Ocean gliders move both horizontally and vertically by changing buoyancy using an oil-filled bladder. Oil is pumped into or out of the bladder to change its volume, which causes the glider to ascend or descend. The glider is steered by an internal computer to follow a pre-programmed course. Periodically the glider will rise to the surface to determine its location and transmit a summary of its data and diagnostic information to a satellite. Then it will receive commands to descend again and cruise to its next programmed location. Gliders are frequently used by biological and physical oceanographers to investigate ocean conditions such as temperature, salinity, and currents.

        Acoustic gliders (yellow instrument shown above) use hydraulic buoyancy changes to alter the vehicle’s density in relation to the surrounding water thereby causing the vehicle to either float or sink. The glider periodically surfaces, so that it can relay (to a satellite or a ship) a navigational fix, transmit its data, and receive further instructions for command and control. Image courtesy of Teledyne Webb Research.

      An acoustic glider is an ocean glider that includes a hydrophone to record acoustic signals. Acoustic gliders work continuously in all weather conditions for long time periods, possess a suite of high-resolution oceanographic sensors, and operate silently (because they do not have an engine or propeller). They move very slowly, at about 0.50 knots (about 25 cm/s). Researchers use acoustic gliders to study whale distribution, behavior, and habitat. Scientists are also using gliders to study acoustic propagation.

      Since acoustic gliders can be deployed for very long periods of time (days to months), travel long distances (up to thousands of kilometers) if needed, and operate silently, they are an ideal tool for passive acoustic monitoring of whales. Acoustic gliders have been used to study sei whales,North Atlantic right whales, and beaked whales. hey have also been used to track acoustically tagged fish. Typically, tagged fish are monitored by stationary hydrophones, but given that many fish are migratory animals, they may not always be in the area of the hydrophone. This may create large geographic data gaps in a data set. As the gliders actively follow tagged fish, scientists can learn about where the fish go and also measure physical, chemical, and biological aspects of the ocean at the same time.