atmospheric correction algorithm_igarss.pptx
TRANSCRIPT
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
1. Introduction _ Atmospheric Correction
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
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
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
2. Process of Atmospheric Correction _
- 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)
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
)()()( 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)
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
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
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
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 : 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
Atmospheric Correction
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
Atmospheric Correction
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
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) = bb
w(NIR)+bbchl(NIR) + bb
nc(NIR)
- a(NIR) = aw(NIR)+ achl(NIR) + anc(NIR
ρW (865nm) ρW (865nm)
2. Process of Atmospheric Correction _Step 4. Remove Aerosol Signal
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
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 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
2. Process of Atmospheric Correction _Step 5. Apply Diffuse Transmittance
- 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
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
)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)
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
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)
3. Result & Validation _ Result
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
4
6
8
10
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