29th Review of Atmospheric Transmission Models Meeting

Aspects of Polarized Radiative Transfer, June 14

A Vector Version of the 6S Radiative A Vector Version of the 6S Radiative Transfer Code for Atmospheric Transfer Code for Atmospheric

Correction of Satellite DataCorrection of Satellite Data

Svetlana Y. Kotchenova & Eric F. VermoteDepartment of Geography, University of Maryland,and NASA GSFC code 614.5

Purpose of the CreationPurpose of the Creation

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Movie credit: Blue Marble Project (by R. Stöckli)Reference: R. Stöckli,  E. Vermote, N. Saleous, R. Simmon, and D. Herring (2006) "True Color Earth Data Set Includes Seasonal Dynamics", EOS, 87(5), 49-55. www.nasa.gov/vision/earth/features/blue_marble.html

The 6S code is a basic RT code used for the calculation of look-up tables (LUTs) in the MODIS (Moderate Resolution Imaging Spectroradiometer) atmospheric correction (AC) algorithm.

Vector 6S

LUTs for the AC algorithm

AC of MODIS Level 1B data

MOD09 (surface reflectance)

different applications

History of the CodeHistory of the Code

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1992: development of 5S (Simulation of a Satellite Signal in the Solar Spectrum) by le Laboratoire d’Optique Atmosphérique

1997: development of 6S (Second 5S) by Vermote et al. for further use in AC

2004: modification of the scalar 6S to account for polarization by Vermote → 6SV

2005: release of a -version of 6SV by Vermote & Kotchenova → 6SV1.0B

2007: release of version 1.1 of 6SV by Vermote & Kotchenova → 6SV1.1 (Second Simulation of a Satellite Signal in the Solar Spectrum, Vector, version 1.1)

Technical DetailsTechnical Details

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RT method: successive orders of scattering

Polarization: the Stokes vector {I, Q, U, V = 0}

Language: Fortran 77

Input file:

Output file:

6SV Features6SV Features

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Spectrum: 350 to 3750 nmMolecular atmosphere:

6 code-embedded & 2 user-defined models

Aerosol atmosphere:

6 code-embedded & 4 user-defined models &

AERONET

homogeneous &non-homogeneous with & without directional effect

(10 BRDF + 1 user-defined models)

Ground surface:

AATSR, ALI, ASTER, AVHRR, ETM, GLI, GOES,

HRV, HYPBLUE, MAS, MERIS, METEO, MSS, TM,

MODIS, POLDER, SeaWiFS, VIIRS, & VGT – 19 in total

Instruments:

6SV Accuracy6SV Accuracy

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RT simulations: the number of calculation layers and angles

default: 30 layers and 48 (Gaussian) angles (accuracy 0.4%)

validation work: 50 layers and 148 angles

Aerosol simulations: the number of scattering phase function angles

default: 83 angles (including 0°, 90°, and 180°)

validation work: depends on the case

Accuracy-control file: paramdef.inc

6SV Validation Effort6SV Validation Effort

The complete 6SV validation effort is summarized in two manuscripts:

S. Y. Kotchenova, E. F. Vermote, R. Matarrese, & F. Klemm, Validation of a vector version of the 6S radiative transfer code for atmospheric correction of satellite data. Part I: Path Radiance, Applied Optics, 45(26), 6726-6774, 2006.

S. Y. Kotchenova & E. F. Vermote, Validation of a vector version of the 6S radiative transfer code for atmospheric correction of satellite data. Part II: Homogeneous Lambertian and anisotropic surfaces, Applied Optics, in press, 2007.

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validation against real

measurements

testing against benchmarks

comparison with other RT codes

resolution of previous 6S accuracy issues

Benchmarks (Coulson’s Tables)Benchmarks (Coulson’s Tables)

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Coulson’s tabulated values represent the complete solution of the Rayleigh problem for a molecular atmosphere.

Standard RT code accuracy requirement: 1%Reference: Muldashev et al., Spherical harmonics method in the problem of radiative transfer in the atmosphere-surface system, Journal of Quantitative Spectroscopy and Radiative Transfer, 61 (3), 393-404, 1999.

Benchmarks (Monte Carlo)Benchmarks (Monte Carlo)

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Languages: Fortran, C.

Limitations: large amounts of calculation time and angular space discretization.

The code is written by F.M. Bréon (le Laboratoire des Sciences du Climat et de l'Environnement, France) based on the Stokes vector approach.

Previous 6S Accuracy IssuesPrevious 6S Accuracy Issues

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Raised in: A. Lyapustin, “Radiative transfer code SHARM-3D for radiance simulations over a non-Lambertian non-homogeneous surface: intercomparison study,” Applied Optics, 41, 5607-5615 (2002).

1. Inability to model aerosols with a highly-asymmetric scattering phase function

2. Incorporation of surface BRDF influence through approximate formulas

resolved

resolved

Experimental MeasurementsExperimental Measurements

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IKONOS reflectances corrected using AERONET and 6SV vs. reference tarp (portable brightness targets)

reflectances

Ret

riev

ed ta

rp r

efle

ctan

ceMeasured tarp reflectance

The data were acquired over the Stennis Space Flight Center on February 15, 2002.

Corrected MODIS Aqua water-leaving reflectances vs. MOBY water-leaving reflectances measured at

= {412; 443; 490; 530; 550} nm.

The MOBY data were collected off the coast of Lanai Island, Hawaii, during 2003.The analysis is done by R. Matarrese (University of Bari, Italy).

Effects of PolarizationEffects of Polarization

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Why is it so important to account for polarization?

The maximum relative error is more than 7%.

Example:

6SV Web Site6SV Web Site

http://6S.ltdri.org

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6SV Interface6SV Interface

We provide a special Web interface which can help an inexperienced user learn how to use 6SV and build necessary input files.

This interface also lets us track the number and location of 6SV users based on their IP addresses.

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6SV Users (over the World)6SV Users (over the World)

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Total: 898 users

6SV Users (in Europe)6SV Users (in Europe)

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6SV Users (Distribution per Country)6SV Users (Distribution per Country)

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6S Users per Country

0 50 100 150 200 250

United StatesIsraelIndia

ChinaFrance

ItalyJapanBrazil

CanadaSpain

AustraliaGermany

United KingdomSweden

SingaporeFinlandRussia

ThailandUkraine

HungaryMalaysia

PhilippinesAustria

ColombiaIran

New ZealandNorway

PortugalSouth AfricaSouth Korea

SwitzerlandUnited Arab

DenmarkMexico

NetherlandsAlgeria

BelarusBelgium

ChileCosta Rica

Czech RepublicGreece

PakistanPoland

RomaniaSaudi Arabia

6SV e-mail distribution list: 145 users

Vector RT CodesVector RT Codes

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6SV

RT3

Monte Carlo

MODTRAN

VPD

available (publiclyor by request)

publicly not available

Vector RT codes capable of simulating the reflection of solar radiation by

a coupled atmosphere-surface system (top-of-atmosphere reflectance):

?

Code Comparison Project (Description)Code Comparison Project (Description)

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SHARM(scalar)

RT3

Coulson’s tabulated

values(benchmark)

Dave Vector

Vector 6S

Monte Carlo(benchmark)

All information on this project can be found at http://rtcodes.ltdri.org

Code Comparison Project (Rationale)Code Comparison Project (Rationale)

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All participating codes are used in different remote sensing applications.

6SV: MODIS atmospheric correction & aerosol retrieval

RT3: MODIS aerosol retrieval

VPD: TOMS (Total Ozone Mapping Spectrometer) aerosol inversion

SHARM: MAIAC (Multi-Angle Implementation of Atmospheric Correction for MODIS)

Code Comparison Project (Goals)Code Comparison Project (Goals)

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to illustrate the differences between individual simulations of the codes

to determine how the revealed differences influence on the accuracy of atmospheric correction and aerosol retrieval algorithms (Levy et al., Effects of neglecting polarization on the MODIS aerosol retrieval over land, IEEE Transactions on Geoscience and Remote Sensing, 42 (11), 2004 → no effect on aerosol retrieval)

to create a reference (benchmark) data set

The results will be summarized in a manuscript

Project Web site

scientific report

Code Comparison Project (Results)Code Comparison Project (Results)

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Molecular atmosphere: done

Example: TOA reflectances of the codes vs. Coulson’s tabulated values for τmol = 0.25.

Aerosol atmosphere: results are ready for 6SV and SHARM

Mixed atmosphere: not done yet

Accuracy vs. SpeedAccuracy vs. Speed

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Mode of

6SV1

Time, seconds

Number of scattering phase function angles

83 (default)

123 153 203

vector4.93

(3.85)7.24

(5.62)8.81

(6.65)11.20(8.20)

scalar2.39

(1.13)3.06

(1.35)3.71

(1.52)4.42

(1.69)

Example: Time required to simulate TOA reflectance measured by MODIS band 3 over Midway Islands (values in () show how much time is required when the pre-computed aerosol model is read from a file).

Computer: Pentium 4 CPU 2.80GHz

Reference: Kotchenova & Vermote, 2007

6SV Applications6SV Applications

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Atmospheric correction (MODIS, VIIRS, etc.)

MODIS TOA reflectance MODIS surface reflectance

The MODIS data were collected over Alta Foresta on July 16, 2003.

Reference data sets for product validation

Different atmospheric RT simulations

Theoretical Error BudgetTheoretical Error Budget

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Overall theoretical accuracy of the atmospheric correction method considering the error source on calibration, ancillary data, and aerosol inversion for threeτaer = {0.05 (clear), 0.3 (avg.), 0.5 (hazy)}:

Reflectance/ value value valueVI clear avg hazy clear avg hazy clear avg hazyr3 (470 nm) 0.012 0.0052 0.0051 0.0052 0.04 0.0052 0.0052 0.0053 0.07 0.0051 0.0053 0.0055r4 (550 nm) 0.0375 0.0049 0.0055 0.0064 0.0636 0.0052 0.0058 0.0064 0.1246 0.0051 0.007 0.0085r1 (645 nm) 0.024 0.0052 0.0059 0.0065 0.08 0.0053 0.0062 0.0067 0.14 0.0057 0.0074 0.0085r2 (870 nm) 0.2931 0.004 0.0152 0.0246 0.2226 0.0035 0.0103 0.0164 0.2324 0.0041 0.0095 0.0146r5 (1240 nm) 0.3083 0.0038 0.011 0.0179 0.288 0.0038 0.0097 0.0158 0.2929 0.0045 0.0093 0.0148r6 (1650 nm) 0.1591 0.0029 0.0052 0.0084 0.2483 0.0035 0.0066 0.0104 0.3085 0.0055 0.0081 0.0125r7 (2130 nm) 0.048 0.0041 0.0028 0.0042 0.16 0.004 0.0036 0.0053 0.28 0.0056 0.006 0.0087NDVI 0.849 0.03 0.034 0.04 0.471 0.022 0.028 0.033 0.248 0.011 0.015 0.019EVI 0.399 0.005 0.006 0.007 0.203 0.003 0.005 0.005 0.119 0.002 0.004 0.004

Forest Savanna Semi-aridAerosol Optical Depth Aerosol Optical Depth Aerosol Optical Depth

Reference: Vermote, E. F. & El Saleous, N. Z. (2006). Operational atmospheric correction of MODIS visible to middle infrared land surface data in the case of an infinite Lambertian target, In: Earth Science Satellite Remote Sensing, Science and Instruments, (eds: Qu. J. et al),  vol. 1, chapter 8, 123 - 153.

Performance of the MODIS C5 algorithmsPerformance of the MODIS C5 algorithms

To evaluate the performance of the MODIS Collection 5 algorithms, we analyzed 1 year of Terra data (2003) over 127 AERONET sites (4988 cases in total).

Methodology:

http://mod09val.ltdri.org/cgi-bin/mod09_c005_public_allsites_onecollection.cgi26

If the difference is within ±(0.005+0.05ρ), the observation is “good”.

Subsets of Level 1B data processed using the standard surface

reflectance algorithm

Reference data set

Vector 6S

AERONET measurements

(τaer, H2O, particle distribution)

comparison

Validation of MOD09 (1)Validation of MOD09 (1)

Comparison between the MODIS band 1 surface reflectance and the reference data set.

The circle color indicates the % of comparisons within the theoretical MODIS 1-sigma error bar:green > 80%, 65% < yellow <80%, 55% < magenta < 65%, red <55%.

The circle radius is proportional to the number of observations.

Clicking on a particular site will provide more detailed results for this site.27

Validation of MOD09 (2)Validation of MOD09 (2)

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Example: Summary of the results for the Alta Foresta site.

Each bar: date & time when coincident MODIS and AERONET observations are available

The size of a bar: the % of “good” surface reflectance observations

Scatter plot: the retrieved surface reflectances vs. the reference data set along with the linear fit results

Validation of MOD09 (3)Validation of MOD09 (3)

In addition to the plots, the Web site displays a tablesummarizing the AERONET measurementand geometrical conditions, and showsa browse image of the site beforeand after atmospheric correction.

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Percentage of good:

band 1 – 86.62% band 5 – 96.36%

band 2 – 94.13% band 6 – 97.69%

band 3 – 51.30% band 7 – 98.64%

band 4 – 75.18%

NDVI – 97.11% EVI – 93.64%

TOA

MOD09-SFC

Similar results are available for all MODIS surface reflectance products (bands 1-7).

DrawbacksDrawbacks

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Ground Surface

Ozone, Stratospheric Aerosols

8 Km

20 Km

Molecules (Rayleigh Scattering)

H2O, Tropospheric Aerosol

2-3 Km

O2, CO2

Trace Gases

1. Compared to MODTRAN:

MODTRAN

Vertical variationof aerosol type

6SV

Only vertical variation of aerosol profile

2. Compared to SHARM:

Speed

never heard any complaints except from the SHARM developer

Future PlansFuture Plans

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Non-spherical particles

Solution: combination of 6SV and software for the simulation of non-

spherical particles (developed by T. Lapyonok et al. (AERONET group));

preliminary combination work is done by R. Levy (AERONET group).

Polarized surface model

Solution: in process

Thanks!Thanks!

Thank you for your attention!Thank you for your attention!

Questions: 6S@ltdri.org

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