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.