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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.

{ 3 comments… read them below or add one }

. November 10, 2008 at 6:17 pm

Aspidistra was a World War II man-in-the-middle attack. The vulnerability that made it possible was that German broadcast stations were mostly broadcasting the same content from a central source; but during air raids, transmitters in the target area were switched off to prevent them being used for radio direction-finding of the target.

The exploit involved the very powerful (500KW) Aspidistra transmitter, coupled to a directional antenna farm. With that power, they could make it sound like a local station in the target area.

With a staff of fake announcers, a fake German band, and recordings of recent speeches from high-ranking Nazis, they would smoothly switch from merely relaying the German network to emulating it with their own staff. They could then make modifications to news broadcasts, occasionally creating panic and confusion.

. June 19, 2011 at 11:54 am

In the early years of the Iraq war, the U.S. military developed a technology so secret that soldiers would refuse to acknowledge its existence, and reporters mentioning the gear were promptly escorted out of the country. That equipment – a radio-frequency jammer – was upgraded several times, and eventually robbed the Iraq insurgency of its most potent weapon, the remote-controlled bomb. But the dark veil surrounding the jammers remained largely intact, even after the Pentagon bought more than 50,000 units at a cost of over $17 billion.

Recently, however, I received an unusual offer from ITT, the defense contractor which made the vast majority of those 50,000 jammers. Company executives were ready to discuss the jammer – its evolution, and its capabilities. They were finally able to retell the largely-hidden battles for the electromagnetic spectrum that raged, invisibly, as the insurgencies carried on. They were prepared to bring me into the R&D facility where company technicians were developing what could amount to the ultimate weapon of this electromagnetic war: a tool that offers the promise of not only jamming bombs, but finding them, interrupting GPS signals, eavesdropping on enemy communications, and disrupting drones, too. The first of the these machines begins field-testing next month.

. September 6, 2015 at 9:05 pm

Because of the inaccuracy of celestial navigation for precise target location in a fast moving aircraft, the Luftwaffe developed radio navigation devices and relied on three major systems: Knickebein (“Crooked leg”), X-Gerät (X-Device), and Y-Gerät (Y-Device). This led the British to develop countermeasures, giving rise to the “Battle of the Beams“.

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