Space-Based ADS-B Collision Model Analysis Using Beamforming Techniques

In aviation, the development of the Communications, Navigation and Surveillance (CNS) framework has allowed to constantly improve the overall performance of flight operations (environmental impact, capacity, fuel efficiency) while ensuring high standards of safety. Regarding surveillance, one of the most promising initiatives is space-based Automatic Dependent Surveillance - Broadcast (ADS-B). This system relies on a spaceborne architecture to receive, process and download the information of ADS-B squitters gathered from the aircraft under coverage to the ground surveillance data centers. Space-Based ADS-B (SBA), like its air-ground counterpart, also suffers from the congestion of the 1090 MHz frequency band, which is shared with other aviation systems such as Traffic Collision Avoidance System (TCAS) and Secondary Surveillance Radar (SSR) Modes A/C/S. This congestion can lead to degradation of all these systems, with all messages colliding among themselves. From the space-based ADS-B point of view, this would potentially mean that the aircraft position is not updated on the air controller's display, who would increase the separations between aircraft for the sake of safety, causing the airspace to reduce its capacity and efficiency. The purpose of this document is to provide an analysis of the potential performance of SBA and discern, in a preliminary basis, whether this new architecture is able to fulfil the requirements currently set by aviation authorities. For this objective, this paper starts modelling the ADS-B transmission from a statistical standpoint, which can be compared to a pure ALOHA network. By introducing some additional assumptions, a modified Pure ALOHA model can be adapted to the 1090 MHz situation, where TCAS and SSR Modes A/C/S messages are tagged as False Replies Unsynchronized In Time (FRUIT), that is, potential colliders with ADS-B squitters. This new model enables the study of the ADS-B squitters' collision probability. Once the mathematical model is set and modified considering the impact of FRUIT, it is applied to a real scenario with two variants: one implementing beamforming techniques and another without them. An analysis of the impact of beamforming techniques on the performance of SBA system is also provided, highlighting the improvement of the non-collision probability thanks to the smaller areas that each beam has to cover in comparison to the single beam case. These two variants (with and without beamforming) are analyzed in a single location with different parameters (radar configurations, bottom-antenna visibility, satellite location, aircraft type) to achieve a full scope of the non-collision probability for aircraft horizontal position, which is then compared against the current regulations to determine the fulfilment of the safety requirements. Eventually, this document presents the results, showing that the SBA's performances are in line with the literature: the introduction of beamforming techniques helps to achieve and comply with the requirements of the aviation authorities.