The digital mobile revolution began to happen around 1993. Around this time, GSM, the Global Standard for Mobiles was created. This was to be a global digital cellular phone network, but of course it didn't quite work out like that.
Being digital, it's much more difficult to snoop on conversations than with a TACS phone - though GSM also uses encryption techniques. GSM phones originally appeared on 900MHz networks, but are now available in the UK as GSM1800 (1800 MHz) on two networks.
Rumours indicate that it is possible to decipher the GSM signal into intelligible speech, using some computer equipment and a scanner, though I have not seen this equipment anywhere.
The older analogue-based mobile telephone networks, are obsolescent in most countries now, having been superceeded by GSM or other digital based networks. The UK system, TACS, is analogue, that is, the speech is sent using ordinary FM radio signals. Granted, the call will hop between cells as necessary, but it's still plain speech that being transmitted.
The TACS phones use two frequency bands, one between 850MHz and 870MHz and the other between 935MHz and 950MHz. On these frequencies, anyone with a radio able to tune in to these frequencies, can listen to any analogue cellphone call that happens to be using a base station within range.
So what has all this got to do with Big Brother? Where were you today at 8:30am? In the car on your way to work? On the bus? In the office? Did you have your mobile phone switched on? If you did, your mobile phone network knows where you were, or at least it knows what cell your phone was logged in to, whether you made or received a call or not.
As the range of the cells is quite limited, your approximate location can be determined. If triangulation information from 2 or more sites is available, your exact position could be located.
There is very little to stop the authorities getting this information from the cellular telephone companies. Once they have that information, they know at least roughly where your phone was at any given moment.
According to an article published in The Guardian (London) newspaper - mobile telephone companies have been keeping records of customers whereabouts for some time now. One in particular, Virgin Mobile, has been keeping these records for the past two years.
According to the article: "When calls are made or received on a mobile phone, the call is automatically logged at the nearest base station through a "locator code", allowing the networks to track the geographical usage pat terns of their customers. In urban areas where there is a high density of base stations, the information is currently accurate to within a few hundred metres. When the new breed of 3G - third generation - phones comes on stream, probably next year, they will enable the users' location to be pinpointed to within a couple of metres."