Airspace surveillance is one of the classic applications of radar technology. Existing systems are focused on large-scale surveillance of commercial airspace. This research presents a prototype of a small, mobile, passive radar system that for example can be used to monitor aircraft and UAVs in the airspace over maritime borders. The system is based on a coherent, software defined receiving system for data acquisition. The signal processing is subsequently performed in the digital domain. The paper gives an introduction to all the required fundamentals and all the relevant steps of signal processing are shown, starting from digital beamforming, through the generation of the range Doppler matrices, all the way to the tracking of targets using Kalman filters. Furthermore, a novel system for the detection of transmitter locations is presented, which allows the use of such a system in previously unknown scenarios. Finally, measurements are presented to show what such a system can achieve under real world conditions.