These rules of engagement were based on the situation in 1968, when North Vietnam's surface-to-air missiles were controlled by fire control radars with a high-pulse recurring frequency, which keyed an alarm in the USAF aircraft. By late 1971, shortly after Lavelle had taken command of 7th Air Force, the North Vietnamese had begun networking their long-range search radars with the missile sites. These additional sources of radar data allowed North Vietnam to turn on SAM radar at the last second, giving US aircrews virtually no warning.