The BBC's Sue Nelson reports: "At midnight the GPS time system will re-set to zero"

Millions rely on the Global Positioning System (GPS) satellites to give them their exact location on the Earth's surface.

But sailors, pilots and outdoor sports enthusiasts across the world have been warned that as from midnight GMT on Saturday night their equipment may cease to work or, worse still, give inaccurate readings.

They have been urged to check their GPS receivers before setting out on their journeys.

The problem, known as the "end-of-week rollover", comes from the way the satellites keep track of time.

(Click here to see how GPS works)

They count the number of weeks since their launch at the start of 1980 up to a maximum of 1,024 weeks (19 years, eight months). They then return to week zero.

The main commercial and government users of GPS are thought to be well prepared and the US Air Force reported no problems after the rollover.

BBC's Richard Hollingham: Warning may be too late for some

Most concern lies with ageing GPS receivers used by people who have become reliant on a system originally developed by the US military for targeting its missiles.

Many amateur sailors for instance have abandoned all other navigation equipment and techniques because they find GPS so easy - and apparently reliable - to use.

And in gadget-obsessed Japan, many drivers may find their in-car navigation systems will be no use in downtown Tokyo's chaotic maze of narrow streets.

Potential problems

People whose devices were built before 1994 may find they:

• cannot locate satellites, resulting in the receivers not working
• take more time than usual to locate the satellites
• appear to work normally but display inaccurate positions, times or dates

Pre-publicity targeted yachtsman, walkers, lorry drivers, the pilots of light aircraft and even cab drivers.

Air alert

Rollover-compliant car navigation systems on sale in a Tokyo store

Most large commercial airlines do not rely on GPS but some smaller operations do.

The Australian Government advised its citizens against travelling on smaller airlines in the Asia-Pacific region over the weekend.

The South Korean Government said it would be on alert this weekend for incidents involving aircraft.

However airlines in the Solomon Islands and Fiji have moved to allay fears saying their equipment complies with international standards and follows the same specifications as larger airlines.

In the UK the Royal Yachting Association (RYA) said it expected the weekend to pass off with few, if any, incidents as there are few GPS sets which would not reacquire the data stream when the satellites come back on.

However the RYA's Jon Challis says people should not rely on GPS as their only point of reference.

Rollover compliant

GPS satellites will experience what is termed a "rollover"

"Anyone who is afloat at that time should have the use of other forms of navigation as well," he says.

Like many organisations, including the UK Government's Maritime and Coastguard Agency, the RYA have been urging GPS users for some months to contact equipment suppliers and check the devices are compliant.

A US recreational boating organisation, Boat US, says anyone who is unsure whether their receiver is rollover-compliant can check with the manufacturer's specifications listed on their Website at www.boatus.com.

The US Coast Guard has also set up a Website at www.navcen.uscg.mil.

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