The Air Force is pushing back the launches of two Lockheed Martin [LMT]-built satellites that were supposed to lift off in fall 2017.

Each spacecraft – the fourth Advanced Extremely High Frequency (AEHF-4) protected-communications satellite and the fourth Space Based Infrared System (SBIRS) geosynchronous Earth orbit (GEO Flight-4) missile-warning satellite – is slated to ride on a United Launch Alliance (ULA) Atlas 5 rocket from Cape Canaveral Air Force Station in Florida. ULA is a joint venture of Boeing [BA] and Lockheed Martin.

The AEHF-4 launch has been delayed from October until sometime in 2018 due to a technical glitch with the satellite, the Air Force wrote in a recent response to questions.

“An issue was discovered with a power regulator unit, and the issue requires a modification to ensure the system meets requirements,” the Air Force said. “The schedule impact is still under evaluation, but the program office is projecting a launch in calendar year 2018.”

AEHF satellites are replacing the aging Milstar constellation. AEHF-1 was launched in 2010. It was followed by AEHF-2 in 2012 and AEHF-3 in 2013. AEHF-5 and AEHF-6 are in production, and the Air Force projects they will be available for launch in fiscal year 2019.

Meanwhile, the SBIRS GEO Flight-4 launch has been moved from November to January “due to an updated prioritization of resources across multiple SBIRS mission-enhancement efforts,” the Air Force said. “There are no issues with the space vehicle.”

SBIRS GEO Flight-3 was launched in January. It was preceded by GEO-1 in 2011 and GEO-2 in 2013. GEO-5 and GEO-6 are in production. The Air Force plans to buy long-lead parts for two more SBIRS satellites in FY 2018 and begin full production in FY 2019.

The SBIRS program, the successor to the aging Defense Support Program satellites, has also placed three sensor payloads on classified host satellites in a highly elliptical orbit (HEO). A fourth payload was delivered to its classified host in 2015.