What are underwater gliders?

Gliders are small autonomous underwater vehicles which were developed to carry out in-situ observations of the upper 1km of the ocean filling the gaps left by the existing observing systems. Different glider models have been developed, among with the ones developed by the 3 groups in the USA who have first developed operational ones:

• the Seaglider by University of Washington ;
• the Slocum by Webb Research Corp ;
• the Spray by Scripps Institution of Oceanography.
• the Exocetus by Exocetus Autonomous Systems.
• the SeaExplorer by Acsa-Alcen.
• the SeaWing by the China's Academy of Sciences Institution of Oceanology.

Although the designs are different, they have many features in common. They all have a small size (about 1m50 long and 20cm in diameter) and their weight is around 50kg in air (and +/-200g in water). 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 the bottom of the ocean or maximum depths of typically 100-1000m (and recently 6000m depth), along re-programmable routes (using two-way satellite link). There is around ~2-6 km between surfacing when diving to 1km depth. They achieve vertical speeds of 10-20cm/s and forward speeds of 20-40 cm/s thanks to a ballast pump, wings, and rudders, and can be operated for a few months before they have to be recovered (Davis et al., 2002) - see also “how does a glider work?”. They can record temperature, salinity, pressure data and depending on the model some biogeochemical data, such as dissolved oxygen, different fluorescences and/or optical backscatters by using miniaturized sensors on-board.

First conceived in 1986 (Webb, 1986), the idea emerged in 1989 in a 'science fiction' paper (Stommel, 1989) that any glider user must have read (see Stommel's vision). The first prototype flight tests were carried out in 1991 (Simonetti, 1992) and after a 'teenager' period starting around 2002, gliders have now reached a mature stage and are being incorporated into the operational technology portfolios of numerous research institutions and agencies.

It has been now demonstrated that gliders are able to carry out high resolution measurements of not only physical (temperature and salinity as well as average velocities over a dive, and velocity profiles if equipped with small ADCP) but also biogeochemical parameters like dissolved oxygen, fluorescences (Chla, CDOM, phycoerythrin,…) and optical backscattering at various angles/wavelengths (turbidity, particle size…). They can also be equipped with acoustic modems, echosounders, and hydrophones for underwater positioning, underwater data telemetry, estimating stocks of zooplankton, track fish or listening to sea mammals. Other sensors have been integrated for measuring microturbulence, nitrates, ph/pCO2, hydrocarbons, ambient noise, scintillation, and imagery.

The objective is not only to participate to the global ocean observing system but also to enable frontier science through innovative sampling. Applications include in particular the simultaneous use of a fleet of gliders. Such concerted deployments enable novel multidisciplinary 3-dimensional surveys of the upper ocean interior.