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“GPS signals are not difficult to jam because they are weak in the first place and a very, very long way away,” said Todd Humphreys of the Radionavigation Laboratory at the University of Texas in Austin.

Global positioning satellites hovering 22,000 miles above the planet produce the signals that ground-based detectors use to triangulate locations on Earth’s surface.

Even military GPS signals, which are 10 times stronger than civilian ones, can be jammed easily, at least in small areas, with cheap commercially available equipment, Mr. Humphreys said.

He noted that Chinese-made jammers are advertised for sale on the Internet — pocket-sized devices that block GPS signals from several feet to more than 100 feet away.

And it is not just rogue nations such as North Korea that are interested in jamming GPS signals.

In Britain, a clandestine government-sponsored network of 20 roadside GPS monitoring posts this year found dozens of incidents of jamming with small-scale devices. Most were caused by truckers trying to defeat the electronic surveillance devices that tell their employers how long they drive and how fast, and when they stop for mandatory rest breaks.

But some of the jamming appeared to be caused by thieves seeking to disable security tracking devices in commercial vehicles, according to Charles Curry, whose company Chronos Technology Ltd. set up the network.