In situ data extraction from ocean sensors

Abstract

There are three basic techniques commonly employed to extract data from an undersea sensor: 1. via a fixed wired link to a surface expression. 2. via a "pop-up buoy, usually incorporating an RF link. 3. via wireless undersea communications. We suggest that the first is impractical in deep or rough conditions (consider the recent oil platform disaster in the Gulf of Mexico). The second consumes considerable energy, especially when winching down the buoy, and it is susceptible for many reasons to mechanical failure. Wireless communications, which encompasses any or all of magnetic, RF, optical, and, especially acoustic methods, has distinct throughput constraints, but it overcomes the limitations of the first two. We focus on this third option, but consider systems in which the communications function and infrastructure is a tightly integrated component of the sensing system itself. The historical focus for wireless communications was primarily on developing assets to support existing sensors - devices that themselves were designed for wired connectivity. This focus led directly to never-ending efforts to maximize communications data rate, even when the information content of the data might require only a modest signaling rate. High data rate signaling generally requires high SNR and/or a very limited multipath environment - or highly complex and energy-demanding signal processing resources. We suggest that the focus of the discussion change to in situ data and signal processing and compression to support appropriate telemetry. This can provide savings in energy consumption and more appropriate allocation of resources. We describe intelligent, autonomous cross-over among several modes of underwater communications, each designed for an appropriate physical range, or data rate, or appropriate level of energy consumption. We discuss these issues through reference to three real-world examples: diver-based sensors and position tracking, a UUV-based data truck which relies on acoustic communications for precision navigation and data recovery, and long term deployment of ocean sensors. In the diver case, we describe a diver-borne integration of RF and acoustic communications (acomms) and the use of the latter for autonomous monitoring of diver location and status. The through-water RF link provides telemetry between a wrist-mount device, several diver-status monitors, and a backpack-mounted acomms modem. The Slocum Glider UUV is now integrated with an acomms modem which provides autonomous guidance to and through remote undersea networks linked by acomms. The UUV may visit any modem-equipped sensor, but in particular is used as a data truck with our Sonar Modem, a combined acomms modem and subsea surveillance sensor. ©2010 IEEE.