Gliders are small autonomous underwater vehicles which were developed to carry out in-situ observations of the upper 1 km of the ocean. They enhance the capabilities of profiling floats by providing some level of maneuverability and hence position control. They perform saw-tooth trajectories from the surface to depths of 1,000 m, along reprogrammable routes (using two-way satellite link). There is around ~2-6 km between surfacing when diving to 1 km depth. They achieve vertical speeds of 10-20 cm/s and forward speeds of 20-40 cm/s and can be operated for a few months before they have to be recovered (Davis et al. 2002). They can record temperature, salinity, pressure data and depending on the model some bio-geochemical data, such as dissolved oxygen and fluorescence/optical backscatter-ing at various angles/wavelengths (Chl-a, CDOM, phycoerythrin, turbidity, etc.). They can also be equipped with acoustic modems and hydrophones for underwater positioning and underwater data telemetry.

Gliders can "fly" underwater along slightly inclined paths without propeller. A change in volume (generated by filling an external oil bladder) creates positive and negative buoyancy. Because of the fixed wings, the buoyancy force results in forward velocity as well as vertical motion. So gliders move on a sawtooth pattern, gliding downward when denser than surrounding water and upward when buoyant. Pitch and roll can be controlled by modifying the internal mass distribution and gliders automatically align the positions of the center of buoyancy and the center of gravity to achieve desired angle of ascent/descent. Either a rudder or a roll control is used for navigation through lists of waypoints. The high efficiency of the propulsion system enables gliders to be operated for several months during which they may cover thousands of kilometers.

Davis et al. (2008) have operated gliders over many years in the eastern pacific to perform repeat sections. Similar long sections along the coasts of the USA in the Pacific, and in the Atlantic (Castelao et al. 2008; Glenn et al. 2008; Perry et al. 2008) demonstrated the capacity of gliders to carry out, over years, measurements of the local vertical structure of the ocean over 0-200 or 0-1,000 m from the near-shore environment (10-100 m depth) to the open sea (hundreds of kilometer offshore). Other important aspects of gliders are (1) the longest glider section ever done with one set of batteries is 6,000 km long (Eriksen and Rhines 2008) and (2) crossing very high currents is possible (such as the Gulf Stream, Nevala 2005). The Australian National facility of ocean Gliders (ANFOG) under IMOS uses gliders to observe the boundary currents and shelf processes around Australia.

Glider technology is advancing quickly, and will be ideal for monitoring water masses and currents in a variety of oceanic regimes. In regions of divergence zones and the boundary currents near the continental slope or steep topographic features, gliders contribute immensely to measuring sub surface parameters. This will also facilitate in understanding meso and sub-meso scale processes. Presently, the assimilation of glider data is already operational for T-S, in regional and global models (Testor et al. 2009). The real time data are being archived at the Coriolis Data Center, Brest, France.

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