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Both ground-based ATC (Air Traffic Control) and airborne CAS (Collision Avoidance Systems) are needed now and in the future to cope with the problem of collision aloft. They should be considered mutually complementary. The limited form of the CAS known as a PWI (Proximity Warning Indicator) is believed not to be sufficient for safety. The two essential elements of a CAS are 1) an indication of the effective maneuver to secure adequate miss distance, and 2) effective means for discovering and assessing the original risk. Miss distance produced by a maneuver never exceeds maneuver displacement. The ratio of the two may be low unless escape time is of the order of 30 seconds or more. In nearly all cases increasing escape time is more effective to insure adequate miss distance than is increasing maneuver severity. The preferred form of CAS would be self-sufficient, but this form appears not to be feasible for basic physical reasons. Cooperative CAS presents many problems, but is believed to have sufficient promise of success to justify careful study and substantial development effort.