In late 1971, the North Vietnamese took several actions which vastly improved and augmented their tracking capability. The most important was netting of their early warning and surveillance radar and their anti-aircraft artillery radar with SAM missiles. In that netted mode, the Fan Song (radars) which alerted U.S. pilots to the surveillance never came up, as the surveillance could all be conducted with the other radars. General Lavelle believed that, with those mutually supporting radar systems transmitting tracking data to the firing sites, the SAM missile system was activated at U.S. aircraft at any time they were over North Vietnam.