High sensitivity fixed tuned direct conversion receiver for FM band commensal radar

Growing congestion of the EM Spectrum is pushing radar operators and regulators to consider sensors based on the Commensal form of Passive Radar i.e. using broadcast and data transmissions, to provide illumination for target detection by a receive only function. Our interest in the FM Broadcast Band (88 to 108 MHz) for utilisation in the Developing World to assist Air Traffic Management (ATM) requires low cost solutions. To this end, we have examined fixed tuned, direct conversion receivers. Commensal Radar (CR) relies on cross correlating the uncooperative transmitter signal (reference) with a signal that contains Doppler shifted versions of the reference, due to motion of the targets, as well as clutter. The target echo signal power is 60 to more than 100 dB lower than that of the reference signal, and any other spurious signals that might be generated in the receiver will reduce overall system sensitivity. A direct sampling receiver, while appealing in terms of system cost, must be carefully designed to preserve dynamic range. This paper recognises that a fixed frequency receiver system is suitable for a CR used for the ATM function, and this means that ultra narrowband filters using piezo-electric technology are very suitable to ensure spectral purity of each frequency utilised. In this paper, we demonstrate the operation of such a system, after showing where the major contributions to spurious signals originate, and how the narrowband filter improves receiver performance. This is done by means of theory, simulation and measurement, including field trials with aircraft detection at bistatic ranges of more than 300 km.