# A Performance-Based Airspace Model for Unmanned Aircraft Systems Traffic Management
## 2 UTM Airspace Design and Operational Concepts  
Airspace structure design is an important aspect of UAM/UAS integration and to date remains an open research gap in the literature. Although lessons learned in conventional ATM are useful, many of its solutions cannot be scaled down and directly applied to the local management of UAM and UAS [12]. Table 1 summarizes the main airspace structure concepts that have been proposed in the literature so far. Four concepts are readily discernible: full mix, layers, zones and tubes. These are described in detail in [23; 24]. In the full mix concept, the airspace is essentially unstructured, with vehicles using a direct route between their origin and destination, with optimised flight altitudes and velocities to minimise fuel burn and other related costs. In the layered concept, the airspace is divided into sections stacked on top of each other. Each layer is typically dedicated to a type of aerial vehicle and a specific heading. The zone concept divides the airspace into different regions on the basis of a set of criteria. This typically includes risk exposure to people and property. Lastly, tube-based airspace structures are centred on fixed corridors; vehicles must follow a common speed limit, thus ensuring safe separation and minimizing conflicts. The so-called \"stacked-layer\" concept is also gaining traction, with one proposal to split each layer into so-called sky lanes in urban centres that mimic the organisation of streets [13]. Each lane is essentially a corridor that encompasses a reference nominal trajectory with sufficient buffers to account for potential off-nominal situations. The separation assurance from other vehicles and static obstacles is maintained by onboard autonomous systems and/or remote pilots. The flow of traffic is essentially managed by offline or online design/redesign of the lanes. Situations in which lanes intersect are managed by a time-based prioritization for each lane, conceptually similar to how traffic lights are applied to road traffic. Another layered airspace concept which is analogous to road traffic is presented in [25; 26], where roundabouts are used at the junction of multiple sky lanes, in place of time-based separations. The zone concept has also been proposed in the literature as a means of dividing the airspace into operational volumes. In several proposals in the literature, the division is typically performed on the basis of the level of risk exposure to the general public. Alternatively, the division can also be performed on the basis of the services provided, and on the level of overall system performance required to support a given category of operation. Access to a particular zone is then contingent on the UAS meeting the level of system performance stipulated for that zone. This mirrors the implementation of PBN for manned aircraft, wherein the employed navigation systems are required to meet a certain level of performance depending on the region and phase of flight. In fact, PBN has been endorsed by NASA [29], as a potentially viable concept to adopt and apply to the airspace structuring problem in the UTM context. This is also emphasised in the NASA AAM initiative [30]. Navigation performance for UAS can vary to a greater extent than manned aircraft. Most UAS navigation systems employ a Global Navigation Satellite System (GNSS) receiver as the primary source of positioning in a global frame. This is typically supplemented by fusion with other sensors including inertial sensors, visual sensors and lidar to obtain a full navigation state estimate. The achievable performance is dependent on the individual sensor characteristics as well as the employed fusion algorithm. Performance is also dependent on environmental characteristics that are different from those encountered in manned aircraft operations. For example, GNSS performance is highly degraded in urban environments owing to signal multipath and obscuration relative to conventional manned aircraft operations.