upon the outbreak of world war ii, the bbc closed the regional and national programmes and replaced them with a single channel known as the bbc home service. the main transmitter network was synchronized between just two groups, using 668 and 767 khz, each of which could be turned off during air raids to prevent its signals being used as navigational beacons (with listeners required to retune in such an event to a low-powered single-frequency network on 1474 khz).