"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 wont 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 its under attack. New receivers called Precision Lightweight GPS Receivers (PLGRS), or "pluggers," have been made for use with pseudolites.
Pseudolites and backup inertial navigation systems mean smart bombs and cruise missiles are likely to reach their intended targets as long as the U.S. military controls the air space over a battlefield. But in a conflict in which pseudolites cannot be safely deployed, ground forces could go astray or misdirect their fire if they encounter a minefield of expendable, hockey-puck-size GPS jammers, each of which could disrupt the GPS signal within a one-kilometer radius. Iraq reportedly also purchased as many as 400 small GPS jammers from Aviaconversiya prior to the outbreak of hostilities in the region, although it is not known whether any of these were used.
"It's a serious threat," says Jim Hendershot, president of Radio Design Group, Inc., a maker of GPS jamming gear used for training purposes, based in Grants Pass, Ore., "because these small jammers can screw up the guy on the ground. Soldiers' GPS receivers don't have backup navigation systems. They would be deprived of their ability to navigate."
Just how dependent ground forces can be on GPS was inadvertently revealed to Greek authorities in August 2000, when the U.S., Britain and France competed for a $1.4-billion tank contract. As each country's tank entry demonstrated its prowess, it became clear that U.S. and British tanks could not acquire a GPS signal for navigation. Sometime later and to the amusement of Greek defense officials, reports the journal Military Review, it was revealed that French agents were remotely activating small, one-foot-high GPS jammers to disrupt the GPS signal when British and U.S. tanks were in the field. Such GPS jamming tactics should not have come as a surprise considering the fact that the U.S. and Australian militaries were jointly conducting research on GPS jammer locators in remote Woomera, Australia, as far back as March 2000.
In fact, small GPS jammers can be built by any hobbyist with a spare $400 to invest in electronic components, using plans supplied by the online hacker magazine Phrack, for example. Such devices can easily disrupt a commercial GPS signal and possibly a military GPS signal, even though the latter is encrypted with a code that changes on a regular basis. The military signal is a little harder to disrupt," Hendershot says, "but it's still easy."
That's why the military is lobbying for the launch of 20 new GPS satellites starting next year. These new satellites would transmit a GPS signal eight times stronger than the current signal, which means that any potential jammers would have to increase in size and complexity. Until then, note experts like Hendershot, "GPS is very vulnerable."
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