Surveillance Frequency Monitoring through Radar-to-Reply Allocation

Aeronautical civil surveillance relies on cooperative sensors that exchange messages between aircraft and ground infrastructure. These exchanges occur on two frequencies -one for uplink and one for downlink - shared across all deployed systems, making these frequencies a scarce resource that can be overloaded if surveillance interrogators are not properly configured. As part of its Network Manager Function, EUROCONTROL is responsible for monitoring aviation infrastructure performance. To this end, EUROCONTROL has deployed a network of approximately 80 receivers across Europe that passively record all surveillance data exchanges between aircraft and ground infrastructure. However, because interrogators direct their messages towards aircraft in the sky and the monitoring receivers are ground-based, few uplink interrogations fall within the receivers' coverage. Conversely, most downlink replies are collected and can be used to monitor the load on surveillance frequencies. An allocation process linking downlink replies to interrogators is necessary to identify each interrogator's contribution to the overall frequency load. This paper introduces a new data-driven approach for assigning downlink replies to interrogators. Given that rotating Mode S radars are the primary contributors to the frequency load, the algorithm employs a simplified rotating radar beam model, defined by the radar's position, antenna rotation dynamics, and beam width. The algorithm first estimates these parameters before allocating the replies that fall within the rotating beam. Two groups of surveillance interrogators are defined: fixed interrogators for which the positions are known and mobile interrogators with indeterminate positions. Fixed interrogators only need their rotational parameters to be estimated while mobile interrogators also need their positions to be determined. This paper proposes an advanced method to locate mobile interrogators to equally integrate them into the radar-to-reply allocation process. After validating the algorithm's accuracy with labeled test flight data, the solution was tested on 24 hours of downlink replies collected across Europe. Nearly all replies were successfully allocated to a radar in the core areas of the continent, demonstrating the method's potential for various 1030/1090 MHz monitoring applications.