# Unmanned Aerial Traffic Management System Architecture for U-Space In-Flight Services
## 2 Background
### Related Work  
The development of completely operational UTM systems is still at an early stage, even though it has recently become a growing field. The authors in [16] define what a UTM system is, and they give an overview of both a physical UTM architecture and a UTM software manager based on automated services. Big companies are one of the major parties interested in boosting the deployment of UTM. For instance, Google has proposed an ecosystem [17] where all UAS should be equipped with communication and _sense & avoid_ technologies in order to perform cooperative flights when encountering other UAS or manned aircraft. In their proposal, the separation and planning services would be provided by an _Airspace Service Provider_. Furthermore, Amazon has put forward a one-operator-to-many-vehicle model [18], where the decision-making authority gets significantly distributed among the operators. Additionally, there exist several commercial UTM system applications in the market. They implement most pre-flight services, but just partially a few in-flight services. For instance, Airmap [19] has its focus on UAS registration, geographic information systems, flight communication, traffic monitoring, and user interfaces. The Unify platform [20] connects authorities with pilots to safely integrate UAS into the airspace. On the one hand, the authorities can visualize and approve the UAS flights, as well as manage _No Flight Zones_ in real time. On the other hand, the pilots can manage their UAS (e.g., with the E-registration, E-identification, and Flight plan management services) and they can plan and receive flight approvals aligned with international and local regulations. Another framework is the Thales ECosystem UTM [21], which integrates UAS and pilot registration. ECosystem provides a flight planning functionality, using airspace rules and situational awareness as guidelines. It also includes tools to manage map overlays and 3D terrain views. The aforementioned UTM applications offer pre-flight UTM services and some in-flight capabilities such as UAS tracking. Even though they are capable of publishing real-time information about the UAS, they do not manage operations autonomously during the flight phase. Moreover, it is important to highlight that all those applications are commercial products that are not available for the community as open software. The scientific community has also been putting effort into functional UTM frameworks; a recent review of related works can be seen in [22]. A prototype UTM for flight surveillance has recently been proposed in Taiwan [23]. One of its core properties is the capability to monitor vehicles, being the ADS-B (_Automatic Dependent Surveillance Broadcast_) technology used for surveillance. There is a pre-flight procedure to schedule and approve flights, and then the UTM system can send surveillance alerts during the operation, though all the decisions for conflict resolution are up to the pilot. Another UTM system has been presented in Sweden [24]. It incorporates a complete toolkit to manage traffic, geofences, flight altitude segregation as in the general aviation, and complex visualization. This research has also identified problems that dense traffic in the low-level airspace will bring to the city users, by simulating the future urban airspace. In general, the functionalities of the aforementioned systems have only been demonstrated through simplistic simulations, and quite a few works have been devoted to field flight campaigns for preliminary tests [25,26]. We have also proposed in a previous work [27] a more realistic simulator for UAS operations based on the ROS middleware and the 3D simulation suite Gazebo ([[http://gazebosim.org](http://gazebosim.org)]([http://gazebosim.org](http://gazebosim.org)), accessed on 26 April 2021). In that work, we introduced a preliminary definition of our in-flight services and a tool for mission validation. In the current paper, we go beyond by implementing a complete UTM architecture.