Underwater GPS

GPS does not work underwater-the radio signals on which it depends cannot pass through water. So submariners have yet to benefit from the revolution in navigation that it allows.

Now engineers working for the US Office of Naval Research think they have found a simple way to let submarines and divers get an accurate GPS fix.

A base station is tethered to the sea bed at known depth and known GPS location. A submersible anywhere in the area sends out a sonar request pulse to which the base station replies with a signal which gives its GPS position and depth as well as the bearing angle from which the submersible’s request arrived.

The submersible then uses its own depth, which is easily measured, plus the round trip pulse time and the bearing angle sent by the base, to calculate its own position. Simple.