## 4 Integration into Airspace (U-Space)  
As indicated in [25], the expected number of drone operations in the European market is expected to reach 10 billion euros per year by 2035 and more than 15 billion euros per year by 2050. This volume of operations will pose safety, security and airspace integration issues in European airspace, especially at low flight levels (which are currently defined as altitudes below 150 m) with drones belonging mostly to open and specific categories. Many organizations are iteratively maturing their approach to address this volume of drone operations, as outlined in various concepts developed by bodies such as the Single European Sky Airspace Research (SESAR) Joint Undertaking (JU) through the CORUS project [26], the Federal Aviation Administration (FAA) [27], and global standardization bodies such as the Global UTM Association (GUTMA) [28]. In Europe, safe drone traffic management and its safe operation within the existing air traffic environment in a harmonized manner across the European airspace are ensured by U-space (outside Europe, this concept is commonly referred to as unmanned traffic management or UTM). In [29], SESAR JU defined U-space in a blueprint as a set of services based on high levels of digitization, as well as automation of functions and procedures designed to guarantee access to airspace for a large number of drones in a safe and efficient way, with an initial focus on operations at very low levels (VLL), with a maximum height above ground level of 150 m. Then, U-space is a technological framework designed to facilitate any type of operation, in all classes of airspace and in any environment, even the most congested, while providing an appropriate interface for manned aviation and air traffic control. The ConOps (concept of operations) for U-space was developed in the CORUS project and published on 30 September 2019 [26]. CORUS was focused on drones belonging to open or specific categories that operate in VLL, which are split into three types according to the services provided (see Figure 3):  
* X: No conflict resolution service is offered;* Y: Only preflight conflict resolution is offered;
* Z: Preflight conflict resolution and in-flight separation are offered. Type Y airspace will be available from U2 and will facilitate VLOS, EVLOS (extended visual line of sight) and BVLOS flight. The risk mitigations provided by U-space mean that the Y airspace is more amenable to other flight modes than X. Type Z airspace may be subdivided into Zu and Za, controlled by UTM and air traffic management (ATM), respectively. Za is simply a normal controlled airspace and is therefore immediately available. Zu airspace will be available from U3. Because U-space provides more risk mitigations for Z type, it is more amenable to other flight modes, and allows higher density operations than Y airspace. Z allows VLOS and EVLOS and facilitates BVLOS and automatic drone flight. Finally, EASA issued Opinion No 01/2020 [30] on 13 March 2020, which proposed a draft of the high-level regulatory framework for U space, closely linked to the two existing drone regulations [21; 22]. The opinion proposed a first set of what were considered by EASA as the minimum necessary rules, which are to be complemented later with further provisions enabling a more mature state of airspace integration. The objective of the opinion was to create and harmonize the necessary conditions for manned and unmanned aircraft to operate safely in the U-space airspace focusing on strategic and pretactical traffic management techniques in order to mitigate the risk of collisions by requiring adapted services and sharing essential traffic information. In this sense, the opinion acknowledged that ensuring that U-space participants are cooperative is required until further development of detect and avoid (DAA) systems to ensure safe operations, in particular for BVLOS operations.