Circling the Seas: Design of Lagrangian Drifters for Ocean Monitoring

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Abstract
Lagrangian drifters are oceanographic devices used to study circulation patterns in the ocean. The devices are typically passive, which means that they have no actuators to initiate motion. They are mainly driven by the ocean currents. A drifter, as depicted in Figure 1, in principle, consists of a surface float and a drogue connected via a tether. The surface float provides the necessary buoyancy to hold the drogue at a certain depth, whereas the drogue (and, thus, the whole drifter) is carried by the currents prevailing at that depth of the ocean. A main focus of oceanographers' and marine biologists' research is the complex dynamics of the ocean, the circulation patterns of water, and the induced movements of effluents, larvae, and other microorganisms [1]. Drifters are used to tag and track the ocean currents and can help to better explain both oceanographic as well as biological phenomena. Our particular interest is the monitoring task of tracking wastewater plumes in the coastal ocean. In this article, we describe the design of low-power, low-cost Lagrangian drifters for ocean monitoring, developed at the Robotic Embedded Systems Laboratory (RESL) at the University of Southern California (USC). We discuss the design challenges and present the overall system characteristics of the RESL drifters. The drifters are augmented by radio communication, which simplifies the task of drifter recovery by a research vessel and enables setting up a bidirectional wireless sensor network between drifters, autonomous underwater vehicles (AUVs), vessels, and base stations.

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