The U.S. Air Force may be unable to launch new global positioning system (GPS) satellites in time to meet the goals of a scheduled modernization of the system, according to the federal government's watchdog agency.

A report (PDF) by the Government Accountability Office (GAO) noted several challenges to the launch of the number of satellites needed to fully meet the needs of everyone using the GPS. They include a ground system that won't be ready until after the launch of the satellites it will support and delays in technology deployment for all three facets of the new system -- space, ground control and user equipment.

If the Air Force fails to solve these issues, it could result in an interruption of GPS services as modernization project takes place, according to the report. The Department of Defense (DoD) began GPS modernization efforts in 2000. The Air Force is currently in the process of launching two sets of satellites -- IIF and IIA -- to complete the full constellation of satellites to support the GPS.

The IIF satellites add a third civil signal for transportation-safety requirements to the GPS, while the IIA satellites add a stronger military signal to improve jamming resistance as well as a fourth civil signal that is compatible with foreign signals.

The first IIF satellite launched nearly 3 1/2 years later than planned, on May 27; this type of delay is typical for the DoD, which has never met its planned schedule for deploying satellites, according to the GAO.

While the first deployed IIF satellite seems technically sound, there are problems with the program that could affect the performance of the satellite in orbit, as well as future IIF launches, according to the GAO. Primarily, the report noted that there hasn't been sufficient testing of how the satellites will perform once in orbit, and if there are problems, it could delay the launch of those not deployed already because redesign might be required.

The IIA program, too, faces some challenges, although the GAO found that program directors has developed and implemented best practices in an attempt to avoid mistakes made on the IIF program.

For example, the program has maintained stable requirements, used mature technologies and is providing more oversight than the IIF program, the report found.

However, delays in the effective deployment of IIA satellites also may be on the horizon due to an overly ambitious procurement schedule and problems in the development of the satellites' ground system, according to the GAO.

The latter is especially a concern, as the first segment of the ground system -- called the Next Generation Control System (OCX) -- is scheduled to be operational in the fourth quarter of 2015, more than a year after the launch of the first IIA satellite.

Other satellites are scheduled to be launched in rapid succession after that, so a delay could create a domino effect in preventing the full system from being operational on time, according to the report.