Published in IET Radar, Sonar and Navigation Received on 7th October 2008 Revised on 2nd January 2009 doi: 10.1049/iet-rsn.2008.0159 Special Issue – selected papers from IEEE RadarCon 2008 ISSN 1751-8784 Adaptive beamforming for high-frequency over-the-horizon passive radar G. Fabrizio 1 F. Colone 2 P. Lombardo 2 A. Farina 3 1 Defence Science and Technology Organization (DSTO), Adelaide, Australia 2 INFOCOM Department, University of Rome ‘La Sapienza’, Italy 3 SELEX Sistemi Integrati, Rome, Italy E-mail: joe.fabrizio@dsto.defence.gov.au Abstract: Target detection and tracking systems using emitters of opportunity have received significant interest recently, especially those which exploit VHF and UHF broadcasts as signal sources in so-called passive radar systems. Here, the authors discuss an experimental system in the high-frequency (HF) band, where due to long-distance ionospheric propagation of radio waves in the 3–30 MHz spectrum, the illuminator may be located well beyond the line-of-sight. In this study, live data was recorded by a high dynamic range multi- channel digital receiver connected to a two-dimensional (L-shaped) antenna array, and signals from an uncooperative HF over-the-horizon (OTH) radar transmitter have been captured and analysed. As a preliminary step towards the development of a general HF-OTH passive radar system, the scope of this work is to compare the performance of conventional and adaptive spatial processing techniques in terms of their ability to cancel direct-wave interference and protect useful signal echoes to detect a small cooperative aircraft target. In particular, an alternative adaptive beamforming method specifically tailored to this application is proposed, and its practical performance is compared with classical and standard adaptive beamforming approaches. GPS data measured on-board the cooperative aircraft provided accurate ground truth of the flight path, enabling target profiles in bi-static range, Doppler frequency and direction-of-arrival (azimuth/elevation) to be calculated as a function of time. This information permitted the different processing schemes to be evaluated with a high degree of confidence. The experimental system and live data analysed are exclusively from the HF Radar program of the Defence Science and Technology Organisation (DSTO), Australia. 1 Introduction In recent years, aircraft target detection and tracking systems that exploit signal waveforms transmitted by uncooperative sources of opportunity have attracted significant attention and resources. The extensive effort has resulted in the fielding of a number of passive radar systems, also referred to as passive bistatic radar (PBR), passive covert radar (PCR) or passive coherent location (PCL) systems. The well-known advantages of such systems with respect to a conventional monostatic radar include low cost, small size, immunity to ECM threats, no additional demand on spectrum resources, and potentially higher target RCS due not only to their inherently bistatic configuration, but also the lower frequency regimes often exploited by PCL systems. While the notion of passive radar was conceived some time ago (circa 1935) [1], there is currently a strong resurgence of interest in PCL systems from industry, defence and academic communities [2, 3] due to the confluence of a number of contributing factors, including; (i) the development of high dynamic range receiver technology and growth in computing and digital signal processing capabilities that enable real-time operation of practical systems, (ii) the availability of illuminators that combine high bandwidth and power to provide more suitable waveforms for PCL systems, mainly introduced by an increasing shift towards digital broadcasting networks, but also due to sources for navigation and communication and (iii) the remarkable progress in high performance adaptive processing techniques, particularly the maturity of robust array processing algorithms studied within the radar community for target detection and parameter estimation 384 IET Radar Sonar Navig., 2009, Vol. 3, Iss. 4, pp. 384–405 & The Commonwealth of Australia 2009 doi: 10.1049/iet-rsn.2008.0159 www.ietdl.org