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Seventeen years after it was filed, the Royal Navy thinks it’s safe to tell us what GB 2376298 contains. The classified patent describes how Thorn EMI improved the accuracy with which submerged hydrophones help locate submarines. Hydrophone signals are sent up a cable to a buoy. The buoys can be accurately located by satellite positioning systems, but the position of the hydrophones – and hence of the hunted sub – is not known as precisely because their cables allow them to wander. To pinpoint the hydrophones’ position, Thorn EMI suggested calibrating their position by fitting them with receivers for an extra low frequency (ELF) time signal sent from eight underwater transmitters strategically placed around the world.