The Entry of the AGM-78 Standard ARM
While the Shrike was an advanced weapon for its day, its limited speed and turning capability had a negative impact on the Wild Weasel’s ability to combat the North Vietnamese surface-to-air missile (SAM) threat. The crew could try to prevent the SAM from locking on to its target by destroying the radar site from which it was launched, but this would almost surely be a losing battle.
North Vietnam had received deliveries of Soviet SA-2 Guideline surface-to-air missile, and the U.S. was using AGM-45 Shrikes to try to destroy the SAM's guiding radar installations. The SA-2s were faster than the Shrikes and outranged them by a good five miles. North Vietnamese radar operators were keenly aware of this advantage and learned to capitalize on it by shutting down just before the Weasels entered firing range. Once the radars were down, Weasel crews had to resort to the difficult and dangerous job of searching for potential surface-to-air missile threats visually.
Wild Weasel airmen were on the lookout for suspicious indications of the presence of a SAM site. Examples were roads going in to some place that didn't make sense for a road to enter, or for signs of trucks or construction activity which seemed out of place in a village. The greatest challenges were presented by sites buried beneath trees that grew some 300 feet tall. Sometimes crews couldn’t really find what they were looking for unless they descended low enough to get hit by anti-aircraft artillery.
Enter the AGM-78 Standard Anti-Radiation Missile
The Weasel’s ability to thoroughly destroy SAM installations took a gigantic leap forward in May of 1968, with the introduction of the AGM-78 Standard anti-radiation missile. In order to save on development costs, "off-the-shelf" components were used. General Dyamics took their RIM-66 "Standard" surface-to-air missile and created an air-launched variant, attaching the Shrike's anti-radar seeker head to its front. Thus was created the AGM-78 "Standard" anti-radiation missile, or STARM.
The AGM-78 Standard was enormous for an air-launched missile, measuring 15 feet in length and weighing nearly 1,400 pounds. The expanded volume enabled the delivery of a 210 pound blast-fragmentation warhead; a vast improvement over the Shrike's 149 pound warhead. The STARM provided Weasel crews with previously unheard of long-range capabilities. Working in conjunction with an F-105 radar receiver, the AGM-78 Standard could lock on to and strike enemy radars from a distance of more than 50 miles — assuming that the enemy radar cooperated by continuing to transmit. In any case, Wild Weasel crews could finally launch their weapons from beyond the maximum range of North Vietnamese SA-2 batteries.
Another advantage of the STARM was that its anti-radar seeker was mounted on a gimbal, permitting a wider range of maneuvers for the Wild Weasel aircraft. The enemy quickly learned that shutting off the Fan Song fire control and tracking radars guiding the SA-2s left U.S. anti-radiation missiles without a signal to home in on. The Wild Weasel missile would then wander off "aimlessly" and finally land somewhere with a loud, feckless kaboom! For that reason, upgraded versions of the AGM-78 Standard incorporated a simple memory circuit, allowing the STARM to guide itself to the last-known enemy radar location.
In The Hunter Killers: The Extraordinary Story of the First Wild Weasels, Dan Hampton explains the limitations of this circa-1968 inertial navigation technology. The "drift error" of those upgraded AGM-78 Standards was slightly over 50 feet per minute, so if launched from 20 miles away, the STARM could be expected to impact well over 100 feet away from its target. A reliable improvement on radar-seeking technology had yet to be discovered.