Bug in computer could force aircraft to fly blind

PILOTS MAY be given a rehearsal of the effects of the "millennium bug" a few months early. Navigation systems used by pilots, sailors and lorry and taxi drivers could be thrown into chaos at the peak of the summer travel season because of a computer hitch.

Senior civil servants have admitted that the international Global Positioning System (GPS) of 28 satellites could fail at midnight on 22 August, leaving pilots to fly blind. David Rowlands, director-general for railways, aviation and shipping at the Department for the Environment, Transport and the Regions, said automatic systems that pinpoint aircraft positions could fail.

Mr Rowlands said that the issue "causes concerns". But he pointed out that pilots and air crew were trained to fly without properly working navigation equipment. "Essentially what they do if the whole lot is down is fly by a combination of compass and watch, radio communication with the ground and visual identification out of the window," he told MPs on the Commons Transport Select Committee late last year.

The problem arises because early versions of computers designed to receive the signal from GPS were programmed on a 1,024-week cycle. The cycle, which began on 6 January 1980, will end at midnight on 22 August - which the receivers will either read as 7 January 1980 or an invalid date.

GPS is a network of military navigation satellites set up by the United States and the former Soviet Union, which civilian airlines and other transport firms can use. Any receiving system that has not been recalibrated will be unable accurately to compute its position on 22 August. Mr Rowlands said that the Maritime and Coastguard Agency had drawn the problem to the attention of the shipping industry and other users such as Railtrack were aware of it.

The Civil Aviation Authority said that it believed all aircraft in service had been checked and approved. It said the failure of GPS receivers would merely involve the loss of one element of onboard navigation. A spokesman for the authority said: "All of the GPS navigation equipment used in all the world's commercial aeroplanes has been tested capable of withstanding this August date changeover without any trouble."

He said GPS was only one tool that was available to a pilot as part of the navigation equipment. It was both inaccurate - because the military had deliberately degraded the civilian signal - and unreliable. The signal had been switched off before without warning and caused no problem.

Peter Mellor, lecturer at the Centre for Software Reliability at City University in London, said there were "sound technical reasons" for believing many GPS systems would suffer problems in August.

"Some manufacturers of GPS kit will have been aware of the rollover and designed resilient software. Others may not have been so far-sighted, and some may not even be aware of the problem," he said.