The U.S. Air Force’s fourth Space Based Infrared System (SBIRS) geosynchronous Earth orbit (GEO-4) missile-warning satellite lifted off late Jan. 19 on a United Launch Alliance (ULA) Atlas 5 rocket from Cape Canaveral Air Force Station in Florida.

Later that evening, SBIRS GEO-4 began responding to commands from operators with the 460th Space Wing at Buckley Air Force Base in Colorado, according to Lockheed Martin [LMT], which built the satellite in Sunnyvale, Calif.

The spacecraft is expected to take about 10 days to reach its final location about 22,000 miles above the Earth. It will then deploy its solar arrays and antennas to begin on-orbit testing.

The first three SBIRS GEO satellites were launched in 2011, 2013 and 2017 as part of an effort to replace aging Defense Support Program (DSP) satellites. GEO-4 will complete the initial SBIRS constellation, providing global coverage.

While DSP “continues to provide phenomenal data,” the newer SBIRS sensors improve the Air Force’s ability to detect “dimmer targets and keep pace with the missiles that our adversaries are fielding day to day,” said Col. Dennis Bythewood, director of the Remote Sensing Systems Directorate at the Air Force’s California-based Space and Missile Systems Center.

Two more SBIRS satellites, GEO-5 and GEO-6, are in production. They are scheduled for delivery in 2020 and 2021 and launch in 2021 and 2022. To ensure a robust constellation on orbit, the Air Force plans to launch them even if the first four GEO satellites are still fully operational.

The SBIRS program has also placed several sensor payloads on classified host satellites in a highly elliptical orbit (HEO).

In November, the Air Force issued a request for information (RFI) on industry capabilities that could support a SBIRS follow-on. The RFI cited “an unusual and compelling urgency” for new satellites that can “counter emerging threats while operating in a contested environment.” The Air Force envisions that five new satellites would be launched from fiscal year 2025 to FY 2029.

The GEO-4 launch was the Atlas 5’s 75th mission. The 76th, slated for March 1 at Cape Canaveral Air Force Station, will carry the Lockheed Martin-built Geostationary Operational Environmental Satellite-S (GOES-S) satellite for NASA and the National Oceanic and Atmospheric Administration (NOAA).

Separately, ULA announced Jan. 22 that it has assumed responsibility from Lockheed Martin for marketing and selling Atlas 5 commercial launches. ULA, a joint venture of Boeing [BA] and Lockheed Martin, has been conducting all Atlas 5 launches since it was formed in 2006.