Aging Satellites Spark GPS Concerns
by James Figueroa
Concern about the future of the Global Positioning System (GPS) mounted this month when the US Air Force announced an operation delay for an advanced technology satellite it recently sent into orbit. The news comes on the heels of a government report warning that aging satellites could threaten GPS functionality by 2011, although the Air Force insists that GPS will continue to operate smoothly.
The Space and Missile Systems Center (SMC), the Air Force's space systems development arm, announced on 16 June that the GPS IIR-20 satellite launched last March wouldn't be introduced into GPS until at least October. The delay is related to distorted signals that don't match other IIR satellites, which were designed to replace older satellites currently in use. GPS IIR-20 is the first to use an L5 signal, an improved GPS frequency specifically developed for civil applications such as navigation. The satellite successfully transmitted on that signal in April, but the Air Force said it had traced the signal distortions to the L5 interface. Engineers expect to correct the problem in the next few months.
Delays and cost overruns were at the heart of a US Government Accountability Office (GAO) report to Congress in May, which stoked fears that the Air Force wouldn’t be able to replace enough satellites before they reach the end of their life spans.
"If the Air Force does not meet its schedule goals for development of GPS IIIA satellites [the design block currently in planning stages], there will be an increased likelihood that in 2010, as old satellites begin to fail, the overall GPS constellation will fall below the number of satellites required to provide the level of GPS service that the US government commits to," the report stated. "Such a gap in capability could have wide-ranging impacts on all GPS users, though there are measures the Air Force and others can take to plan for and minimize these impacts."
Following the report, the Air Force has worked hard to assure the public that GPS isn't in any danger. "GPS is alive and well and not going to fall out of the sky," said Master Sgt. Matt Gilreath, an Air Force spokesman.
In a Twitter session on 21 May to address GPS concerns, Air Force Space Command Col. Dave Buckman acknowledged a small risk that GPS performance might falter, but emphasized that the Air Force wouldn't let the number of functioning GPS satellites fall under 24, the minimum required to maintain an adequate service level. There are currently 30 operating GPS satellites.
"We have the largest GPS constellation in history and active plans to mitigate any potential gap in coverage," Bruckman wrote. "The issue is not whether GPS will stop working. There's only a small risk we will not continue to exceed our performance standard."
Mitigation measures could include extending the life of current orbiting satellites.
The GAO report also criticized the Air Force for overly optimistic development plans that often run into technical problems, delaying schedules and requiring extra funds that must be taken from other projects. The satellite program that will follow IIR, called IIF, exceeded costs by roughly $870 million and is currently slated to begin deployment in November, three years later than planned, the GAO said.
Next in line, the IIIA program will feature several improvements over older designs, including a new civil signal, cross-linked architecture for updates to the entire constellation from a single ground station, and interoperability with Galileo, Europe's global satellite navigation system planned for operation by 2013. The IIIA satellites will be built by incoming contractor Lockheed Martin and are currently projected to begin launching in 2014, a timeframe GAO questioned because its development would be three years faster than the IIF program.
"GAO's analysis found that this schedule is optimistic, given the program’s late start, past trends in space acquisitions, and challenges facing the new contractor," the report said.
To alleviate its concerns, GAO recommended consolidating GPS development with a single authority to make the process smoother. Currently, the Air Force divides responsibility with several divisions, including SMC, Space Command, and the 50th Space Wing. The Department of Defense also oversees development.
According to the Air Force, GPS provides global navigation information for military and civilian use at extremely accurate measurements, including time within a millionth of a second, velocity within a fraction of a mile per hour, and location within 100 feet. Satellites orbit the globe every 12 hours, emitting constant signals. The system began operations in the 1970s and became globally functional in 1994.