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 _ 1.Introduction _ -Atmospheric Correction -Atmospheric Algorithms of the GOCI > Standard NASA Algorithm > SGCA > SSMM 2.Process of Atmospheric Correction _ -Standard NASA Algorithm -SGCA -SSMM 3.Result & Validation _ -Result -Validation 4.Conclusion _ Ocean Color

1. Introduction _ Atmospheric Correction Atmospheric Correction Atmospheric Correction L TOA (555nm)Rrs(555nm) Atmospheric Correction Atmospheric Correction *L : radiance *Rrs : remote sensing reflectance

1. Introduction _ Atmospheric Correction Clear water / thin aerosol case *Lr: Radiance of molecular scattering La : Radiance of aerosol scattring *Lw : Radiance of Ocean Case 1 water : L W is 1~7% of L TOA

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

1.Introduction _ 3 atmospheric 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)

2. Process of Atmospheric Correction _ Geometric Corrected TOA Radiance Image L TOA (λ) Geometric Corrected TOA Radiance Image L TOA (λ) Raw Image Reflectance of TOA Image ρ(λ)=ρ‘ (λ) + ρ R (λ) Reflectance of TOA Image ρ(λ)=ρ‘ (λ) + ρ R (λ) Reflectance of Ocean + Aerosol Image ρ‘ (λ) = T d (λ)ρ W (λ) + ρ A (λ) + ρ RA (λ) Reflectance of Ocean + Aerosol Image ρ‘ (λ) = T d (λ)ρ W (λ) + ρ A (λ) + ρ RA (λ) Reflectance of Ocean Image ρ W (λ) Reflectance of Ocean Image ρ W (λ) Level 2 Product Chl, SS, CDOM, Kd490, … Level 2 Product Chl, SS, CDOM, Kd490, … Radiometric Calibration & Geometric Correction Downward Solar Irradiance Normalization  Longitude, Latitude, Time, SZA, VZA, AZA Remove Rayleigh & Sun-glint Reflectance & Mask  Radiative Transfer Equation, Cox&Munk Model Remove Aerosol Reflectance  Radiative Transfer Equation, Aerosol Model Underwater Algorithm Reflectance of Ocean Image Rrs(λ) Reflectance of Ocean Image Rrs(λ) Atmospheric Correction Standard NASA Algorithm SSMMSGCA

2. Process of Atmospheric Correction _ Step 1. Downward Solar Irradiance Normalization Downward Solar Irradiance Normalization L TOA (λ) cos(θ S ) * θ S : solar zenith angle F0(λ) : Extraterrestrial spectral irradiance ρ TOA (λ)

Process of Atmospheric Correction _ - Slot Correction of Solar Irradiance Normalization cos(θ S ) Step 1. Downward Solar Irradiance Normalization

2. Process of Atmospheric Correction _ Step 2. Remove Rayleigh Signal ρ TOA (443nm)ρ R (443nm) ρ‘ (443nm)

2. Process of Atmospheric Correction _ - 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 Step 3. Remove Rayleigh & Sun-glint Reflectance Scattering off a rough sea surface Molecular scattering

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

2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signal ρ‘ (555nm)ρ A (555nm)+ρ RA (555nm)ρ W (555nm)

2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signal - Standard NASA algorithm  Basic Assumption : ρ W (NIR) = 0 (GOCI’s NIR Band : 745nm, 865nm) Atmospheric Correction Select 2 Aerosol Type Multiple Scattering to Single Scattering for all Aerosol Types Get Two Aerosol Models (model1/model2) ε model1 (B7, B8) < ε ave (B7, B8) < ε model2 (B7, B8) Look-up Table from RTE (6S) Calculate Multiple Scattering of Specific Aerosol type Get ε (λ, B8) for all band Calculate Single Scattering of 2 Specific Aerosol type Calculate Single Scattering Reflectance for all Band  ρ as model (λ) 2 Aerosol Models sza/vza/aza ρ as model1 (λ) ρ as model2 (λ) Get ρ a (λ) + ρ ra (λ) and t(λ) of 2 models Interpolate ρ a (λ) + ρ ra (λ) and t(λ) of 2 models Calculate Rayleigh Scattering

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) ε model1 (B7, B8) < ε ave (B7, B8) < ε model2 (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

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%) Band 8 signal (aerosol signal) Aerosol model selection result Aerosol removed signal (pure ocean signal : ρ w (443)) East sea

2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Reflectance - SSMM (Spectral Shape Matching Method)  Assumption : ρ W (NIR) = 0 (GOCI’s NIR Band : 745nm, 865nm)  Assumption : ρ aerosol_model_1 (λ) + ρ aerosol_model_2 (λ) = 0  Use reference site’s spectrum shape Atmospheric Correction LUT Reflectance of Specific Aerosol type 2 Aerosol Models sza/vza/aza ρ a (λ) + ρ ra (λ) and t(λ) Calculate Rayleigh Scattering Reference site Get Aerosol reflectance Get Two Aerosol Models & mixing ratio from LUT

ρ TOA (NIR)=ρ r (NIR) + ρ a (NIR) + ρ ra (NIR) + t(NIR) ρ f (NIR) + t(NIR) ρ w (NIR) ρ r (λ)  calculated by RTE ρ a (λ) + ρ ra (λ)  calculated by LUT t(NIR)  calculated by LUT + RTE ρ f (NIR)  calculated by Cox&Munk’s Eq ρ r (λ)  calculated by RTE ρ a (λ) + ρ ra (λ)  calculated by LUT t(NIR)  calculated by LUT + RTE ρ f (NIR)  calculated by Cox&Munk’s Eq ρ w (λ)  chl, ss Atmospheric Correction Underwater Algorithm CHL, TSM  ρ w (NIR) Ocean Color Model ρ w (λ), chl  corrected ρ w (λ) BRDF 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Reflectance - Iterative Method of NASA Standard Algorithm & SSMM  Turbid water : ρ W (NIR) ≠0

2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signal - Iterative Method of NASA Standard Algorithm & SSMM  Rrs(NIR) = f/Q*b b (NIR)/(a(NIR)+b b (NIR)) - Bb(NIR) = b b w (NIR)+b b chl (NIR) + b b nc (NIR) - a(NIR) = a w (NIR)+ a chl (NIR) + a nc (NIR ρ W (865nm)

2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signal ρ‘ (λ) Td(λ) ρ W MOD (λ) + ρ A (λ)+ρ RA (λ)+ error (λ) ρ W MOD parameters (λ, chl, Bb S ) ρ Aerosol MOD parameters (C 0, C 1, C 2 ) Min-error (λ) Final value (chl, C0, C1, C2) ρ W (λ) - SGCA (Sun-glint Correction Algorithm)  Basic Assumption : ρ W MOD (λ) is valid  Polynomial fitting : ρ W MOD (λ) & ρ Aerosol MOD (λ)  ρ W MOD (λ) : Using Biogenic optical model (by A.Morel)  ρ Aerosol MOD (λ) : C 0 + C 1 λ -2 + C 2 λ -4

B1 2. Process of Atmospheric Correction _ Step 5. Apply Diffuse Transmittance - Extract Rayleigh diffuse transmittance  Generic Rayleigh diffuse transmittance model  τ r (λ) : use H.R.Gordon’s model B3B4B8 Td r cos(Ф) Model’s Td r RTE’s Td r

2. Process of Atmospheric Correction _ Step 5. Apply Diffuse Transmittance - Extract Rayleigh diffuse transmittance  A simple Rayleigh diffuse transmittance model C6C6 C5C5 C4C4 C3C3 C2C2 C1C1 C0C0 412nm E E E E E E E nm E E E E E E E nm E E E E E E E nm E E E E E E E nm E E E E E E E nm E E E E E E E nm E E E E E E E nm E E E E E E E-01

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

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

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

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

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

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

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