Intruder F/O 1, 2 (NOSS-4 1, 2) ?

NOSS-3 (Naval Ocean Surveillance System), which is likely not the official designation, is the third generation of NOSS satellites, which are low orbit SIGINT satellites. In 2013, the designation Intruder leaked out. These satellites carry equipment to track ships and aircraft via their radio transmissions. Positions of the origin of the transmissions are determined by triangulation. Each launch consists of two satellites of similar size in the same orbit. Both satellites maneuver to stationkeep relative to one another.

In contrast to the earlier first generation (Parcae) and second generation (improved Parcae) constellations, it features only two satellites. Notably absent is also the dispenser unit, which deployed the earlier generations. Like the earlier generations, it features the same orbit. Each pair has reportedly a combined weight of 6500 kg. These satellites combine the capabilities of the improved Parcae and Farrah satellites.

The fourth launch was left in a slightly too low orbit, as the Centaur upper stage shust down too early during the second burn. The payload was able to reach its intended orbit on its own propulsion system. Due to this problem the launch vehicle configuration was changed for the next launch from Atlas-5(401) to Atlas-5(411)¹ to allow a single burn mission for greater reliability. After confidence was restored, the launches reverted back to the Atlas-5(401) version.

The ninth launch (Intruder 17/18, NOSS-3 9, NROL 85) is listed tentatively here, as the observed orbit is fitting for an Intruder payload. But in this case no second payload was observed, which might either hint towards an other different type of payload or a seperation failure.

Note: The NROL designations refer to the launch, not to the payload.