Active drifters: Sailing with the Ocean Currents

The ocean is a highly dynamical system in both temporal and spatial dimensions, which makes oceanographic and marine biological studies challenging. One important class of tasks for gaining deeper understanding of the ocean is the tracking of an advection of ocean phenomena, such as algae blooms or oil spills, caused by ocean currents. One approach to this tracking task is to tag a current through passive Lagrangian drifters. A drifter is a buoy that consists of a surface float, i.e., a sealed container floating on the surface, and a drogue, which is fixed at a certain depth. The drogue plays basically the role of an underwater “sail” that makes the drifter travel passively with the ocean current at the corresponding water layer (see Fig. 1). Traditionally, a drifter acts as a simple reference point which takes no or just a small part of measurements. More measurements can be carried out with the help of an additional active vehicle like a ship or an autonomous underwater vehicle (AUV) [1]. However, this approach comes with the disadvantage of increased complexity and significant financial expenses due to high ship and AUV operating costs. This may affect the scope of scientific studies and lead to shortened temporal domains and sacrifices in the spatial resolution of collected data. Thus, oceanographers and marine biologists can profit from a simple and inexpensive platform which nevertheless offers enough autonomy to carry out long missions and make decisions actively. Such a platform does not have to be capable of executing rapid maneuvers; agility can be traded off for endurance. For this reason, we suggest to turn a traditional passive drifter into an active drifter. By actuating the drogue to adjust it in depth (see Fig. 1 on the right), the drifter is capable of exploiting the stratification of the ocean, i.e., it can actively select from the different layers of ocean currents, each of which potentially provides a different current vector. This enables the system to 1) measure the vectors of ocean currents directly at varying depths on site, and 2) obtain (limited) control capability. Recently, a few alternative active drifter systems have been presented. Following a similar idea to ours, [2] introduces a small profiling drifter that can raise and lower its drogue via a winch, and [3] uses a free-floating drogue vehicle which is capable of submerging by a change in buoyancy. Based on a similar principle, larger and more complex Lagrangian profilers can also be operated as active drifter systems [4, 5]. antenna