Air Force Considers GPS Architecture Changes

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The U.S. Air Force is weighing the risks and benefits of changing the architecture of the Global Positioning System (GPS) as it transitions to GPS-3, the next generation of the U.S. satellite constellation that provides civil and military users alike with position and timing data.

A recently completed study by a Defense Science Board task force recommended changing the number of orbital planes in the constellation from six to three and increasing the total number of satellites from 24 to 30.

“We’ve been doing some analysis with our independent review team looking at similar recommendations, and doing some additional analysis on how we’d go about doing that, and what the appropriate timing would be, and what the cost and schedule would be,” Air Force Col. Alan Ballenger, a system program director for the Navstar Global Positioning System joint program office, said in a Dec. 1 telephone interview.

Though no official decision has been made, Ballenger said Air Force officials are weighing scenarios that would move away from the current arrangement of satellites, and that it has even considered transitioning to a new setup as early as with the launch of its next GPS satellite, which is slated for early 2006. He acknowledged, however, that it is unlikely the transition would begin that soon.

“At this point, we’re not on a path to transition from a six- to a three-plane system, but I would not rule that out,” Ballenger said. He said that while a transition as early as the next satellite launch is less realistic, scenarios for moving to a three-plane system before GPS 3 are being given more attention.

While the U.S. GPS satellites are arranged in six orbital planes, the European Space Agency’s planned Galileo satellite navigation system, by contrast, is being designed to operate in three orbital planes with 10 satellites per plane.

The more satellites you add to a constellation, the less necessary a six-plane arrangement becomes, because the loss of one particular satellite becomes more negligible, Ballenger said.

“Once you hit around 27 to 30 on orbit, the six planes is no longer a strongly preferred option,” Ballenger said.

The biggest concern when deciding when to make the change is the effect the transition would have on the service to the end user, Ballenger said.

The task force report, “The Future of the Global Positioning System,” which was released Nov. 22, concluded that a constellation of at least 30 satellites is needed to make up the entirety of GPS 3 in order to create enough coverage to support ground operations.

“That’s a great recommendation in terms of improving service to users,” Ballenger said. “What it kind of comes down to, though, in our business, is all kinds of requirements and funding.”

Ballenger said the Air Force would like to operate more than 24 satellites and has operated as many as 30 at one time since the inception of the GPS program, but that 24 satellites represents what it will guarantee in terms of performance to both military and civil customers.

The Air Force has been working with the civil community to look at updating its performance standards so it can guarantee more to customers, not just in terms of the number of satellites, but other variables such as how many meters of horizontal and vertical accuracy the system can provide, Ballenger said. New performance standards potentially could be issued in less than a year, he added.

Ballenger was a participant in a GPS Civil Focus Day in October, which brought together participants from the military and other offices such as the Department of Transportation to discuss GPS issues affecting both user communities.

GPS 3 is being designed with additional anti-jamming capabilities, though the DSB report warned those capabilities may not be sufficient to protect GPS satellites from an attack that occurs before 2013.

Ballenger said the Air Force is working on additional anti-jamming capabilities in the meantime. Approximately 100,000 receivers have been produced which incorporate Selectively Available Anti-Spoofing Modules, which provide additional security against unauthorized access, and more are on the way. A dditional help will arrive when receivers are produced that accept the new military signal known as the M code, he said. Prototype cards with this capability should be available in 2008 or 2009, with working models in the field by 2010 to 2012, he said.

The request for proposal for GPS 3 will likely be issued in January 2006, Ballenger said, with awards allotted that summer. Lockheed Martin of Bethesda, Md. and Boeing Company of Chicago are the two leading contenders for the contract.