 However, problems with the engines of the P-120 Malakhit forced the Soviets to design a sub-launched missile based on the P-15M Termit (SS-N-2C 'Styx') as a stopgap measure for the first batch of Charlie submarines.This became the P-20L, later renamed the P-70 Ametist.The P-15M was fitted with an L band active radar homing sensor and a new radar altimeter both developed for the 'Siren', but there was no room for a datalink in the smaller 'Styx'.