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What is it and why do we use it?



How does it work?






What platforms are needed?


Advantages and limitations?




RAFOS floats are designed to take measurements of temperature, salinity, and pressure in layers of ocean water one to three thousand meters below the surface. Each weighs about 10 kilograms, and can be carefully lowered over the side of a vessel by one person. Often two people deploy it using ropes around the glass housing and a quick release 'launching clip' that frees the ropes once the float reaches the water.

The float's electronics are housed in a 6.5 foot (two meter) glass tube. Glass is used to house the instrument because it is relatively inexpensive and is extremely strong under pressure. The glass used for making RAFOS floats is about ? inch (7 mm) thick. An additional benefit of using glass is that the antenna can be contained inside the glass and still transmit data to the satellites.

An acoustic hydrophone 'listens' for sound signals in the ocean, recording the time the signals take place. These sounds can come from anything; ship noise or moored sound sources. The recorded signals are then compared, or correlated, with a signal pattern that matches that of the sound sources. The best matching signals from each listening cycle (varying from a few hours to a few days) are stored in the float?s memory, for transmission at mission's end.

The saved sound signal data are called times-of-arrival (TOAs), and by knowing the positions of the sound sources, the sound velocity in the ocean, and the lengths of time it took to hear each source, the float's position can be triangulated for each listening cycle. These float positions are used to reconstruct the float's journey along deep ocean currents.

The float is ballasted by a stainless steel weight. At the end of a mission, lasting from nine months to two years, the float drop its weight by sending an electrical pulse to a wire that burns and releases the weight. The now-buoyant float rises to the ocean surface and beams its data via the antennae to orbiting satellites.
RAFOS FLOAT IN ACTION - RAFOS floats are deployed using one of two methods. Some floats are attached to a small anchor and dropped over the side of a research vessel, as shown in the first picture. They sink to the bottom and remain there, in a dormant state, until a preprogrammed time when they release the anchor and rise up to their target depth to start their drifting mission.

After completing their drifting mission, these floats release a second weight and rise to the sea surface, where they transmit all their stored data to orbiting satellites which then re-broadcast the information to ground stations, as seen in picture three.

Most floats however begin their drifting missions immediately after deployment from a research vessel or ship of opportunity (shown in picture two). At the end of their drifting mission, they also transmit the data they collected to satellites that relay the information to the scientist.
RAFOS FLOATS IN THE MEDITERRANEAN - The trajectories of four different floats are shown in the formation of Mediterranean eddies, also known as 'meddies' off the southwestern corner of the Iberian Peninsula. The floats were released in 1993 to 1994 south of the Iberian Peninsula into the Mediterranean undercurrent. This is a subsurface current that carries salty water from the Mediterranean (which has left through the Strait of Gibraltar) along the continental slope of the Iberian Peninsula.

