During the Second World War, the German air force used a system called Knickbein to guide bombers to targets in Britain. Essentially, it consisted of a two radio beams which the planes could fly, allowing them to drop bombs on blacked out cities, deal with bad weather, and otherwise manoeuvre accurately. The beam to one side broadcasts ‘dot’ signals, while the other sends ‘dash’ signals. Hearing a continuous tone means you are in the middle. Hearing one or the other tells you to divert your course somewhat. Intersecting beams over targets told pilots where to drop their ordinance:
Estimates of the accuracy of the Knickebein suggested that it was capable of putting a 300 m x 300 m box around an intended target, which when saturated with bombs from a mere 40 aircraft would put the bombs down on average 17 metres apart.
The beams were 400-500 metres wide and were in operation (at a frequency of 31.5 Hz) as of 1940.
The British came up with progressively more sophisticated means of blocking the technology. Operation ‘Headache’ sought to jam the beams using radio chatter, itself produced using “modified hospital electro-diathermy units used for cauterising wounds.” Later, more sophisticated approaches were employed. According to David Khan, the British were eventually able to ‘bend’ the guidance beams using transmitters of their own, causing bombers to “unload most of their high explosive… into empty fields and the Channel.” A jamming system used against the more advanced Y-Geraet system in 1941 apparently prevented 80% of bombers employing it from dropping their bombs at all, as they were prevented from getting accurate ranging signals.
No doubt, dramatically more advanced systems and countermeasures exist today, in spite of the existence of cheap and accurate satellite-based positioning technology.