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Atmospheric Correction Algorithm for the GOCI

Atmospheric Correction Algorithm for the GOCI. Jae Hyun Ahn * Joo-Hyung Ryu * Young Jae Park* Yu-Hwan Ahn * Im Sang Oh** Korea Ocean Research & Development Institute Seoul National University. I n d e x _. Introduction _ Atmospheric Correction Atmospheric Algorithms of the GOCI

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Atmospheric Correction Algorithm for the GOCI

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  1. Atmospheric Correction Algorithmfor the GOCI Jae Hyun Ahn* Joo-HyungRyu* Young Jae Park* Yu-Hwan Ahn* Im Sang Oh** Korea Ocean Research & Development Institute Seoul National University

  2. I n d e x _ • Introduction _ • Atmospheric Correction • Atmospheric Algorithms of the GOCI • > Standard NASA Algorithm • > SGCA • > SSMM • Process of Atmospheric Correction _ • Standard NASA Algorithm • SGCA • SSMM • Result & Validation _ • Result • Validation • Conclusion _ Ocean Color

  3. 1. Introduction _ Atmospheric Correction Atmospheric Correction M(λ) *LTOA(λ) *Rrs(λ) Chl SS CDOM … Radiometric Calibration L2 algorithms LTOA(555nm) Rrs(555nm) Atmospheric Correction *L : radiance *Rrs : remote sensing reflectance

  4. 1. Introduction _Atmospheric Correction Clear water / thin aerosol case Case 1 water : LWis 1~7% of LTOA *Lr: Radiance of molecular scattering La : Radiance of aerosol scattring *Lw : Radiance of Ocean

  5. 1. Introduction _Atmospheric Correction Issue : GOCI has longer optical path than the polar orbit satellite Observation area • Earth GOCI equator 26˚ < Satellite zenith angle < 55˚ (MODIS : 0˚ < Satellite zenith angle < 40˚)

  6. Introduction _ 3atmospheric Algorithms of the GOCI • Standard NASA algorithm • A classical standard atmospheric correction algorithm • Developed by M.Wang & H.R.Gordon • Aerosol selection, turbid-water iterative method, diffuse transmittance models are updated by J.H.Ahn • SSMM (Spectral Shape Matching Method) • Developed by Y.H.Ahn & P.Shanmugam • Using reference site • Aerosol models updated by J.H.Ahn • SGCA (Sun-Glint Correction Algorithm) • Developed by HYGEOS • Removing sun-glint & atmospheric signal • Polynomial fitting algorithm (ocean color & atmospheric model)

  7. 2. Process of Atmospheric Correction _ Raw Image Radiometric Calibration & Geometric Correction Geometric Corrected TOA Radiance Image LTOA(λ) Downward Solar Irradiance Normalization  Longitude, Latitude, Time, SZA, VZA, AZA Reflectance of TOA Image ρ(λ)=ρ‘ (λ) + ρR (λ) Remove Rayleigh & Sun-glint Reflectance & Mask  Radiative Transfer Equation, Cox&Munk Model Reflectance of Ocean + Aerosol Image ρ‘ (λ) = Td(λ)ρW(λ) + ρA(λ) + ρRA(λ) Atmospheric Correction • Remove Aerosol Reflectance • Radiative Transfer Equation, Aerosol Model Reflectance of Ocean Image ρW(λ) Standard NASA Algorithm SSMM SGCA Reflectance of Ocean Image Rrs(λ) Underwater Algorithm Level 2 Product Chl, SS, CDOM, Kd490, …

  8. 2. Process of Atmospheric Correction _ Step 1. Downward Solar Irradiance Normalization LTOA(λ) ρTOA (λ) Downward Solar Irradiance Normalization cos(θS)* • θS: solar zenith angle • F0(λ) : Extraterrestrial spectral irradiance

  9. 2 1 0 3 7 5 6 4 8 9 11 10 15 13 14 12 2. Process of Atmospheric Correction _ Step 1. Downward Solar Irradiance Normalization • Slot Correction of Solar Irradiance Normalization cos(θS)

  10. 2. Process of Atmospheric Correction _ Step 2. Remove Rayleigh Signal ρTOA(443nm) ρR(443nm) ρ‘ (443nm)

  11. 2. Process of Atmospheric Correction _ Step 3. Remove Rayleigh & Sun-glint Reflectance • Remove direct & sun-glinted Rayleigh reflectance • Computed by radiative transfer equation • Integrate with GOCI bands’ spectral response • Using pre-computed LUT • Wind speed : 0~16 m/s Scattering off a rough sea surface Molecular scattering

  12. 2. Process of Atmospheric Correction _ Step 3. Land & Cloud Masking • Using threshold of Band8 (865nm) • Masking 5x5 around the above threshold pixel

  13. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signal ρ‘ (555nm) ρA(555nm)+ρRA (555nm) ρW (555nm)

  14. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signal • Standard NASA algorithm • Basic Assumption : ρW(NIR) = 0 (GOCI’s NIR Band : 745nm, 865nm) Atmospheric Correction Calculate Rayleigh Scattering Select 2 Aerosol Type Multiple Scattering to Single Scattering for all Aerosol Types Get Two Aerosol Models (model1/model2) εmodel1(B7, B8) < εave(B7, B8) < εmodel2(B7, B8) • Calculate Single Scattering of 2 Specific Aerosol type Get ε(λ, B8) for all band Calculate Single Scattering Reflectance for all Band  ρasmodel(λ) • Calculate Multiple Scattering of Specific Aerosol type 2 Aerosol Models sza/vza/aza ρasmodel1(λ) ρasmodel2(λ) Look-up Table from RTE (6S) Get ρa(λ) + ρra(λ) and t(λ) of 2 models Interpolate ρa(λ) + ρra(λ) and t(λ) of 2 models

  15. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signal • Standard NASA algorithm • Aerosol model selection (Modified) Select 2 Aerosol Type Multiple Scattering to Single Scattering for all Aerosol Types Get Two Aerosol Models (model1/model2) εmodel1(B7, B8) < εave(B7, B8) < εmodel2(B7, B8) Average all aerosol models’ ε(B7, B8) Select 4 aerosol models Average 4 aerosol models’ ε(B7, B8) Select 2 aerosol models Get weight of 2 aerosol models

  16. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signal • Aerosol models • Maritime (RH 50%, RH 80%, RH 99%) • Urban (RH 50%, RH 80%, RH 99%) • Continental (RH 50%, RH 80% RH 99%) East sea East sea East sea East sea Band 8 signal (aerosol signal) Aerosol removed signal (pure ocean signal : ρw(443)) Aerosol model selection result

  17. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Reflectance • SSMM (Spectral Shape Matching Method) • Assumption : ρW(NIR) = 0 (GOCI’s NIR Band : 745nm, 865nm) • Assumption : ρaerosol_model_1(λ) + ρaerosol_model_2(λ) = 0 • Use reference site’s spectrum shape Atmospheric Correction Calculate Rayleigh Scattering • Reflectance of Specific Aerosol type 2 Aerosol Models sza/vza/aza LUT ρa(λ) + ρra(λ) and t(λ) Reference site Get Aerosol reflectance Get Two Aerosol Models & mixing ratio from LUT

  18. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Reflectance • Iterative Method of NASA Standard Algorithm & SSMM • Turbid water : ρW(NIR) ≠0 ρTOA(NIR)=ρr(NIR) + ρa(NIR) + ρra(NIR) + t(NIR) ρf(NIR) + t(NIR)ρw(NIR) Atmospheric Correction ρr(λ) calculated by RTE ρa(λ) + ρra(λ) calculated by LUT t(NIR) calculated by LUT + RTE ρf(NIR) calculated by Cox&Munk’sEq BRDF ρw(λ), chl  corrected ρw(λ) Underwater Algorithm ρw(λ) chl, ss Ocean Color Model CHL, TSM  ρw(NIR)

  19. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signal • Iterative Method of NASA Standard Algorithm & SSMM • Rrs(NIR) = f/Q*bb(NIR)/(a(NIR)+bb(NIR)) • Bb(NIR) = bbw(NIR)+bbchl(NIR) + bbnc(NIR) • a(NIR) = aw(NIR)+ achl(NIR) + anc(NIR ρW (865nm) ρW (865nm)

  20. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signal • SGCA (Sun-glint Correction Algorithm) • Basic Assumption : ρWMOD(λ) is valid • Polynomial fitting : ρWMOD(λ) & ρAerosolMOD(λ) • ρWMOD(λ) : Using Biogenic optical model (by A.Morel) • ρAerosolMOD(λ) : C0 + C1λ-2 + C2λ-4 ρWMOD parameters (λ, chl, BbS) ρAerosolMOD parameters (C0, C1, C2) Min-error(λ) Final value (chl, C0, C1, C2) Td(λ) ρWMOD(λ) + ρA(λ)+ρRA(λ)+ error(λ) ρW(λ) ρ‘(λ)

  21. 2. Process of Atmospheric Correction _ Step 5. Apply Diffuse Transmittance • Extract Rayleigh diffuse transmittance • Generic Rayleigh diffuse transmittance model • τr(λ) : use H.R.Gordon’s model Tdr B1 B3 B4 B8 cos(Ф) Model’s Tdr RTE’s Tdr

  22. 2. Process of Atmospheric Correction _ Step 5. Apply Diffuse Transmittance • Extract Rayleigh diffuse transmittance • A simple Rayleigh diffuse transmittance model

  23. 2. Process of Atmospheric Correction _ Step 5. Apply Diffuse Transmittance • Get aerosol diffuse transmittance from AOT • Aerosol model, single scattering reflectance, single scattering albedo, phase function  Get aerosol optical thickness • A simple aerosol diffuse transmittance model (Hajime Fukushima, 1998) • Using Aerosol+Rayleigh LUT (Future work) • A generic data driven method

  24. 3. Result & Validation _ Result • Comparison images of GOCI & MODIS (NASA Standard Algorithm) GOCI with NASA standard 2011/03/17 03:16 (UTC) MODIS with NASA standard 2011/03/17 05:05 (UTC)

  25. 3. Result & Validation _ Result • Comparison spectrums of GOCI & MODIS (with NASA Standard Algorithm) B1 : 412nm B2 : 443nm B3 : 490nm (MODIS : 488nm) B4 : 555nm (MODIS : 551nm) B5 : 660nm (MODIS : 667nm) B6 : 680nm (MODIS : 678nm) GOCI MODIS GOCI MODIS

  26. 3. Result & Validation _ Result • Comparison images of SSMM & MODIS (NASA Standard Algorithm) GOCI : SSMM 2010/09/17 04:16 (UTC) SSMM Rrs(412nm) SSMM Rrs(443nm) SSMM Rrs(490nm) SSMM Rrs(555nm) MODIS : NASA Standard Algorithm 2010/09/17 04:45 (UTC) MODIS Rrs(412nm) MODIS Rrs(443nm) MODIS Rrs(490nm) MODIS Rrs(555nm)

  27. 3. Result & Validation _ Validation • Comparison nLw spectrums of SSMM & SGCA & MODIS (NASA Standard Algorithm) SSMM nLw(555nm): 2010. 08. 20 04:16 (UTC) SGCA nLw(555nm): 2010. 08. 20 04:16 (UTC) MODIS nLw(555nm): 2010. 08. 20 04:25 (UTC) SSMM SGCA NASA Standard (MODIS)

  28. 4. Conclusion _ • NASA Standard Algorithm for the GOCI • Basic schema is all implemented. • Need to improve the ocean color model • Add more good arrangement aerosol models • Need to consider the new aerosol model for the GOCI observation area • Change to the look up table based diffuse transmittance estimation • Aerosol model selection and weight method update • SSMM • Looks reasonable but needs more tuning • Better result high turbidity water and blue absorption aerosol case • Also consider about horizontal aerosol type changes • Collect more reference site • SGCA • Relatively good matching at the high optical thickness case • Improvement for turbid water • Needs more local tuning

  29. THANK YOU

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