Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site Highlights ► Societies increasingly depend on timely information on ecosystems and natural hazards. ► Data is needed to improve climate-related uncertainty and geo-hazard early warning. ► Observatory networks coordinate and integrate the collection of standardised data. ► Ocean observatories provide opportunity for ocean science to evolve.

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.

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