# Implementing and Testing a U-Space System: Lessons Learnt
## 3 The U-Space Services
### Mission Management  
* **Operation Plan Preparation/Optimization**. Using this service, operators can submit flight plan approval requests or cancel already approved ones. To help operators to visualize the approved plans, the Web app makes it possible to display one or more plan trajectories in a 2D/3D map view that can be rotated, tilted or zoomed over a clean topographic map background or an orthophoto (the CesiumJS [27] open source library was used for geo-spatial visualization). The trajectories are rendered as a list of waypoints. If one is clicked, the 4D constraints on it are displayed. These features are extremely useful for research purposes, for example, to analyze the separation decisions of the path planner (Figure 12). The functionality determined by the ConOps was that, when the Operation Plan Processing service receives a flight plan approval request, a series of checks and services sequentially look over the constraints and factors that can determine the feasibility of the flight. The reply would be the acceptance of the flight plan or an informed rejection, to provide to the operator the clues to submit a feasible alternative. However, a validation mode is suggested, where the checks are applied, but the plan is not scheduled if found feasible; this mode can help the operator to optimize the flight. In Labyrinth, this optimization was included in the approval request step. Operators sent their flight requests as a draft of the desired trajectory and a set of preferences, and, if the path planner found no conflicts, returned a deconflicted, feasible--in terms  
Figure 11: In cases of severe non-compliance, pilots received a visual and aural warning. Figure 10: A drone inside a geo-cage. of the drone capabilities--and optimized flight plan based as close as possible on the draft. The flexibility of the trajectory design and allowed preferences were determined by the path planning sub-service (Figure 13). Such flexibility will be key to satisfying the needs of the final users. Labyrinth's path planner calculated trajectories for drafts defined as: origin and destination; origin, an area to scan and destination (the planner returned a pattern to scan the area (Figure 14)); or as origin, a list of waypoints and a destination. However, it was learnt, after the exercises with the first responders, that an option defining origin, a geo-cage and destination should be added. Geo-cages were requested when trajectories in the area could not be decided beforehand since they depended on the needs of each situation. An example would be a drone arriving at an area of interest to follow the unpredictable movements of a mass of people. Figure 12: In the Web interface, selecting a waypoint of the trajectory displays its 4D constraints. Figure 13: UC3M’s path planner made it possible to specify the required altitude in origin and destination waypoints, and preferred cruise altitude and speed. * **Operation Plan Processing**. It handles the list of pending flights and processes new ones. When this service receives a new request for flight permission, after successful syntactic and semantic checks, it adds the geo-fences and traffic affecting the requested trajectory at the moment of the flight, the drone capabilities--max. and min. speed, climb/descend rate, type (VTOL/fixed-wing)-and forwards the request to the Strategic Conflict Resolution Service, which subsequently calls the path planner sub-service. The planner returns, if found, a feasible and optimized trajectory with the hard constraints applied and the soft ones when possible. Paths are really treated like a tube by the planner, providing a radius of separation along it to consider possible relatively light and unavoidable deviations due to navigation inaccuracies or wind. The resulting flight plan is sent to the operator, who must verify whether it fits their needs and accept or reject it. To ease this decision step, it is important to provide a clear and informative display of the plan returned.