Criminal gangs are reported to use increasingly sophisticated devices called jammers in order to hinder satellite navigation signals that guide sat-navs and other navigational devices with potentially devastating consequences.

"GPS gives us transportation, distribution industry, 'just-in-time' manufacturing, emergency services operations - even mining, road building and farming, all these and a zillion more", David Last, a consultant engineer and former President of the Royal Institute of Navigation, told the conference.

GPS devices make use very weak signals from four satellites circling the earth with each of them generating a less output power than a 25W bulb more than 20,000 Km from the Earth surface. Hence, it is quite easy to literally flood nearby GPS even with very low power jammers.

David Last at the National Physical Laboratory in Teddington yesterday explained that the current trend is used by car thieves to use jammers to hamper the tracking security systems embarked on expensive cars and lorries.

However, the possibility that it's more powerful could be used by terrorists to cause havoc around airports by inundating the timing signal that is crucial to some onboard positioning systems.