WASHINGTON — After a five-month delay and despite lingering questions about the Delta 4 rocket’s upper stage engine, the U.S. Air Force launched its fifth satellite in the GPS 2F series of positioning, navigation and timing satellites Feb. 20.

The launch aboard a United Launch Alliance Delta 4 rocket from Cape Canaveral Air Force Station in Florida had been delayed since October by questions surrounding an upper stage engine anomaly that occurred during the launch of a GPS satellite aboard a similar rocket in 2012. Liftoff occurred at about 8:59 p.m. Eastern time following a brief delay related to concerns about solar radiation.

GPS 2F-5, built by Boeing Space and Intelligence Systems of El Segundo, Calif., originally was slated to launch Oct. 17, but that date was pushed back to Oct. 23. Then, on Oct. 15, Air Force officials said the launch had been delayed again, but did not initially specify a reason.

Denver-based ULA said at the time “updated conclusions” about the October 2012 launch, in which the Delta 4’a upper stage experienced a thrust glitch, prompted the postponement. Despite the thrust issue — the rocket’s RL-10 upper-stage engine underperformed — the rocket ultimately was able to deliver its payload, also a GPS 2F satellite, to its proper orbit using reserve fuel.

The ensuing investigations, one by ULA and one by an independent accident review board, delayed at least two other ULA missions, in part because of similarities between the upper stage engines on the company’s Delta 4 and Atlas 5 rockets. Both vehicles have long since resumed flying — the Atlas 5 delay was very brief– but nagging questions nonetheless put the GPS F5 mission on hold.

The Air Force and ULA in February elected to proceed with the mission after modifications to the RL-10 were tested to their satisfaction. “Additional investigation activities have confirmed that there is not a systemic issue with the Delta 4 second stage RL10B-2 engine,” Jim Sponnick, ULA vice president for Atlas and Delta programs said in a conference call with reporters Feb. 14. “Additionally the investigation results have reconfirmed that the system improvements that were implemented following Phase 1 of the investigation were appropriate and both ULA and our Air Force customer have approved proceeding with the launch of the GPS 2F-5 mission.”

Several weeks after the 2012 incident, Gen. William Shelton, commander of Air Force Space Command, said the Air Force could ill afford to lose a satellite in a launch failure and that it was therefore important to determine the precise cause of the engine thrust anomaly. But just two months later, in January 2013, Shelton said the investigations might never conclusively determine a root cause.

Both ULA and the Air Force investigations are expected to wrap up in April, said Col. William Hodgkiss, the launch systems director at Air Force Space and Missile Systems Center in Los Angeles.

The GPS 2F satellites provide better accuracy and resistance to jamming than the previous generation of GPS satellites, most of which are still in operation. The launch also helps bolster a GPS fleet whose satellites are beginning to show their age, Air Force officials say.

The new satellite will be one of 31 active satellites in the constellation. Once it checks out on orbit, the Air Force is expected to move one of its older GPS 2A series satellites into a reserve mode, said Col. Bill Cooley, head of the GPS directorate at the Space and Missile Systems Center. That would bring the number of satellites in reserve mode to six, he said.

The GPS 2A satellites had a life expectancy of 7.5 years. The older satellite targeted for placement in reserve mode is 16 years old, Cooley said.

The Air Force has taken a series of steps in recent years to extend the life of GPS satellites, including changing the way it charges the batteries on some satellites.

The Air Force expects to launch at least two more GPS 2F satellites before the end of the year. Follow Mike on Twitter: @Gruss_SN

Mike Gruss is a senior staff writer for SpaceNews. He joined the publication in January 2013 to cover military space. Previously, he worked as a reporter and columnist for The Virginian-Pilot in Norfolk, Va. and The Journal Gazette in Fort Wayne, Ind. He...