CAPE CANAVERAL, Fla. — A United Launch Alliance (ULA) Delta 4 rocket successfully launched the third in the U.S. Air Force’s series of Boeing-built GPS 2F navigation, positioning and timing satellites Oct. 4 from Cape Canaveral Air Force Station here.

The launch took place at 8:10 a.m. local time from Space Launch Complex 37, according to Denver-based ULA, a Lockheed Martin-Boeing joint venture. GPS 2F-3 was placed into a roughly 17,600-kilometer orbit, where it will undergo a test and checkout period before replacing an older satellite in the constellation.

While the rocket placed the satellite into the correct orbit, controllers received unexpected data readings from the vehicle’s upper stage during the launch, ULA spokeswoman Jessica Rye said. “Per our normal, rigorous post-flight processes we will look into this data signature to understand the cause,” Rye said via email Oct. 5.

The GPS constellation currently consists of 31 operational satellites spanning four generations, plus three to four decommissioned spacecraft that could be reactivated if needed, according to, a U.S. government website.

According to the website, an earlier-generation GPS 2RM satellite launched in March 2009 has been declared unusable. That satellite, the second to last of the GPS 2RM series built by Lockheed Martin Space Systems of Denver, carried an experimental L5 payload for civil navigation services that created interference issues with the primary payload.

Lockheed Martin spokesman Michael Friedman said the satellite “remains a test and reserve satellite that could potentially be brought into service if needed in the future. We are working closely with the U.S. Air Force to determine the appropriate next steps for the satellite.”

Boeing Space and Intelligence Systems of Seal Beach, Calif., is under contract to build 12 GPS 2F satellites, which provide more robust and jam-resistant signals than the earlier-generation satellites. The newer satellites also have the operational L5 civil signal, which is used for commercial aviation as well as search and rescue operations.

The GPS 2F satellites have 12-year design lives and will last longer than previous-generation spacecraft, according to the Air Force.

During an Oct. 3 media briefing here, Paul Rusnock, Boeing vice president of government space systems, said the second of the GPS 2F satellites experienced a problem with an atomic clock — which is at the heart of the satellite’s ability to discern position location — that forced controllers to switch to a backup system. The satellite was launched in July 2011 and the problem was discovered during on-orbit testing, he said.

The GPS 2F satellites include two rubidium clocks and one cesium clock, Boeing spokeswoman Paula Shawa said. The problem, which occurred with the cesium clock, was due to trapped air inside a xenon bulb that caused a pump failure that in turn created maintenance issues, she said.

Faced with “higher than desired clock maintenance from the ground crew,” program officials elected to transfer the cesium clock applications to a rubidium clock and there was no impact to the satellite’s performance, Shawa said. A manufacturing change has been made to the nine remaining GPS 2F satellites to limit future risk, she said.

Rusnock said engineers tweaked the xenon bulb on GPS 2F-3, which was too close to launch when the issue was discovered to make the manufacturing change.

Col. Steve Steiner, chief of the GPS directorate at Air Force Space and Missile Systems Center in Los Angeles, said the constellation is “healthy, stable and robust.” The operational constellation currently consists of 10 GPS 2As, 12 GPS 2Rs, seven GPS 2RMs and two GPS 2Fs, but many of the older craft are well beyond their design lives, he said. The Air Force has been launching GPS satellites at a rate of one per year but hopes to increase that starting next year, he said.

Tony Taliancich, director of ULA’s customer program office, said the next GPS 2F satellite will be launched in May aboard an Atlas 5 rocket.

The Air Force is studying launching the next-generation GPS 3 satellites two at a time aboard Atlas 5 rockets. Taliancich said the dual-launch mode is only being considered for the Atlas 5 because intermediate-lift versions of that vehicle have more capability than comparable versions of the Delta 4.