
Two types of METOCEAN Data Systems'
drifters were deployed in the Southern
Ocean: physical drifters and optical
drifters.

Physical drifters

The physical drifters used were WOCE
SVP GPS drifters, which measure
location and sea surface temperature.
The basic drifter design is shown in
the schematic to the right.

GPS location data are acquired once
per hour.  Sea surface temperature
measurements are made just before and
after each GPS fix, and averaged.
These hourly data are then transmitted
to ARGOS every 400 s.  ARGOS
positioning occurs if 5 or more GPS
fix attempts have failed.  The typical
lifetime of these drifters in the
Southern Ocean is 4.5 months.

Optical drifters

The optical drifters used
were WOCE/OCM/GPS (Ocean Color
Monitor) Lagrangian drifters.  The
basic drifter design is similar to
that of the physical drifters
(schematic to the right), except that
optical sensors are included in the
surface unit (diagram below) and the
drogue dimensions are different (40 cm
diameter, 12.27 m long) to decrease
its influence on the optical
measurements.

    


These drifters contain sensors that measure the water temperature
and optical sensors from Satlantic, Inc., that measure
downwelling irradiance above the sea surface and upwelling
radiance just below the sea surface .  The irradiance is measured
at a center wavelength of 490 nm and the radiances are measured
at seven wavebands centered at:

            412, 443, 490, 510, 555, 670, and 683 nm.

The 683 nm sensor has a bandpass of approximately 10 nm. The
remaining sensors have a bandpass of 20 nm.   These optical
sensors were calibrated by Satlantic, Inc., before deployment.

Optical measurements were made every 100 s and averaged over a
one hour period.  Sea surface temperature measurements were made
alternately with GPS location fixes each hour, hence SST and GPS
location data are not available at the same time.  Each set of
hourly data was transmitted to ARGOS every 400 s.  The typical
lifetime of these drifters in the Southern Ocean is 3 months.


