By John Brandon
 - FOXNews.com

An electronic device small enough to fit in a shirt pocket and big enough to conceivably bring down an airplane can be easily purchased over the Internet. All a terrorist needs is a credit card and $49.

With car thieves in the United Kingdom using GPS jammers to aid their getaways, experts say it's only a matter of time until crooks -- and, ominously, terrorists -- in the United States catch on.

Jammers transmit a low-power signal that creates signal noise and fools a GPS receiver into thinking the satellites are not available. They can be used to confuse police and avoid toll charges, and some pranksters use them to nettle unsuspecting iPhone users.

But the real threat is the unknown. Criminals could use them to hide their whereabouts from law enforcement -- and some experts fear terrorists could use high-powered jammers to disrupt GPS reception on an airplane or in military operations.

The devices pose serious societal risks, and they're unquestionably illegal to buy and use in the United States. The FCC is bullish about pursuing anyone who buys a GPS jammer and will prosecute and jail anyone who uses one. Yet they're easily bought online, and their proponents say they should stay that way. Fox News was able to buy GPS jammers for as little as $50 from numerous online sources.

"GPS is so embedded in the transportation, manufacturing industries and economies of our societies that the risk is high," said David Last, an Emeritus Professor of Bangor University in the U.K. and a well-known authority on criminal use of GPS jammers.

"It's especially so in telecommunications: GPS is the ultimate source of timing for most of our telephone systems, the Internet and, in the U.S., phone cells."

All those systems are potential prey for jammers, and that's largely why they are illegal. But the devices' proponents say they can serve a purpose, and that people should have the right to buy them. And, for the time being, they can.

Jammer-Store.com, a company based in Sweden, sells the GJ6 jammer for $430. Brando Workshop, based on Hong Kong, sells the Car Cigarette Anti-GPS System for $49. Jammer-Store.com touts free worldwide shipping via UPS, FedEx and others as a perk for shoppers; one site even cited U.S.-specific models.

Michael Kharkovoy, the CEO at Jammer-Store, told FoxNews.com that GPS jammers can be stowed easily in a car or a bag and can help avoid spy detection -- say, from a spouse who suspects infidelity and plants a GPS tracking device like the Zoombak in a car.

"GPS jammer will help you protect your personal privacy," said Kharkovoy. "Our new GPS jammer model GJ6 was created to block all possible tracking systems and also all civil GPS systems including GPS L1, GPS L2, and GPS L5. To run the GPS jammer you simply turn on the switch at the top of the jammer."

But that, says Bruce Romano, the legal adviser at the FCC's Office of Engineering and Technology, is not a good argument for using a jammer. Anyone, he says, can hire a detective to perform a sweep of a car or personal belongings to look for GPS receivers.

"Besides being illegal, or [criminals] thinking they can get away with using them because customs will not detect them, there are a wide variety of critical devices that could be affected, and there could be unintended consequences that cause problems, and you have no idea you are causing them," Romano said.

The Air Force -- tasked with deploying and maintaining GPS satellites -- acknowledges that GPS systems are vulnerable, since they are widely available for public use.

"GPS design has incorporated measures to ensure signal availability to users in a war fighter environment," said Andy Roake, chief of current operations at Air Force Space Command Public Affairs. "An element of signal availability is jam resistance, and that has been a key focus in the development of the satellite constellation, the ground segment, and military user equipment.

"It is an important part of what we've done with our GPS constellation, and we continually work to improve jam-resistant capability. However, we cannot discuss technical elements of how we achieve this due to the sensitivity of revealing capabilities to any potential adversary."

While government agencies will not discuss how they detect or dissuade jamming equipment, or how next-gen GPS satellites will be improved to make jamming more difficult, Last said there was one step the Bush Administration took in 2008 to counteract the jamming risk -- a high-power, ground-based system called Enhanced Loran (eLoran), which was designed to be a fall-back for GPS jamming.

"So far, the current administration has not announced any intention to proceed with eLoran," Last said, "... leaving the U.S. without the principal defense it had announced it wished to deploy."

Of course, GPS and cell phone jammers are not exactly state of the art. The devices, which cause signal confusion and disruption, are actually similar to illegal cell phone jammers.

The risk is low for airplanes, which use ground-based radars for guidance and have a back-up navigation system that does not depend on satellites. Military personnel use a private GPS network. But GPS jamming could nonetheless cause confusion in the cockpit as pilots have to switch to back up navigation systems. And maritime shipments that rely on GPS coordinates for finding port locations could face problems as well.

Ronald Repasi, the FCC's Deputy Chief for the Office of Engineering and Technology, said selling, importing, owning, or using a GPS jammer in the U.S. is illegal, and he said the agency actively pursues those who use the devices. He said GPS jammers could pose a potential risk if used negligently.

"It goes to the capability of the jamming device," said Repasi.  "Higher power devices will have greater range and greater potential for interference over a wider area than lower power devices."