The Maginot Line of the 21st century's first war

SOME experts are now calling it America's Maginot Line in the waragainst terrorism, after the supposedly invulnerable Frenchfortifications that Hilter's armies simply outflanked in 1940.

It is the network of global positioning satellites (GPS) on whichthe world relies for its airline and shipping navigation and whichAmerica is now using for the first time as a hi-tech bomb-aimingsystem in Afghanistan.

But evidence is fast emerging that not only is GPS highlyvulnerable to jamming, but that a major terrorist assault on thenetwork could wreak far worse havoc than either hijacked airlinersor anthrax.

On 5 October, the US department of transportation held anemergency conference in Washington to review the vulnerability ofGPS to terrorist jamming. Government GPS expert Jim Carroll revealedthat "many jammer models exist". A Russian firm, AviaConversia,offers a $40,000 device, not much bigger than a pack of cigarettes,that could disrupt satellite signals over a 150-mile radius.

There is also evidence that Iraq has been producing military GPSjammers, some of which could have made their way to Afghanistan.

The first global positioning satellites were launched in 1978.Measuring the time it takes to receive pulses sent from thesatellites, each of which remains fixed in an exact spot overhead,can tell a person, plane, ship or vehicle on earth exactly wherethey are, provided they have the right receiving equipment.

GPS means no-one every needs to get lost. America has introducedthe so-called GPS satellite bomb in the Afghanistan conflict. Itreplaces the laser bomb used in the Gulf War and Kosovo.

Britain and America discovered in the Kosovo conflict of 1999that modern laser-guided bombs could easily be outsmarted. The laserbeams used to lock on to ground targets such as Serbian tanks couldnot penetrate ordinary low clouds in the wet Balkans, so most bombsmissed.

The GPS bomb uses signals from the GPS network to navigate to anexact point on the ground. Provided a spy satellite or a ground-based forward observer has identified a suitable target, and keyedin its co-ordinates, GPS will put the bomb on the spot.

The advantage is that pin-point attacks can be made at night orin any weather.

However, the GPS network is relatively easy to jam over shortdistances, because the signals emitted from the satellites areextremely weak - the equivalent power of a 25-watt light bulb - andare thus easy to swamp, sending bombs miles off target.

GPS vulnerability also exists in everyday electronic systems thatuse its high-accuracy timing capability - computers, mobile phones,automated bank transfers, electrical power grids and the internet.