 In the event of a suspected catastrophic attack on Britain, submarine captains, in addition to other checks, check for a broadcast signal from Radio 4 on 198 longwave to verify the annihilation of organised society in Great Britain.The BBC Home Service was the predecessor of Radio 4 and broadcast between 1939 and 1967.It had regional variations and was broadcast on medium wave with a network of VHF FM transmitters being added from 1955.