Perimetr PTS dates back to 1974, in response to Soviet fears of devastating nuclear strikes from US Ballistic Missile submarines. It became operational in January 1985, with SS-17 launch silos in Vypolzovo (Yedrovo) and Kostroma, each 100 and 150 miles north-west of Moscow respectively. The system was built with multiple layers of redundancy, in case multiple layers of communications were destroyed in the initial strike. Launch authorization would be transmitted by UHF radio, by transmitters buried deep underground. Development continued over the years with the system incorporating the new RT-2PM2 Topol-M ICBMs in December 1990, and further refinements in 1996.