In the late 1980s the World Ocean Circulation Experiment
(WOCE) required global coverage with subsurface floats. This
precluded acoustic tracking and resulted in the development
of the Autonomous Lagrangian Circulation Explorer (ALACE),
a neutrally buoyant float that would surface at regular intervals
by inflating an external bladder, transmit temperature/pressure
data and be positioned by the ARGOS satellite system and then
return to its operating depth. A similar buoyancy-change mechanism
is used in the ALFOS floats (are tracked as is the RAFOS).
Companion of the ALACE is the SLOCUM that derives the energy
needed for buoyancy change from the temperature difference
between the ocean surface and the ambient temperature at its
operating depth. A prototype has been built.