The US Air Force has awarded a $1.5bn contract to Lockheed Martin to build the third highly elliptical orbit (HEO-3) payload and the third geosynchronous orbit (GEO-3) satellite.
Under the contract, the company will also build associated ground modifications for the space-based infrared system constellation.
The air force satellites aim to provide early warning of missile launches and support missile defence, technical intelligence and battlespace awareness missions.
While GEO-1 undergoes thermal vacuum testing, the spacecraft bus has been integrated with the infrared sensor payload for GEO-2.
The US Air Force also has plans to add a fourth HEO and GEO satellite and is expected to award the contract to Lockheed later this year.