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[] UNWR 21.10.02: U.S. News & World Report October 21, 2002,

U.S. News & World Report October 21, 2002

Keeping Our Bearings

The coming war over the global positioning system

By David Whitman

The war against the Taliban made heroes of soldiers and pilotsand of 24 
small satellites, broadcasting feeble navigation signals from thousands of 
miles up. The global positioning system, or GPS, guided bombs to their 
targets with stunning accuracy and helped ground forces get their bearings 
and call in airstrikes in unfamiliar terrain. Now planners for a possible 
war in Iraq anticipate that Saddam Hussein's forces will try to neutralize 
that advantage by jamming the GPS signals, potentially sending bombs astray 
and disorienting soldiers.

A 2001 report from a commission chaired by Donald Rumsfeld before he became 
defense secretary concluded that the Iraqi military has jamming technology, 
and foreign news accounts suggest that Baghdad has already tried to jam air 
patrols in Iraq's no-fly zones. Last month the Wall Street Journal reported 
that a $39.99 jammer available on the Internet might be enough to make U.S. 
precision-guided bombs and missiles miss their targets. Military analyst 
James Zumwalt has even predicted GPS jammers could soon "have the same 
impact as did the stone used by David to slay Goliath."

Most military experts, however, think that GPS jamming may prove less 
deadly in practice than David's slingshot. Thanks in part to existing 
antijam features, new technology, and some simple tactical maneuvers, the 
military may be able to protect its alleged Achilles heel. Even successful 
jamming wouldn't necessarily hobble U.S. forces, say officials. "You can't 
buy a $40 jammer and send a JDAM [a GPS-aided bomb] awry,'' says Col. 
Douglas Loverro, the Air Force's Navstar GPS systems program director in El 
Segundo, Calif. "Just because the GPS is jammed, the bomb still fallsall we 
are arguing about is how close [it hits]."

Swamped. GPS, perhaps the biggest advance in navigation since the compass, 
is easy to jam because the signals from the distant satellites are so 
weakakin, say, to a 25-watt bulb from 11,000 miles away. A more powerful 
local source easily swamps them. Mario Casabona of Electro-Radiation Inc. 
in Fairfield, N.J., a company that builds low-cost antijamming devices, 
says his "rule of thumb is that if a country has a RadioShack, it will have 
a jammer. You can build a jammer for about $500." A low-power 4-watt jammer 
built by the Russian firm Aviaconversia can, with a clear line of sight, 
block a receiver from picking up GPS signals for up to 124 miles away. The 
U.S. military, meanwhile, has become far more dependent on GPS since the 
Gulf War in 1991, when troops used GPS receivers to navigate the trackless 
Iraqi desert. Today, the armed services have more than 500,000 GPS 
receivers, most of them on precision-guided missiles, bombs, and other 

Many GPS-equipped weapons have some built-in protection. GPS satellites 
broadcast on both a military frequency and a commercial frequency used by 
civilians. And while military GPS receivers usually use the civil code to 
acquire the military "P" frequency, it is much harder to jam a military 
receiver once it has locked into P code, because that signal is more 
robust. "It is probably 1,000 times harder to knock you off signal than to 
block you from acquiring GPS," says Colonel Loverro, which means that a 
jammer that can block GPS acquisition for 100 miles can jam the military 
signal for a few miles at most. Typically, planes carrying JDAMs lock on to 
the military frequency well outside of jamming range.

Even if the Iraqis did succeed in jamming the military signal, the JDAM has 
a backup inertial navigation system that doesn't rely on GPS. Guided by 
inertial navigation alone, a JDAM's accuracy decreases from an average of 
about 40 feet to about 100 feet. In Baghdad, 60 feet might make a tragic 
difference between bombing a school and blowing up a military installation. 
But for targets that require a surgical strike, other precision weapons 
that don't depend on GPS, such as laser-guided bombs, could be used.

Null and void. Besides these backup technologies, the military has ways to 
actively thwart jammers. One might be to track the source of the jamming 
signal and destroy it. Another is a technology added to many planes since 
the Gulf War: "nulling" antenna arrays. The antenna arrays can block 
reception of signals from the direction of the jammer, allowing the 
receiver to "hear" GPS signals from other directions.

Lockheed Martin and Rockwell Collins have developed an even more powerful 
countermeasure called G-STAR, which will be installed in one missile type 
starting next year. The G-STAR, which sounds like something fresh out of 
Star Wars, partly or fully blocks the signal from a jammer and then 
redirects the GPS receiver through "beam steering" to look for other 
nonjammed satellites. In the coming decade, the military will also field a 
new generation of more powerful GPS satellites, which will reduce the 
jamming threat.

For now, GPS jamming may hamper ground troops more than pilots. Soldiers 
lugging hand-held GPS receivers or maneuvering GPS-guided artillery and 
tanks won't have the luxury of locking on to the military signal before 
they come into jamming range. In most cases, though, a jammed GPS receiver 
simply won't work, alerting the troops not to trust it. Soldiers are also 
trained to practice crude versions of high-tech antijamming measures. They 
can crawl behind a jeep or humvee, using its body to partly block the 
signal coming from a jammer on the other side. Or they can stick their GPS 
receiver in a coffee can so they can "beam steer" toward satellites away 
from the jammer.

As Colonel Loverro points out, "ground troops did maneuvers for years 
without GPS," relying on visual clues, scouts, photographs, maps, and 
military intelligence. With no foolproof means to prevent jamming, either 
on the battlefield or in the sky, troops may sometimes have to improvise.

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