IFREMER -French Research Institute for the Exploitation of the Sea -is currently operating manned submersibles, towed sonars and ROVs up to 6,000 m. In most cases, long base line (LBL) a r e to be replaced by a new Ultra Short Base Line (USLB) positioning system called POSIDONIA 6000. This system was developed by Thomson Marconi Sonar and Mors Environment. It reduces operators time and overall costs by improving accuracy, reliability in noisy environments and reducing multipath effects. After a complete description of the system, the paper presents results of sea operations with SAR (deep water side scan sonar), VICTOR 6000 (the IFREMER ROV) and NAUTILE (deep manned submersible). These vehicles have been operated a t depths reaching up to 5,400 m in various combinations. A precision better than 1% of water depth was obtained, even in very bad weather conditions.