The initial contribution of radio to navigation was the transmission of time signals and these became established the 1920s. Shipboard direction-finding equipment of the period was cumbersome but efforts to circumvent this drawback gave rise to a long-lasting radio/acoustic technique for determining distance. Experience with groundbased D/F as an aid to airship navigation in the 1914—18 war had taught important lessons and led to the evolution of the rotating-beacon type of system. Some prophetic inventions were published during the decade and the fourcourse radio range took practical shape.