I can't hear you. You're breaking up.

Ultra Wide Band (UWB) radio communications is cropping up more and more in the media these days. It is portrayed as an open source use of the electro-magnetic spectrum against the vested interests of mobile phone companies, broadcasters and the military. This sounds very admirable and sure everyone wants to connect to the Internet with amazing data rates, but it ignores one important point; how it affects the passive users of the spectrum such as astronomers and geophysicists.

Spectrum allocation in the US

IMAGE: NTIA Office of Spectrum Management

What is the spectrum? You will already have some experience of it in the numbers that your favourite radio station tells you every few minutes or so. Think taglines such as "97-99 FM. Radio 1","Key 103 Manchester" or "96.3 Aire FM". Those numbers actually represent the frequency - in this case they are in Mega Hertz (MHz) - of the radio waves used to transmit the station to you. Unless you only like one station (or have a DAB radio) you will probably be familiar with moving through the spectrum when you turn the dial to get to another station. The radio spectrum is much wider than your radio set can cope with though as it extends from about 3 kHz (3,000 Hz) up to 3 GHz (3 billion Hertz). It is usually managed by governments and international treaties so that everyone doesn't interfere with everyone else. The allocation of the spectrum in the US is shown above (click on the image for a large PDF version). Radio astronomy occupies a very small part of the spectrum (a specific shade of yellow in the image) but unlike the other users, it doesn't transmit; it just 'listens'.

The UWB technology plans to spread the transmissions thinly across wide swathes of spectrum. The proponents claim that the amount of power in these transmissions is lower than the natural noise level and so isn't a problem. But the wide spread use of this technology will actually raise the noise 'floor'. Between radio stations you will be used to hearing 'white noise' which you might think is the natural noise level. Well, it isn't. It is mainly due to the fact that your radio set is relatively warm. Warm electronics means more energetic electrons and so more electronic noise in your receiver. Radio Astronomers combat this by cooling their receivers to extremely low temperatures. My receiver gets cooled down to about 15 Kelvin (close to -260 C), so is probably much cooler than your radio set! After taking all the trouble to make things as quiet as possible (sometimes at great cost), it is very annoying when someone sticks a whopping great signal such as a mobile phone nearby. This is just like someone turning on a floodlight next to your nice optical telescope.

To defend the scientific use of the spectrum, we have good people such as Jim Cohen at Jodrell Bank Observatory, who have been fighting for years to keep the astronomical parts of the spectrum quiet while dealing with underhand tactics by other spectrum users. Despite all the efforts, radio astronomers are facing increasingly difficult times.

Posted in astro blog by Stuart on Thursday 12th May 2005 (13:37 UTC) | Permalink
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