atmospheric correction algorithm_igarss.pptx

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

M(λ)*LTOA(λ

)*Rrs(λ)

Chl

SS

CDOM

…

Radiometric

Calibration

Atmospheric

CorrectionL2 algorithms

LTOA(555nm) Rrs(555nm)

Atmospheric

Correction

*L : radiance

*Rrs : remote sensing reflectance


0

10

20

30

40

50

60

70

80

B1 B2 B3 B4 B5 B6 B7 B8

0

10

20

30

40

50

60

70

80

B1 B2 B3 B4 B5 B6 B7 B8

Lr

La

Lw

Clear water / thin aerosol case

*Lr: Radiance of molecular scattering

La : Radiance of aerosol scattring

*Lw : Radiance of Ocean

Case 1 water : LW is 1~7% of LTOA


Issue : GOCI has longer optical path than the polar orbit satellite

(MODIS : 0˚ < Satellite zenith angle < 40˚)

26˚ < Satellite zenith angle < 55˚

Observation area

EarthGOCI

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

LTOA(λ)

Raw Image

Reflectance of TOA Image

ρ(λ)=ρ‘ (λ) + ρR (λ)

Reflectance of Ocean + Aerosol Image

ρ‘ (λ) = Td(λ)ρW(λ) + ρA(λ) + ρRA(λ)

Reflectance of Ocean Image

ρW(λ)

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)

Atm

osp

he

ric

Co

rre

cti

on

Standard

NASA

Algorithm

SSMM SGCA

2. Process of Atmospheric Correction _Step 1. Downward Solar Irradiance Normalization

Downward Solar Irradiance

Normalization

LTOA(λ)

cos(θS )*

•θS : solar zenith angle•F0(λ) : Extraterrestrial spectral irradiance

ρTOA (λ)

)(0)cos(

)()(

F

L

S

TOATOA

Target Area

20.00

25.00

30.00

35.00

40.00

45.00

50.00

110.00 115.00 120.00 125.00 130.00 135.00 140.00 145.00 150.00

Longitude (deg)

La

titu

de (

deg

)

P1P2

P3P4

0 1 2 3

457 6

98

12131415

1110


- Slot Correction of Solar Irradiance Normalization

cos(θS )

Step 1. Downward Solar Irradiance Normalization

2. Process of Atmospheric Correction _Step 2. Remove Rayleigh Signal

)()()( RTOA' ρTOA(443nm) ρR(443nm)

ρ‘ (443nm)


- 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

)()()( gmmR

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

M M M

M M M M M

M M M M M

M M M M M

M M M

2. Process of Atmospheric Correction _Step 4. Remove Aerosol Signal

)(

)()()()(

Td

' RAAw

ρ‘ (555nm) ρA(555nm)+ρRA (555nm) ρW (555nm)


- Standard NASA algorithm

Basic Assumption : ρW(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)

εmodel1(B7, B8) < εave(B7, B8)

< εmodel2(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

ρasmodel(λ)

2 Aerosol Models

sza/vza/aza

ρasmodel1(λ)

ρasmodel2(λ)

Get

ρa(λ) + ρra(λ)

and t(λ)

of 2 models

Interpolate

ρa(λ) + ρra(λ)

and t(λ)

of 2 models

Calculate Rayleigh Scattering


- 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


- 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 : ρw(443))

East sea East sea East seaEast sea

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


LUT

Reflectance of Specific Aerosol type

2 Aerosol Models

sza/vza/aza

ρa(λ) + ρra(λ)

and t(λ)

Calculate Rayleigh Scattering

Reference site

Get Aerosol

reflectance

Get Two Aerosol Models

& mixing ratio from LUT

ρTOA(NIR)=ρr (NIR) + ρa(NIR) + ρra(NIR) + t(NIR) ρf(NIR) + t(NIR) ρw(NIR)

ρr (λ) calculated by RTE

ρa(λ) + ρra(λ) calculated by LUT

t(NIR) calculated by LUT + RTE

ρf(NIR) calculated by Cox&Munk’s Eq

ρw (λ) chl, ss


Underwater Algorithm

CHL, TSM ρw (NIR)

Ocean Color Model

ρw (λ), chl corrected ρw (λ)

BRDF

2. Process of Atmospheric Correction _Step 4. Remove Aerosol Reflectance

- Iterative Method of NASA Standard Algorithm & SSMM

Turbid water : ρW(NIR) ≠0


- Iterative Method of NASA Standard Algorithm & SSMM

Rrs(NIR) = f/Q*bb(NIR)/(a(NIR)+bb(NIR))- Bb(NIR) = bb

w(NIR)+bbchl(NIR) + bb

nc(NIR)

- a(NIR) = aw(NIR)+ achl(NIR) + anc(NIR

ρW (865nm) ρW (865nm)


ρ‘ (λ) Td(λ) ρWMOD(λ) + ρA(λ)+ρRA(λ)+ error(λ)

ρWMOD parameters

(λ, chl, BbS)

ρAerosolMOD parameters

(C0, C1, C2)

Min-error(λ) Final value

(chl, C0, C1, C2)ρW(λ)

- SGCA (Sun-glint Correction Algorithm)

Basic Assumption : ρWMOD(λ) is valid

Polynomial fitting : ρWMOD(λ) & ρAerosol

MOD(λ)

ρWMOD(λ) : Using Biogenic optical model (by A.Morel)

ρAerosolMOD(λ) : C0 + C1λ

-2 + C2λ-4

B1

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

)cos(2

)(

)(

r

eTdr

0

0.1

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0.5

0.6

0.7

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0.9

1

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

B3 B4 B8

Tdr

cos(Ф

)

)00013.0()0113.0(0.1008569.0)( 424 r

Model’s Tdr

RTE’s Tdr


- Extract Rayleigh diffuse transmittance

A simple Rayleigh diffuse transmittance model

6

0

)cos()()(n

n

nr CTd

R² = 1

R² = 1

R² = 1

R² = 0.999

R² = 0.999

R² = 0.999

R² = 0.999

R² = 0.999

0.45

0.55

0.65

0.75

0.85

0.95

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Band1 (412nm)

Band2 (443nm)

Band3 (490nm)

Bnad4 (555nm)

Band5 (660nm)

Band6 (680nm)

Band7 (745nm)

Band8 (865nm)

C6 C5 C4 C3 C2 C1 C0

412nm 2.446662E+00 -8.426278E+00 1.091486E+01 -5.986775E+00 3.424127E-01 1.212632E+00 3.582148E-01

443nm 2.439042E-01 6.214171E-02 -2.343571E+00 4.741604E+00 -4.368938E+00 2.218751E+00 3.401276E-01

490nm -3.409564E+00 1.368336E+01 -2.270315E+01 2.024385E+01 -1.059768E+01 3.364536E+00 3.456215E-01

555nm -6.190158E+00 2.375412E+01 -3.712744E+01 3.049661E+01 -1.420755E+01 3.801402E+00 4.276636E-01

660nm -6.027454E+00 2.276901E+01 -3.481947E+01 2.770477E+01 -1.228477E+01 3.025252E+00 6.094426E-01

680nm -5.722233E+00 2.158916E+01 -3.295611E+01 2.615090E+01 -1.154451E+01 2.820577E+00 6.416646E-01

745nm -4.680227E+00 1.760824E+01 -2.677182E+01 2.111729E+01 -9.234431E+00 2.219140E+00 7.273351E-01

865nm -3.040593E+00 1.140555E+01 -1.727012E+01 1.354123E+01 -5.866066E+00 1.386646E+00 8.353374E-01


- 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

)cos(

)()()(1 0

)(

aerosol

eTdaerosol

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)


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

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0.0045

B1 B2 B3 B4 B5 B6 B7 B8

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0.0045

0.005

B1 B2 B3 B4 B5 B6 B7 B8

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)


Comparison images of SSMM & 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)

Comparison nLw spectrums of SSMM & SGCA & MODIS (NASA Standard Algorithm)

0

2

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12

14

412 443 490 555 660 680 745 865

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

THANK YOU

atmospheric correction algorithm_igarss.pptx

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