NASA Educational Workshop
SeaWiFS: Atmospheric Correction
We want to measure the "color" of the ocean, but we actually measure ocean + atmosphere. The atmosphere is 90% of the signal, and it must be accurately modeled and removed.
Effects of the Atmosphere
Gaseous absorption (ozone, water vapor, oxygen).
Molecular scattering (air molecules), also referred to as Rayleigh scattering. Scattering efficiency decreases with wavelength as l-4. Reason for blue skies and red sunsets.
Aerosol scattering and absorption (haze, dust, pollution). Whitens or yellows the sky.
Light Paths to the Sensor
Scattering and Attenuation
Atmospheric Correction Equation
) + tL
+ t L
is the quantity we wish to retrieve at each wavelength.
is Sun glint, the direct reflectance of the solar radiance from the sea surface. This effect is avoided through tilting.
is the contribution due to "white"-capping, estimated from statistical relationship with wind speed.
is the contribution due to molecular (Rayleigh) scattering, which can be accurately computed.
is the contribution due to aerosol and Rayleigh-aerosol scattering, estimated in NIR from measured radiances and extrapolated to visible using aerosol models.
SeaWiFS "True-Color" Image Near Hawaii
Retrieval of Lw from typical pixel near Hawaii
SeaWiFS Derived Chlorophyll Near Hawaii
SeaWiFS Calibration and Validation Process
SeaWiFS Lunar Observations
Lunar Viewing Geometry
SeaWiFS field-of-view = 1.6 x 1.6 milliradians
Distance to Earth = 705 km
Distance to Moon = 384400 km
Nadir pixel size on Earth = 705 x 0.0016 = 1.1 km
Nadir pixel size on Moon = 384400 x 0.0016 = 615 km
Diameter of Moon = 3478 km (approximately 6 pixels)
SeaWiFS Lunar Calibration
Correction for Long-term Detector Degradation
Comparison with in situ (field) measurements
SeaWiFS Project Home Page