Interesting article in Wired about "Numbers Stations" that broadcast on Shortwave. These are high-powered transmitters across the planet are are broadcasting strings of numbers, letters, backwards music, or even the noise of a fruit machine.
Spectrographic analysis of the signals has revealed that modulated data bursts are sometimes contained within the transmissions. A subculture of obsessive listeners has built up around the stations, despite the fact that they have little hope of ever decoding the signals.
Undoubtably some are used by intelligence agencies around the world for transmitting one way signals to agents, but another use is probably to reserve the frequency for emergancy use. By broadcasting noise on a particular wavelength it prevents others from using that signal.
The tactics involved in the use of the EM spectrum can sometimes be quite fascinating. Recently there was quite a bit of fuss about the EU's Galileo system broadcasting it's unencrypted positioning signals too close to the frequency used by the US for its encrypted GPS signals meaning that the US would not be able to jam the EU signal in times of war). During the recent US led attack on Falluja in Iraq, mobile and satallite communications were jammed in the area.
All this just goes to show how much electronic warfare goes on and how little we usually hear about it.
