or more sound sources, and the previous location of the float, its current location can easily be determined to considerable (<1 km) accuracy. For instance, the float will listen for three sources and store the time of arrival for the two largest signals heard from each source. The location of the float will be computed onshore. Technical characteristics Mechanical characteristics The floats consist of 8 cm by 1.5 to 2.2 m long glass pipe that contain a hydrophone, signal processing circuits, a microprocessor, a clock and a battery. A float weighs about 10 kg. The lower end is sealed with a flat aluminium endplate where all electrical and mechanical penetrators are located. The glass thickness is about 5 mm, giving the float a theoretical maximum depth of about 2700 m. The external ballast is suspended by a short piece of wire chosen for its resistance to saltwater corrosion. By dissolving it electrolytically the 1 kg ballast is released and the float returns to the surface. Electrical characteristics The electronics can be divided into four categories: a satellite transmitter used after surfacing, the set of sensors, a time reference clock, and a microprocessor. The clock is essential in locating the float, since it is used as reference to calculate the time travel of the sound signals from the moored emitters. It is also useful to have the float work on schedule. The microprocessor controls all subsystems except the clock, and stores the collected data at a regular schedule. The satellite transmitter is used to send data packages to orbiting satellites after the surfacing. It usually takes three days for the satellite to collect all the dataset. The isobaric model An isobaric float aims to follow a constant pressure plane, by adjusting the ballast's weight to attain buoyancy to a certain depth. It is the most easily achieved model. To achieve an isobaric float, its compressibility must be much lower than that of seawater. In that case, if the float were to be moved upwards from equilibrium, it will expand less than the surrounding seawater, leading to a restoring force pushing it downwards, back to its equilibrium position. Once correctly balanced, the float will remain in a constant pressure field. The isopycnal model The aim of an isopycnal float is to follow the density planes, that is to attain neutral buoyancy for constant density. To achieve this, it is necessary to remove