A Lagrangian method for the acoustic observation of ocean currents in the interior of the ocean. The main components of a RAFOS system are a minimum of three fixed sound sources moored at intermediate depths and free-drifting floats labeling a certain water mass. The term RAFOS (SOFAR spelled backwards) was selected for this technology to indicate that the sound transmission direction is the opposite of the SOFAR technology. In both cases, the minimum in vertical profiles of sound velocity in the ocean is utilized for communication between the sources and receivers. RAFOS floats are expendable receivers that listen for coded signals from the moored RAFOS sound sources. After their underwater mission is terminated (duration up to two years), RAFOS floats drop a ballast weight, ascend to the surface, and transmit their internally stored data via satellite link to a shore-based receiving station. The RAFOS technology enables acoustic observations of ocean currents to within the millimeter per second range.