Firms that rely on the US Global Positioning System (GPS) should ensure that they have a fallback plan, as one of the UK's top navigation experts has warned that the system may prove unreliable.

Speaking at a Cambridge University conference in September, Norman Bonnor, a retired RAF Air Commodore and president of the Royal Institute of Navigation, said the GPS signal is more fragile than many users might suppose. "Too many people don't understand its foibles and vulnerabilities," he added.

GPS is widely used in systems ranging from car navigation to emergency services co-ordination; and for time-keep- ing in utilities, finance and telecoms sectors.

The system relies on a network of satellites, which cannot be repaired once launched and have a limited lifespan. Sixteen of the present 28 satellites were built to last seven and a half years, but are now between eight and 14 years old. Twenty-four satellites are required for full coverage.

Bonnor said launches of new satellites are "only just keeping up" with current losses of around two satellites per year. But the failures could accelerate due to the number of satellites that have used up their redundant hardware. "Many are on their last legs," he explained.

The first of eight modernised satellites, built to last 10 years, was due to launch yesterday after a series of delays. A European alternative to GPS, called Galileo, is not expected to be complete until 2010.