- Iraq has jamming equipment potentially capable of knocking
some of America's "smart" weapons off course, according to a
U.S. government official with access to intelligence reports.
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- The official said this week the Iraqis have obtained
at least a limited number of transmitters that can jam signals from the
satellite-based global positioning system or GPS. The satellite signals
are used to guide bombs called JDAMs (Joint Direct Attack Munitions) to
their intended targets with precision.
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- "The question is how will they (the Iraqis) implement
this and how effective they would be" at sending U.S. bombs astray,
said the official, who asked not to be identified.
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- "Of course we are concerned," said Air Force
Maj. Gen. Dan Leaf, director of operational capability requirements for
the Air Force. But, he added, "Any adversary or potential adversary
of the United States betting their future on GPS jammers is making a serious
miscalculation." He said he was confident the Pentagon has effective
means to counter the jamming threat, including alternate weapons such as
laser-guided bombs.
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- The use of precision-guided bombs and missiles has been
cited by Pentagon officials as a way to minimise civilian casualties in
any attack on Iraq, particularly in densely populated areas like Baghdad.
If Iraq were able to send some of the GPS-guided weapons off course, it
could increase civilian casualties and spur regional criticism of a U.S.
attack on Iraq.
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- Specialists, in and outside of government, have long
noted the potential vulnerability of GPS-guided bombs to jamming. "If
I was going to be bombarded with weapons guided by GPS, I would be out
there looking for jammers too," said John Beukers, a radionavigation
expert in Vero Beach, Fla. "It's not rocket science to build these
things."
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- A Russian designer unveiled a small, 4-watt jammer at
the 1997 Moscow Air Show that is said to be capable of disrupting GPS signals
over an area about 100 nautical miles in radius.
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- The global positioning system, developed for the U.S.
military, consists of a constellation of 24 satellites that continuously
beam navigation signals that can be used by anyone with the proper receiver.
The satellites transmit two signals, one available to civilian users, including
commercial airliners, to determine their position within a distance of
a few metres. The second signal, for use by the military, is encrypted.
The Air Force placed GPS receivers on previously "dumb" gravity
bombs as a way to enhance their accuracy. Once dropped from an aircraft,
the bombs use GPS signals to locate a specific geographical location up
to 15 miles from the drop site. GPS-guided JDAMS bombs were used in Kosovo
and also in Afghanistan.
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- The GPS signals from the satellites are weak and can
be overwhelmed by a broadband transmitter that generates enough "noise"
at the right wavelengths, experts said. "I would tend to believe that
you could jam those encrypted signals with enough power," said Linn
Roth, president of Locus Inc. of Madison, Wis., a maker of radionavigation
products. "Those signal levels are so low."
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- The military also uses the civilian signal to help home
in on the military signal, another complication that could make the GPS-guided
bombs vulnerable, specialists said.
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- Leaf said the Air Force has given GPS jamming "a
lot of thought for a long time." He said the Air Force has run tests,
including two recently, to help better understand how aircraft and weapons
perform in the presence of jammers.
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- While JDAMS are "GPS-aided," Leaf said, they
also have backup inertial navigation systems that "will still be precise
enough for almost any target that we face." But he said the Air Force
prefers the extra precision that GPS provides. John Pike, director of GlobalSecurity.org,
a military research group, said an inertially guided bomb has an accuracy
of about 100 feet, substantially less than GPS.
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- Leaf said jamming can be countered by changing the location
of GPS-receiver antennas on the bombs to make them less vulnerable. Pike
said placing directional antennas on the bombs "will give you a hundred-fold
improvement" in ability to receive the weak satellite signals. "It
is not clear which, if any, platforms or munitions have been fitted with
such antennas," Pike said. Leaf declined to go into detail on specific
countermeasures.
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- Pike said the military also could use airborne sensors
to locate any GPS jammers and quickly destroy them with homing missiles,
he said. Leaf said there are methods for silencing jammers. "If my
jersey said GPS jammer operator, I wouldn't be real secure of my future,"
Leaf said.
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- A longer-term fix is to substantially increase the power
of the GPS signal to make it less susceptible to jamming, Pike said. But
that will require launching a next generation of satellites, a costly proposition
that has not been budgeted.
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