GPS designers understood from the beginning that the system’s weak signals would be vulnerable to inadvertent or deliberate interference, with the threat formally recognized by the DOT’s Volpe Center in Cambridge, Mass., on Sept. 10, 2001�one day before 9/11. Since that time, the Department of Defense has run annual all-altitude tests�over the Western U.S. last month and currently over the Southeast�of the jam-resistance of military GPS equipment. In 2009, Newark Liberty International Airport installed a Honeywell GPS ground-based augmentation system (GBAS) to enhance terminal operations, and Continental Airlines has since equipped 18 of its Boeing 737s with GBAS avionics, with the FAA providing GBAS support. But system tests last year revealed intermittent and unpredictable interference, causing the ground station to shut itself down. The culprits were small GPS jammers used by drivers of passing 18-wheelers to disable the tracking devices installed by trucking companies. Reportedly, thousands of these jammers have been sold, with one model selling for only $22.95 online. The FAA and Honeywell are now investigating protective measures, but in the meantime Newark’s GBAS cannot be certified for instrument landings.