Northrop Grumman Electronic Systems said Oct. 4 that it has completed payload integration and initial functional testing of a U.S Air Force missile warning payload that will fly on a classified satellite in a highly elliptical orbit, or HEO.

Two HEO payloads and a dedicated geosynchronous satellite have already been launched as part of the Space Based Infrared System (SBIRS) missile warning constellation that Sunnyvale, Calif.-based Lockheed Martin Space Systems is building for the Air Force.

Northrop Grumman said in a press release that the recently completed round of functional tests clears the HEO-3 payload to enter the next phase of testing, an environmental test regimen that includes bombarding the payload with sources of electromagnetic interference and seeing how it behaves in a thermal vacuum chamber.

The payload is scheduled for delivery in 2013.