RAFOS float

A RAFOS float is a specific type of float used to map ocean currents in a [Lagrangian description|Lagrangian] way. It is a self-sustaining device using the SOFAR (SOund Fixing And Ranging) channel as a waveguide for sound, allowing it to determine its position as a function of time. It derives from SOFAR floats, wich emitted from the ocean to moored recievers, but instead recieves the signals from the moored devices, thus allowing a much lower fabrication cost. That is why it is called RAFOS: it is SOFAR spelled backwards.

Introduction

Of the importance of measuring Ocean Currents

The underwater world is still majoritally unknown. The main reason for it is the difficulty to gather information in situ, to experiment, and even to reach certain places. But the ocean nonetheless is of a crucial importance for scientists, as it covers about 71% of the planet. Knowledge of ocean currents is of crucial imprtance. In important scientific aspects, as the study of global warming, ocean currents are found to greatly affect the Earth's climate since they are the main heat transfer mechanism. They are the reason for heat flux between hot and cold regions, and in a larger sense drive almost every understood circulations. These currents also affect marine debris, and vice versa. In an economical aspect, a better understanding can help reducing costs of shipping, since the currents would help boats reduce fuel costs. In the sail-ship era knowledge was even more essential. Even today, the round-the-world sailing competitors employ surface currents to their benefit. Ocean currents are also very important in the dispersal of many life forms. An example is the life-cycle of the European Eel.

The SOFAR channel

The SOFAR channel (short for Sound Fixing and Ranging channel), or deep sound channel (DSC), is a horizontal layer of water in the ocean at which depth the speed of sound is minimal, in average around 1200 m deep. It acts as a waveguide for sound, and low frequency sound waves within the channel may travel thousands of miles before dissipating. The SOFAR channel is centered on the depth where the cumulative effect of temperature and water pressure (and, to a smaller extent, salinity) combine to create the region of minimum sound speed in the water column. Near the surface, the rapidly falling temperature causes a decrease in sound speed, or a negative sound speed gradient. With increasing depth, the increasing pressure causes an increase in sound speed, or a positive sound speed gradient. The depth where the sound speed is at a minimum is called the sound channel axis. This is a caracteristic that can be found in optical guides. If a sound wave propagates away from this horizontal channel, the part of the wave furthest from the channel axis travels faster, so the wave turns back toward the channel axis. As a result, the sound waves trace a path that oscillates across the SOFAR channel axis. This principle is similar to long distance transmission of light in an optical fiber. In this channel, a sound has a range of over 2000 km.

The RAFOS float

The global idea

To use a RAFOS float, one has to submerge it in the specified location, so that it will get carried by the current. Then, every so often (usually every 6 or 8 hours) a 80 seconds signal is sent. Using the fact that a signal transmitted in the ocean preserves its phase structure (or pattern) for several minutes, it has ben thought to use signals in which the frequency increases linearly of 1.523 Hz from start to end centered aound 250 Hz. Then recievers would listen for specific phase structures, by comparing the incoming data with a reference 80 second signal. The detection sheme can be simplified by keeping only the information of positive or negative signal, allowing to work with a single bit of new information at each time step. This method works very well, and allows the use of small micro-processors, enabling the float itself to do the listening and computing, and a moored sound source. From the arrival time of the signals from two or more sound sources, and the previous location of the float, it's current location can easily be determined to considerable (<1 km) accuracy. For instance, the float will listen for three sources every so often (usually at 8 h intervals) and store the time of arrival for the 2 largest signals heard from each source. The location of the float will be computed onshore.

Technical caracteristics

Mechanical characteristics

The floats consist of 8 cm by 1.5 to 2.2 m long glass pipe that contain a hydrophone, signal processing circuits, a microprocessor, a clock and a battery. A float weighs about 10 kg. The lower end is sealed with a flat aluminium endplate where all electrical and mechanical penetrators are located. The glass thickness is about 5 mm, giving the float a theoretical maximum depth of about 2700 m. The external ballast is suspended by a short piece of wire chosen for its resistance to saltwater corrosion. By dissolving it electrolytically the 1 kg ballast is released and the float returns to the surface.

Electrical characteristics

The electronics can be divided into four categories: an satellite transmitter used after surfacing, the set of sensors, a time reference clock, and a microprocessor. The clock is essential in locating the float, since it is used as reference to calculate the time travel of the sound signals from the moored emettors. It is also of use to have the float work on schedule. The microprocessor controls all subsystems exept the clock, and stores the collected data at a regular schedule. The satellite transmitter is used to send data packages to orbiting satellites after the surfacing. It usually takes 3 days to collect all the dataset.

The isopycnal model

The isobaric model

Measures end projects

The Argo project

Other results

References

The RAFOS system, T. Rossby D. Dorson J. Fontaine, Journal of atmospheric and oceanic technology, v.3 p.672-680
Particle pathways in the Gulf Stream , T. Rossby A.S.Bower P-T Shaw, Bulletin American Meteorological Society, vol 66,n 9

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