System for positioning an underwater device (105, 110, 115, 120), comprising: - at least two surface transponders (125, 130, 135), each surface transponder comprising a receiver of radio signals transmitted by at least two satellite sources (165, 170, 175) of signals of a geolocation system, - each surface transponder comprising: - a means for estimating at least one radio pseudo-distance between the surface transponder and at least two sources of signals from the geolocation system, - an attachment to a float configured to have neutral buoyancy at a specific depth, and - a means for communicating information representative of the radio pseudo-distances to the underwater device, and - an underwater acoustic transmitter synchronized to the time of the geolocation system configured to transmit an acoustic signal to the underwater device, - the underwater device comprising: - a means for receiving information representative of the radio pseudo-distances transmitted by at least two surface transponders, - an acoustic signal receiver configured to receive the acoustic signals transmitted by at least two surface transponders, - a means for determining one or more acoustic pseudo-distances between at least two underwater acoustic transmitters and the underwater device, and - a means for calculating the position of the device in a terrestrial frame of reference centered on one of the surface transponders, the means for calculating the position comprising a computing unit executing a probabilistic estimation process performing this estimation by means of at least the following measurements: - a selection of information representative of the radio pseudo-distances between the surface transponder and at least two sources of signals of a geolocation, received by the reception means, - a selection of acoustic pseudo-distances determined by the determination means. - at least two surface transponders (125, 130, 135), each surface transponder comprising a receiver of radio signals transmitted by at least two satellite sources (165, 170, 175) of signals of a geolocation system, - each surface transponder comprising: - a means for estimating at least one radio pseudo-distance between the surface transponder and at least two sources of signals from the geolocation system, - an attachment to a float configured to have neutral buoyancy at a specific depth, and - a means for communicating information representative of the radio pseudo-distances to the underwater device, and - an underwater acoustic transmitter synchronized to the time of the geolocation system configured to transmit an acoustic signal to the underwater device, - a means for estimating at least one radio pseudo-distance between the surface transponder and at least two sources of signals from the geolocation system, - an attachment to a float configured to have neutral buoyancy at a specific depth, and - a means for communicating information representative of the radio pseudo-distances to the underwater device, and - an underwater acoustic transmitter synchronized to the time of the geolocation system configured to transmit an acoustic signal to the underwater device, - the underwater device comprising: - a means for receiving information representative of the radio pseudo-distances transmitted by at least two surface transponders, - an acoustic signal receiver configured to receive the acoustic signals transmitted by at least two surface transponders, - a means for determining one or more acoustic pseudo-distances between at least two underwater acoustic transmitters and the underwater device, and - a means for calculating the position of the device in a terrestrial frame of reference centered on one of the surface transponders, the means for calculating the position comprising a computing unit executing a probabilistic estimation process performing this estimation by means of at least the following measurements: - a selection of information representative of the radio pseudo-distances between the surface transponder and at least two sources of signals of a geolocation, received by the reception means, - a selection of acoustic pseudo-distances determined by the determination means. - a means for receiving information representative of the radio pseudo-distances transmitted by at least two surface transponders, - an acoustic signal receiver configured to receive the acoustic signals transmitted by at least two surface transponders, - a means for determining one or more acoustic pseudo-distances between at least two underwater acoustic transmitters and the underwater device, and - a means for calculating the position of the device in a terrestrial frame of reference centered on one of the surface transponders, the means for calculating the position comprising a computing unit executing a probabilistic estimation process performing this estimation by means of at least the following measurements: - a selection of information representative of the radio pseudo-distances between the surface transponder and at least two sources of signals of a geolocation, received by the reception means, - a selection of acoustic pseudo-distances determined by the determination means. - a selection of information representative of the radio pseudo-distances between the surface transponder and at least two sources of signals of a geolocation, received by the reception means, - a selection of acoustic pseudo-distances determined by the determination means.