A sensor capable of making acoustic measurements using the echo principle (difference between the emission and reception of the acoustic wave) is essential for determining accurately the speed of propagation, or velocity, of the sound wave in water.

To this end, the ships of the fleet are fitted with a Sippican Mk21 data acquisition system which profiles this velocity throughout the water column via eXpendable BathyThermograph probes launched from the ship.

An XBT probe is a small projectile with a lead tip and a plastic shell containing a coil of fine copper wire.

This probe is inserted into a handheld launcher which fires it into the sea.

As the probe sinks into the water column, the wire coil unwinds and transmits the measurements made by a thermistor to a PC-type microcomputer fitted with a MK 12 card.

The program installed on the PC enables the operator to monitor the T=f(P) measurement in real time, and record the curve which will then be injected into the acoustic instruments after formatting by the Doris program.