The introduction of sea-skimming missiles dramatically increased the threat against these ships. Unlike the earlier generation of anti-ship missiles (ASMs), sea-skimmers approached at low level, like an attack aircraft, hiding themselves until the last moment. The missiles were relatively small and much harder to hit than an attacking aircraft. While the older defenses might be considered a credible threat to a large aircraft at low altitude or a missile approaching at higher altitudes, against a sea-skimming missile they were useless. To successfully counter this threat, ships needed new weapons able to attack these targets as soon as they appeared, accurately enough to give them a high first-attempt kill probability - there would be little time for a second attempt.