Well known problems with propulsion of United States Navy communication satellite MUOS-5 are still not explained even after performing visual inspection by one of the GSSAP satellites.
Since first problems with MUOS-5 were reported to public in July 2016, U.S. Navy has been trying to solve mysterious issue with propulsion. To remind: MUOS-5, representing next generation of military communication satellites, was launched on 24th June, 2016 and reached GTO orbit with perigee at 3829 km and apogee 35758 km. Unfortunately it failed (probably after conducting 3 or 4 from planned seven burns) to reach correct orbit for further tests performed from ground station placed at Hawaii. It has remained on orbit with apogee of 5398 km over Pacific with its IHI BT-4 thruster cut off. U.S. Navy announced in the beginning of August, that MUOS-5 will be using its additional 18 thrusters fueled with N2H4 to reach correct orbital position. In the beginning of September MUOS-5 started to use thrusters and slightly changed its orbit for 300 km in the perigee (from 15750 km to 16102 km). Still pace of the orbit change is very slow and probably satellite will fail to keep the schedule with reaching GEO orbit on December 2016. It is worth to mention, that on August 18, 2016, USAF declared that one of the high orbit space surveillance satellites will be commanded to perform visual check of the MUOS-5. This task was performed on August 19 by one from two operational Geosynchronous Space Situational Awareness Program satellites operated by Joint Functional Component Command for Space at Vandenberg Air Force Base in California (first pair of GSSAP satellites were launched in 2014, second pair was launched on August 20, 2016). 1st Space Operations Squadron was ordered to perform rendezvous of GSSAP satellite and took high resolution pictures which could be useful in evaluating reasons of the problem. Unfortunately in spite of general announcement about performed maneuver no details were unveiled. At the moment U.S. Navy or Lockheed Martin, manufacturer of the satellite, are still not able to evaluate condition of their spacecraft.
On picture above: launch of the MUOS-4