Atmospheric Correction forOcean Color Remote Sensing

Geo 6011

Eric Kouba

Oct 29, 2012

Ocean Color Overview

Measured data Top of Atmosphere Radiance

Need to do Atmospheric Correction

Desired signal Water-Leaving Radianceor Remote Sensing Reflectance

****************************************************************************Use other equations and methods...

Proxy parameter e.g. Chlorophyll-A concentration

Biological parameter Phytoplankton primary productivity

Desired goal Information about health of the ocean

Sunlight to Surface to Sensor

1. Solar spectrum at top of atmosphere

2. Atmospheric absorption, scattering, etc

3. CloudsThin clouds allow some visibility

4. Reflection from top layer of oceanCase 1 clear waters (tens of meters)Case 2 turbid waters (less penetration)

5. Atmospheric absorption, scattering, etc

6. Radiance measured by satellite sensorOnly 10% to 20% of signal comes from ocean waters

Ocean Color Sensors and Satellites

CZCS sensor on Nimbus-7 satellite

SeaWiFS sensor on OrbView-2 satellite

MODIS sensor on Terra satellite

MERIS sensor on ENVISAT satellite

MODIS sensor on Aqua satellite

ETM+ sensor on LANDSAT satellite

Sensors and Satellites - Pastwww.ioccg.org

Sensors and Satellites - Currentwww.ioccg.org

Sensors and Satellites - Plannedwww.ioccg.org

Major types of correction methodsKnow which one your software uses

Dark object subtraction

Invariant object subtraction

Histogram matching

Cosine estimation of atmospheric transmittance

Contrast reduction

Path extraction

Spectral shape matching method -> CAAS

Others

SeaWiFSseadas.gsfc.nasa.gov

SeaDAS Atmospheric CorrectionWavelength dependent equation

ρT(λi) = ρR + ρA + ρC + ρSG + ρWC + transmittancesurf-sensor * ρW

ρT Top of atmosphere reflectance SensorρRA Rayleigh scattering Not difficultρAS Aerosol scattering DifficultρCPL Coupled Rayleigh-Aerosol effects Assume zeroρSGSun glint Assume zeroρWC Whitecaps (wind on sea surface) Less than 7 m/sρW Water-leaving reflectance Desired

Basically a dark object subtraction, with additionsAssumes NIR (765 and 865 nm) should be zero

12 aerosol models -> 25000 simulation runs -> Lookup tablesWorks well for deep, clear, and low growth waterFails in shallow, turbid, and high growth water

Options when standard correction fails

Flag and ignore regions that are difficult to process

If available, use SWIR instead of NIR for dark subtractionbut MODIS has SWIR signal to noise problem

Simultaneous spectroradiometer measurements in field of viewbut Not practical for daily operations

Take spectroradiometer measurements nearbybut Atmospheric parameters vary in time and space

Use other algorithm with standard atmospheresbut Atmospheric parameters vary in time and space

Use algorithm to get aerosol correction from within image datae.g. Shanmugam (2012)

aeronet.gsfc.nasa.gov

CAAS - Basic Equation

LT(λi) = LR + LA + LC + transsun-surf * LSG + LWC + transsurf-sensor * LW

LT Top of atmosphere reflectance SensorLR Rayleigh scattering Not difficultLA Aerosol scattering EstimatedLC Coupled Rayleigh-Aerosol effects EstimatedLSGSun glint EstimatedLWC Whitecaps (wind on sea surface) Ignored for nowLW Water-leaving reflectance Desired

Use Rayleigh-corrected Radiance to derive Aerosol correction

Spectral shape matching method

See Shanmugam (2012) pg 205-207 for math discussion

Conclusions

NIR dark subtraction fails in shallow, turbid, and high growth waterLike counting worldwide trees, but failing in the jungle

Aerosol modeling remains difficult

Atmospheric correctionis important...

Choose wisely

Questions?