In this talk I will address aviation security and secure surveillance/control that are crucial for the safety of air traffic. Two wireless systems widely deployed in this context are the Global Positioning System (GPS) and ADS-B (Automatic Dependent Surveillance Broadcast). I will discuss why wireless systems like GPS and ADS-B are hard to fully secure and I will present novel mechanisms to improve air-Traffic-related security at the intersection of these two systems. In particular, I will talk about (1) Crowd-GPS-Sec, a system to detect and localize GPS spoofing attacks on moving airborne targets such as UAVs or commercial airliners, as well as (2) ADS-B-Trust, an approach to leverage machine learning for ADS-B and GPS attack detection. These systems demonstrate the potential of sensor-based crowdsourcing and wireless witnessing for attack detection purposes in a typically rather closed industry sector.