pectroradiometerMODIS) level B
radiances re corrected or atmospheric ffects o generate he
surface eflectanceproduct.Atmosphericcorrection equires
inputs hat describehe variableconstituentshat influence he
signalmeasured t the top of the atmosphereseeFigure 1 and
Tables a and lb) and a correctmodelingof the atmospheric
scattering nd absorption i.e., a band absorptionmodel and
multiple-scatteringectorcode). n addition,an accurate or-
rection requiresa correction or the atmospheric oint spread
function for highspatial esolution ands)and coupling f the
surfaceBRDF and atmosphere ffects.
Department f Geography, niversity f Marylandand NASA
GoddardSpaceFlight Center, Greenbelt,Maryland.
2NASAGoddard pace lightCenter,Greenbelt, aryland.
3Science,ystem nd Applicationsnc., NASA GoddardSpace
Flight Center, Greenbelt,Maryland.
4Laboratoire e Physique ppliqufieaux Milieux Oceaniques
C6tiers, Station Marine-Universitfi du Littoral, France.
SLaboratoire'Optique tmosphfirique,niversitfiesSciencest
Techniques e Lille, Villeneuved'Ascq,France.
Copyright 997by the AmericanGeophysical nion.
Paper number 97JD00201.
0148-0227/97/97 JD-00201 $09.00
Over the past few years, considerable ffort has been put
into the modeling of atmospheric ffects at the Laboratoire
d'Optique Atmospheriqueof Lille, France. Recently, he Sec-
ond Simulationof the Satellite Signal n the Solar Spectrum
(6S) radiativecodewas released Vermote t al., 1997] and is
well suited or various emote sensing pplications nd is fully
documented. It includes simulation of the effects of the atmo-
sphericpoint spread function and surfacereflectancedirec-
tionality.We are currentlyusing he 6S code as the reference
to enable ntercomparison f algorithms nd to verify the cor-
rect implementation of the MODIS atmosphericcorrection
algorithm.For example,Tables la and lb illustrate the appli-
cationof the code to show he relative atmospheric ffectson
examplesof existingenvironmentalsensors.
Our plan is to use MODIS atmospheric roductsand ancil-
lary data setsas nputs o the operationalatmospheric orrec-
tion. Collaboration n the area of aerosol etrieval has already
been nitiatedwith the MODIS aerosolgroup [Kaufmanet al.,
this ssue].An importantaspectof the operationalalgorithm s
to accommodate he large amount of input data and the nec-
essary ast turnaroundof products,without compromisinghe
scienceobjectivesor the accuracygoals.
2. Theoretical Background
The surface eflectance roductwill be computed rom the
MODIS calibrated adiancedata (level B) in bands1 through
7 (centeredat 648, 858, 470, 555, 1240, 1640, and 2130 nm,
respectively). he product s an estimateof the surface pectral
reflectance for each band as it would have been measured at
ground evel f there were no atmospheric cattering r absorp-
tion. The correction scheme includes corrections for the effect
of atmospheric ases, erosols, nd thin cirrusclouds,and it is
17,131
This article is a U.S. government work, and is not subject to copyright in the United States.
8/10/2019 Atmospheric Correction of Visible to Middle-Infrared EOS-MODIS Da
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17,132 VERMOTE ET AL.: ATMOSPHERIC CORRECTION OF EOS-MODIS DATA
G2,
I 20km
IMoeculesRayleighcaterng)
Ozone,tat sphercAerosol
I H20,roposphericerosol
Ground Surface
Figure l. Descriptionof the components ffectinga satellite
remote sensingsignal n the 0.4-2.5/zm range.
applied to all noncloudy evel lB pixels hat pass he level lB
qualitycontrol.The surface eflectance roduct s the input for
generation or severaland products: egetation ndices VIs),
BRDF/albedo, thermal anomaly, snow/ice, and FPAR/LAI.
Therefore it is an important and essentialproduct. The at-
launch version will be fully operational.An Algorithm Theo-
retical BackgroundDocument ATBD) for the surface eflec-
tance product and the approach adopted for atmospheric
correctionof MEDIS data [Vermote t al., 1995] s availableon
Home Page of the EOS ScienceProjectOffice (SPO) (http://
eospso.gsfc.nasa.gov/atbd/pgl.html,TBD-MeD08).
Rather than reproducing he ATBD contents, his paper
focuseson several mportant and recent improvements o the
algorithm,namely, he atmospheric oint spread unctioncor-
rection and an atmosphereBRDF couplingcorrection.One
critical aspectof MEDIS algorithmdevelopment s the need to
optimize the algorithm for efficient global processing.n the
sections hat follow, the practical aspectsof global data pro-
cessingor the algorithmare described.
2.1. Atmospheric Point Spread Function Correction
The case of nonuniform ground boundary conditionshas
been addressed y several esearchersTanr et al., 1981;Kauf-
man, 1982; Mekler and Kaufman, 1980]. The correction ap-
proach is to assume hat the signalreceivedby the satellite s
a combinationof the reflectanceof the target pixel and reflec-
tances from surroundingpixels, each weighted by their dis-
tance rom the target. Because he apparentsignalat the top of
the atmosphere f a pixel comesalso rom adjacentpixels, his
effect is also called adjacency ffect. The correction nvolves
inverting he linear combinationof reflectanceso solve or the
reflectanceof the target pixel.
We presenthere the proposedoperational mplementation.
It shouldbe noted that this effect will be less mportant for
MEDIS with a 250-500 m pixels han it is for higher spatial
resolutiondata, for example,30 m pixel from Thematic Map-
per. The correctionprocedurestems rom the modelingwork
by Tanr et al. [1981] and is simplified n this code for opera-
tional application.
In the caseof an infinite uniform Lambertian target of re-
flectance ps, the reflectanceat the top of the atmosphere,
PTOAcould be written as
PTR+A(s) R+A(Iv)
PTOA-- R+A'- 1 -psUSRA (1)
Table la. Order of Magnitude of AtmosphericEffects
for AVHRR Bands 1 and 2 and NDVI =
(Band 2 - Band 1)/(Band 2 + Band 1)
Ozone Water Vapor Aerosol V:
0.247-0.480, 0.5-4.1, Rayleigh, 60-10 km
cm/atm g/cm 1013mbar Continental
Pl
620 _+ 120 nm 4.2-12%
P2
885 _+ 195 nm
NDVI
(bare soil)
pl = 0.19,
P2 = 0.22
NDVI
(deciduous
forest)
p = 0.03
P2 = 0.36
0.02-0.06
0.7-4.4% 0.02-0.06 0.005-0.12
7.7-25% 0.006-0.02 0.003-0.083
0.011-0.12 0.036-0.094 0.006-0.085
0.006-0.017 0.036-0.038 0.086-0.23 0.022-0.35
The proportional ffect transmission)s givenaspercentage%) of
increase ) or reduction ) of the signal.All othereffects swell as
the effect on the normalizeddifference ndex (NDVI) are given n
absoluteunits;p is the reflectanceobserved n the advanced ery high
resolution adiometer (AVHRR) band 1; /92 s the reflectanceob-
served in AVHRR band 2.
Table lb. Order of Magnitude of AtmosphericEffects
for TM Bands 1-5 and 7 and NDVI --
(Band 4 - Band 3)/(Band 4 + Band 3)
Ozone Water Vapor Aerosol V:
0.247-0.480, 0.5-4.1, Rayleigh, 60-10 km
cm/atm g/cm 1013mbar Continental
Pl
P2
P3
P4
P5
P7
490 _+ 60 nm
575 _+ 75 nm
670 _+ 70 nm
837 _+ 107 nm
1692 +__ 78 nm
2190 _+ 215 nm
NDVI
(bare soil)
P3 = 0.19,
/94 ' 0.22
1.5-2.9% ". 0.064-0.08 0.007-0.048
5.2-13.4% 0.5-3% 0.032-0.04 0.006-0.04
3.1-7.9% 0.5-3% 0.018-0.02 0.005-0.034
" 3.5-14% 0.007-0.009 0.003-0.023
. 5-16% 0.000-0.001 0.001-0.007
. 2.5-13% "' 0.001-0.004
0.015-0.041 0.015-0.06 0.03 0.006-0.032
The proportional ffect transmission)s givenaspercentage%) of
increase r reductionof the signal.All other effects swell as he effect
on the NDVI are given n absoluteunits.
8/10/2019 Atmospheric Correction of Visible to Middle-Infrared EOS-MODIS Da
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VERMOTE ET AL.: ATMOSPHERIC CORRECTION OF EOS-MODIS DATA 17,133
where PR+A is the intrinsic atmospheric eflectancedue to
Rayleigh and aerosol scattering,TR+A(l%) (respectively
T + ( v)) is the total downwardrespectivelypward)atmo-
spheric ransmission, + is the sphericalalbedo of the at-
mosphere,% (respectively/Zv)s the cosineof the Sun (re-
spectively iew) zenith angle.
When taking nto account he adjacency ffect, the signalat
the top of the atmosphere an be rewritten by decoupling he
photonsoming irectlyrom he target e /v) from hose
coming rom areasadjacent o the target and then scattered o
the sensor t d(v) ):
psTa+A(s)-/v + {ps} a+A(s)td(v)
/'TOA IR+A 1 - (ps)S+ ' ""
r is the atmospheric pticaldepth, a(/, v) is the diffuseupward
transmission, is the pixel reflectance, nd {p} is the contri-
bution of the pixel background o the top of the atmosphere
signal hat is computedas
(Ps) = f(r(x, y))p(x, y) dx dy,
(3)
wherex, y denotes he coordinate o a local referencecentered
on the target, and (r) is the atmospheric oint spread unc-
tion.
We can see hat if we considerhat 1/(1 - (9s)S+)
1/(1 - p}'S+), then Ps and p}' (from (1)) are related
through he followingequation:
e-'/ v td(p,O
Os= Os +a(l(Os>+a(lv)' (4)
Thereforewe cancorrect he reflectance, btained rom (1), p}'
for the adjacency ffect using
psUT+n(v) (ps)td(tXv)
Ps e-,/v (5)
In practice, Ps} s computedrom a subzone f (2n + 1) x
(2n + 1) pixelsof the original magecenteredon the pixel to
be corrected sing p}' i, j) is used nsteadof Ps i, j) since he
latter is not available; he error introducedcan be reducedby
usingsuccessiveterationsbut is small [Putsay, 992])
(Ps) f(r(i, ))psU(i,), (6)
j=-n i=-n
with r(i, j) representinghe distance etween he pixels i, j)
and the center of the zone.
As part of a NASA-funded collaborationwith the National
ScienceFoundation NSF) Long-TermEcologicalResearch
(LTER) sitenetworkon atmosphericorrection alidation,we
applied the techniquepreviouslypresented or correction of
the atmosphericpoint spread function. This techniquewas
applied to the LandsatTM data (30 m resolution)up to a
distanceof 20 pixels around the viewed pixel. Two optimiza-
tionswere necessaryo arrive at an acceptable rocessingime
(about wice he amountof time of a simplecorrection). irst,
we generated over the range of expected alues)a look-up
table of the product (r(i,/))p}'(i, j). By using he look-up
table,we do not performanymultiplicationo compute 6) and
reduce he processingime significantly. econdly, he compu-
tation for the pixel of coordinate k, l)(ps(k, l)) was com-
puted from its neighborat coordinate k, l - 1)(ps(k, l -
1), the advantagehere being that the atmospheric oint
spread unction s rather smoothand that the differencematrix
between wo adjacentpixelscontainsmany zeroes.By sorting
the differencematrixvaluesand eliminating he zero elements,
we were further able to reduce he number of operations o be
performed by roughly a factor of 10. Figure 2 illustrates he
resultsof the adjacency ffect correction or TM band 1. The
scenewas acquired over a coastalarea in the eastern United
States.On the left side s the corrected mage;on the right side
is the originaldigital count mage.The top part represents he
full subset1000 x 1000pixels); he bottompart represents n
enlargementof a part of the scene.The correctedsceneap-
pears o showmore contrast han the uncorrected cenedue to
the correction of atmospheric eflectance and transmission
terms.The enlargeddetail shows he impactand correctionof
the adjacency; he small dark area in the original scenewas
previously essvisiblebecause t was surrounded y brighter
pixels.
For MODIS the surfaceadjacency ffect correctionwill be
made up to a distanceof 10 pixelsusing he same technique
developed or the TM. Because he atmospheric oint spread
functionvaries with the view angle as illustratedby Figure 3,
we will use precomputed ablesas a functionof view angle.
2.2. BRDF Atmosphere Coupling Correction
If the surface is not Lambertian, the result of the correction
using 1) is inexactdue to the couplingbetween he surface
BRDF and the atmosphereBRDF [Lee and Kaufman, 1986]
which the equationdoesnot take into account.For example,
Figure 4 shows he differencebetween he true ground reflec-
tance (9s), and the reflectance omputedwithout coupling
(psa) can each .02 or off-nadir iews.
An approach to model/correct his effect stems from the
work of TanrE t al. [1983].The contribution f the target o the
signalat the top of the atmospheres decomposed s the sum
of four terms: (1) the photonsdirectly ransmitted rom the
Sun o the targetand directly eflectedback o the sensor, 2)
the photonsscatteredby the atmosphere hen reflectedby the
target and directly ransmittedo the sensor, 3) the photons
directly transmitted o the target but scatteredby the atmo-
sphereon their way to the sensor, nd finally (4) the photons
havingat least two interactionswith the atmosphereand one
with the target. The equation is written as [Vermoteet al.,
1997]:
PTOA(s, -Lv,Os- 0v) = PR+A
(1)
+ e-'/'e-'/Sps(tZs,Zv,Os- Ov)
(2) (3)
+ + '
(4)
+ +
(v)S (b)
1 -Sb
(7)
with Os (resp.0v) the Sun (resp.view) azimuthangle,and
8/10/2019 Atmospheric Correction of Visible to Middle-Infrared EOS-MODIS Da
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17,134 VERMOTE ET AL.: ATMOSPHERIC CORRECTION OF EOS-MODIS DATA
..
.
:"' ? ''. .:" ...
:. : . =======================================================:.:::,:...;:::::..............:.:...::.::::::::::.::=====================
....'-;:i::'::i::ii4..,,.......-.i:,
..... ......
.:. :::..:..::.:
..........
....::::::::::::::::::::::::::::::::::::::::
Figure2. Comparisonf ThematicMapper TM) band1 datacorrectedor atmosphereleft side) o
uncorrectedountsrightside).The correctionncludes djacencyffectcorrection.
L+,(-,, R, ,, , qb')ps(,,, qb'- ck)dldck'
fo2L+,(,,,s,,'dz k'O
(8a)
where - (resp. .q) are the Rayleigh resp.aerosol) ptical
depth, and
p' (s, , q,) = (, m, q),
(8b)
= p'(s, , ), (8c)
1f2*rfs([l,,.1,',[}.l,.l,did'arkl
0 J0
. (8d)
1f2*rf[I,LI,'[l,'([l,
0 J0
In our approach, we use the ratio between the estimated
BRDF (p}n) coupledwith the atmospherend the actualsur-
face BRDF (p) to correct he measured alues; hat is,
iOTOA(/- 's,- 'v,) = PR+A-e-'/e-*/*ps(ts,, rk)
+ ps(s, , )
with
(9a)
pm([- ,s,- ,v, )
P*(s,, rk) psm(s,, rk) (9b)
P'*(s, , rk)= p*(, s, rk), (9c)
= : pm(ldl's'dl,,
P*(s,, rk) psm(s,, rk)' (9d)
The solving f (9a) for p is doneby solving second-order
equation hat only has one positivesolution.The rationale
behind his approachs that only the "shape"of the BRDF
influenceshe correction rocess nd not the actual"magni-
tude" of the estimatedBRDF. This approachgivesmore
weight to the actual observation han to the estimatedBRDF
used.Both the surfaceBRDF (p) and the atmosphereou-
pledBRDF terms 1o , i0m) canbe precomputedndstored n
tables.
We are currently nvestigatingwo alternativeapproaches
for obtainingmodeledBRDF inputs.One approachs to use
the inearBRDF modelparameterserivedrom he previous
16 dayperiod,generated spartof the MODIS BRDF product
[Strahler t al., 1995].The otherapproachelieson the BRDF
8/10/2019 Atmospheric Correction of Visible to Middle-Infrared EOS-MODIS Da
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VERMOTE ET AL.' ATMOSPHERIC CORRECTION OF EOS-MODIS DATA 17,135
associated ith land covercategories sed n the MODIS LAI/ o.35 .
FPAR product [Running t al., 1994] and simulatedusing a
three-dimensionalanopy odelMynenital., 1992].n the 0.3-
latter case,BRDF and coupling ermsare stored n tablesas a
0.25
functionof Sun-viewgeometry,atmospheric ptical depth, bi-
ome type,and LAI. The biome s determinedby the land cover o.2
map,and he LAI is selectedy minimizinghe difference a.
betweenhespectralependencef observedndmodeled 0.5
reflectance n MODIS bands 1, 2, 3, and 4. The advantage n
usinghe irstapproachs hat heBRDF shapesmore inked 0.
to actualobservations;he advantage n the secondcase s that
0.05 _
the couplingcorrectioncan be estimated n real time. A com-
parisonof the relativemeritsof theseapproaches ill be made
0
in the near future. -0
3. Implementation for Global Processing
3.1. ProcessingScheme
As currentlydeveloped, he atmospheric orrectionprocess
ingests he MODIS-calibrated radiances nd estimates he sur-
face reflectances. In addition to the estimates of the surface
reflectance he data product contains he following quality
assuranceQA) information or eachpixel: 1) integrityof the
surface eflectance stimate; 2) successfulompletionof the
correction cheme;3) presence f cloud clear,cloudy,mixed,
shadow); 4) presence f cirruscloud no cirrus, ow, average,
high); 5) sourceof aerosol nf(rmation:MODIS aerosol, li-
matology; 6) presenceof aerosol low, average,high); (7)
sourceof water vapor information:MODIS water vapor, cli-
matology; 8) sourceof ozone information:MODIS ozone,
climatology;9) whether he datum s from land or water.
0.5
-- 50 km
Figure 3. Isolines of the pixel backgroundcontribution to
the signalat the top of the atmosphere or a pure molecular
case. he energy ources 10 W andeachpixel s considered
to have a Lambertian reflectance of 1. The contribution of the
backgrounds presented s the numberof watts coming rom
eachcell (201 cells x 201 cells).The plain lines are for nadir
viewing; he broken lines are for a view angle of 70 [from
Vermote t al., 1997].
[] p2(0,0, ) 'n.. ct
OTOA(0s,, b'qb
3=670nm, =0.23
a
SUN _
-t0 -2'0 0 2b 4b 6b
0 [deg]
v
Figure 4. Illustration of the atmosphere-bidirectional istri-
bution function BRDF) couplingeffect correction.The sur-
face reflectancePs is computedby the Hapke model (the
parameters are inverted from surface reflectance directional
data taken over a plowed field by Kimes et al. [1986]) and
correspondso the interrupted ine. The plain ine corresponds
to the top of the atmosphere ignal,P:o^,at 670 nm, assuming
average atmospheric urbidity. The surface reflectances e-
trieved singheLambertianypothesisr equation1), psa,
are representedby the open squares.
The current version of the software is the first that reads
MODIS synthetic ata (A. Fleig and K. Yang, personalcom-
munication, 995) andwritesMODIS products sing he HDF
(hierarchical ata format) structure.n this codewe were able
to successfullymplement the correction or the atmospheric
point spread function and the coupled atmospheresurface
BRDF. Each of these corrections an be selectively ctivated.
Figure 5 givesa global overviewof the current software unc-
tionality, data volumes,and central processing nit (CPU)
requirements.
3.2. Developing Aerosol Climatology
Because he aerosolproductbeinggeneratedby the MODIS
atmospheregroup is a critical input to the atmospheric or-
rectionscheme Tanr et al., 1992],validationand prototyping
of the aerosolproduct and correctiondeservespecialcooper-
ation between he atmosphereand the land groups.Software
developmentactivities are currently focused on the aerosol
climatology.We need to understand on a global basis the
impactof aerosols.We have thereforebasedour approachon
the advanced eryhighresolution adiometer AVHRR) data.
This activity s important for MODIS for three different rea-
sons: 1) it provides he default nput to the atmospheric or-
rection algorithm should he MODIS aerosolproduct be un-
available, 2) it can contribute o the validationof the aerosol
algorithm or MODIS, and (3) it validates he aerosolatmo-
spheric correction process or AVHRR data. We have cur-
rently developeda prototypeapproach o generate his clima-
tology.Plate 1 shows ver the tropicalbelt area a composite f
the aerosoloptical depth during irst week of September1993.
Smoke rom biomass urning Brazil and SouthAfrica) and
dust from arid area (Red Sea and East Africa) aerosolcan
clearlybe observed.An important sourceof perturbation or
vegetationmonitoring s the stratospheric erosols fter a sig-
nificantvolcaniceruption.The stratospheric erosols ave ong
lifetime and thereforecouldbe eliminated n monthlycompos-
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17,136 VERMOTE ET AL.: ATMOSPHERIC CORRECTION OF EOS-MODIS DATA
- ...... -;, .... . ;, . ' ,. . ,-. _. ....
..- .... , , .. -r e ..:- .. . .. -. %.. [' : - .
r:,,' . ?:- .-.' ," " . :.;. " *" . ', 7
. . S .. :. -: ,- .. ,- ,..: ,,*... . - ' ,.. s ,. ,- . .-
..' .. :.:- . ' ,," ' , -" ' - . .... ";.,&'"Y':5'
} - ..' '&'/ '. 5, 'S'.l. " ... : ,. ;--
i i
';: .....
Plate 1. Maximum aerosoloptical hickness t 550 nm during the week September3-9, 1993, derived rom
NOAA AVHRR global area coverage GAC) data.
ites aswell as ropospheric erosols. ince1981, wo important
volcaniceruptionsaffected global vegetationmonitoring, El
Chichonand Mount Pinatubo,as illustratedby Plate 2.
4. Validation Plan
Validation activities anbe divided nto prelaunch alidation
of the algorithmand postlaunch alidationof the product.The
MODIS surface eflectanceproductvalidationwill use acom-
bination of ground-basedmeasurements, irborne measure-
ments,comparisonwith other sensordata, and imageanalysis.
4.1. Methodology
Validation of the reflectanceproductwill be undertaken or
selected lobal est sitesaspart of the proposedEOS Test Site
Program,where a suite of measurementsor atmosphere, ur-
face reflectance,and other surfaceparameterssuch as LAI,
FPAR, and net primary production NPP) will be collected
I I
outh El Chichon
50
1990
Pinatubo
1981
Plate 2. Monthly averageof the stratospheric erosoloptical depth deduced rom the advanced ery high
resolution adiometer AVHRR) data showingmajor eruptionsof E1 Chichon 1982) and Pinatubo 1991)
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220 '
" I uniform surface I
[Climatology
j ft ompton7minutesPU
VI computation 3 minutes CPU
249
Figure 5. Current moderateresolution magingspectroradi-
ometer (MODIS) atmospheric orrectionprocessing hread
flowchart.
[Runninget al., 1994]. These measurementswill be made at
different temporal frequencies s appropriate or the param-
eters. t is important to considerseveral actorswhen evaluat-
ing the atmosphericcorrection, or example under changing
Sun/sensor eometry,atmospheric tate, and surfacestate.As
part of the surface eflectance alidation,we propose o con-
duct a maximum of four campaigns year correspondingo
seasonal hangesn surfacestate and Sun geometry.For each
of these campaigns,we will collect surface reflectance and
atmosphericstate data with simultaneous atellite coverage.
Both clear and turbid atmospheres re desirable or eachcam-
paign as well as data gatheredat four or five different viewing
geometries nadir, backscattering,orward scattering).Dark
and bright surface measurementswill be collected for each
campaign,given that the success f the correction or a dark
surfacedependsmainly on the correctestimateof the intrinsic
atmospheric eflectance,whereas or a bright surface t de-
pendson the correctestimateof the transmittance.Critical to
this plan is the continuation and expansionof AERONET
[Holbenet al., 1997],whichprovidesdata on opticaldepth and
aerosolas well as size distributionon a continuous asis see
Figure 6 for current locations).The total number of sites
needed or validation s related to two requirements: 1) the
expected ariabilityof atmospheric onditions;2) spectral ig-
naturesand anisotropic ropertiesof the different and covers.
We expect o need approximately -3 sites n the polar envi-
ronment polar regions),7-8 sites n the boreal environment
(Canada, Greenland, Norway-Sweden,Russia), 5-6 sites in
North America, 5-6 sites n western Europe, and 5-6 sites n
easternEurope. More siteswill be needed n the tropical areas
becauseof the importanceof aerosolvariability and transport
and the rapid temporal change n land cover (e.g., at desert
boundaries).There are currently 18 Sun photometers nstalled
in the tropics hat mainlycoverBrazil (12) with some n Africa
(6). Ideally,we need at least12 more n Africa and 12 more for
the rest of the tropics o coverAsia. The southernhemisphere,
includingSouthAmerica, southernAfrica, and Australia, will
need at least 7-8 sites with 2-3 sites n the southern polar
region.On the basisof this allocation he total number of sites
is between 69 and 74. Further details on validation methods
and sites are available on the internet through the EOS Vali-
dation Office MODIS summarycharts.
Limiting errors n surface eflectance o less han 0.005 rms
for clear conditions nd 0.01 rms for turbid conditions 5 km
visibility) will be considered a success or the atmospheric
correction.To meet this goal, MODIS radiometriccalibration
(relative o the Sun) shouldhaveat least2% accuracy. ecause
finite cloud fields modify the downward radiation field, we
expect a decrease n the performanceof the algorithm under
these conditions. Cloud situations should be included in the
validation data collection to assess he change n algorithm
performance.
4.2. Prototyping Activities
Two tasksneed to be addressedwhen generating/validating
a global "product"before launch. The first task is to validate
how the algorithmperformswith a similar data set ("science
validation"). For this product he approachwill be to continue
and expandon the LTER atmospheric orrectionproject. The
LTER project is particularly relevant as the TM data used in
this study ncludesseveral of the spectralbands planned for
MODIS. AERONET Sun photometer data are collected at
these sites simultaneouslywith the satellite data. Figure 7a
illustrates he validationof the aerosol etrieval algorithmwith
TM data. In this case,optical depth deduced rom band 1 and
3 "dark targets" eflectances re compared o Sun photometer
measurements. he "dark targets" are selectedby using the
pixelswhose eflectance t 2.14/m are between0.015 and 0.05
and whosenormalizeddifferencevegetation ndex (NDVI) is
greater than zero (to eliminate water bodies). The average
"dark target" reflectance n band 1 is computedon a 50 km x
50 km area around the Sun photometer, and the surfacere-
flectance in band 1 is assumed to be one third of the reflec-
-1......... [ . I 89.50
... ' .... ....
..... :*'.' ...' "$"':.....**.... 45.00
-.-- - -;- . 0.50
' .a -.
;= .. ,:-44.00
0.50 S0.75 1812 50
Figure 6. Sun photometernetwork ocations source,B. N.
Holben, NASA GSFC). The vertical axis is the latitude in
degree south s negative,north s positive), he horizontalaxis
is the longitude n degree west s negative,east s positive).
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1.00
0.80 -
0.60 -
0.40 -
0.20-
0.00 -
0.0
y -0.11831.2937x= .96417OG-ISLAND,2 ul
M SON, Jun
I
0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80
Sunhotometer470nm)
Figure 7a. Retrieval of opticaldepth n TM band 1 using he
"dark targets"approach band 7 at 2.14/xm is used or "dark
targets"detection)comparedwith Sun photometerdata.
0.5... .......
.4 ..........
0.2 ..................................................---- ..................................................................................
i i i
0.1-
0
0 0.1 0.2 0.3 0.4 0.5
unphotomete/670nm)
Figure 8a. Retrieval of aerosolopticaldepth using he dark
target techniquewith the 2.14/xm from MODIS airbornesim-
ulator (MAS) data during he SCAR A experiment Roger t
al., 1994].
tanceobserved t 2.14/xm. Figure 7a shows omparisonsf the
derivedopticaldepthusinga continentalaerosolmodelversus
the measured pticaldepth.Despitesomevery ow and slightly
negative values observed or the Sevilleta, New Mexico, site
that may be due to the topography f the area, the agreement
is encouraging.More encouragings the comparison hown n
Figure 7b. These histograms f band 1 reflectance or the Hog
Island, Virginia, LTER site for clear and hazy daysshow hat
Histogram of the reflectanceobservedover Hog Island
20.00 .... I .... I .... I .... I ....
..... Turbid day (corrected)
..... Clear day (Corrected)
Turbid day (top of the atmosphere)
15.00 ClearayTopfhetmosphere)
10.00
0.00 .... "',"''-,'.r--=--_.,-- ..... I ....
0.0 0.050 0.10 0.15 0.20 0.25
TM band I reflectance
Figure 7b. Test of the result of the atmospheric orrection
procedure includingaerosol etrieval and atmospheric oint
spread unctioncorrection) or a 1000 x 1000TM pkels area
of the Hog Island site for the clear and ha day.
the atmospheric orrectionprocess learly removed he arti-
facts ntroducedby the aerosol emporalvariability.
A further approach o validation s to use high-altitude ir-
borne data from MAS as we did during the Sulfate Cloud
AerosolReflectance-AtlanticSCAR A) experiment Roger t
al., 1994]. Using the MODIS airbornesimulator MAS), we
tested he 2.14/xm approach or detecting dark targets"over
six estsitesusing he same est as describedor TM. In Figure
8a, aerosol etrievalwasperformed or "dark targets"using he
0.67/xm MAS band (a reflectanceof 0.015 was assumedor
dark targetsat 0.67/xm) and a continental erosolmodel.The
Table 2. Resultsof the Inversionof Top of the
Atmosphereand Top of the CanopyUsing
SCAR A MAS Data Set
Leaf
Area Leaf Leaf Angle
Index Reflectance Distribution
rms
Green band top of 0.4 0.010
atmosphere
reflectance
Green band top of 9.2 0.105
canopy
reflectance
Red band top of 1.2 0.150
atmosphere
reflectance
Red band top of 2.7 0.070
canopy
reflectance
Green band >3 0.05-0.25
expected alues
Red band >3 0.03-0.16
expected alues
planophile 0.26X 10 3
erectophile0.44X 10 4
erectophile0.135X 10 4
erectophile0.137X 10 4
erectophile
erectophile
SCAR A MAS, Sulfate Cloud Aerosol Reflectance-Atlantic MODIS
airborne simulator.
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- , .NT
........q%:;.........
_P}T
.....?,-;{.
Figure 8b. Grey level image of the MAS scene 0.87
used n the BRDF inversion Figure 8c and 8d). Annotations
on the imagespoint to the foresthot spot and water sunglint
features.
optical depth measurementsrom AERONET comparedwell
to MAS retrievals. In that case, the effect of the aerosol model
is more sensitive han for TM as the scatteringangle of the
differentobservationsaries from 10 o 150).For SCAR A
MAS data, strongdirectionalsurface eflectanceeffectscould
be observedover forest with a particularly sharp hot spot
phenomenon Figure 8b). We were able to fit the MAS data
using physically asedmodel Myneni t al., 1992].The results
are shown in Figures 8c and 8d. In the green band, where
at..mosphericcattering s larger, the atmosphericcorrection
definitely mproves he fit of the data both in backscatter nd
0.10
0.08
0.06
0.04
0.02
0.00
-40
Canopy Reflectance Model Fit of MAS Band 1
0.14...................................................
Top of Atmosphere Reflectance
O. 2 a /*' 0 Atmospherically....edeflect...
x - Vegetationodelit
-Vegetationodelit
-__
................................................
-30 -20 -10 0 10 20
View Zenith Angle (deg)
Figure 8c. Fits of MAS principal plane data usingMyneni et
al. model [1992] usingcorrected plain lines, diamonds)and
top of the atmosphere dashed ine, triangles) or the green
band (0.55/xm).
in forward scatteringdirections Figure 8c). In the red band,
both corrected and uncorrected data are fit well. The differ-
ence s important or the retrieved surfaceparameters Table
2), whether the top of the atmosphere eflectanceor the sur-
face estimated reflectanceor top of the canopy s used. The
parametersderived from top of the canopy eflectancevalues
are closer o the expectedvalues than the ones derived from
top of the atmosphere eflectance or this orestcase.The LAI
is higher n the greenband than in the red band,but it is quite
difficult to retrieve LAI values above 3 because of the satura-
tion of the signal,and therefore LAI errors are commonwith
densecanopies.
The second ask concerns alidating he global applicability
and robustnessof the algorithm. We can test the algorithm
data flow by using he MODIS syntheticdata set being devel-
oped by the MODIS sciencedata support eam (SDST), al-
though the sciencecannot be thoroughly ested using these
data becausewe are comparingone model output with an-
other. The globalprocessingssuehas o be addressed ith real
data. We plan to start to test the global applicabilityof the
MODIS algorithm using AVHRR time series data. Collabo-
ration will be developedwith the NASA EOS AVHRR Path-
Canopy Reflectance Model Fit of MAS Band 2
0.08.............................................
A Top of Atmosphere eflectance
.-Z x O Atmosphericallyorrectedeflectance
Vegetotlonodel it
0.06 ,,-. Vegetationodelit
-30 -20 -10 0 10 20
View Zenith Angle (deg)
c
0.02
0.00
-40
Figure 8d. Fits of MAS principal plane data usingMyneni et
al. model [1992] usingcorrected solid lines, diamonds)and
top of the atmospheredashed ine, triangles) or the red band
(0.67 m).
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0.5
0.4
0.3-
0.2-
0.1-
0.0
0 0.1 0.2 0.3 0.4 0.5
'CSu (550nm)
hotometer
Figure 9a. Comparison of retrieved optical depth using
AVHRR data and the dark target approach and measured
optical depth from the Sun photometer network in August
1993 over eastern United States.
2.00
1.50-
1.00-
0.500-
0.00
0 0.5 1 1.5
: (630nm)
Sun photometer
Figure 9c. Same as Figure 9b but in 1993.
finder 2 Project, whose goal is to improve on the current
AVHRR Pathfinderby designing nd testingglobalprocessing
of AVHRR 1 km and 4 km land data as a pathfinder or
MODIS. This MODIS prototyping using the AVHRR will
include operationalcorrection or Rayleigh,ozone,water va-
por, and stratospheric erosoland will investigatehe possibil-
ity of correcting or tropospheric erosols.These corrections
will be conducted at a global scale over a range of actual
atmosphericand surface conditions.As an example of the
tropospheric erosolcorrectioneffort, we obtainedsomevery
encouraging esultsby usingAVHRR data to retrieve aerosol
at specific and sites. n these cases he 3.75 /am reflectance
[Rogerand Vermote,1996] was used n lieu of the 2.14/am to
detectdark targetsusinga thresholdof 0.03. Figures9a and 9b
show he resultsof AVHRR optical depth retrieval during he
SCAR A campaigns nd the 1992AERONET Brazil data set.
In both casesa continentalaerosolmodel was used.Figure 9c
shows the results of AVHRR retrieval over Brazil using
AERONET 1993August-September ata and a smokemodel
derived from AERONET aerosol size distribution.
0.8
0.4
0.2
i I
A A
0.2 0.4 0.6 0.8 1
Sunhotometer550nm)
Figure 9b. Same as Figure 9a but in 1992 over Brazil.
5. Conclusion
The presentpaper describeshe approach dopted or com-
puting the surface reflectance n MODIS visible to middle-
infrared land bands.The processing f the top of the atmo-
spheresignal o obtain surface eflectance s not perfect but
does ncorporatestateof the art techniques.We are currently
workingon operationalprototypes singexisting ata sources
for the prelaunchalgorithm and productvalidation phase. n
addition o AVHRR and LandsatTM we are planning o use
test data sets rom the polarizationand directionalityof the
Earth's reflectances POLDER) instrument and the sea-
viewingwide field-of-viewsensor SeaWiFS) nstrumentcor-
respondingo our selected est sites.Postlaunch,he advanced
spacebornehermal emissionand reflectionradiometer (AS-
TER), the multi-angle maging spectroradiometerMISR),
and the Landsat7 EnhancedThematicMapper Plus ETM+)
data will be used duringvalidationcampaigns.Moreover, the
limited numberof test siteswill make t practical o systemat-
ically cross-compare ifferent sensors.MISR will definitely
improve our knowledgeof BRDF and ASTER and Landsat
7/ETM+ shouldenable us to extrapolate ocal ground-based
measurementso the MODIS pixels.
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(ReceivedMarch 21, 1996; revisedDecember3, 1996;
acceptedDecember 3, 1996.)
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