glint-aerosol discrimination using nir-swir wavelengths€¦ · glint-aerosol discrimination using...
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Glint-aerosol discrimination using NIR-SWIR wavelengths
Young-Je Park1, Arnold Dekker, Eric Hochberg and HyspIRI Sun Glint Subgroup
24-26 Aug 2010 HyspIRI Science Workshop
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Outline
• HyspIRI glint compared to MODIS• Existing glint correction techniques• Aerosol and glint discrimination approach• AVIRIS example• Discussion• Application to algal bloom monitoring
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Introduction
0°
60°
MODIS-Terra glint center
10:30
HyspIRI glint center
11:30
HyspIRI swath
MODIS-Terraswath
MODIS Terra image of 29 Nov 2006 over Queensland, Australia
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Existing glint correction techniques
• For Medium-Coarse spatial resolution data (MODIS, MERIS)• Wind based surface slope model: Cox and Munk 1954
• High spatial resolution image• NIR-VIS linear relationship (Hochberg 2003)• NIR subtraction technique (Gao)• Uniform spectral offset approach (Goodman and Ustin, 2007)
• Why looking for new approach?• Use of adequate glint spectra normally decreasing to the blue • Automated processing
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Glint-aerosol decoupling approach
• Top of atmosphere (at-sensor) reflectance
TOAw
TOAgart ρρρρρ +++=
• Atmospheric-glint correction is meant to remove the reflectances due to air molecules, aerosols and glint from satellite measured reflectance.
Air molecules
aerosol
Glint(sunglint + skyglint)
Water body
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• At wavelengths where water reflectance is zero
rtTOAga ρρρρ −=+
known
• Aerosol reflectance and glint reflectance can be decoupled • 1) using a spectral matching technique if we know the spectral
shapes of aerosol and glint reflectances:
OR
• 2)Using a look-up table for as function of aerosol and wind speed as well as sun-sensor angles
ρa(λ) = ρa0 ⋅ A(λ)
ρgTOA(λ) = ρg0 ⋅ G(λ)
TOAga ρρ +
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Glint-aerosol discrimination approach
ρa(λ) = ρa0 ⋅ A(λ)
ρgTOA(λ) = ρg0 ⋅ G(λ)
• RT (6S) simulations for HyspIRI east and west edges
• Aerosol and glint reflectance have different spectral shapes• TOA glint spectrum is the surface reflectance multiplied by a
two-way atmospheric transmittance
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Verification with HyspIRI simulation data
• ALOHA-East edge U=10m/s
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AVIRIS example: French frigate shoals
• Apr. 18 2000 on French Frigate Shoals, Hawaii
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After correction
Before correction After correction
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After correction with smoothed aerosol reflectance
Before correction After correction
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Transect
0
0.1
0.2
0.3
0 100 200 300 400 500 600
pixel number
refle
ctan
ce a
t 443
nme
before correctionafter correction
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Spectra at a glint affected pixel
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
400 500 600 700 800 900 1000
wavelength (nm)
refle
ctan
ce Rayleigh correctedrho_arho_g (model)rho_wrho_g (image)
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Discussion
• Aerosol optical properties = type and optical thickness• Suggested to apply a low pass spatial filter to the retrieved aerosol
optical thickness
• Can aerosol model (spectral slope) be derived? Difficult where sun glint is significant.
• For operational use, vicarious calibration of satellite data against model would be required for glint correction.
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Glint correction for algal bloom detection
• Intense blooms of the brown algae, Hincksia sordida in Southeast Queensland recently have substantially reduced the recreational value. Wide spread algal blooms confirmed by aerial overflights and in situ observations were not visible in true colour image from the MODIS Rapidfire website.
Hincksia SordidaNon-toxic filamentous brown algae(E. Abal et al., 2006)
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Top Of Atmosphere MODIS reflectance spectra for bloom and non-bloom (=ocean water) pixels
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0 500 1000 1500 2000 2500
wavelength (nm)
TOA
refle
ctan
ce (n
o di
men
sion
)
pixel #1pixel #2pixel #3pixel #4pixel #5pixel #6pixel #7pixel #8
• Spectra from Terra image of Oct 3 2006• Blue curves for non-bloom pixels and red curves for bloom pixels.• Reflectance at 869nm should indicate the bloom intensity if the atmosphere and sea surface conditions are same
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TOA RGB
R - 645nmG - 555nmB - 469nm
(≈ NASA rapid response imagery)
Floating Algae (FA) enhanced
TOA RGB
R - 859nmG - 645nmB - 469nm
water reflectance (Rw)
RGB
R - 645nmG - 555nmB - 469nm
(TENTATIVE)
Floating Algae (FA) enhanced
Rw RGB
R - 859nmG - 645nmB - 469nm
(TENTATIVE)
TOA RGB normal TOA RGB FA Rw RGB normal Rw RGB FA
Satellite & Local Time
Before atmospheric correction After glint atmospheric correction
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TOA RGB
R - 645nmG - 555nmB - 469nm
(≈ NASA rapid response imagery)
Floating Algae (FA) enhanced
TOA RGB
R - 859nmG - 645nmB - 469nm
water reflectance (Rw)
RGB
R - 645nmG - 555nmB - 469nm
(TENTATIVE)
Floating Algae (FA) enhanced
Rw RGB
R - 859nmG - 645nmB - 469nm
(TENTATIVE)
TOA RGB normal TOA RGB FA Rw RGB normal Rw RGB FA
Aqua 2006 Oct 2 (275) 13:30
Before atmospheric correction After glint atmospheric correction
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Summary
• HyspIRI sun glint should be substantially reduced by shifting the equator crossing time from 11.30 to 10.30 AM.
• Glint-aerosol discrimination algorithm has been verified with simulation data and the results from application to AVIRIS and MODIS images look promising.
• It will be important to make sure consistency between satellite data and radiative transfer model that will be used for atmospheric-glint correction. Need vicarious calibration or similar.
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Thank you
Environmental Earth Observation GroupCSIRO Land and WaterYoungJe ParkEmail: [email protected]