(Bloomberg) — A $6.1 billion Raytheon Technologies Corp. system of 17 worldwide ground stations to control the Pentagon’s GPS satellite constellation has newly discovered software flaws, delaying its delivery and initial operations yet again.

There’s no new timetable for delivery of the system, which most recently had been projected for December after it had passed acceptance tests in November, Air Force officials told Bloomberg News. The latest plan had been to declare it ready for operations by April. It was originally supposed to be up and running in October 2016.

Once called the Defense Department’s “No. 1 troubled program” by a Pentagon official, the ground stations are intended to provide improvements, including access to more secure, jam-resistant software, for the military’s use of the GPS navigation system, which is also used by civilians worldwide.