WASHINGTON — According to the U.S. Air Force, a SBIRS missile-warning satellite still under construction will launch ahead of one that’s already finished.

Lockheed Martin Space Systems, prime contractor for the Space Based Infrared System satellite program, is storing the recently completed SIBRS GEO-3 satellite in Sunnyvale, California, until the Air Force needs it for a 2017 launch.

GEO-4, the fourth of six geosynchronous SBIRS satellites Lockheed Martin is under contract to build for the Air Force, will be shipped once it’s finished to Cape Canaveral, Florida, for a 2016 launch.

“Originally, GEO-3 would have launched in the summer of 2016,” said Air Force spokeswoman Tina Greer. “However, due to early delivery of this satellite and the projected, on-time delivery of GEO-4,” it makes sense to launch GEO-4 next, she said.

Instead of paying to transport and store two satellites, the Air Force will launch the fresh satellite and only pay for GEO-3’s storage.

GEO-3, Greer said, will launch no earlier than September 2017.

GEO-3 and GEO-4 are designed to complement a suite of satellites and sensors that constitute the Air Force’s $19 billion SBIRS missile-warning system, which will replace older Defense Support Program warning satellites.

GEO-1 launched in 2011 and GEO-2 launched in 2013. Both are operational.

In addition to the dedicated geosynchronous satellites, the SBIRS constellation includes missile-warning sensors hosted aboard military satellites in elliptical orbits. Three of the four sensors have been launched. The final hosted sensor was shipped in May for installation on a classified satellite.