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36 st / June 2016 www.sea-technology.com transport and extreme events. Although sea state variables are presently measured by meteo-oceanographic moored buoys, remote sensing, HF radars and shipborne sensors, wave observations still remain sparse relative to the size of oceans, and present best sea state assessments are obtained from model systems. A coordinated and sustained global observing effort for sea state is still missing. A networked, scalable and portable technology for in-situ observation of sea state conditions would signifcantly beneft from the re- alization of such observational effort. Recently, it has been proposed that observations of sea state could beneft from the coverage of glider networks initially deployed for monitoring the ocean's interior. The approach contemplates deriving sea state variables when gliders periodically surface to reposition and establish com- I n 1989, the famous oceanographer H. Stommel envisioned by 2021 the exis- tence of a World Ocean Observing System capable of monitoring the ocean by using a feet of small, intelligent and relatively cheap platforms that he called Slocums. These were defned as "neutrally-buoyant foats…that draw their power from the tem- perature stratifcation of the ocean". In his vision, Slocums migrated vertically through the water column by changing buoyancy, while being steered horizontally by glid- ing on wings. They could operate for years providing, in near-real time and in an un- manned fashion, data about ocean temper- ature, salinity, oxygenation, nutrients and geochemical tracers. Observational ocean- ography would then be transformed from ship-based capabilities to a network of sen- sor nodes. Scalability and rapid portability are the major advantages of this networked observational concept. Thanks to recent technological develop- ments, Stommel's fction has become real and many research organizations around the world operate oceanographic platforms similar to those proposed by him. These vehicles, called underwater gliders, are ro- botic systems with a hydrodynamic shape that make use of buoyancy changes and lifting surfaces to carry out saw-tooth pattern motions be- tween the surface and a predefned depth with a net hori- zontal displacement at about 0.4 m/s. Glider motions are controllable to some degree and their endurance can reach several months. During a mission, a glider periodically sur- faces to reposition and establish communications with the base station. Time at the surface is programmable and usu- ally ranges from a few minutes up to an hour depending on the amount of information to be transmitted. Glider-based observing networks are continuing to develop. Observations of sea state are not only particularly rel- evant to coastal and offshore human activities, but they also infuence knowledge about air-sea fuxes and provide com- plementary information of relevance to monitoring changes in the marine environment, e.g., in winds, storms, sediment Slocum Gliders as Relocatable Waveriders of Opportunity Increasing Underwater Glider Sensing Capabilities With Smart Payloads By Bartolome Garau • Dr. Alberto Alvarez Slocum glider at sea surface.