Three research efforts are presented which develop systems to increase radionavigation robustness. These three efforts constitute stand-alone chapters. The first of these describes an alternative position and navigation system that estimates absolute aircraft position and velocity based on time-series of accumulated delta range measurements from Distance Measuring Equipment (DME) ground transponders. The second presents a method to mitigate multipath in GNSS signals that rely on an antenna array. The developed method can be used as part of an anti-spoofing system. The third discusses a novel radionavigation system based on passive reflectors in orbit and ground transmitters. The proposed system's ability to change the signal broadcast relatively easily in case the system comes under attack provides a robust navigation system.