What is it and why do we use it?
The name RAFOS is the word SOFAR spelled backwards. SOFAR is an acronym for SOund Fixing And Ranging, which refers to the way these floats track the motion of water in the ocean. In large numbers, the floats can be used to map the mean currents over a wide area, and for statistical studies of dispersion and mixing. The acoustic tracking provides relatively high-resolution trajectories (with position fixes several times each day). This can be especially valuable in studies of eddies and boundary currents.
Some RAFOS floats are also called Deep Lagrangian Drifters, or DLDs.
How does it work?
A drifting RAFOS float listens and records sound signals from stationary acoustic beacons. At the end of their pre-programmed mission, the floats drop their ballast weights, rise to the surface, and beam their data to orbiting satellites.
A RAFOS float weighs about 22 pounds (10 kilograms) and can be dropped over the side of a small boat by one person, although they are most commonly deployed from large oceanographic research vessels. The float's electronics are housed in a 6 and a half foot (two meter) glass tube. After assembly, the float is placed in a tank that can replicate the pressure of a given depth and is then ballasted with stainless steel weights that are attached at the bottom of the float.
A float's mission can be up to two years in duration. At the end of its mission, a pre-programmed command from the float's microprocessor releases the float's ballast weight. The float rises to the sea surface and beams the stored acoustic tracking data to two satellites, which pass the information to ground receiving stations, which then send it over the Internet to researchers' computers.
RAFOS floats can carry multiple sensors for measuring ocean properties, including pressure, temperature, and dissolved oxygen. They can be made "isopycnal", or density-following, so that they more accurately follow water parcels in regions where density surfaces are steeply sloped.
The predecessor to the RAFOS float, the SOFAR float (no longer used in large numbers but sometimes for special applications), made low-frequency signals periodically, and the signals were heard and recorded by moored listening stations. The backwards spelling of RAFOS refers to the reversed pathway of the sound in tracking these floats. In this case, the moored instruments make the sound, and the drifting floats listen and record the signals.
What platforms are needed?
The moored sound beacons used for tracking the RAFOS floats are usually deployed from oceanographic research vessels by trained mooring technicians. A special version of the float can be temporarily anchored to the ocean floor, and released from the anchor at a pre-set time. This allows for deployment in remote locations and at various times without the need for a research or other vessel.
Advantages and limitations?
The acoustic tracking provides high-resolution trajectories that are particularly valuable for resolving eddy motions and narrow boundary currents. As with other floats, Lagrangian instruments are useful for mapping currents over a large spatial area since they tend to spread out after deployment.
One disadvantage is that the scientist must wait until the end of the float mission, up to two years, for the data to be available. This is balanced by the advantage that by staying submerged, the floats can closely follow the motions of water particles for extended periods of time.
A Mediterranean Undercurrent Seeding Experiment (AMUSE): Part I I : RAFOS Float Data Report May 1993 - March 1995 by Heather D. Hunt, Christine M. Wooding, Cynthia L. Chandler, and Amy S. Bower