Oko (Russian: Око meaning Eye in old Russian),[note 1] is a Russian (previously Soviet) missile defence early warning programme consisting of satellites in Molniya and geosynchronous orbits. Oko satellites are used to identify launches of ballistic missiles by detection of their engines' exhaust plume in infrared light, and complement other early warning facilities such as Voronezh, Daryal and Dnepr radars. The information provided by these sensors can be used for the A-135 anti-ballistic missile system which defends Moscow. The satellites are run by the Russian Aerospace Defence Forces, and previously the Russian Space Forces.
Development of the Oko system began in the early 1970s under the design bureau headed by AI Savin, which became TsNII Kometa. The spacecraft element was designed by NPO Lavochkin. The first satellite was launched in 1972  but it was not until 1978 that the overall system became operational and 1982 before it was placed on combat duty. The system had a major malfunction in 1983 when it mistakenly identified sunlight on high altitude clouds as a missile attack. Stanislav Petrov, on duty at the new control centre in Serpukhov-15, Moscow Oblast, discounted the warning due to the newness of the system and the lack of corroboration from ground-based radar.
The vast majority of the satellites launched (86 out of 100 as of March 2012 ) have been the first generation US-K satellites which operate in molniya orbits. Seven first generation satellites were launched into geosynchronous orbits, called US-KS, starting in 1975. A decree of 3 September 1979 led to the creation of the second generation satellites US-KMO which had their first launch in 1991. In total, 101 satellites have been launched.
The US-K satellites, were launched by Molniya-M carrier rockets with Blok 2BL upper stages from Plesetsk Cosmodrome. The US-KS and US-KMO operate in geosynchronous orbits and were launched by Proton rockets with DM-2 upper stages from Baikonur.
The system has two dedicated control centres. The western centre is at Serpukhov-15 (Russian: Серпухов-15) near Kurilovo outside Moscow  ( ) and the eastern centre is at Pivan-1 (Russian: Пивань-1)  ( ) in the Russian Far East. The centre at Serpukhov-15 burnt down in 2001  which caused the loss of contact with currently orbiting satellites.
- also known as SPRN (Russian: СПРН from спутник для предупреждения о ракетном нападении meaning satellite for warning on rocket attack) according to one source, but SPRN is more commonly used to describe the whole early warning system, система предупреждения о ракетном нападении (system of missile warning), which includes Oko but also includes early warning radar.
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- Kosmos 520 in the Lavochkin museum - photograph from Novosti Kosmonavtiki, telescopes, infrared telescope, antenna