Kernel Density Estimation for The Detection and Synchronization of Interfered Mode S / ADS-B Preamble

The global Air Traffic Control faces challenges over the 1090 MHz Secondary Surveillance Radar downlink channel, mainly due to high occupancy rates. Several waveform types are concurrently used, among which one can find Mode A, C and S. A growing proportion of Mode S usage, comprising the Automatic Dependent Surveillance-Broadcast (ADS-B) is also observed. Countermeasures against the resulting co-channel interference are usually proposed for Mode S payload decoding and bit correction, while assuming correct signal detection and time-synchronization. However, such strategies yield limited advantage if the 4-pulse preamble pattern itself is not detected. Indeed, methods using cross-correlation would show highest detection rates on low SNR situations but are less effective in a classical interfered airport environment, where the probability of signal collision over a preamble is non-negligible. Other methods such as the "Enhanced Preamble Detection" proposed in the RTCA DO-260B normative document focuses on pulse edges detection, displaying better performance in case of Mode A/C interference. Nevertheless, it fails to address the more and more likely overlapping Mode S cases, especially when a preamble is overlapped by a higher amplitude signal.This paper proposes a method to detect and synchronize Mode S preambles, with constraints over Signal-to-Noise Ratio and interference rates, for applicability in current ground stations and future satellite-based traffic surveillance contexts. Simulation results show that a 5.6dB sensibility gain over DO-260B is achievable. Moreover, significant improvement in high interference environments is obtained when comparing to any known preamble detection method. Lastly, field experiments using an omnidirectional antenna tend to support the simulation results.