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.

Gaseous Absorption

Transparency Windows

Light Paths to the Sensor

Scattering and Attenuation

Atmospheric Correction Equation

L_t = L_r + (L_a + L_ra) + tL_wc + TL_g + t L_w

L_w is the quantity we wish to retrieve at each wavelength.
TL_g is Sun glint, the direct reflectance of the solar radiance from the sea surface. This effect is avoided through tilting.
tL_wc is the contribution due to "white"-capping, estimated from statistical relationship with wind speed.
L_r is the contribution due to molecular (Rayleigh) scattering, which can be accurately computed.
L_a + L_ra 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

rainbow line

SeaWiFS Project Home Page

Bryan Franz(franz@seawifs.gsfc.nasa.gov)