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 transmitter network was synchronized on 668 and 767 khz in order to use the other frequencies for propaganda broadcasts in foreign languages. each transmitter group would be turned off during an air raid to prevent their signals being used as navigational beacons and listeners were required to retune to a low-powered single-frequency network on 1474 khz.