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Navy craft use very low frequencies, giant antennas and high power to broadcast radio signals underwater. Now Flight Refuelling of Dorset has come up with a better way to send messages short distances between remotely controlled submersibles and surface craft (WO 02054635). Each craft has a pair of underwater antennas, one clamped to its body and the other trailing on a 30-metre-long shielded cable. Because radio waves are strongly absorbed by water, the trailing antenna picks up a signal of a different strength to that of its fixed partner. Measuring the difference in signal strength between the two allows the receiver to decode a low-power message sent on frequencies of several hundred hertz over distances of up to a kilometre.