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Atmospheric Correction Algorithm for the GOCI

Atmospheric Correction Algorithmfor the GOCIJae Hyun Ahn*Joo-Hyung Ryu*Young Jae Park*Yu-Hwan Ahn*Im Sang Oh**

Korea Ocean Research & Development InstituteSeoul National UniversityLet me introduce before this presentation, Im Jae Hyun Ahn, who studying atmospheric correction at the Korea Ocean Satellite Center of KORDIAnd now Ill talk about the atmospheric correction of the GOCI.You must know that GOCIs atmospheric correction is still on going process,And need more tuning before release GOCI data processing system(GDPS)1I n d e x _Introduction _Atmospheric CorrectionAtmospheric Algorithms of the GOCI> Standard NASA Algorithm> SGCA> SSMM

Process of Atmospheric Correction _Standard NASA AlgorithmSGCASSMM

Result & Validation _ResultValidation

Conclusion _

Ocean ColorOrder of presentation is as in the following

Introduction,Process of algorithm,ResultAnd conclusion21. Introduction _Atmospheric CorrectionM()*LTOA()*Rrs()ChlSSCDOMRadiometric CalibrationAtmosphericCorrectionL2 algorithms

LTOA(555nm)Rrs(555nm)AtmosphericCorrection*L : radiance*Rrs : remote sensing reflectanceOK, First.

So, What is an atmospheric correctionIs a process of extracting the exact ocean color radiances or reflectance from the satellite that located top of atmosphere

For an ease understanding, just see the following sequence.We can get Top of atmosphere radiance image from a Raw satellite image, after the radiometric calibration and geometric calibration.And after the atmospheric correction process, we can acquire remote sensing reflectance called Rrs that contains only exact color of ocean.31. Introduction _Atmospheric CorrectionClear water / thin aerosol case*Lr: Radiance of molecular scattering La : Radiance of aerosol scattring*Lw : Radiance of OceanCase 1 water : LW is 1~7% of LTOABut accurate correction is not so easy.You can easily understand why, by watching this bar graph.

This is clear water, thin aerosol, no sun-glint, a fortune case that, we can simply remove atmospheric signals.( )Here, from band1 to band8 is visible bands of the GOCI, that 412nm to 865nmThis green bar is molecular scattering radiance of atmosphere that called Rayleigh scattering.This red bar is aerosol scattering signals of atmosphere.And this small blue bar is just ocean signal radiance that we hope to know.Once again, this is the easiest case of the atmospheric correction.

41. Introduction _Atmospheric CorrectionIssue : GOCI has longer optical path than the polar orbit satellite (MODIS : 0 < Satellite zenith angle < 40)26 < Satellite zenith angle < 55Observation areaEarthGOCIequatorFurthermore, a geostationary orbit case is more difficult.Lets see this image.Unlike polar orbit satellites, GOCIs viewing zenith angle is 26 degrees to 55 degreesOther polar orbit satellites viewing angle is from 0 to 40 degrees.So, GOCIs optical path is much more extended than polar orbit satellites.

5Introduction _3 atmospheric Algorithms of the GOCIStandard NASA algorithmA classical standard atmospheric correction algorithmDeveloped by M.Wang & H.R.GordonAerosol 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.ShanmugamUsing reference siteAerosol models updated by J.H.Ahn

SGCA (Sun-Glint Correction Algorithm)Developed by HYGEOSRemoving sun-glint & atmospheric signalPolynomial fitting algorithm (ocean color & atmospheric model)

Atmospheric correction is a difficult part of ocean color remote sensing,Fortunately, several good atmospheric correction algorithms are already exist.GOCI adopts these 3 atmospheric correction algorithms.

GOCI data processing system, GDPS adopts 3 atmospheric correction methods.

First is a Standard algorithm of NASA.

This is a most common algorithm of atmospheric correction.This algorithm is now mainly adopted NASAs ocean color satellite, named MODIS,And mainly developed by Menghua Wang who studies atmospheric correction at the NOAA.In the case of GOCI, I personally updatedA turbid water iterative method, aerosol selection way and a diffuse transmittance calculation method.

Second atmospheric correction method is,A Spectral Shape Matching Method called SSMM,Developed by KORDI.This is more simple method than a standard algorithm.SSMMs key concept is:Firstly find reference target area the water leaving radiances spectrum shape doesnt change. And using the spectrum shape of the reference site.This shows relatively good result for turbid water case or blue band absorption aerosol case.But if we cant find out reference site for an example cloud covers, we cannot use this method.So this method is now an optional atmospheric correction of GDPS until we secure enough reference sites.

The last method is Sun Glint Correction Algorithm, SGCA.SGCA removes sun-glint and atmospheric signal at once.This is also an optional atmospheric correction method of the GDPS.

SGCA is the simplest atmospheric correction algorithm.The key concept of SGCA is the polynomial fitting between two models that the atmospheric model and the ocean color model.Especially a very thick aerosol case , this algorithm shows a good result than the other algorithms.

62. Process of Atmospheric Correction _Geometric Corrected TOA Radiance ImageLTOA()Raw ImageReflectance of TOA Image()= () + R ()Reflectance of Ocean + Aerosol Image () = Td()W() + A() + RA()Reflectance of Ocean Image W()Level 2 ProductChl, SS, CDOM, Kd490, Radiometric Calibration & Geometric CorrectionDownward Solar Irradiance Normalization Longitude, Latitude, Time, SZA, VZA, AZARemove Rayleigh & Sun-glint Reflectance & Mask Radiative Transfer Equation, Cox&Munk ModelRemove Aerosol ReflectanceRadiative Transfer Equation, Aerosol Model

Underwater AlgorithmReflectance of Ocean Image Rrs()Atmospheric CorrectionStandardNASAAlgorithmSSMMSGCAIt was a long introductionNow Ill simply introduce the process of atmospheric correction algorithms.Atmospheric correction algorithms process is as following.The Key processes are( )downward solar irradiance normalization,Removing Molecular scattering signalRemoving aerosol scattering signalNext page, Ill explain about these algorithms in detail72. Process of Atmospheric Correction _ Step 1. Downward Solar Irradiance Normalization

Downward Solar IrradianceNormalizationLTOA()cos(S )*S : solar zenith angleF0() : Extraterrestrial spectral irradiance

TOA ()

First step is Downward solar irradiance normalizationYou should expect simply solar zenith angle correction like this right image

///////////////////////////Lets see this image.

This image acquired local-time 5 pm in winterIn the image solar zenith angle of each pixels are different.Thats why the east side of image is darker than the west.

After this process, we can acquire a reflectance image that called p_toa()In this image we can see the brightness of image evenly regulated8

01234576981213141511102. Process of Atmospheric Correction _ Slot Correction of Solar Irradiance Normalization

cos(S ) Step 1. Downward Solar Irradiance NormalizationThere is an important issue that calculating solar zenith angle.In the case of GOCI, stitching of each 16 slot images are required to make a full scene image,And this slot images is getting sequentially with about 100 second intervals, not at the same time.During this interval, sun moves constantly, we must consider this and recalculate solar zenith angles per slotSo the output result of this process changes like this. (click)92. Process of Atmospheric Correction _ Step 2. Remove Rayleigh Signal

TOA(443nm)R(443nm) (443nm)Next is the removing Rayleigh scattering

Like this picture, top of atmosphere reflectance minus Rayleigh reflectance,

than we can acquire the Rayleigh removed image,

That means only the ocean and the aerosol signal remain.

Generally using radiative transfer equation to estimate amount of Rayleigh signal.102. 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 surfaceMolecular scatteringIn the case of GOCI, viewing angle rarely make a sun-glint issue.But, when considering about Rayleigh scattering phase function has a big lateral scattering,Including the sea surface roughness is better when calculate with a radiative transfer equation.To estimate sea surface roughness, We used the climatological wind-field data in place of the realtime meteorological data.

To reducing the calculation time of the radiative transfer equation.We use the pre computed Look-up table.11M2. Process of Atmospheric Correction _ Step 3. Land & Cloud Masking

Using threshold of Band8 (865nm) Masking 5x5 around the above threshold pixel

MMMMMMMMMMMMMMMMMMMMMNext is the land and cloud masking.There are so many methods of masking.Here I adopted the simplest way.Masking the pixel whose near infrared reflectance is greater than a reference value.

Additionally, I used only threshold of one band,But, the cloud continuously moves during the time-gap of the other band.For that reason, ( ) Masking more 20 pixels around the above threshold pixel122. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signal

(555nm)A(555nm)+RA (555nm)W (555nm)Removing aerosol signal is a core part of the atmospheric correction.First, lets see the image,( )The first image contains aerosol patterns and ocean color patterns at the


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