Mobile phones: your own personal tracking beacon

When your mobile phone is switched on, your cellular network provider knows exactly where you are in the world to within a hundred metres or so.

Similar technology is used to track down lost aircraft and yachts through their radio beacons. It's not identical, because most radio beacons use satellites and cell phones use land-based aerial arrays, but the principle is the same.

At any one time, your phone is usually able to communicate with more than one of the aerial arrays provided by your phone network. They're ten or twenty kilometers apart (less in cities) and it's usually within range of at least three of them. So by comparing the signal strengths (and possibly the time lags, though they're mere microseconds) for the signals at each station, your network can triangulate your position and work out where you are.

This is true whenever your phone is switched on, whether you're using it or not, because every so often it sends out a check signal to make sure everything is working as it should.

Not surprisingly, the phone network companies are shy about admitting they have this ability. But within the industry, it's seen as just one more piece of data. There is even talk of selling the data to Internet content providers so they can send information to your Internet phone based on your location, for example reviews of nearby restaurants.

The triangulation capability of cell phone network companies varies according to the age of their equipment. A few can only do it manually with a big drain on skilled manpower. But these days most companies can generate the information automatically, which makes it cheap enough to sell.

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