Float Group Instrumentation
New instrumentation are always under development within the Float Group. The equipment that presently makes up the instruments that the group maintains on a regular basis are -
RAFOS Floats are neutrally buoyant, free drifting instruments, which are launched from a ship in the research area Once deployed the float will settle to a predetermined depth (typically from 300-4000 meters) and remain there for up to two years. The float contains an acoustic receiver which listens for signals from an array of moored sound sources. The sound sources transmit a unique signal at 260 Hz at a specific time. That signal is received by the float after some period of time which is proportionate to it's distance from the source. Knowing that the speed of sound in water is approximately 1500 M/sec, we can calculate the distance. RAFOS floats track multiple sound sources. If you know the range to each of the three sources we can then triangulate the position of the float on a given day. By connecting the daily positions of the float we get a plot of the floats movements that are driven by the oceans current.You can check the progress of an ongoing experiment using RAFOS floats here.
In addition to suppling data on ocean currents, each float also measures it's depth and the temperature of the seawater at each of the daily positions. When a float has completed it's mission, a weight is dropped and it rises to the surface. Once on the surface, all of the data it has collected during it's mission is transmitted to an orbiting Argos satellite which relays the data back to the Labratory in Woods Hole.
If you would like to learn more about how sound reacts in the ocean, click HERE to vist the "Introduction to Underwater Acoustics" page from the Whale Acoustics Project at NOAA.
Freely drifting subsurface float which performs, during it's time at depth, a series of temperature measurements, rises to the surface and sends the data and it's position via the ARGOS satellite system back to the lab. Lifetime, several years or 80 round trips. Depth range 0 to 2000 meters.
ALFOS Floats are very similar to there cousins the RAFOS float with one major difference. ALFOS Floats have the ability to adjust their ballast by means of an external oil filled bladder. When the float wishes to rise to the surface it simply pumps oil from an internal reservoir into the expandable external bladder. This changes the volume of the float and effectively makes it lighter causing it to rise. When the float descends, it simply opens a valve which allows the oil to return to the internal reservoir. Each time it surfaces the float transmits via Argos, the data it has collected. This enables the scientist to receive data on a predetermined schedule as opposed to having to wait two years as with a RAFOS.
The PALACE ( a modification of an ALACE Float) is a free drifting, naturally buoyant profiler. These versatile floats measure conductivity ( salinity ), temperature, pressure ( depth) and current flow for periods of more than a year. Much like the ALFOS float, it to can adjust it's buoyancy and rises to the surface every seven to ten days to transmitted back it's data via Argos satellites. Unlike the ALFOS it does not track it's position daily using sound sources, instead it's position is logged at the spot that it surfaces. By connecting the points of each of the surfacing, scientist get a mean current plot. As the instrument starts to ascend it begins to collect conductivity (C), temperature (T), and pressure (Depth) data. Over the course of the mission more than a hundred CTD profiles can be collected.
SOLO Floats are the latest addition to the Float Group. Very similar to the PALACE instrument in size and shape, they incorporate a few new features. SOLOs will utilize a new form of satellite communications from Orbcomm® that will increase the amount of data transfered, decrease the cost and enable us to collect data more frequently. These instruments can provide temperature data, conductivity (salinity) data, pressure data and can be acoustically tracked, muck like a RAFOS, to get a better profile of current movement as compared to a PALACE. Click here to see a animation of a SOLO deployment. SOLOs will be used extensively in the up coming Argo experiment.
Vertical Current Meter
The VCM is the latest addition to our array of floats. Identical to the PALACE in every way, it also has the ability to measure vertical current flow. Attached to the pressure case is a fixed rotor that, as water flows past it vertically, causes the entire instrument to rotate. Internally, a flux gate compass counts the rotations allowing a flow rate to be calculated.
Numerous other types of specialized instrumentation also reside with the Float Group. Presently, we are working on an new form of satellite communications for not only floats but other types of oceanographic equipment. The advantage to this new system is higher data throughput, lower cost ( than Service Argos ) and bidirectional communications ( Argos is transmit only ).
Another new instrument that has shown a lot of promise is the Neutrally Buoyant Sediment Trap. Unlike conventual sediment traps that are moored to the sea floor, this instrument moves freely with the current, de-coupling it from any mooring movement or tilt. A buoyancy controls unit keep the trap within 10 meters of it's target depth. Four NBST have been built for scientist at the Bermuda Biological Station by the WHOI Float Operations Group, and have been deployed for testing from the R/V Weatherbird with success.
NBST Technicial Report - in pfd format. Requires Acrobat Reader.
If you would like information on how you could utilize the WHOI Float Group's services, please contact Jim Valdes at:
Woods Hole Oceanographic Institution
56 Water Street
Woods Hole, MA 02543
MS 30 Attention Jim Valdes