United Launch Alliance’s Atlas V has launched on her seventy-fifth flight, lofting the SBIRS GEO-4, a missile early warning satellite. Following a scrub on Thursday, Friday’s attempt was issue free, launching at the opening of the window at 19:48 Eastern time (00:48 UTC on Saturday).
SBIRS GEO-4 is the fourth geostationary satellite in the Space Based Infrared System (SBIRS), a constellation of satellites that use infrared sensors to detect and track missile launches.
Replacing the Defense Support Program (DSP), a fleet of satellites that began watching for missile launches in the 1960s, SBIRS is designed to provide the United States with advance warning of an enemy nuclear strike, while also allowing the country to monitor other missile and rocket launches around the world.
In addition to its geostationary satellites, SBIRS also uses sensors mounted aboard satellites in highly elliptical orbit (HEO). Piggybacked on the National Reconnaissance Office’s Trumpet-class signals intelligence satellites, these SBIRS-HEO sensors provide additional observations of Earth’s polar regions, which are less visible from geostationary orbit.
SBIRS – via Lockheed Martin
SBIRS was also to have incorporated satellites in low Earth orbit, SBIRS-LOW, however these were canceled long before deployment was to have begun. The Space Tracking and Surveillance System (STSS) was developed instead, using hardware that had originally been built for SBIRS.
As well as detecting missile launches, SBIRS is also used for intelligence-gathering, helping to identify and characterize events that result in the emission of infrared radiation and to improve general battlefield awareness.
The SBIRS GEO-4 satellite was constructed by Lockheed Martin. Based on the A2100M platform, it carries two infrared sensors: a scanning sensor which watches the full disc of the Earth for infrared events and a “staring” sensor to detect smaller short-range missiles which do not produce as much infrared radiation. The satellite has a mass of about 4,500 kilograms (9,920 lb) and is intended for a twelve-year mission.
SBIRS GEO-4 – Photo by Lockheed Martin/ULA
GEO-4 was the third geostationary SBIRS satellite to be built, Satellite Vehicle 3 (SV-3). Construction of the satellite was completed before it was required to launch, so the spacecraft was placed into storage. The US Air Force later opted to launch Satellite Vehicle 4 (SV-4) first as SBIRS GEO-3, saving the cost of putting the newly-completed SV-4 into storage and additional testing that would be needed upon taking it back out.
SBIRS GEO-4 will join the three existing geostationary satellites and four SBIRS HEO sensors in orbit. The previous geostationary satellites were deployed in May 2011, March 2013 and January 2017. Two further GEO satellites, GEO-5 and GEO-6, were ordered in 2012 for launch in the early 2020s.
