BEIJING, May 18 -- A few weeks ago a man turned up at my front door, having been guided there by a GPS satellite navigation system on his dashboard. He was a bit surprised when I told him that the place he was looking for was 10 kilometers away.

    I was less surprised because I have been testing these potentially wonderful devices for nearly five years and have yet to come across a single one reliable enough to justify shelling out hundreds of dollars for something that is often less reliable than a map.

    One still marvels at a technology able to pinpoint your position and guide you to restaurants and hotels nearby - when it is working properly. I meet people who swear by them, though if you probe further they often encounter bugs such as being directed the wrong way down a one-way street or being given instructions on the motorway seconds after the turnoff has been passed.

    They work brilliantly when you don't really need them - in the open country. They work much less well in crowded cities such as London, where high buildings interfere with satellite signals. Now they're being incorporated into mobile phones.

    When I reviewed the admirable Nokia N95 recently I was surprised at how spasmodic the GPS reception was and how sparse the information was on the very few occasions it linked up with the satellites. The nearest restaurant mentioned was 1.5 kilometers away, despite my being in central London.

    As I was going on a three-city European holiday, I asked for the loan period to be extended so I could test it more extensively. In case I had been using a rogue model, they biked around another one. The new one couldn't be persuaded to receive any signals despite e-mail exchanges with Nokia representatives.

    Back in London, I was about to start writing when I borrowed another N95 from a visitor who happened to have one (it is selling well) but who hadn't been able to get the GPS to work.

    This one worked fine after a 10-minute wait for an initial connection, though it was difficult to follow the streets on the phone's screen, especially as not all of them were named.

    When I searched for nearby restaurants they were all franchises such as McDonalds or Pizza Hut and all of the pubs were a single company's. It picked up local art galleries and some hotels, but was disappointing if you were looking for something different.

    It is worth mentioning all this because while I don't doubt that GPS on phones will be successful, it is not enough to get the technology right. The experience has to be user - not producer - driven, and phone companies are not very good at that. We want to know what other people think about local attractions, not just what big brands are advertising.

    The possibilities for GPS are endless - including keeping track of aging relatives and discovering which of your MySpace friends is in the area (if you are up for that sort of punishment).

    Blind people can be helped to navigate through towns, and the market for tagging children (and pets) could be huge. HP Laboratories has just announced a prototype for GPS phones to trigger images, text, sounds and video clips relevant to where you are going. A lot of positional data will be used by corporations and governments for often dubious reasons.

    But opposition won't halt the march of the surveillance society. Accident victims located by GPS, as happens in the US, will defuse criticism. There is a perverse principle at work: Criticism of surveillance is inversely related to the speed of its spread.

    (Source: China Daily)