A neutrally buoyant float designed to study the subsurface flow in the oceans and making use of the SOFAR (SOund Fixing and Ranging) acoustic channel, present at about 1 000–1 300 metres in tropical and sub-tropical oceans. It was originally named the Swallow float after the inventor. Floats were fitted with a piezoelectric transducer, and the sounds, which were found to propagate over hundreds of kilometres, were recorded by a moored, autonomous listening system (ALS). The system was used successfully over the whole of the Atlantic basin.
Subsequently, the system was greatly improved by introducing floats that recorded signals from fixed sources. These RAFOS floats (SOFAR spelled backwards to indicate that the direction of the signals was reversed) are much simpler, lighter, and cheaper. Three or four moored SOFAR floats are used as sources and the positions of RAFOS floats are obtained by triangulation, with an accuracy of a few kilometres. At the end of the mission, the floats drop ballast and return to the surface, and transmit their data by satellite link. Further developments have produced isopycnal floats, which remain within a specific parcel of water, regardless of its temperature or salinity; floats that follow bottom water that spills from one ocean basin to another; and floats that may be used in areas of strong upwelling.
Subjects: Meteorology and Climatology.