The lone Su-15 proved incapable of intercepting the F-100s, because its radar lacked `look-down/shoot-down’ capability.
Closed in 1949, the Sukhoi Design Bureau was reborn in 1953 to meet an urgent demand for a fast interceptor that would counter the threat posed by NATO bombers. It wasted no time developing a succession of missile-armed, Mach 2 interceptors characterized by delta wings; the single-engined Su-9 (NATO reporting name: Fishpot-B) entered service in 1960, followed by the up-armed Su-11 (NATO reporting name: Fishpot-C) in 1964 and the twin-engined Su-15 in 1967. Though built in modest numbers, the three types became an important asset for the Soviet air defence force—particularly the more capable Su-15 (NATO reporting name: Flagon), which outlasted the Soviet Union, the last being retired in 1996.
As told by Yefim Gordon & Dmitriy Komissarov in their book Sukhoi Interceptors: The Su-9, Su-11, and Su-15: Unsung Soviet Cold War Heroes, throughout the 1970s the southern borders of the Soviet Union perpetually received the attentions of hostile aircraft coming from Turkey.
On Sep. 7, 1972, a flight of Turkish Air Force (THK: Türk Hava Kuvvetleri) North American F-100 Super Sabres entered Soviet airspace near Leninakan, Armenia (the city is called Gyumri). Despite flying at ultralow altitude, the intruders were detected by air defence radars in timely fashion. Another ploy of the ‘bad guys’ worked, however—the fighters flew in close formation, appearing on the radarscopes as one heavy aircraft (the USAF had used this tactic against North Vietnamese interceptors during the Vietnam War); hence only a single 166th Fighter Aviation Regiment (IAP) Su-15 scrambled from Sandar AB in neighbouring Georgia to intercept `it’. The GCI command post operators did not realise they were dealing with multiple targets until the Turkish fighters swept over the place with a roar.

The lone Su-15 proved incapable of intercepting its quarry, because its radar lacked `look-down/shoot-down’ capability. As a result, the F-100s flew over Leninakan and were fired on by a heavy machine gun, providing antiaircraft protection for the PVO’s radar site, but got away unscathed.
On May 23, 1974, another THK F-100 intruded into Soviet air-space over the Caucasus region with impunity. A Su-15 standing on QRA duty scrambled from the airbase in Kyurdamir, Azerbaijan, but was not directed toward the target because the latter had unwisely intruded into an area defended by an SAM regiment. A missile was fired at the F-100 but missed due to a malfunction in ‘m the guidance system.
Eventually, however, the Turks fell victim to the rule ‘pride goeth before the fall’. On Aug. 24, 1976, Soviet AD radars detected a target moving in Turkish airspace toward the Soviet border. This was soon identified as a pair of F-100s flying in close formation. No fewer than three Su-15s scrambled this time (two from Kyurdamir and one from Sandar AB), but again they did not manage to get a piece of the action. The fighters had again rashly flown right into a nest of SAMs; this time the PVO crews on the ground did their job well, and one of the Super Sabres was shot down. Unfortunately the wreckage fell on the wrong side of the border, and the pilot, who ejected, also landed in Turkish territory; the following day the Turks raised hell, accusing the Soviet Union of the ‘wanton destruction of a Turkish fighter’.
Sukhoi Interceptors: The Su-9, Su-11, and Su-15: Unsung Soviet Cold War Heroes is published by Schiffer Publishing and is available to order here.

Photo credit: U.S. Air Force and Filip.vidinovski via Wikipedia