Underwater gliders are autonomous vehicles that profile vertically by controlling buoyancy and move horizontally on wings. Gliders were described by Henry Stommel in a visionary article (Oceanography, 1989) that might best be characterized as highly informed science fiction. The first-person narrative, written as if the year were 2021, discussed the first quarter century in the use of floats that “migrate vertically through the ocean changing ballast ... steered horizontally by gliding on wings ... broach the surface six times a day to ... transmit their accumulated data and receive instructions telling them how to steer through the ocean ... [at a] speed [of] generally half a knot.” Stommel’s vision is a reality today nearly exactly as he imagined it would be.

Russ Davis, Jeff Sherman and the Instrument Development Group at Scripps Institution of Oceanography have developed the glider Spray (Figure 1). Spray is 2 m long and weighs 50 kg. It communicates to shore using Iridium, and navigates with GPS. Spray steers by changing its center of mass through the movement of internal heavy battery packs. Spray carries sensors to measure a number of variables, including pressure, temperature, salinity, optical properties, and velocity.

Figure 1. The underwater glider Spray at the surface just after initial deployment.

In typical use, Spray cycles from the surface to 1000 m, traveling 6 km in the horizontal in 6 hours. Spray’s horizontal velocity is thus about 0.25 m/s, and its vertical velocity is roughly 0.1 m/s. GPS and Iridium antennas are in Spray’s wings, so when Spray is on the surface it rolls 90° to navigate and communicate. During communication, Spray sends data to shore, and shore-based pilots can change mission parameters such as waypoints and dive depth. Typical deployment duration is 3-5 months, depending on sensor suite, stratification, dive depth, and profiling speed.

Our Spray operations have grown over the last several years to the point where we are among the world leaders in the number of gliders in the water at any one time. A useful metric is to sum the total amount of time all gliders are in the water during a month and to divide by the duration of that month. The resulting units are glider-days/day, giving the average number of gliders in the water during that month (Figure 2). We typically average over 10 gliders in the water during the past few years.

Acknowledgements. Spray development and operation have been supported by the Office of Naval Research, the National Science Foundation, and the National Oceanic and Atmospheric Administration.

Figure 2. Spray operations measured in terms of glider-days/day, that is the average number of gliders in the water during each month.