By the late 1950s, however, anti-aircraft surface-to-air missiles (SAMs) could threaten high-altitude aircraft, as demonstrated by the 1960 downing of Gary Powers's U-2. The USAF Strategic Air Command (SAC) were aware of these developments and had begun moving its bombers to low-level penetration even before the U-2 downing. This tactic greatly reduces radar detection distances through the use of terrain masking; using features of the terrain like hills and valleys, the line-of-sight from the radar to the bomber can be broken, rendering the radar (and human observers) incapable of seeing the target. Even at somewhat higher altitudes, radar systems of the era were subject to "clutter" from stray returns from the ground and other objects, requiring a minimum angle above the ground to be effective. Bombers flying at low altitudes could remain under these angles simply by keeping their distance from the radar sites. This combination of effects made SAMs of the era ineffective against low-flying aircraft. The same effects also meant that low flying aircraft were difficult to detect by higher flying interceptors, since their radar systems could not readily pick out opposing aircraft against the clutter from ground reflections (lack of look-down/shoot-down capability).