Time-critical coordination tools for unmanned systems can be employed to enforce the type of temporal constraints required in terminal control areas, ensure minimum distance requirements among vehicles are satisfied, and successfully perform coordinated missions. In comparison with previous literature, this paper presents an ampler spectrum of coordination and temporal specifications for unmanned systems, and proposes a general control law that can enforce this range of constraints. The constraint classification presented considers the nature of the desired arrival window and the permissible coordination errors to define six different types of time-coordination strategies. The resulting decentralized coordination control law allows the vehicles to negotiate their speeds along their paths in response to information exchanged over the communication network. This control law organizes the different members in the fleet hierarchically per their behavior and informational needs as reference agent, leaders, and followers. Examples and simulation results for all the coordination strategies presented demonstrate the applicability and efficacy of the coordination control law for multiple unmanned systems.Recent developments in unmanned systems (UxSs) technologies have opened vast opportunities in scientific research, 1-3 remote monitoring and control, 4 rapid assessment of catastrophic events, 5 search and rescue operations, 6, 7 cartography, 8, 9 and cinematography, to name a few examples. Not only have UxSs eliminated the constraints and limitations of manned vehicles, but also their relatively low cost, miniaturization levels, and ease of deployment have popularized their use in the aforementioned areas. Moreover, fleets of cooperating UxSs can be used to exploit synergistic behaviors among vehicles, remove the risk of single-point failures, improve flexibility, increase redundancy and reliability, achieve blanket coverage of larger areas, and often reduce mission execution times.A fundamental component to achieve these goals is the availability of cooperative control strategies that are robust to temporary communication dropouts, packet loss, and external disturbances. Additionally, these cooperative strategies shall ensure safe inter-vehicle separation, while being able to adapt to changing mission goals, fleet size, and vehicle diversity. Technical advances in time coordination have the potential to realize new transportation paradigms that hold promise for revolutionary changes in other fields such as air traffic management, 10 mail services, and ground transportation systems.Inspired by these challenges, previous work 11, 12 developed a set of algorithms for time-critical cooperative path-following control of a fleet of Unmanned Aerial Vehicles (UAVs) that exchange information over a communication network. The resulting framework divides the fleet into leaders and followers and guarantees that the UAVs follow their assigned trajectories, while meeting temporal and spatial constraints. Moreover,