 By the 1960s, however, advances in ground-based radar, guided missiles, radar-guided anti-aircraft guns, and fighter planes greatly decreased the odds that bombers could reach their targets, whether they used the traditional high-altitude or newer low-altitude approach.One 1964 study of British V bombers estimated that a bomber that did not use chaff or other countermeasures would encounter an average of six missiles, each with a 75 percent chance of destroying its target.The study thus stated that "the bomber will not always get through", and advocated Britain emphasize the Polaris submarine missile instead.