These satellites are part of the Oko programme and are in geosynchronous orbit 35,750 km above the Earth's equator.

[2] They complement ground-based early warning radars and the US-K satellites which are in molniya orbits.

The first prototype satellite was launched on 8 October 1975, atop a Proton-K/DM-2 carrier rocket from Baikonur Cosmodrome.

[3] As of December 2015, the entire Oko programme is being replaced by the new EKS system.

They feature a 1-metre diameter infrared telescope with a 4.5 metre hood which identifies missiles by their exhausts.