U.S. and South Korean military commanders will be on the lookout for North Korean efforts to jam GPS signals as they take part in exercises on the divided peninsula this week and next.
North Korea repeatedly has jammed GPS signals in South Korea, which has "very serious implications" because U.S. and South Korean military system rely on the navigation system, said Bruce Bennett, a North Korea scholar for the California think tank Rand Corp.
The jamming also underscores the vulnerability of a satellite-based tool on which civilian systems from car navigation to air traffic control rely upon.
North Koreans have used Russian-made, truck-mounted jamming gear near the border to disrupt low-power GPS signals in large swaths of South Korea. By broadcasting powerful radio signals on the same frequencies as the satellites, the jammers drown out the GPS signals.
Mr. Bennett said the jamming has occurred three times in the past two years and has coincided with joint U.S.-South Korean military exercises.
The timing strongly suggests the jamming was "an experiment ... a test ... to let [the North Koreans] see what effect it would have and maybe disrupt the exercises," he said.
Defense officials declined to comment on the jamming, or discuss what measures U.S. forces are taking to guard against further incidents during this week's exercises, which end Aug. 31 and involve more than 80,000 troops from the United States and South Korea, plus observers from seven other countries.
"The U.S. Department of Defense takes all jamming seriously," said Air Force Lt. Col. Damien Pickart, a spokesman for U.S. Pacific Command.
For North Korea, the jamming is an attempt to turn the tables on the more technologically advanced U.S. and South Korean forces.
"Neutralizing those [technological] advantages could have big psychological benefits in peacetime and major military benefits in war," Mr. Bennett said.
He added that the jamming equipment is easy to locate because of the powerful signals it broadcasts. "If you use this kind of weapon, you must assume that sooner or later, the other guy is going to destroy them," he said.
Determining how North Korea might use the jamming as a weapon is difficult because its military does not produce publications, unlike China's, which publishes academic journals and policy documents, Mr. Bennett said.
"The way we try to understand North Korean [military] doctrine is watching how they train and exercise," he said.
In September, there were reports that North Koreans were developing their own, more-powerful jamming technology. Mr. Bennett said it appears that this new equipment was being tested in the most recent jamming incidents in March and April.
The incident in April caused the most significant disruption, even though the jammers were switched on only intermittently. The South Korean capital, Seoul, is only a few miles from the border, and its airport, Incheon International, was badly affected by the jamming. Aircraft had to rely on alternative navigation aids, and even cars in the city's northern suburbs found their GPS equipment affected.
"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.
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