It wouldn't take a lot to push the Global Positioning System below the minimum threshold of 24 operational satellites necessary to ensure a high standard of signal availability and accuracy, says the Government Accountability Office.

In a report dated Sept. 15, the GAO notes that Air Force officials have given prime contractor Lockheed Martin only 72 months to build the first of the Block III satellites in order to meet a planned 2014 launch date. The Defense Department had earlier intended to give the contractor 84 months from contract award to complete the first of the advanced capability GPS satellites, but a three year delay in awarding the contract was matched by an only two year postponement to the original planned launch date, the GAO report states.

Although the GPS IIIA program so far has kept requirements stable and used nothing but mature capabilities for critical technologies, yet uncompleted complex tasks associated with the design and integration phases will make the 72 month deadline difficult to meet, GAO auditors say in the report. No major DoD satellite program in the past decade has met its schedule, auditors add. GPS IIIA satellites will carry a new civilian-use signal, the L1C, and increase the jamming resistance of the military signal.   

Right now there are 31 working GPS satellites in orbit, with the first of the IIF block of GPS satellites having gained orbit in May 2010. But, a delay in the launch of the first GPS III satellite could reduce the size of the constellation below the 24 baseline number, the report warns. Were that to occur, most users would continue to receive GPS signals, although accuracy and availability "could diminish in some locations for brief periods." GAO auditors found that none of the military services have studied how a diminished GPS constellation might impact operations.

Possible delays in the new ground segment control system necessary for the operation of Block III satellites--the Next Generation Control Segment--might also disrupt the planned launch dates of several IIIA block satellites. The Next Generation Control Segment, abbreviated as OCX, isn't meant to become operational until the late summer of 2015, more than a year after the first GPS IIIA satellite is schedule to be in orbit. Until OCX comes online, Air Force officials want to limit GPS IIIA launches to just one. Were OCX to become delayed, an Air Force plan to launch an additional five GPS IIIA satellites within two years after the first launch might have to be scrapped.

UPDATE: This article was modified on Sept. 24 to reflect the fact that there are 31 healthy satellites in the GPS constellation, but 32 operational satellites. One satellite, SVN-49, is operational but unhealthy and its signals are unavailable to GPS users.

For more:
- download the report, GAO- 10-636 (.pdf)

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