WASHINGTON — The U.S. Air Force launched its sixth GPS 2F positioning, navigation and timing satellite May 16 amid confusion over the overall GPS constellation’s health that has led Congress to request information on the service’s fleet replenishment plan.

GPS 2F-6, built by Boeing Space and Intelligence Systems of El Segundo, California, launched aboard a United Launch Alliance Delta 4 rocket from Cape Canaveral Air Force Station in Florida. Liftoff occurred at about 8:03 p.m. local time.

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 helps bolster a GPS fleet whose satellites are beginning to show their age, Air Force officials say.

Two more GPS 2F satellites are scheduled to launch before the end of the year.

The overall health of the GPS constellation, however, is unclear, in part because of what the Air Force acknowledges have been its own seemingly contradictory statements on the matter.

In its budget request for 2015, the Air Force slowed its procurement of GPS 3 satellites, citing budget pressures and the fact that earlier-generation GPS satellites are lasting longer on orbit than expected.

The constellation currently consists of 31 active satellites and as many as six semi-retired satellites that could be used in an emergency. The system requires a minimum of 24 working satellites to provide full global coverage.

At the same time, officials have talked about the age of many satellites in the constellation, some of which are 21 years old.

“Some of them are old enough to vote,” Gen. William Shelton, commander of Air Force Space Command, quipped during a Feb. 7 breakfast here. “They’re all doing great. … But they’re getting a little fragile. We’re a little bit concerned about the long-term viability of some of the satellites there.”

The House Armed Services Committee questioned the Air Force’s decision to delay buying more GPS 3 satellites in its version of the 2015 defense authorization bill, which was approved May 8 and is awaiting a vote on the House floor.

“While the committee is aware that the Air Force may have made some technical changes to enable better power management of on-orbit satellites, this does not affect the overall constellation fragility as characterized by factors such as satellite age and technical state of internal redundancy or lack thereof,” the bill said. “The committee is concerned with the revised Air Force plan and has not seen any detailed analysis to support the significant changes to the schedule.”

In a briefing with reporters May 8, Air Force Col. Steve Steiner, chief of the service’s GPS space system division, said the confusion is understandable. 

“We have a lot of satellites up there; that’s why you hear the phrase at times that the constellation is robust. However, this is seemingly  a conflicting message,” Steiner said. “We launched a bunch of [satellites]in order to get the constellation up and running in the early days. So you have a large number of satellites that are all about the same age that are susceptible to old age. There is always the possibility that you can have a large amount of failures in that age group at one time.”

Steiner said the Air Force’s plan to launch the GPS 2Fs through 2016, when the next-generation GPS 3 satellites, being built by Lockheed Martin Space Systems of Denver, are expected to begin launching, will help guard against a mass die-off in the constellation.

“It certainly supports good programmatic sense and making sure we don’t fall prey to a bunch of failures of our earlier satellites,” he said. “Our current fly out profile for 2F represents a balance between programmatics and sustainment of the constellation.”

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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...