WASHINGTON — The U.S. Air Force awarded Lockheed Martin Space Systems a $1.3 billion contract modification Dec. 15 that will allow the Sunnyvale, Calif.-based company to build a fourth Advanced Extremely High Frequency (AEHF) secure communications satellite.

Lockheed Martin, the AEHF prime contractor, began ordering long-lead parts for AEHF-4 under a $22 million contract awarded in September 2009. The total contract value associated with that satellite is about $1.6 billion following the latest modification.

The AEHF constellation will replace the current-generation Milstar satellites for handling the U.S. military’s most critical satellite communications traffic, including nuclear command and control. The total value of the AEHF contract, including the four satellites and the ground segment, is about $6.6 billion.

The first AEHF satellite was launched in August aboard a United Launch Alliance Atlas 5 rocket, but has yet to enter service due to an on-board propulsion system failure that forced the Air Force to rely on a backup plan for slowly nudging AEHF-1 into its operational orbit using the craft’s xenon-ion electric thrusters. The cause of that failure is still under investigation. The Air Force said in October that orbit-raising maneuvers were proceeding on schedule and that the satellite would reach its geostationary orbit between June and August 2011.

The second AEHF satellite, which recently completed testing, is expected to launch sometime next year.

Although it is the fourth satellite in the series, the AEHF-4 construction program includes “unique systems engineering; system level factory test; system database management functions; systems level support equipment; and program management,” the Pentagon said in the contract announcement. According to Lockheed Martin officials, there was a four-year production break between the third and fourth AEHF satellites and because of the resulting component obsolescence, there is only 70 percent parts commonality between the two.

The Pentagon at one time planned to buy just three AEHF satellites before moving on to the futuristic Transformational Satellite (T-Sat) program. But T-Sat was canceled in late 2008, meaning the AEHF satellites will continue to handle the Pentagon’s most crucial communications for the foreseeable future.

The Air Force plans to purchase at least two more AEHF satellites. Lockheed Martin has been urging the service to allow the company to order long lead components for the fifth and sixth satellites simultaneously, arguing that doing so would be cheaper than ordering the parts under separate contracts.

Brian Berger is editor in chief of SpaceNews.com and the SpaceNews magazine. He joined SpaceNews.com in 1998, spending his first decade with the publication covering NASA. His reporting on the 2003 Space Shuttle Columbia accident was...