The space-based Global Positioning System (GPS) signal that guides smart bombs and cruise missiles to their targets underpins U.S. technological superiority on the battlefield. Yet because it is relatively easy to jam, it is also the Achilles' heel of U.S. military might. Although the integrity of the GPS signal was maintained in the war with Iraq, enemy attempts to corrupt it underscored the need to protect GPS-dependent weapons and navigation systems. Against a more capable enemy, GPS might find itself among the first casualties of any new conflict.

"It's generally recognized the GPS signal is vulnerable," says Jack Spencer, a defense analyst with the Washington, D.C.-based Heritage Foundation, which is lobbying for GPS to be declared a critical infrastructure that would be placed under the protective umbrella of homeland security. GPS is a system of 24 satellites that continuously beam navigation signals to Earth. Each satellite transmits two signals, one for civilian use and another, encrypted version for military purposes. But a broadband transmitter generating electronic "noise" at the right frequencies can overwhelm these signals, which are weaker than the average radio signal people listen to every day.

Indeed, it was the potential vulnerability of GPS to jamming that prompted Iraq to purchase a number of GPS jammers from Aviaconversiya Ltd., a Russian company that has been hawking GPS jammers at military hardware shows since 1999. The high-priced and high-powered GPS jammers offered by Aviaconversiya were said to be able to jam GPS signals for a radius of several miles. The Iraqi military used at least six of these high-powered GPS jammers, which cost $40,000 or more each, during the war. All six were quickly eliminated by U.S. forces over the course of two nights. Officials won¿t provide details, but considering the speed with which they tackled the problem, the Iraqi GPS jammers may initially have been somewhat effective.

The failure of the Iraqi military to continually throw GPS-equipped Joint Direct Attack Munition (JDAM) bombs and Tomahawk cruise missiles severely off course can be attributed to several factors. Among the most important was the installation of backup inertial navigation systems (INS) that could keep the bombs and missiles on target if the GPS signal was compromised. INS systems are slightly less accurate than GPS, however, so there is a greater risk that INS-guided weapons might be a tad off course, raising the risk of collateral damage and casualties in densely populated areas. Still, the danger is likely to be short-lived: high-powered GPS jammers can easily be traced back to their origin, effectively painting themselves with a bull's-eye. "In fact, we destroyed a GPS jammer with a GPS weapon," U.S. Major General Victor Renuart told reporters at a briefing in Qatar that described the attacks against GPS jammers.

Iraqi efforts at jamming may also have been thwarted by a novel, signal-boosting technology--deployed in Iraq under a shroud of secrecy--that overwhelmed the GPS jammers. Airborne pseudo satellites, nicknamed "pseudolites," installed on Global Hawk or Predator unmanned drones would have created a miniature GPS constellation over Iraq. These pseudolites would have captured the weak GPS signals from space and then relayed them, at substantially higher power and at closer range, to airborne bombs and missiles or to forces on the ground. Like the satellites in space, four pseudolites would be required to plot a navigational solution. Tests performed in April 2000 by the Defense Advanced Research Projects Agency (DARPA), the Pentagon's research arm, convinced the military that the psuedolites were powerful enough to overcome jamming. Specially developed beam-forming antennas and signal processors allow the pseudolites to acquire the space-borne GPS signal even when it¿s under attack. New receivers called Precision Lightweight GPS Receivers (PLGRS), or "pluggers," have been made for use with pseudolites.

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