# Implementing and Testing a U-Space System: Lessons Learnt
## 1 Introduction
### The Scope of Labyrinth  
The present work was part of the project Labyrinth [12; 13], a three-year Horizon 2020 project ending in May 2023. Its main goal was to develop and test the algorithms behind two of the key U-space services: Strategic Conflict Resolution and Tactical Conflict Resolution. The Robotics Lab research group at the University Carlos III of Madrid (UC3M) developed the algorithms to find a feasible path, which was returned as a four-dimensional (4D) trajectory. The details of these algorithms have already been described in other articles [14; 15; 16; 17; 18; 19; 20; 21], where their potential can be better discovered, since only part of their possibilities was used in the real flights during the project. The task of the algorithms was to provide the drone operators with a deconflicted trajectory based on their flight intentions, capabilities and some existing constraints:  
* 1] waypoints, is considered available for other traffic. * Geo-fences. * Elevation map including buildings. * Maximum and minimum speed of the aircraft. * VTOL/fixed-wing capabilities regarding turns and take-off/landing maneuvers. These constrains that UC3M's algorithms took as input were just part of all the constraints that a system like this should consider, especially if used for relevant traffic densities flying over population. But to develop a strategic or tactical deconfliction service able to integrate more factors we first need services providing such sources of information. Most of those services do not exist yet and would deserve their own project to be developed due to their complexity. As an example, the case of the elevation data used during the project. There were used the Digital Surface Models (DSMs) based on LiDAR (LaserImaging Detection and Ranging) point clouds available in the web of the Spanish National Geographic Institute. The algorithms used in the path planner had a high computational demand, being limited in the size of the area or work, which had to be previously cut from the DSMs and converted, which is not a quick step. As a result, the tests during the project were limited to some specific areas in Spain and Italy (Figure 1). This is not an option for a service that had to be used in a wide area containing at least a city and its surroundings, or connecting two cities. Moreover, while the resolution of the LiDAR samples was acceptable (5 m), the datasets available were slightly aged ([2009,2012] depending on the sheet), which means that some buildings or vegetation was not reflected in the data. In one of the flight tests later described, it can be seen that the trajectory finally flown had more altitude than that approved by the UTM; the reason was a group of tall trees, which probably were not there or not that high when the dataset was captured; then, the altitude of the approved plan was increased to avoid risks. For a safe and efficient deconfliction service, we would need a service able to rapidly provide specific pieces of recent elevation maps. This is just an example of the many potential business models that U-space allows. UC3M's algorithms, in the same way as in the approach of [22], follow a First-Come, First-Served assignment of the airspace. But it would be more convenient to have flights with priorities to support missions like police tasks or medical services. In this case, the submission of a flight plan could mean modifying other scheduled or ongoing flights. But priorities could be associated to criteria like penalties or reputation points in case the authorities decide to motivate the good practices of the airspace users; priorities could be also just a matter of price. In any of these cases, one service should exist to provide the conflict resolution services with the priority of the flight in the deconfliction decision. The complexity to develop any of the mentioned U-space services, policies or procedures together with their coordination explains why the services implemented in Labyrinth only had the intention to provide a basic functionality.