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Pentagon urged not to
use ability to degrade GPS signals
July 17, 2007
A newly formed multiagency advisory board argues
that the Defense Department should never again intentionally degrade the performance of the Global Positioning System.
At its first meeting held in March, the National Space-Based Positioning Navigation and Timing (PNT) Advisory Board
decided that, although the Air Force has the ability to degrade signals from the constellation of GPS satellites through a
process known as "selective availability," board chairman and former Defense Secretary James Schlesinger says he
"cannot conceive any scenario in which SA has any credibility today," according to minutes of the meeting, which
the board released this month.
The last time Defense intentionally degraded the civilian signal was in 1990, and
its reason for doing so was not made public. The Air Force intends to add the ability to degrade the signal in its next-generation
GPS III satellites, which it plans to launch in 2013.
The PNT board includes members from the departments of Defense,
Transportation, Commerce, State, Homeland Security, and the Joint Chiefs of Staff and NASA, as well as representatives from
academia and U.S. industry. Representatives from Australia, India, Japan, Norway, Switzerland and Great Britain also sit on
the board.
The Air Force started development of GPS during the Cold War and included the ability to degrade the
accuracy of signals sent for civilian use, which are globally available, to an accuracy of about 100 meters instead of the
current 10 meter or better level of accuracy. In May 2000, President Clinton declared that the United States would no longer
degrade the civilian GPS signal.
President Bush issued a revised PNT policy in December 2004, which promised "uninterrupted
access" to civilian GPS signals. But Bush added that the United States would include capabilities to deny hostile use
of the GPS system "without unduly disrupting civil and commercial access" to GPS signals.
Chet Huber,
a PNT board member and president of OnStar, a vehicle navigation and security system that relies on the GPS system, told the
meeting that the 2 million users of OnStar need assurance of signal stability. GPS-equipped OnStar units help responders locate
emergencies more quickly, Huber said, and "there would be a high price due from applying SA."
Retired
Air Force Gen. James McCarthy, a former pilot who currently serves as a national security professor at the Air Force Academy
and is a PNT board member, told the meeting that SA can be eliminated "with the right set of arguments, which have not
yet been made or articulated." McCarthy added that in his view there is no need for SA, although he would not have said
that five years ago.
The United States turned off its ability to degrade the GPS signal seven years ago, but James
Miller, a senior GPS technologist with NASA, told the board meeting that many countries still do not trust GPS because of
"the international perception that continuing with SA capability enables GPS to be turned off at any time."
Accuracy for both military and civilian users also will be improved by insuring that the GPS constellation remains at its
current level of 30 satellites, according to Schlesinger and other board members. Schlesinger said the Air Force only guarantees
24 satellites. An increase would boost GPS accuracy, and he said 30 satellites are the minimum needed to support ground forces
operating in varied terrain.
Aviation users also say they need 30 satellites to support aircraft navigation. Capt.
Joseph Burns, director of flight standards and technology at United Airlines and a board member, said he is concerned about
accuracy being degraded by interference.
Board member Timothy Murphy, a technical fellow with the Boeing Commercial
Airplane Group, said the company's vision is "tightly wrapped" around the notion that there will be a "robust"
satellite navigation system based on 30 satellites.
Retired Air Force Maj. Gen. Robert Rosenberg, chairman of the
Air Force Space Command GPS Independent Review Team, told the board that the GPS system must be robust enough to work in challenging
environments, such as mountains and urban canyons, which require more than 24 satellites. Rosenberg said all users today are
used to the service that 28 to 30 satellites provide, and a reduction in that number of satellites would have a potentially
"enormous adverse impact."
But it may be difficult to keep that number of fully functional satellites
in orbit in the near term, he said. By next year, 11 GPS satellites will have reduced capabilities and the number of satellites
in the constellation may have to be cut.
Future funding for GPS may be limited and "more must be done with
the same or less finding," Rosenberg said. Col. Allan Ballenger, commander of the Air Force GPS Wing, which is the acquisition
arm for the GPS program, says his funding runs between $900 million and $1 billion a year.
Source: govexec.com
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