SeaWiFS

• Sea-viewing Wide Field-of-view Sensor.
• It is a sensor on board the SeaStar spacecraft launched by NASA.

What are its specifications?

• SeaStar spacecraft has 705 km circular noon sun-synchronous orbit.
• SeaWiFS sensor is an 8-channel optical instrument operating in the visible spectrum. 
• Equator crossing at noon + 20 mins, descending.
• Telemetry 2 streams, LAC/HRPT (Local Area Coverage/ High Resolution Picture Transmission) and GAC (Global Area Coverage). 
• Swath width of 2801 km in LAC/HRPT stream and 1502 km in GAC stream.
• Spatial resolution of 1.1 km LAC and 4.5 km GAC.
• Data rate 665 kbps and digitization of 10 bits.

How does SeaWiFS measure the radiances?

• SeaWiFS has a folded telescope and rotating half-angle mirror that rotates at half the speed of the telescope.
• Incoming radiation is collected by telescope and reflected onto mirror.
• Radiation is split into 4 wavelength intervals containing 2 spectral bands each.
• 4 aft-optics assemblies separate the collected radiation into 8 separate wavelength channels and image the channels onto 4 detectors aligned in the scan direction.
• Scanner has a tilt mechanism to avoid sunglint.
  

 What correction/calibration is required to process the SeaWiFS data?

• Calibration: converting voltage measured to corresponding brightness or radiance values.
• Solar calibration using solar radiation diffuser which controls the amount of sunlight falling on the sensor by changing the aperture of the diffuser.
• Lunar calibration using spacecraft maneuver so that the sensor gets enough light during the night time part of its orbit.
• Atmospheric Correction Equation
          Lt = Lr + (La + Lra) + t Lwc + T Lg + t Lw

            Where,
                    Lt = Radiance measured.
                    Lr = contribution due to molecular (Rayleigh) scattering, which can be accurately computed.
                    L_w = quantity we wish to retrieve at each wavelength.
                    TLg = Sun glint, the direct reflectance of the solar radiance from the sea surface. This effect is avoided through tilting.
                    tLwc = contribution due to "white"-capping, estimated from statistical relationship with wind speed.
                    La + Lra = contribution due to aerosol and Rayleigh-aerosol scattering, estimated in NIR from measured radiances and                                                       extrapolated to visible using aerosol models.

• Atmospheric corrections:   
• Gaseous absorption (gases, water vapor, oxygen)
• Molecular/ Rayleigh Scattering
• Aerosol scattering and absorption (haze, dust, pollution)
                         

How do we finally obtain chlorophyll data from the radiance data?

• SeaWiFS has a default chlorophyll algorithm, ‘chl_oc4’ which outputs product ‘chlor_a’.
• Algorithm inputs are the retrieved remote sensing reflectances, ‘Rrs_nnn’ given by
              Rrs_nnn = nLw_nnn / F0_nnn
      where F0 is mean solar irradiance and nnn is wavelength.
• Chlorophyll concentration is calculated by

For more information, refer to the SeaWiFS website.


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