From TOpAs to ESIT: Estimating Transponder Load using Digital Twins of Surveillance Infrastructure

Aircraft are equipped with transponders to respond to interrogations from cooperative surveillance systems such as secondary radar systems, multilateration systems, military surveillance systems and the Traffic Collision Avoidance System (TCAS). The efficiency and safety of the European Air Traffic Management (ATM) Network depends on the flawless operation of these systems. However, the large number of surveillance systems deployed in Europe, combined with the high density of aircraft has led to an increased rate of interrogations received by aircraft transponders. This large amount of interrogations can have a detrimental impact on the behavior of these transponders and could, in the worst case, lead to transponder failure resulting in a loss of aircraft surveillance. In response to that threat, the European Commission has issued regulation (EU) 2017/386 which obligates the Member States to check the transponder load in their airspaces and to take measures to reduce and limit interrogation rates to a safe level. To accomplish this, Austro Control GmbH ordered the Transponder Occupancy Analysis (TOpAs) tool from the Institute of Microwave and Photonic Engineering (IHF), Graz University of Technology. When EUROCONTROL decided to implement a European wide analysis tool, Austro Control, with IHF and SeRo Systems GmbH as subcontractors, won the call for bids for the development of European Simulator of Interrogators and Transponders (ESIT) in 2021. This paper intends to demonstrate the evolution of those tools and the functionality they provide.