pressure induced restoring forces, thus the float has to have the same compressibility as the surrounding seawater. This is often achieved by a compressible element, as a piston in a cylinder, so that the CPU can change the volume according to changes in pressure. An error of about 10% in the setting can lead to a 50 m depth difference once in water. This is why floats are ballasted in tanks working at high pressure. Measures and projects Computing the float's trajectory Once the float's mission is over and the data collected by the satellites, one major step is to compute the float's route over time. This is done by looking at the travel time of the signals from the moored speakers to the float, computed from the emission time (known accurately), the reception time (known from the float's clock and corrected if the clock had moved). Then, because the speed of sound is known to 0.3% in sea, the position of the float can be determined to about 1 km by an iterative circular tracking procedure. The doppler effect can also be taken into account. Since the float's speed is not known, a first closing speed is determined by measuring the shift in time arrival between two transmissions, where the float is considered not to have moved. The Argo project The Argo project is an international collaboration between 50 research and operational agencies from 26 countries that aims to measure a global array of temperature, salinity and pressure of the top 2000m of the ocean. It uses over 3000 floats, some of which use RAFOS for underwater geolocation; most simply use the Global Positioning System (GPS) to obtain a position when surfacing every 10 days. This project has greatly contributed to the scientific community and has issued many data that has since been used for ocean parameters cartography and Global change analysis. Other results Many results have been achieved thanks to these floats, on the global mapping of the ocean characteristics, or for example how floats systematically shoal (upwell) as they approach anticyclonic meanders and deepen (downwell) as they approach cyclonic meanders. On the left is a typical set of data from a RAFOS float. Today, such floats remain the best way to systematically probe the ocean's interior, since it is automatic and self-sufficient. In recent developments the floats have been able to measure different amounts of dissolved gases,