Recent efforts to modernize aviation traffic control have mandated the gradual replacement of the existing analogue radar system with a next-generation (NextGen) digital one. Part of this NextGen system is the Automatic Dependent Surveillance-Broadcast (ADS-B) standard. ADS-B aims at improving aviation safety by enabling aircraft broadcast navigation information. However, the current ADS-B standard does not provide mechanisms for verifying the integrity of navigation broadcasts. Consequently, aircraft trajectories can be easily spoofed. In this paper, we address the problem of verifying the navigation information of ADS-B transmissions. Fundamentally, this is a classical message integrity problem that can be addressed with cryptographic methods. However, cryptographic primitives are not part of ADS-B, primarily due to standardization and key management challenges. To address the shortcomings of ADS-B, we propose a PHY-layer verification method that exploits RF attributes of ADS-B transmissions to verify the aircraft's velocity and position. Specifically, we exploit the short coherence time of the wireless channel and the Doppler spread phenomenon to detect spoofed ADS-B messages broadcasted by a rogue ground station. We show that the security offered by our verification method is equivalent to the hardness of under defied quadratic equation systems, which are used in public-key cryptography.