The role of manned point defense designs was reassigned to unmanned interceptors—surface-to-air missiles (SAMs)—which first reached an adequate level in 1954–1957. SAM advancements ended the concept of massed high-altitude bomber operations, in favor of penetrators (and later cruise missiles) flying a combination of techniques colloquially known as "flying below the radar". By flying terrain masking low-altitude nap-of-the-earth flight profiles the effective range, and therefore reaction time, of ground-based radar was limited to at best the radar horizon. In the case of ground radar systems this can be countered by placing radar systems on mountain tops to extend the radar horizon, or through placing high performance radars in interceptors or in AWACs aircraft used to direct point defense interceptors.