# Implementing and Testing a U-Space System: Lessons Learnt
## 3 The U-Space Services
### Conflict Management  
* **Strategic Conflict Resolution** This service is in charge of returning deconflicted 4D flight plans, where the fourth component was given by the ToA at the waypoint or the speed on it. The reason to provide two constraints to achieve the 4D compliance (speed/ToA) was that, depending on the platform, the UAV was capable to consider and try to comply with one or the other value; e.g., speed in the case of INTA's DJIs. The Strategic Conflict Resolution depends on the path planner sub-service, whose capabilities and flexibility will have a substantial impact on this and other services. In Labyrinth, the trajectory was not only deconflicted from the traffic and geo-fences at the moment of the flight, but it was also considered the elevation map of the area--a Digital Surface Model. The availability and reliability of these maps is key for optimized and safe trajectory planning. For this project, we had the support of the Spanish National Geographic Institute (IGN) [28]; different types of elevation maps of almost the whole country can be found freely available in their website. No distinctions between the airspace users were implemented during the project, in the sense that, for example, the airspace was not partitioned in areas or levels for different UAV capabilities, and it was assigned in a first-come, first-served basis. But, when existing, these policies should also be taken as input by the path planner. Finally, there was a difference between the plans calculated for the fixed-wing and those for the VTOL UAVs. The first waypoint for the fixed-wing would be at a certain altitude, and the drone would autonomously calculate the maneuver to reach it. Therefore, in this case, two segregated volumes around the take-off and landing points were considered, big enough for the aircraft to execute its climbing/descend maneuver. Figure 14: A flight plan request as an origin, an area to scan and a destination receives a trajectory with a pattern to cover the area of interest. * **Tactical Conflict Resolution**. Some events could have an impact on the ongoing flights; aircraft unable to comply with the expected 4D constraints, malfunctions or sudden geo-fences (Figure 15) are some examples. When this happens, thanks to this service, the affected traffic automatically receives instructions to avoid the conflict. This service also makes use of the path planner, which, together with the new constraints, should consider which kind of instructions is the aircraft able to execute during the flight due to its capabilities or status. The importance of this was seen while testing with a drone connected to the U-space via satellite communications (satcom) (Figure 16). Together with Telefonica I + D, and the Network Technologies research group from UC3M, part of the project was focused on identifying requirements and suggesting an infrastructure--both ground and airborne--to support a reliable connectivity of the drones. A smart link switcher was developed [29], and DLR studied the impact of satcom users in the environment and the need for any special measure for them. From the tests, it was learnt that it should be avoided to send large portions of data, e.g., an entirely new trajectory, to a satcom-linked drone. In this case, only simpler instructions like position holding, changes in altitude or speed were feasible. Sending a detailed trajectory could take so much time that the drone could not be able to receive and execute it on time to avoid the conflict. Also important is to include in the conflict resolution procedure the estimation of the time for the instruction to reach, be accepted and applied by the pilot. An example is illustrated by the instructions to overwrite the remaining trajectory: if the first new waypoints are close to the current position (consider a case like that of Figure 15), while the instruction is _wilceed_ and loaded, the drone could have overflow them already, forcing it to backtrack.