Taxis could be left driving around aimlessly, over-protective mothers risk losing track of wayward offspring, and skiers could well whizz off-piste never to be seen again.

The reason: millions of navigation devices around the world could fail as counters in the Global Positioning Satellite (GPS) system roll back to week zero.

The system's 24 satellites circling the Earth track time by counting the number of weeks since the network was launched in 1980 - but only compute up to 1024 weeks (about 20 years) before going full circle.

Golf balls - GPS can find the bunker

The GPS sends out two signals - one for civilians and one for the US military. The civilian signal allows positions to be determined to about 100 metres but the encrypted military signal allows an accuracy of 20m, according to the Pentagon. Unofficially the accuracy is believed to be as good as two metres.

(For more details on how GPS works, click on "Navigation bug fears" in Relevant Stories.)

Ironically, the Pentagon has allowed the more accurate signal to be accessed by everybody during major conflicts such as the Gulf War. With too few military receivers to cope with demand, they had to rely in part on civilian receivers.

Now the £25bn navigation device has been applied to everything from golf balls to wristwatches, which pick up the signals through cellular phone frequencies.

Here are just 10 of the thousands of things which could lose their way.

Children can be under the satellite's watchful eye

1) Fashion designers have fitted tracking devices into clothing, so mothers can find lost toddlers and search parties can find lost skiers. And security-concious women will soon be able to don a GPS-fitted bra.

2) In the much-heralded cashless society to come, police will be able to track stolen credit cards - by satellite.

3) An Australian golf club has GPS devices in its buggies and golf balls so players can locate their wayward shots. Players will also be able to tell how far they are from the hole and which club to use.

4) US farmers can use a GPS-fitted combine harvesters to produce detailed yield maps for individual fields. The maps can then be used to tailor the application rates for seed and fertilisers.

London cabbies can get a helping hand

5) Monitors can be fitted to mobile phones to act as satellite-aided private detectives or nursemaids. The devices - retailing from £200 up - monitor the movements of vulnerable people such as elderly relatives or children out at play. For a jealous husband or wife, it can keep watch on a partner suspected of errant behaviour - not simply pinpointing his or her location, but revealing whether he or she is standing up or in a horizontal position.

6) Scientists are using GPS to monitor ground movement in earthquake-prone and ground distortion around active volcanoes.

7) Nato is using the network to gather forensic evidence against suspected war criminals in Kosovo. As well as examining bullet wounds and footsteps around massacre sites, GPS equipment will record the exact location of every relevant object.

8)Motorists can buy cars with special consoles that allow the GPS system to monitor their location and send the driver news of traffic jams.

The Casio watch to tell you when and where you are

9) Some taxis in London and Shanghai have GPS devices to tell the drivers exactly where they are on electronic maps, helping out when their Knowledge is a bit lacking