A software and security update by Lockheed Martin will improve the U.S. Air Force's GPS ground station used to communicate with the GPS 2F satellites, pictured above.

WASHINGTON — New information stemming from the investigation into an engine issue during a Delta 4 launch a year ago led United Launch Alliance (ULA) to delay the upcoming launch of the GPS 2F-5 positioning, navigation and timing satellite aboard a similar rocket, the company said.

The GPS 2F-5 satellite originally was slated to launch aboard a Delta 4 rocket from Cape Canaveral Air Force Station, Fla., on Oct. 17. But that date was pushed back to Oct. 23 in recent weeks. Then, on Oct. 15, Air Force officials said the launch had been delayed again, but did not specify a reason.

In an update posted on its website Oct. 18, Denver-based ULA said “updated conclusions” about the October 2012 Delta 4 launch, in which the vehicle’s upper stage experienced a thrust glitch, led to the postponement. Despite 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 problem and ensuing investigations delayed at least two subsequent missions. The Delta 4 returned to flight the following May with the successful launch of an Air Force Wideband Global Satcom communications satellite and has since conducted two more successful missions.

In the meantime, the Air Force in May successfully launched its fourth GPS 2F navigation satellite from Cape Canaveral atop a ULA Atlas 5 rocket.

ULA in December said a fuel leak was behind the Delta 4 upper-stage thrust anomaly during last year’s GPS mission. In its Oct. 18 statement, ULA said the ongoing investigation has led to “updated conclusions related to dynamic responses that occurred on the engine system during the first engine start event.”

As a result, the GPS 2F-5 launch was delayed to determine whether more modifications to the launch vehicle are warranted, ULA said. The rocket “has completed the standard and checkout on the launch pad and will be maintained in a ready state for spacecraft mate and launch pending completion of this assessment,” ULA said.

A new launch date will be determined once the assessment is completed in the coming weeks, ULA said.

The October 2012 glitch triggered separate investigations by the Air Force and by ULA and RL-10 engine maker Aerojet Rocketdyne of Sacramento, Calif.

Several weeks after the 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, Shelton said the investigations might never conclusively determine a root cause.

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Mike Gruss covers military space issues, including the U.S. Air Force and Missile Defense Agency, for SpaceNews. He is a graduate of Miami University in Oxford, Ohio.