How might Cuba be jamming US transmissions to Iran?

In theory all they have to do is deafen the US satellite transmitting to Iran. Whether it's happening or not is uncertain. The US is investigating claims that its station, Voice of America, has been blocked since launching a daily Persian-language programme aimed at Iran. The Cubans deny jamming but the US state department suspects a jamming signal is coming from a monitoring complex outside Havana built by the Soviets during the Cold War to spy on the US.

There are dozens of different types of communication satellites in use today but they generally rely on the same principle. Launched into a geostationary orbit (where the satellite maintains a fixed position relative to the Earth), the satellite receives information from the broadcaster, which it then beams back down to a target area on the ground. Anyone in that target area with an antenna and the appropriate decoder can tune in.

To jam the transmission at a particular frequency, you send a powerful transmission to the satellite at that frequency. If it is strong enough and contains only white noise, the satellite will get confused. "It overloads the satellite and it can't tell the difference between that signal and the [correct] signal," says an expert in radio communications at the British National Space Centre (who asked not to be named). "It's like you're sitting in a pub and somebody is shouting very loudly. You just can't hear the conversation of the person sitting next to you."

Finding the satellite is easy. "All their positions are listed and known internationally," says the BNSC source. "It's on an open database - what does what and where." The aim of the database is for countries to coordinate their transmissions and avoid interference. Of course, anyone planning to block signals can use the information. It's not all simple, though. Sending up a signal strong enough to jam a satellite transmission needs a lot of power and very large ground antennae. During the cold war, western broadcasters regularly changed the frequencies of their transmissions to Russia to get around the Soviet government's jamming.

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