Direction of Arrival Estimations for a Secondary Surveillance Radar

Abstract

In aviation, Secondary Surveillance Radar (SSR) systems are used to generate situational images of an airspace. Through standardized communication protocols, aircraft reply to interrogations from SSR stations with their identity. By time-of-flight calculations and Direction of Arrival (DOA) estimations performed on the reply, the aircraft’s position can be determined. Since the frequencies used for the SSR communications are heavily trafficked, SSR experiences many problems related to interfering signals. This thesis investigates the DOA estimation performance of an airborne SSR system, where a digital hardware architecture is compared to an analog hardware architecture, exploring their potential in relation to the occurrence of interfering signals. The analog architecture, used as a benchmark, employs a phase monopulse algorithm for DOA estimation, while a few different algorithms are analyzed for the digital architecture. Through Monte-Carlo style simulations of Mode S replies, incorporating noise and randomized interfering signals, the performance of each architecture and algorithm is studied. ESPRIT is concluded to be the most prominent algorithm used with the digital architecture. Although not delivering as precise DOA estimations as the analog architecture, the digital architecture outperforms the analog in terms of simultaneous coverage and probability to decode the signals. The results suggest that, by leveraging the digital architecture’s broader field of view by making additional estimates, further improvements of the digital architecture’s estimate precision are possible. Future research could for example focus on methods to enhance digital DOA accuracy under practical operational constraints.