Acoustical localisation system for the tracking of underwater drifting floats

The investigation of vertical motions in winter during the formation of wide patches of homogeneous water in the Gulf of Lions was made possible by the use of neutrally buoyant floats of the type developed in the WHOI. Those floats are equipped with an acoustic projector delivering a 5 kHz pulse every 4 seconds. The necessity of both a permanent tracking for Comparison with data collected by the surface ship, and of free motions of the ship in the surveyed area has led to the development of a moored array of acoustic buoys with radio transmission to the ship and to the shore. Each mooring is equipped with a deep-sea hydrophone, 15OO meters below the sea level. The acoustic signal, locally amplified, is transmitted on a single conductor armored steel cable to a subsurface float and from there to the surface buoy through a floating cable. The signal is then radioed to the boat (via a FM 30 MHz 1 W radio link) where it is recorded on a graphic scanning recorder with a sweep of 4 seconds. Each buoy is activated in turn 6 minutes every 30 mn by an internal clock. By measuring the difference of propagation time for two of the buoys it is possible to locate the float in an hyperbolic network analog to those of classical radio navigation. The operational area is roughly that of the triangle determined by the buoys, and it can be extended by the use of a shipboard clock synchronized on the floats. In that case the propagation time of the acoustic pulse is measured and only two buoys need be in acoustical range. Acoustical range for a sea state 6 is 15 nautical miles. The radio range at sea level is 30 nautical miles. The ground Station located on a mountain at 2400 m above sea level can receive buoys as far as 100 nautical miles, range that covers all the area of interest.