Principles of Oceanographic Instrument Systems -- Sensors and Measurements 2.693 (13.998), Spring 2004 Albert J. Williams, 3rd Lagrangian Current Measurements and Integrating Current Meters An important concern of physical oceanography, geochemistry, and planktonic distribution of organisms is "where does the water go?" Is there a physical variable that is directly related to this question? A simple-minded idea is that the physical variable that answers this question is the track of a particle of water or of a parcel of water. This is a Lagrangian concept, the description of a particle path. The problem is that in a large Reynolds number flow (the ocean is a large Reynolds number fluid, say 1,000 km / (0.01s*10cm/s) =109) particle trajectories are likely to be variable and only in a statistical sense describe the flow. Even if one assumes that turbulence will smear the picture on the small scale but a mean current will advect these smeared trajectories in a clear way across the ocean, it is difficult to get a reasonable picture with a small number of marked particles. The physics of fluids depends on a pressure field to accelerate fluid. This gives rise to a velocity field, the more commonly measured quantity. The displacement field is yet farther from the basic physical variable and thus more complex. Yet Lagrangian measurements are chosen for certain types of problems, generally ones with long integration times. Surface drifters are the crudest kind of current meters. The surface water advects a float and its position is monitored. The drift bottles of the 1960's, released in large numbers from light vessels with notes inside to determine where they washed up, were useful for showing the onshore - offshore seasonal variations in surface drift. Satellite tracked floats with drogues at some modest depth have been useful in tracking the Gulf Stream and its rings. Deep free-drifting Swallow floats exploded the notion of the level of no motion in the late 1950's. The SOFAR float program was an outgrowth of the Swallow float programs of the early 1970's. The principle is that a neutrally buoyant float at some midwater depth will be a nearly perfectly tagged water particle. It can be tracked acoustically. As a sensor, this more nearly tracks the physical variable (water particle displacement) than does the surface drifter with its windage and wave response. Initially and occasionally still, these Swallow floats were tracked by ship. However in the 1970's, low frequency sound was used to signal over long ranges to fixed bottom-mounted hydrophones. The low frequency sound was attenuated little and the spreading loss was cylindrical rather than spherical because it was ducted in the SOFAR channel. Precise timing permitted the determination of position from two tracking stations and when a third station was available, clock drift could be checked. To bring this report of Swallow floats up to the year 2003, PALACE or Profiling Autonomous Lagrangian Circulation Experiment drifters have been deployed by the hundreds. Russ Davis, to measure the current around Antarctica, has deployed many ALACE (not profiling) drifters in Davis Straits. These are not tracked acoustically but return to the surface periodically to transmit their locations by Argos satellite. The ALPS program is presently deploying 3000 such floats uniformly around the world oceans. Each reports its position every 30 days and is expected to survive for about 4 years.The Swallow Float is less compressible than seawater (generally 1.5ppm/psi versus 3ppm/psi for water) yet can be made to float in aluminum cylinders capable of 2000m depth range or spheres of glass capable of any ocean depth. It must carry energy to power its signaling device and its sound transducer and electronics. This scales the package. For short ranges and several weeks' duration, a 13-inch diameter glass sphere has been used. For 1500km ranges and two-year duration, 8-foot long 12-inch cylinders are required. Doug Webb's SOFAR floats use a tuned cavity resonator to convert electrical power to sound power. As the frequency is lowered and the weight of the structure held constant, the efficiency of this sound projector drops. However the attenuation of the lower frequency sound also drops and for long ranges this dominates the equation for loudness at the receiver. It has been optimized at about 2% efficiency for a 220Hz sound source. If the energy can be spread over a longer time interval, the power can be lower but the timing can remain as precise. This does require that the bandwidth be broad. But it need not be broad at every instant, only over the entire pulse. Recently a frequency slide has been employed in which a very narrow band sound source has had a variable tuned cavity, like a trombone, in which the frequency is slowly varied during the transmission. Precise timing is possible with the received signal correlated against a pattern of the transmitted frequency slide. Moving away from the bottom hydrophone arrays, another receiver was needed. Al Bradley designed an autonomous listening station, ALS, to receive and decode the SOFAR float sounds. The individual floats are kept track of by a window when the signals are expected (four times a day) and a correlation is performed to determine the most probable arrival time of the chirp signal. The integration of the system is a nice illustration of instrument design. The physical variable to be detected is an acoustic signal. The ambient noise in the ocean is the background that must be rejected. A single loud noise such as an explosion would be above ambient, but is hard to generate from a lightweight sound source. Equivalently, a continuous tone with the same total energy would be equally detectable but not contain the timing information necessary to determine position. A chirp permits both objectives to be met. Because the sound source is so narrow band, a chirp going from 220Hz to 221Hz in 2 minutes is used. Inverted SOFAR tracking has become common with the RAFOS floats. Instead of lots of powered sound sources floating around and a few fixed receivers, a few fixed sound sources and lots of drifting receivers are used. Tom Rossby developed this small Swallow receiver that pops up and dumps its positions over the last submerged period to a satellite, then dies. Self-propelled floats are also possible. The power to move faster than the current is