Collaborative Research: The Next Generation RAFOS Float: A More Capable, Cost-Effective Subsurface Drifter for Observing Deep Ocean Currents

University Of Rhode Island, Kingston RI

Investigators

Abstract

For over 40 years, acoustically tracked subsurface drifters have provided a unique and valuable perspective on deep ocean currents. The most recent generation?called the RAFOS float (SOFAR, or Sound Fixing And Ranging spelled backwards)-- presently provides the only means by which to observe the variable pathways of narrow currents, the orbiting motion of eddies, and dispersion of pollutants below the surface. With additional sensors, RAFOS floats can monitor biological processes, measure vertical gradients and observe vertical velocity while following a specific patch of water. With their high-resolution tracking (provided by an array of moored sound beacons) and inherent tendency to spread over large horizontal areas, acoustically tracked floats complement the widely used moored current meter and Argo float. Arguably, without acoustically tracked floats, there would be much less awareness of the richness of coherent structures in the subsurface ocean and their impact on the transport of heat, salt, nutrients and other water properties. In spite of the many insights gained from the acoustically tracked float, its future is uncertain. Some of the core electronics have not been modernized for many years. Cost and sophisticated ballasting requirements limit more widespread use and development of new enhancements. With this grant, the principal investigators will develop the next generation RAFOS float to be smaller, lower-cost, more efficient, more capable and easier to use. The heart of the new design will be the recently completed and tested 'Fish Chip' an acoustic receiver on a micro-chip that has been developed to track fish using moored sound beacons. It includes a complete receiver (preamplifier, correlator, time-of-arrival algorithms, and data storage management), as well as a 10-bit digitizer to measure temperature and pressure. As part of this grant, the principal investigators plan to bring together the Fish Chip with the other components necessary for full RAFOS float functionality, including task scheduling and data transmission via iridium. Built in to the new 'Fish Chip (FC) RAFOS' will be accommodations for new sensors, an active buoyancy control unit, and provisions for a small acoustic altimeter to add bottom-following capability. Five prototypes will be constructed and tested in a short-term field demonstration in the Gulf of Mexico. It is anticipated that the FC RAFOS float will attract new users and open up new areas of research in oceanography as a result of the addition of ports for new sensors, lower cost, simpler ballasting and bottom-following capability. With its small size and minimal power requirement, the completed FC RAFOS receiver on a micro-chip could be added to any autonomous vehicle for long-range acoustic tracking.