- Sea-viewing Wide Field-of-view Sensor.
- It is a
sensor on board the SeaStar spacecraft launched by NASA.
Fig. SeaWiFS Sensor and SeaStar Spacecraft
are its specifications?
does SeaWiFS measure the radiances?
- SeaStar spacecraft has
705 km circular noon sun-synchronous orbit.
- SeaWiFS sensor is an
8-channel optical instrument operating in the visible
- 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.
is required to process the SeaWiFS data?
- 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
the collected radiation into 8 separate wavelength channels and image
the channels onto 4 detectors aligned in the scan direction.
has a tilt mechanism to avoid sunglint.
information, refer to the SeaWiFS
Lr = contribution due to
scattering, which can be accurately computed.
Lw = 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
tLwc = contribution due to
estimated from statistical relationship with wind speed.
La + Lra = contribution due to aerosol
Rayleigh-aerosol scattering, estimated in NIR from measured radiances
visible using aerosol models.
converting voltage measured to corresponding brightness or radiance
solar radiation diffuser which controls the amount of sunlight falling
on the sensor by changing the aperture of the diffuser.
calibration using spacecraft maneuver so that the sensor gets enough
light during the night time part of its orbit.
Lt = Lr +
(La + Lra) + t Lwc + T Lg + t Lw
- Gaseous absorption
(gases, water vapor, oxygen)
- Molecular/ Rayleigh
scattering and absorption (haze, dust, pollution)
do we finally obtain chlorophyll data from the radiance data?
- SeaWiFS has a default chlorophyll algorithm,
‘chl_oc4’ which outputs product
- 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