This document describes three atmospheric correction algorithms for the Geostationary Ocean Color Imager (GOCI): the Standard NASA algorithm, the Spectral Shape Matching Method (SSMM), and the Sun-Glint Correction Algorithm (SGCA). It outlines the processing steps for each algorithm, including radiometric calibration, removal of Rayleigh and aerosol scattering, and derivation of remote sensing reflectance. Validation results show SSMM and SGCA provide reasonable matches to NASA standard processing of MODIS data, though all three GOCI algorithms could be improved by updating aerosol and ocean models. The document concludes the algorithms capture the essential ocean color measurement but would benefit from further refinement.

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

2. I n d e x _Introduction _Atmospheric Correction

3. Atmospheric Algorithms of the GOCI> Standard NASA Algorithm> SGCA> SSMMProcess of Atmospheric Correction _Standard NASA Algorithm

4. SGCA

5. SSMMResult & Validation _Result

6. ValidationConclusion _Ocean Color

7. 1. Introduction _ Atmospheric CorrectionAtmosphericCorrectionM(λ)*LTOA(λ)*Rrs(λ)ChlSSCDOM…Radiometric CalibrationL2 algorithmsLTOA(555nm)Rrs(555nm)AtmosphericCorrection*L : radiance*Rrs : remote sensing reflectance

8. 1. Introduction _Atmospheric CorrectionClear water / thin aerosol caseCase 1 water : LWis 1~7% of LTOA*Lr: Radiance of molecular scattering La : Radiance of aerosol scattring*Lw : Radiance of Ocean

9. 1. Introduction _Atmospheric CorrectionIssue : GOCI has longer optical path than the polar orbit satelliteObservation areaEarthGOCIequator26˚ < Satellite zenith angle < 55˚(MODIS : 0˚ < Satellite zenith angle < 40˚)

10. Introduction _3atmospheric Algorithms of the GOCIStandard NASA algorithmA classical standard atmospheric correction algorithm

11. Developed by M.Wang & H.R.Gordon

12. Aerosol selection, turbid-water iterative method, diffuse transmittance models are updated by J.H.AhnSSMM (Spectral Shape Matching Method)Developed by Y.H.Ahn & P.Shanmugam

13. Using reference site

14. Aerosol models updated by J.H.AhnSGCA (Sun-Glint Correction Algorithm)Developed by HYGEOS

15. Removing sun-glint & atmospheric signal

16. Polynomial fitting algorithm (ocean color & atmospheric model)2. Process of Atmospheric Correction _Raw ImageRadiometric Calibration & Geometric CorrectionGeometric Corrected TOA Radiance ImageLTOA(λ)Downward Solar Irradiance Normalization Longitude, Latitude, Time, SZA, VZA, AZAReflectance of TOA Imageρ(λ)=ρ‘ (λ) + ρR (λ)Remove Rayleigh & Sun-glint Reflectance & Mask Radiative Transfer Equation, Cox&Munk ModelReflectance of Ocean + Aerosol Imageρ‘ (λ) = Td(λ)ρW(λ) + ρA(λ) + ρRA(λ)Atmospheric CorrectionRemove Aerosol ReflectanceRadiative Transfer Equation, Aerosol ModelReflectance of Ocean ImageρW(λ)StandardNASAAlgorithmSSMMSGCAReflectance of Ocean ImageRrs(λ)Underwater AlgorithmLevel 2 ProductChl, SS, CDOM, Kd490, …

17. 2. Process of Atmospheric Correction _ Step 1. Downward Solar Irradiance NormalizationLTOA(λ)ρTOA (λ)Downward Solar IrradianceNormalizationcos(θS)*θS: solar zenith angle

18. F0(λ) : Extraterrestrial spectral irradiance21035764891110131514122. Process of Atmospheric Correction _ Step 1. Downward Solar Irradiance Normalization Slot Correction of Solar Irradiance Normalizationcos(θS)

19. 2. Process of Atmospheric Correction _ Step 2. Remove Rayleigh SignalρTOA(443nm)ρR(443nm)ρ‘ (443nm)

20. 2. Process of Atmospheric Correction _ Step 3. Remove Rayleigh & Sun-glint Reflectance Remove direct & sun-glinted Rayleigh reflectance

21. Computed by radiative transfer equation

22. Integrate with GOCI bands’ spectral response

23. Using pre-computed LUT

24. Wind speed : 0~16 m/sScattering off a rough sea surfaceMolecular scattering

25. 2. Process of Atmospheric Correction _ Step 3. Land & Cloud Masking Using threshold of Band8 (865nm)

26. Masking 5x5 around the above threshold pixel2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signalρ‘ (555nm)ρA(555nm)+ρRA (555nm)ρW (555nm)

27. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol SignalStandard NASA algorithm

28. Basic Assumption : ρW(NIR) = 0 (GOCI’s NIR Band : 745nm, 865nm)Atmospheric CorrectionCalculate Rayleigh ScatteringSelect 2 Aerosol TypeMultiple Scattering to Single Scatteringfor all Aerosol TypesGet Two Aerosol Models (model1/model2)εmodel1(B7, B8) < εave(B7, B8) < εmodel2(B7, B8)Calculate Single Scattering of 2 Specific Aerosol typeGet ε(λ, B8) for all bandCalculate Single Scattering Reflectancefor all Band ρasmodel(λ)Calculate Multiple Scattering of Specific Aerosol typeLook-up TablefromRTE (6S)2 Aerosol Modelssza/vza/azaρasmodel1(λ)ρasmodel2(λ)Getρa(λ) + ρra(λ)and t(λ)of 2 modelsInterpolateρa(λ) + ρra(λ)and t(λ)of 2 models

29. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol SignalStandard NASA algorithm

30. Aerosol model selection (Modified)Select 2 Aerosol TypeMultiple Scattering to Single Scatteringfor all Aerosol TypesGet Two Aerosol Models (model1/model2)εmodel1(B7, B8) < εave(B7, B8) < εmodel2(B7, B8)Average all aerosol models’ ε(B7, B8)Select 4 aerosol modelsAverage 4 aerosol models’ ε(B7, B8)Select 2 aerosol modelsGet weight of 2 aerosol models

31. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signal Aerosol models

32. Maritime (RH 50%, RH 80%, RH 99%)

33. Urban (RH 50%, RH 80%, RH 99%)

34. Continental (RH 50%, RH 80% RH 99%)East seaEast seaEast seaEast seaBand 8 signal(aerosol signal)Aerosol removed signal(pure ocean signal : ρw(443))Aerosol model selection result

35. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Reflectance SSMM (Spectral Shape Matching Method)

36. Assumption : ρW(NIR) = 0 (GOCI’s NIR Band : 745nm, 865nm)

37. Assumption : ρaerosol_model_1(λ) + ρaerosol_model_2(λ) = 0

38. Use reference site’s spectrum shape Atmospheric CorrectionCalculate Rayleigh ScatteringReflectance of Specific Aerosol typeLUT2 Aerosol Modelssza/vza/azaρa(λ) + ρra(λ)and t(λ)Reference siteGet Aerosol reflectanceGet Two Aerosol Models & mixing ratio from LUT

39. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Reflectance Iterative Method of NASA Standard Algorithm & SSMM

40. Turbid water : ρW(NIR) ≠0ρTOA(NIR)=ρr(NIR) + ρa(NIR) + ρra(NIR) + t(NIR) ρf(NIR) + t(NIR)ρw(NIR)Atmospheric Correctionρr(λ) calculated by RTEρa(λ) + ρra(λ) calculated by LUTt(NIR) calculated by LUT + RTEρf(NIR) calculated by Cox&Munk’sEqBRDFρw(λ), chl  corrected ρw(λ)Underwater Algorithmρw(λ) chl, ssOcean Color ModelCHL, TSM  ρw(NIR)

41. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signal Iterative Method of NASA Standard Algorithm & SSMM

42. Rrs(NIR) = f/Q*bb(NIR)/(a(NIR)+bb(NIR))

43. Bb(NIR) = bbw(NIR)+bbchl(NIR) + bbnc(NIR)

44. a(NIR) = aw(NIR)+ achl(NIR) + anc(NIRρW (865nm)ρW (865nm)

45. 2. Process of Atmospheric Correction _ Step 4. Remove Aerosol Signal SGCA (Sun-glint Correction Algorithm)

46. Basic Assumption : ρWMOD(λ) is valid

47. Polynomial fitting : ρWMOD(λ) & ρAerosolMOD(λ)

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

49. ρAerosolMOD(λ) : C0 + C1λ-2 + C2λ-4ρWMOD 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)

50. 2. Process of Atmospheric Correction _ Step 5. Apply Diffuse Transmittance Extract Rayleigh diffuse transmittance

51. Generic Rayleigh diffuse transmittance model

52. τr(λ) : use H.R.Gordon’s modelTdrB1B3B4B8cos(Ф)Model’s TdrRTE’s Tdr

53. 2. Process of Atmospheric Correction _ Step 5. Apply Diffuse Transmittance Extract Rayleigh diffuse transmittance

54. A simple Rayleigh diffuse transmittance model2. Process of Atmospheric Correction _ Step 5. Apply Diffuse Transmittance Get aerosol diffuse transmittance from AOT

55. Aerosol model, single scattering reflectance, single scattering albedo, phase function  Get aerosol optical thickness

56. A simple aerosol diffuse transmittance model (Hajime Fukushima, 1998)

57. Using Aerosol+Rayleigh LUT (Future work)

58. A generic data driven method3. Result & Validation _ ResultComparison 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)

59. 3. Result & Validation _ ResultComparison spectrums of GOCI & MODIS (with NASA Standard Algorithm)B1 : 412nmB2 : 443nmB3 : 490nm (MODIS : 488nm)B4 : 555nm (MODIS : 551nm)B5 : 660nm (MODIS : 667nm)B6 : 680nm (MODIS : 678nm)GOCIMODISGOCIMODIS

60. 3. Result & Validation _ ResultComparison 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)

61. 3. Result & Validation _ ValidationComparison 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)SSMMSGCANASA Standard (MODIS)

62. 4. Conclusion _ NASA Standard Algorithm for the GOCI

63. Basic schema is all implemented.

64. Need to improve the ocean color model

65. Add more good arrangement aerosol models

66. Need to consider the new aerosol model for the GOCI observation area

67. Change to the look up table based diffuse transmittance estimation

68. Aerosol model selection and weight method update

69. SSMM

70. Looks reasonable but needs more tuning

71. Better result high turbidity water and blue absorption aerosol case

72. Also consider about horizontal aerosol type changes

73. Collect more reference site

74. SGCA

75. Relatively good matching at the high optical thickness case

76. Improvement for turbid water

77. Needs more local tuningTHANK YOU