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3.2.2 Lagrangian Drifters
Drifting buoys devoid of instrumentation constituted the basis for Lagrangian current measurements for many years before the advent of modern electronics. Current
slip (decoupling of buoy movement from current drift) is minimized using submerged ballasted drogues tethered to the buoy at the depth of interest. Drogues were
often constructed with sheet metal or plywood cross-planes but even aviation parachutes were successfully used. Curiously, piano wire was the tether of choice for
deep water deployments, practicable but laborious, to depths down to 1 km. Drogue
buoys were followed at sea and vessel navigation with the buoy alongside was used
to plot the buoy trajectory.
Visual location and triangulation using a compass and sextant or transit was a
convenient method for obtaining coastal current data by deploying appropriately
drogue-stabilized buoys and carrying out repeated simultaneous readings from two
or more land-based stations of known location, often capes or promontories. This
effort however could only be maintained over periods of hours and nocturnal readings required a light beacon aboard the buoy.
Two global drifter programs contribute to advancing coastal ocean observations
through their use in calibration of satellite temperature data thus providing sea surface temperature maps extending into the coastal zone. The Surface Velocity
Program, an offshoot of the World Ocean Circulation Experiment (WOCE), arose
from the need to accurately determine surface drift in the world oceans. Several
United Nations agencies as well as private and state institutions from member
nations participated during WOCE and continue to do so in the Global Drifter
Program now a part of the Global Ocean Observing System (GOOS). The stated
objective of the GDP is:
To meet the need for an accurate and globally dense set of in-situ observations of mixed
layer currents, sea surface temperature, atmospheric pressure, winds and salinity.
As implied, GDP buoys are currently equipped with temperature, atmospheric
pressure, and conductivity sensors in addition to the original telemetry/navigation
instrument suite. Rather than using the GPS system, these platforms rely on the
Doppler shit of a buoy signal received by the ARGOS satellite constellation allowing precision of between 150 and 1000 m as required. Central to the program is the
Holey Sock Buoy (holey because of the holes in the unusual drogue design consisting
of a wide diameter hoop-reinforced synthetic fabric cylinder with large holes in the
fabric). This design has been shown to minimize slip at an effective depth of 15 m
to 0.7 cm/s in 10 m/s wind speed. For perspective, it is noted that a comparable buoy
that has lost its drogue would slip downwind at 8.6 cm/s under such winds. The
buoys are designed for extended endurance at sea. At this writing 1418 are operationally deployed throughout the world ocean.
Part of the UN-sponsored global GOOS program and of the Global Ocean
Climate Observing System as well, the advanced ARGO profiling buoy system
provides data to depths of 2000 m on temperature, salinity, and, in later versions,
dissolved oxygen. Injecting buoyant fluids from within a cylindrical pressure hull
3.2 Mobile Ocean Observing Platforms
3.2.2 Lagrangian Drifters
Drifting buoys devoid of instrumentation constituted the basis for Lagrangian current measurements for many years before the advent of modern electronics. Current
slip (decoupling of buoy movement from current drift) is minimized using submerged ballasted drogues tethered to the buoy at the depth of interest. Drogues were
often constructed with sheet metal or plywood cross-planes but even aviation parachutes were successfully used. Curiously, piano wire was the tether of choice for
deep water deployments, practicable but laborious, to depths down to 1 km. Drogue
buoys were followed at sea and vessel navigation with the buoy alongside was used
to plot the buoy trajectory.
Visual location and triangulation using a compass and sextant or transit was a
convenient method for obtaining coastal current data by deploying appropriately
drogue-stabilized buoys and carrying out repeated simultaneous readings from two
or more land-based stations of known location, often capes or promontories. This
effort however could only be maintained over periods of hours and nocturnal readings required a light beacon aboard the buoy.
Two global drifter programs contribute to advancing coastal ocean observations
through their use in calibration of satellite temperature data thus providing sea surface temperature maps extending into the coastal zone. The Surface Velocity
Program, an offshoot of the World Ocean Circulation Experiment (WOCE), arose
from the need to accurately determine surface drift in the world oceans. Several
United Nations agencies as well as private and state institutions from member
nations participated during WOCE and continue to do so in the Global Drifter
Program now a part of the Global Ocean Observing System (GOOS). The stated
objective of the GDP is:
To meet the need for an accurate and globally dense set of in-situ observations of mixed
layer currents, sea surface temperature, atmospheric pressure, winds and salinity.
As implied, GDP buoys are currently equipped with temperature, atmospheric
pressure, and conductivity sensors in addition to the original telemetry/navigation
instrument suite. Rather than using the GPS system, these platforms rely on the
Doppler shit of a buoy signal received by the ARGOS satellite constellation allowing precision of between 150 and 1000 m as required. Central to the program is the
Holey Sock Buoy (holey because of the holes in the unusual drogue design consisting
of a wide diameter hoop-reinforced synthetic fabric cylinder with large holes in the
fabric). This design has been shown to minimize slip at an effective depth of 15 m
to 0.7 cm/s in 10 m/s wind speed. For perspective, it is noted that a comparable buoy
that has lost its drogue would slip downwind at 8.6 cm/s under such winds. The
buoys are designed for extended endurance at sea. At this writing 1418 are operationally deployed throughout the world ocean.
Part of the UN-sponsored global GOOS program and of the Global Ocean
Climate Observing System as well, the advanced ARGO profiling buoy system
provides data to depths of 2000 m on temperature, salinity, and, in later versions,
dissolved oxygen. Injecting buoyant fluids from within a cylindrical pressure hull
3.2 Mobile Ocean Observing Platforms