Initially, the interceptor version was equipped with the TL-25 "Smerch-A" (also referred to as "Product 720") radar, a development of the system carried by the earlier Tu-128. While powerful and thus long-ranged and resistant to jamming, the system—due to the age of its design and its intended purpose (tracking and targeting high- and fast-flying US bombers and reconnaissance aircraft)—lacked look-down/shoot-down capability, which limited its effectiveness against low-flying targets. (This is one of the reasons why it was replaced with the Mikoyan-Gurevich MiG-31, whose Zaslon radar has that capability.) By the time the MiG-25 entered service in 1969, this was a serious shortcoming, as strategic bombing doctrine was shifting towards low-level penetration of enemy territory. After Belenko's defection to Japan exposed this flaw to the West, a government decree issued on 4 November 1976 called for urgent development of a more advanced radar. This resulted in the pulse-Doppler radar "Sapphire-25" system fitted to the MiG-25PD variant.