This master thesis presents the work of designing and implementing an advanced software-defined radio receiver for ADS-B/Mode-S messages, broadcasted from aircraft for the use in air traffic control. It presents and argues the design of the software receiver chain and introduces a platform that enables augmenting the basic receiver chain with advanced features. Furthermore, this work leverages the software receiver possibilities to implement and evaluate packet timestamping techniques that produce packet time-of-arrival information necessary for passive position tracking of airplanes. Multiple timestamping techniques are implemented and evaluated including a novel peak-position based approach that achieves precision of impressive 1.2 nanoseconds. Moreover, this thesis produces a method for improving packet decoding by means of packet collision resolution. Proof of concept collision resolution is achieved by facilitating the packet signal cancellation method developed in this work. This thesis also presents improvements for the future that would allow the developed receiver to become cognitive.