An aircraft, comprising: a flight management system; and a trajectory-based sense-and-avoid system configured to detect potential conflicts or collisions based on 4-D trajectories of the aircraft and 4-D constructs, said 4-D constructs representing trajectories of other aircraft or regions from which the aircraft is to maintain separation; characterized in that: wherein potential conflicts or collisions estimated to occur within a first, strategic, time frame are identified using a 4-D trajectory of the aircraft generated by the flight management system and 4-D constructs generated from data obtained from air traffic management data and/or a ground control station; wherein potential conflicts or collisions estimated to occur within a second, tactical, time frame which is less than the first, strategic, time frame are identified using the 4-D trajectory of the aircraft generated by the flight management system of the aircraft and 4-D constructs, some of which are generated at least in part by a combination of data provided by other aircraft or by an on-board sensor suite with air traffic management trajectory data; and wherein potential conflicts or collisions estimated to occur within a third, critical, time frame which is less than the second, tactical, time frame and within which a conflict is deemed to be an emergency are identified based at least in part on data generated by the on-board sensor suite. a flight management system; and a trajectory-based sense-and-avoid system configured to detect potential conflicts or collisions based on 4-D trajectories of the aircraft and 4-D constructs, said 4-D constructs representing trajectories of other aircraft or regions from which the aircraft is to maintain separation; characterized in that: wherein potential conflicts or collisions estimated to occur within a first, strategic, time frame are identified using a 4-D trajectory of the aircraft generated by the flight management system and 4-D constructs generated from data obtained from air traffic management data and/or a ground control station; wherein potential conflicts or collisions estimated to occur within a second, tactical, time frame which is less than the first, strategic, time frame are identified using the 4-D trajectory of the aircraft generated by the flight management system of the aircraft and 4-D constructs, some of which are generated at least in part by a combination of data provided by other aircraft or by an on-board sensor suite with air traffic management trajectory data; and wherein potential conflicts or collisions estimated to occur within a third, critical, time frame which is less than the second, tactical, time frame and within which a conflict is deemed to be an emergency are identified based at least in part on data generated by the on-board sensor suite. wherein potential conflicts or collisions estimated to occur within a first, strategic, time frame are identified using a 4-D trajectory of the aircraft generated by the flight management system and 4-D constructs generated from data obtained from air traffic management data and/or a ground control station; wherein potential conflicts or collisions estimated to occur within a second, tactical, time frame which is less than the first, strategic, time frame are identified using the 4-D trajectory of the aircraft generated by the flight management system of the aircraft and 4-D constructs, some of which are generated at least in part by a combination of data provided by other aircraft or by an on-board sensor suite with air traffic management trajectory data; and wherein potential conflicts or collisions estimated to occur within a third, critical, time frame which is less than the second, tactical, time frame and within which a conflict is deemed to be an emergency are identified based at least in part on data generated by the on-board sensor suite.