guns. On July 24, 1965, four U.S. Air Force McDonnell F-4C Phantoms took part in an airstrike against the Dien Ben Phu munitions storage depot and the Lang Chi munitions factory west of Hanoi. One was shot down and three damaged by SA-75s. This was the first time U.S. aircraft were attacked by SAMs. Two days later President Johnson gave the order to attack known SA-75 positions outside the 30-mile exclusion zone. On the morning of July 27, 48 F-105s participated in the strike, Operation Spring High. The Vietnamese knew U.S. aircraft were coming, and set up many 23mm and 37mm anti-aircraft guns at the two SAM sites. These anti-aircraft guns were lethal at close range. The Vietnamese shot down six aircraft and more than half of the remaining U.S. aircraft suffered damage from ground fire. However, the Vietnamese had replaced the SAMs with white-painted bundles of bamboo. Operation Spring High had destroyed two decoy targets for the loss of six aircraft and five pilots. Between 1965 and 1966 the US developed countermeasures to the S-75 threat. The Navy soon had the AGM-45 Shrike anti-radiation air-to-surface missile in service and mounted their first offensive strike on a site in October 1965. The Air Force fitted B-66 bombers with powerful jammers (which blinded the early warning radars) and developed smaller jamming pods for fighters (which denied range information to enemy radars). Later developments included the Wild Weasel aircraft, which were fitted with AGM-45 Shrike missile systems made to home in on the radar from the threat. The Soviets and Vietnamese were able to adapt to some of these tactics. The USSR upgraded the radar several times to improve ECM resistance. They also introduced a passive guidance mode, whereby the tracking radar could lock onto the jamming signal itself and guide missiles directly towards the jamming source. This also meant the SAM site's tracking radar could be turned off, which prevented Shrikes from homing in on it. New tactics were developed to combat the Shrike. One of them was to point the radar to the side and then turn it off briefly. Since the Shrike was a relatively primitive anti-radiation missile, it would follow the beam away from the radar and then simply crash when it lost the signal (after the radar was turned off). SAM crews could briefly illuminate a hostile aircraft to see if the target was equipped with a