So who's listening in on you?: Hugh Cornwall tunes in to those who scan the airwaves

I SAW my first scanner in 1977. It was called the Bearcat 210 and had been imported from the States by an electronics spook who hinted at former employment at MI5. Scanners come from a marriage of digital technology and radio. Before the scanner, radios that could receive reliably above 100 MHz, the broadcast VHF-FM band, were expensive and temperamental. With a scanner you can, with pinpoint accuracy, tap in the desired frequency.

The scanner can also search between upper and lower frequency limits to discover unknown radio activity. It has memories in which favourite channels can be stored. For a few hundred dollars you can have your own desk-top GCHQ.

The radio-amateur shops first started selling them here, and soon after that the high street emporia. Prices dropped and specifications improved, as with all other high technology. Today, for just pounds 250, you can get a hand-held scanner with 1,000 memories, covering everything from the medium-wave to the low end of the microwave band, and including short-wave broadcasts, the international and local maritime and aeronautical bands, the emergency services, taxis, the military, television sound and, of course, cellular telephones.

The same thing the size of a large matchbox would cost pounds 400, and there are also models that hook up to a personal computer for highly automated searching. Cyril Reenan, the retired bank manager who provided the 'Dianagate' tapes, apparently spent pounds 1,000 on his scanner.

The Scanner Freak Underground - like the network of 'hackers' or computer explorers - is more diffuse than outsiders may think. Scanner owners vary in their preoccupations.

Explorers are the pioneers who identify who uses which frequencies. The airwaves are chock-full of channels and you need a guide. A broad outline of the radio frequency spectrum is sold by HMSO. Thereafter you must track down the openly published frequency listings (which tend to omit anything controversial) and then seek the truly covert publications. Explorers compile their lists using the same techniques as Cheltenham GCHQ analysts: careful listening, logging, and reading technical literature. Often they are more interested in radio technology than the contents of conversations.

For the 999ers, television shows such as Crimewatch and Crime Monthly are insufficient; they want to hear live action. Lists of police, fire and ambulance frequencies have been in circulation for years. The apotheosis of the 999er was the 1980 Iranian embassy siege. There had been days of build-up so hundreds of scanner owners knew where to listen. When the storming took place they turned down Kate Adie's television commentary and heard the police ordered to hold back as 'Nimrod' - callsign for the SAS - was being sent in. By the time of the Libyan embassy affair four years later the police had learnt to use their radios more discreetly. Police frequency lists are now in the hands of almost every petty criminal, and it has been known for years that Northern Ireland terrorists are great users of scanners.

In all of this the Cellular Eavesdropper is a relatively passive creature. On most frequency bands there is always the same user on each channel; with cellular telephones, where computers ensure optimal use of the radio spectrum, the channel is constantly changing.

Although both sides of the conversation can be heard, the owner of an ordinary scanner has no control over whom he is listening to nor any way of targeting an individual. There are over 1,000 cellular channels. Moreover, if the cellphone user is on the move, the frequency will be constantly changing: the eavesdropper must scan through several hundred possible channels to relocate the conversation. Spend enough time listening and eventually you may hear one interesting fragment among much banal trivia.

That is the fate of Cellular Eavesdroppers - they learn that real people talk like Pinter characters.

Mr Reenan, and the eavesdropper who is alleged to have located the Duke of York's ship, were the beneficiaries of good luck. Other lucky hits include Richard Needham's 'that cow' comment on Margaret Thatcher when he was Northern Ireland Minister in 1990 and Sir Michael Edwardes discussing strike- breaking at British Leyland.

Private detectives surveying a specific individual prefer to focus on the domestic cordless phone. These are on one of only eight frequencies; their security features only prevent the owner of a pirate handset using the line. They do not stop eavesdropping.

Last week some of the Explorers were passing on the frequencies for a channel heard across London and operating from 'Eagle Base', on which you could hear over-busy American voices referring to 'Rawhide' and 'Stagecoach'. The guess is that on such depended the security of Cyrus Vance at the London conference on Yugoslavia.

Other Explorers have moved on - eavesdropping digital data, for example. For years telex and picture transmission have used short-wave radio, now there is much computer traffic on the VHF, UHF and microwave bands. With a personal computer and suitable hardware and software you can decode it.

Even this is not enough for the Real He-Men. They are into satellites. Take a satellite tuner, a steerable dish, a specialist television and a scanner and you'll find all sorts of unadvertised and hidden transponders. You can seek the raw news feeds out of Sarajevo, Baghdad or wherever and hear what is being said on those dollars 50,000 satellite phones.

The alleged taped royal conversations have left some MPs and newspapers boiling up for banning legislation. But it already exists. All such eavesdropping is illegal under the 1949 Wireless Telegraphy Act; the problem is enforcement. Would new legislation be any more effective? There are now more than 100,000 scanners in the UK and, unlike illegal CB radios (banned a few years back), there is no transmitted signal for law enforcers to home in on. Besides, there are many legitimate uses for scanners: as radio test equipment, to provide links for radio and televison broadcasters, for the emergency services. To ban sales would lead to an expensive licensing scheme for legitimate users.

The law can only address gross breaches, so you have to look out for yourself. There is no point in worrying if someone you will never meet overhears your personal intimacies. On the other hand no truly confidential conversation should take place on any phone, particularly one that uses radio. As for police radios, when villains started using fast cars the police dumped their push-bikes. The same technological miracles that have made scanners so cheap have produced similar advances in surveillance evasion: scrambling, spread spectrum transmission and frequency-hopping.

Hugo Cornwall is the author of 'The Industrial Espionage Handbook', 'Data Theft' and 'The Hacker's Handbook'.

(Photograph omitted)