The Sukhoi Su-15 (NATO reporting name 'Flagon') was a twin-engined supersonic interceptor developed by the Soviet Union in the 1960s to replace the Sukhoi Su-11, which was becoming obsolete as the United States and Britain introduced newer and more capable strategic bombers. The Su-15 was a major part of the Soviet interceptor force until the fall of the Soviet Union. The aircraft is most famous for being involved in the downing of Korean Air Flight 007.
The Su-15 formed a significant part of the interceptor force of the Soviet Air Defense Forces (V-PVO), and was designed to intercept easier targets such as the American B-52 and U-2, and the British V bombers, leaving the more difficult targets such as the XB-70 and B-58 to the faster MiG-25P.
The Su-15 was optimized for the high-altitude interception role with its fast climb-rate and high speed at high altitude but lacked look-down/shoot-down capability. This eventually led to the MiG-23P, which did have look-down/shoot-down capability, becoming the preferred asset of the V-PVO, especially once NATO switched to low-level penetration tactics. Even so, the Su-15 remained an important part of the V-PVO until the fall of the Soviet Union.
The Taifun radar of the Su-15TM was optimized for counter-countermeasure operation, as opposed to range. As an interceptor, the task of the Su-15TM was to fly under autopilot, using GCI commands sent through the datalink. The radar would only be turned on as the interceptor approached the target in order to provide targeting parameters for the radar homing K-8/R-8/R-98 missiles, the high power of the radar allowing it to 'burn through' enemy ECM signals. If all else failed, IR homing versions of the K-8 would provide a last opportunity to shoot down the intruder, along with any gun pods the Su-15 might be carrying.
Some Western reports indicated that the Su-15TM was also designated Su-21 and the Su-15UM Su-21U. These reports were apparently incorrect. Designation Su-21 was reserved for the Su-17M4, but was never used.
Source: Wikipedia, "Sukhoi Su-15", available under the CC-BY-SA License.