Faced with this problem, military doctrine had already started shifting away from high-altitude supersonic bombing toward low-altitude "penetration". Radar is line-of-sight, so aircraft could dramatically shorten detection distances by flying close to the Earth and hiding behind terrain. Missile sites spaced to overlap in range when attacking bombers at high altitudes would leave large gaps between their coverage for bombers flying at lower levels. With an appropriate map of the missile sites, the bombers could fly between and around the defences. Additionally, early missiles generally flew unguided for a period of time before the radar systems were able to track the missile and start sending it guidance signals. With the SA-2 missile, this minimum altitude was roughly . Flying below this would make the bomber effectively invulnerable to the missiles, even if they happened to fly into range.