Shallow water Lagrangian floats as versatile sensing and imaging platforms

This paper presents results from recent work using a shallow water Lagrangian float for sea floor imaging and water column profiling. The float is a flexible platform that is able to carry a suite of sensors and position itself at any point in the water column using ballast control. The low cost of the float and minimal peripheral requirements for personnel and handling gear make it an attractive monitoring platform. Initial results are shown for a sea floor classification study using the float to collect detailed camera images close to the bottom and a routine phytoplankton study demonstrating flexible profiling and adaptive sampling in coastal waters. Introduction: The float (Fig. 1) is a low cost environmental monitoring tool designed to work near shore [1], [2]. It has a set of capabilities distinct from the more common open ocean float designs [3], and other small [4] and bottom stationing [5] shallow water platforms. It is optimized to operate on time scales of days rather than years, and depths to 100 meters rather than the 2000 meters typical of open ocean Argo floats. The float’s displacement is controlled by a fast acting 450 mL piston type volume changing system. A motor, lead screw and piston change the float’s buoyancy to actively move it up and down in the water column. The control system is based on a feedback linearization approach which compensates for the quadratic velocity of the drag force acting on the float when in motion. Shaped inputs are also used to generate depth reference signals the float is capable of following. A 200 kHz Airmar acoustic altimeter is used to measure the distance to the bottom at all times and has a working range of approximately 100 meters. This allows the float to perform constant depth or altitude drifting, and several profiling functions while controlling its vertical position in the water column. In regions of varying bathymetry the altimeter is required to keep the float from hitting the bottom and allows it to drift at a constant altitude off the bottom even as the total water depth changes. At the completion of a mission the float can drop a one kilogram expendable weight to gain more buoyancy on the surface. The weight will also be dropped during a mission by a dead man timer should the microcontoller have a fault or an excessive internal humidity threshold is detected. A GPS and Iridium system is used to record GPS when the float is on the surface and then report that position back via a satellite phone data message. C. Roman is an Assistant Professor of Oceanography in the Graduate School of Oceanography at the University of Rhode Island, Narragansett, RI 02882 USA [email protected] G. Inglis and C. Tennant are students in the Department of Ocean Engineering at the University of Rhode Island, Narragansett, RI 02882 USA [email protected] and [email protected] 1 meter