Science: Lost? The answer's in space: Satellite navigation could save hikers' lives and speed up 999 crews, but Michael Cross finds governments wary of sharing this military technology

The annual anguish over deaths in Scottish mountains is fuelling interest in a new generation of pocket navigation terminals. Used properly, the devices could save the lives of lost or stranded climbers.

Next month Tom Sackville, the Health Minister, will launch a new idea for speeding up responses to 999 calls. The system, developed in the West Midlands, tracks ambulances by satellite and relays each vehicle's position continuously to controllers.

Climbers and emergency crews are the latest beneficiaries of the Global Positioning System (GPS), a network of military navigation satellites set up to guide the US armed forces.

Experts in traditional navigation have welcomed the new technology. 'It's an excellent thing to have in your back pocket', says Group Captain David Broughton, director of the Royal Institute of Navigation (RIN). But the institute is alarmed that decisions by the US and British governments are preventing its full use.

Portable satellite navigators have been on the market for several years. But only recently have the gadgets appeared in a form that is of use to walkers, rather than pilots or sailors. For about pounds 1,000, it is now possible to buy a device that will display its position to within 100 metres on any map or chart and, even without a map, guide its user back to a starting point.

The gadgets rely on GPS, 24 satellites in interlocking orbits 11,000 miles above the earth. The principle is simple: each satellite continuously broadcasts its identification and the time, from its own atomic clock. The receiver can calculate its distance from the satellite using the time it takes for the signal to arrive. To find its position in three dimensions, the receiver tunes into four satellites, one for each dimension and one to provide a time reference signal, so the receiver does not need its own clock.

Building a working system involved some technical challenges. The engineers had to take account of Einstein's theories of relativity - clocks in the satellites, travelling at 4,000 metres per second, tick more slowly than clocks on the earth's surface. The network, which officially became operational at the end of last year, cost the US government dollars 10bn (pounds 7bn).

Ironically, having invested this money in the highest possible accuracy, the Pentagon then set out to negate it. To stop enemy missiles tuning in, the satellites' signals for civilian users are deliberately degraded, a policy known as 'selective availability'. The RIN opposes the idea. 'Our policy is that anything that enhances the safety of navigation is a good thing, anything that goes against it is a bad thing,' says Capt Broughton. 'But we understand why it's being applied.'

In any case, selective ability has spawned an industry to put the accuracy back into the system. The technology is called 'differential GPS'. A ground station, of an exact known position, monitors signals from passing satellites and broadcasts a correction signal to receivers. This 'fine-tuning' gives GPS receivers an accuracy of up to three metres.

Differential signals are now available in several countries, including the US and much of northern Europe. The service is free except in Britain where users must subscribe to a commercial company, Scorpio.

Despite the restriction, the business is booming. According to one estimate, the market in GPS systems in Britain alone will be worth pounds 280m a year by 1998. Japanese companies are particularly active: electronics firms Sony and Matsushita both market sets that fit in a jacket pocket.

However, the latest development in GPS for walkers comes from the Swedish company Silva. This is a combined GPS set and electronic compass, which can plot its position on any ordinary map. Tony Wale, chairman of Silva UK, says this is a breakthrough: 'The pedestrian navigator today isn't ready for GPS, but he understands the map.'

Mr Wale says the integrated compass is important because it gives constant directions using only a tiny amount of electricity. This overcomes one drawback of conventional receivers that need new batteries every few hours. Mr Wale claims the GPS compass will run for 5,000 hours on one set of batteries. The snag is that at pounds 780 for the GPS compass plus pounds 240 for the map-plotter, the system is only for the well-heeled walker.

One early customer, however, is likely to be the Ministry of Defence. 'All this will be in soldiers' helmets by the year 2000,' Mr Wale says.

For civilians, he foresees GPS becoming part of a personal digital assistant. 'I can envisage personal navigator, telephone and computer in a unit the size of a mobile telephone.'

The arrival of a cheap, accurate and universal navigation might save lives, but it also poses dangers. The first is over-reliance on the technology. A GPS set might tell you the way home, but it won't tell you if there's a sheer drop along the way. And GPS satellites can go out of service.

Capt Broughton dispenses the time-honoured caution of the professional navigator. 'With every form of navigation you should never ever rely on a single source of aid.'

Nevertheless, he concedes that the temptation is there. Aviation authorities are questioning the need to upgrade 'blind-landing' radars because GPS can guide aircraft more cheaply. But this raises the question of responsibility for mishaps, he says. 'What happens if a British aircraft hits a hillside because a GPS satellite runs away without being detected in time? Who's going to claim against whom?'

One solution might be to set up a global system that does not depend on a single benevolent government. A candidate exists: the former Soviet system, Glonass, which works on similar principles to GPS but lacks its accuracy and reliability. Brussels is considering a proposal that the European Union upgrade and maintain the system, so that receivers can tune into either constellation of satellites. The RIN is discussing ways of setting up an international system with sister organisations overseas.

A more subjective qualm about universal satellite navigation is one of morality. Does its availability make it negligent for a mountaineer to go into the wilderness without a GPS receiver? Life inside a cage of satellites might be a lot safer, but it would be duller, too.

(Photographs omitted)