The U.S. Air Force awarded Lockheed Martin Space Systems of Sunnyvale, Calif., a $249 million fixed-priced contract to begin antenna and electronics fabrication and purchase other long-lead flight materials for the fifth and sixth spacecraft in the service’s Advanced Extremely High Frequency (AEHF) constellation of highly secure communications satellites, the Pentagon announced June 25. The work will take place in Sunnyvale and is expected to conclude by Nov. 1.

Lockheed Martin Space Systems is under contract to build up to six AEHF satellites to replace the five-satellite Milstar constellation the United States uses for handling critical military communications. The first AEHF satellite was launched in August 2010 but took more than a year longer than expected to reach its operating orbit due to an on-board propulsion system failure that occurred shortly after liftoff.

The second AEHF satellite was launched successfully in May from Cape Canaveral Air Force Station, Fla., aboard a United Launch Alliance Atlas 5 rocket.