US-K

Upravlyaemy Sputnik Kontinentalny (Russian: Управляемый Спутник Континентальный meaning Continental Controllable Satellite) or US-K (Russian: УС-К) is a series of Russian, previously Soviet, satellites used to detect missile launches as part of the Oko system.^[1] It consists of a constellation of satellites, usually in molniya orbits, designated under the Kosmos system. The satellites are built by the company NPO Lavochkin and are launched on Molniya-M rockets. Oko can be directly translated as the Russian word for eye. As of June 2014, only two of the eight satellites in orbit were still functional, rendering the system inoperable.^[2]

US-K are the first generation of Oko satellites, the first of which was launched in 1972. The vast majority of the satellites launched (86 out of 100 as of March 2012 ^[3]) have been US-K satellites in molniya orbits. Seven first generation satellites were launched into geosynchronous orbits, called US-KS, starting in 1975.^[4] A decree of 3 September 1979 led to the creation of the second generation satellites US-KMO which had their first launch in 1991.^[5] In total, 101 satellites have been launched.

After the dissolution of the Soviet Union, the rate of launches decreased, but Russia remained committed to the programme. A fully functioning constellation with 5 satellites was restored in 2001, but a fire in May 2001 in the Oko control facility at Serpukhov-15 near Kurilovo outside Moscow reduced the number of operational satellites to 2.^[6] A further satellite was launched in the same year. Between 2002 and 2006, there were no Oko launches, until another Oko satellite, designated Kosmos 2422, was launched in July 2006.^[7]

In 2006, Russia had one operational US-KMO (Kosmos 2379) satellite and four US-K satellites (Kosmos 2422, Kosmos 2393, Kosmos 2351, Kosmos 2368).^[7] In June 2011, Kosmos 2422, Kosmos 2430, Kosmos 2446 and Kosmos 2469 were reported to be the only operational satellites.

The Oko satellites are drum-shaped, 2 metres long and with a diameter of 1.7 m. They weigh 1,250 kilograms without fuel and 2,400 kilograms when fully loaded. They have a 350 kg infrared telescope pointing toward Earth, with a 4 m conical sunshield and an instrument bus. The telescope, which is the satellites' main instrument, is able to detect radiation from ascending missiles. Two solar panels provide a total of 2.8 kW of power. There are also several smaller, wider-angle telescopes to supplement the main instrument. The satellites have 16 liquid fuel engines for attitude control and 4 for maneuvering.^[7]

The system requires at least 4 satellites to be fully functional. The danger of false alarms increases when there are fewer than 4 satellites in orbit, due to the inability to double-check a detection.^[7] However, this risk is reduced by the presence of the US-KMO geosynchronous satellites.

The satellites are built by NPO Lavochkin and launched with Molniya-M rockets. The advantages of Oko include that the system is reliable and well-established, and that it uses the relatively inexpensive (but now discontinued) Molniya launcher.^[7]

The last US-K satellite (Kosmos 2469) was launched on 30 September 2010.^[8] As of December 2015, the entire Oko programme is being replaced by the new EKS system.^[9]