Government officials and communications experts are assessing the public safety and security implications of a newly posted online article that provides directions for making cheap devices that can jam Global Positioning System (GPS) signals.

Information in the article that appears in the current issue of the online hacker magazine Phrack potentially puts at risk GPS devices used for commercial navigation and military operations, authorities said.

The Phrack article provides a detailed guide to building a low-cost, portable GPS jammer out of components that can be easily obtained from electronics supply houses. According to the article, the "onslaught of cheap GPS-based navigation (or hidden tracking devices) has made it necessary for the average citizen to take up the fine art of electronic warfare." Electronics and GPS experts who read the article this week called it technically competent and said amateurs with a certain amount of technical skill could build a GPS jammer from the plans.

Although the article said the jammer is designed to work only against civil-use GPS signals broadcast on the frequency of 1575.42 MHz and not the military frequency of 1227.6 MHz, James Hasik, an Atlanta-based consultant and author of the book The Precision Revolution: GPS and the Future of Aerial Warfare, disagreed.

Hasik said that while the Phrack jammer is targeted at civil GPS signals, known as the C/A code, it could also threaten military systems, since "almost all military GPS receivers must first acquire the C/A signal" before locking onto the military signal, known as the P(Y) code.

Hasik said that GPS receivers are especially vulnerable to jamming because of low signal strength after traveling 20,000 miles through space from GPS satellites.

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