Underwater Gliders
A little history of gliders
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Oceanography from 1872 to 2006...
The starting point in oceanography is often set with the first oceanographic research cruise on the HMS Challenger from 1872-1876 leaded by Sir Wyville Thomson. Water samples were collected at 362 stations with simple devices, but in some cases, these devices were not much different than the modern oceanographic equipment.

Remote sensing techniques have improved considerably since then but satellites, airplanes cannot reach more than few meters in the ocean. The ocean has been studied for many years with ship bases measurements and moorings. These have a relatively high cost, which limits their number, and last a little amount of time (from several months for ship cruises to one or two years for moorings). Thus, the ocean has been observed with a very
weak spatial and temporal density.
Developing new technologies...
To increase the resolution of these observations, new technologies have been developed recently such as subsurface floats, Remotely Operated Vehicles (ROV) and Autonomous Underwater Vehicles (AUV).

Autonomous profiling floats
are today very commonly used. They are designed to sink until they reach a certain depth and to resurface every 10 days in order to report an ocean profile via satellite communication. As part of the international Argo program, 3000 of them will be deployed by 2006, thus providing a synoptic view of the ocean. However, as the floats drift with the ocean currents, the location of the measurements impossible to control. Underwater gliders are meant to resolve this problem.
Neutrally buoyant floats to autonomous profiling floats...
The concept of neutrally buoyant float began to be studied in the 1950s by Henry Stommel (USA) and John Swallow (UK) in order to measure ocean currents. The first neutrally buoyant float, the Swallow float was equipped with an acoustic source and circulated in the water at a certain depth communicating with a ship following.

Till the 1980s, several neutrally buoyant floats have been developed throughout the world.

In the
1980s, ALACE (Autonomous LAgrangian Circulation Explorer) and PALACE (Profiling ALACE) floats were developed by Russ E. Davis's group at Scripps Institute of Oceanography and Doug Webb at Webb Research Corporation (WRC). These included GPS and communication via satellite. They could control their depth by controlling their buoyancy to cycle up and down.
Thousands of these floats equipped with CTD sensors were deployed in the 1990s, including for the World Ocean Circulation Experiment (WOCE) and for the Argo array.
The genesis of underwater gliders...
Before the first autonomous profiling floats were deployed, in 1989, Henry Stommel wrote in Oceanography a science fiction article describing the worldwide ocean observation system in 2021. He imagined "a fleet of small neutrally-buoyant floats called Slocums... named after Joshua Slocum, the Yankee skipper who first went around the world singlehanded in a small sailing vessel... They migrate vertically through the ocean by changing ballast, and they can be steered horizontally by gliding on wings at about 35 degrees angle... During brief moments at the surface, they transmit their accumulated data and receive instructions... Their speed is generally about 0.5 knot."

Stommel worked with
Doug Webb in the 1980s in Woods Hole Oceanographic Institute. They were both known to have simple approaches for complex phenomena, simple and novel explanations. Webb imagined a float which could perform thousands of cycle up and down with a thermal ballast engine. With wings and tail, it could control its vertical and horizontal motion. For the thermal engine to be efficient, sufficient thermal differences in the ocean are needed. In 1988, Webb presented his idea to Stommel and they started working on the idea of gliders.
In 1990, Webb and Stommel were awarded a contract by the Office of Naval Technology to develop a battery powered prototype of glider.In January 1991, the first tests of the electric glider SLOCUM were done by WRC. The thermal engine was still being developed in order to be integrated into the glider in future design.

The funding and support by the Office of Naval Research increased in the mid-1990s as part as
Autonomous Oceanographic Sampling Networks (AOSN). Three programs were created to develop the concept of glider : the SLOCUM glider at WRC, SPRAY at the Scripps Institute of Oceanography (named after the name of Joshua Slocum's sailing vessel) and Seaglider at University of Washington.

Today, these glider designs are operational.

Picture by Amos Nachoum
J. Swallow assembling a float on the Discovery II