# Unmanned Aerial Traffic Management System Architecture for U-Space In-Flight Services
## 1 Introduction  
In the last few years, there has been a clear trend to use _Unmanned Aircraft Systems_ (UAS), or drones, for many commercial and civil applications. There are studies [1] estimating that up to 400,000 drones will be providing services in the airspace by 2050, with a total market value of 10 billion euros per year by 2035. Last-mile delivery [2], surveillance [3], infrastructure inspection [4], traffic monitoring [5], media production [6], or managing health emergency situations [7] are just a few examples of the wide spectrum of drone applications. Indeed, the integration of UAS in the civil airspace is probably one of the most revolutionary events for _Air Traffic Management_ (ATM) since the beginning of its implementation. Although ATM has been traditionally based on voice communication through an _Air Traffic Control_ (ATC) entity, its bounded workload and communication capacities turn this centralized resource into a bottleneck for system scalability. Therefore, the rise of UAS operations brings the need for a new paradigm for airspace management, where digital communication will play a key role, and the responsibilities will be shared among different stakeholders instead of a single central actor. There are already some initiatives to integrate UAS into civil airspace and fulfill their operational requirements [8]. The _National Aeronautics and Space Administration_ (NASA) has created the concept for _UAS Traffic Management_ (UTM) [9] to enable safe, large-scale operations with UAS in low-altitude airspace [10]; whereas Europe has extended this UTM concept by proposing the _U-space_ ecosystem [11]. More specifically, an overview of theU-space ecosystem recently proposed by the _European Aviation Safety Agency_ (EASA) [12] is depicted in Figure 1. The idea is to have a _U-space service Provider Platform_, which is a server running on the cloud, as the core component. There, the UTM system consists of a software architecture that provides U-space services to the different actors in the U-space ecosystem using as a bridge the _U-space Service Manager_ (USM), which is a specific module of this UTM system. Currently, the community is in the process of further developing these U-space services. In this paper, we take a first step and propose a novel software architecture that aims to serve as a common framework for implementing and integrating U-space services. Our solution is being developed within the context of the European project GAUSS ([[https://projectgauss.eu](https://projectgauss.eu)]([https://projectgauss.eu](https://projectgauss.eu)), accessed on 26 April 2021), whose main objective is leveraging high-performance positioning functionalities provided by the Galileo ecosystem for U-space operations, including a validation phase with actual fixed-wing and rotary-wing UAS (see Figure 2). We present an architecture that is service-oriented and safety-centered, and that allows the airspace actors to abstract from specific UAS technologies. Besides, we implement a set of U-space services to manage complete UAS operations, but focusing on _in-flight_ services (i.e., those required to handle the operations during the flight phase). Nonetheless, the architecture is modular and flexible enough to be extended with additional functionalities as new services become functional. Our main contributions are as follows. First, we introduce the main concepts and the roadmap for the U-space initiative, and we review other relevant works about UTM (Section 2). Second, we analyze the design properties for our UTM architecture (Section 3). Given a series of desired architectural guidelines (Section 3.1), we propose the open-source _Robot Operating System_ ([[http://www.ros.org](http://www.ros.org)]([http://www.ros.org](http://www.ros.org)), accessed on 26 April 2021) as underlying middleware for our UTM system (Section 3.2). Third, we contribute with a new UTM system architecture implementing the U-space concept (Section 4).