depth the speed of sound is minimal, in average around 1200 m deep. It acts as a wave-guide for sound, and low frequency sound waves within the channel may travel thousands of miles before dissipating. The SOFAR channel is centred on the depth where the cumulative effect of temperature and water pressure (and, to a smaller extent, salinity) combine to create the region of minimum sound speed in the water column. Near the surface, the rapidly falling temperature causes a decrease in sound speed, or a negative sound speed gradient. With increasing depth, the increasing pressure causes an increase in sound speed, or a positive sound speed gradient. The depth where the sound speed is at a minimum is the sound channel axis. This is a characteristic that can be found in optical guides. If a sound wave propagates away from this horizontal channel, the part of the wave furthest from the channel axis travels faster, so the wave turns back toward the channel axis. As a result, the sound waves trace a path that oscillates across the SOFAR channel axis. This principle is similar to long distance transmission of light in an optical fiber. In this channel, a sound has a range of over 2000 km. RAFOS float Global idea To use a RAFOS float, one has to submerge it in the specified location, so that it will get carried by the current. Then, every so often (usually every 6 or 8 hours) an 80-second sound signal is sent from moored emitters. Using the fact that a signal transmitted in the ocean preserves its phase structure (or pattern) for several minutes, it has been thought to use signals in which the frequency increases linearly of 1.523 Hz from start to end centered around 250 Hz. Then receivers would listen for specific phase structures, by comparing the incoming data with a reference 80-second signal. This permits to get rid of any noise appearing during the travel of the wave by floating particles or fish. The detection scheme can be simplified by keeping only the information of positive or negative signal, allowing to work with a single bit of new information at each time step. This method works very well, and allows the use of small micro-processors, enabling the float itself to do the listening and computing, and a moored sound source. From the arrival time of the signals from two