Communication with an underwater vehicle

A method of operating an underwater vehicle, the underwater vehicle comprising an annular hull with a duct;

• and a propulsion system for propelling the vehicle through the water, the method comprising:
• a. operating the propulsion system in a series of thrust pulses separated by drift periods such that the propulsion system operates at a relatively high rate during the thrust pulses and at a relatively low, or zero, rate during the drift periods, wherein water flows through the duct and generates lift during the thrust pulses and during the drift periods;
• b. receiving a series of signal bursts at the vehicle;
• c. timing the drift periods of the propulsion system such that each signal burst arrives at the underwater vehicle during a drift period and not during a thrust pulse;
• and
• d. decoding the signal bursts received at the underwater vehicle to obtain a series of data sets encoded within them.

A method of operating a plurality of underwater vehicles to receive a series of data sets which have been broadcast to them, each underwater vehicle comprising a propulsion system for propelling the vehicle through the water, the method comprising for each vehicle:

• a. operating the propulsion system in a series of thrust pulses separated by drift periods such that the propulsion system operates at a relatively high rate during the thrust pulses and at a relatively low, or zero, rate during the drift periods;
• b. receiving a series of signal bursts at the vehicle;
• c. timing the drift periods of the propulsion system such that each signal burst arrives at the underwater vehicle during a drift period and not during a thrust pulse;
• and
• d. decoding the signal bursts received at the underwater vehicle to obtain the series of data sets encoded within them.