Performance improvements using the separated reference configuration for a multi-static FM broadcast band radar system

A Commensal Radar (CR) uses the transmissions from radiators to detect and track targets. Since there is no requirement for dedicated spectrum allocation, Commensal Radars are gaining interest as an alternative to conventional, monostatic, active radar for applications such as air traffic control. Moves by governments to charge for spectrum usage is a growing concern for operators. Commensal radars have, until recently, been crippled by direct signal interference, limiting dynamic range [1]. Traditionally the CR system will produce bistatic detections by recording the reference and surveillance channels together at the same site with a multi-channel receiver. The multi-channel receiver ensures relative phase stability and inherent synchronicity between the 2 channels for the purpose of cross-correlation, to recover time delay and Doppler shift of the target(s). This configuration makes it difficult to use site selection to reduce the interference in the surveillance channel (e.g. by means of terrain shielding) as the reference antenna always needs to have Line of Site (LoS) to the illuminator. The “Separated Reference” [2] configuration was thus developed to allow the reference and surveillance antennas to be placed at widely separated sites (10s to 100s of km). Each receiver is equipped with a Global Navigation Satellite Systems (GNSS) stabilised oscillator to maintain relative channel synchronicity and provide accurate time stamping to allow the combing of many bistatic baselines. The separated reference configuration allows each antenna's location to be optimised purely for the detection function. This paper reports on some of the results and performance improvements obtained with field testing the separated reference configuration, beyond what was initially published [2].