Working System And Some Key Techniques Of Signal Processing For HF Passive Bistatic Radar

HF radar can see beyond the earth's horizon, which exploits diffractive property of HF electromagnetic waves or uses the ionosphere to reflect its waves back to earth. HF radar has the remarkable advantages of long detection distance, good low-altitude detection performance, countering stealth and anti-radiation missile ability and so on, which is used as important equipment for sovereign nation's strategic early-warning, air defense and protection of the marine rights and interests. However, majority radars in this frequency band now work in active mode, which exploit cooperative transmitters and receive self-transmitting waveforms, whose disadvantages are obvious, such as huge antennas and transmission power needed, very complex radar system, hard to development and deployment, bad mobility and concealment and limited survivability ability in war. HF passive bistatic radar (HFPBR) are a subset of bistatic or multistatic radars operating with non-cooperative transmitters of opportunity in HF band, which passively receive the non-cooperative transmitted HF radio wave scattered by potential targets to realize target detection and track. As a very useful supplement of the traditional radar in this frequency band, HFPBR integrates the multiple advantages of both the HF radar and passive radar, which can effectively improve the radar system's overall detection performance, reduce the research, development and running cost, increase the survivability ability in war, and supply a new detection way for our nation's sovereign nation's strategic early-warning, air defense, protection of the marine rights and interests and environment remote sensing. In recent years, with the digital broadcast and television signals taking place of the traditional ones, passive radar based on the digital broadcast and television signals has become a research hot spot. DRM system is accepted as the only standard for HF band by ITU-R, whose increasing popularity provides conditions for the researches on HFPBR.This paper investigates the working system and some key techniques of signal processing for HFPBR based on the digital shortwave broadcast. The propagation mechanism and signal properties of this radar system are analyzed and the detection mechanism, detection performance and technical feasibility under typical propagation modes are systemically evaluated. Some key techniques of signal processing including the reference signal acquirement, direct-path wave and multipath clutter rejection, radio frequency interference suppression and target information acquisition, are studied in depth. Basing on the induction analysis of the common useful solutions, different ways of reference signal acquirement are quantitatively compared; a new clutter rejection algorithm appropriate for the digital broadcast signal properties are proposed creatively; the influence of synchronization error on the performance of clutter suppression is quantitatively analyzed; and the radio frequency interference's features in passive radar based on digital broadcast waveform are qualitatively analyzed. With the experimental study of HFPBR based on DRM digital shortwave broadcast, the typical detection results under different experimental conditions are presented; different solutions for key techniques in signal processing are tested; the proposed theory and algorithm in this paper are proved and the potential of over-the-horizon passive detection using DRM broadcast is experimentally confirmed, which could form the theory and experimental basis for a further development of HFPBR. The main researches of this paper are as follows:(1) The working system of HFPBR is investigated from the two aspects, including the propagation mechanism and detection signal properties. Firstly, the possible propagation modes of this new radar system are discussed theoretically, based on the propagation features of HF electromagnetic wave and the system configuration of HFPBR; and the detection feasibility and performance of different propagation modes are analyzed using the radar equation. Secondly, the availability of using the digital shortwave broadcast as a radar detection waveform is analyzed; the waveform properties for radar detection are studied and the waveform modification method according to the radar detection demand is proposed. Finally, the radar waveform parameters design for different detection modes is studied.(2) The general procedure of passive radar signal processing is investigated. For some key techniques including the reference signal acquirement, direct-path wave and multipath clutter rejection, match filtering and target detection and track, the common solutions are presented. Different ways of reference signal acquirement are analyzed. Traditional spatial and temporal clutter rejection methods are presented. The implementation method of match filtering convenient for engineering use is studied. The practical problems faced for passive radar systems in different frequency bands are stated. All the above form the basis for the study and implementation of HFPBR target detection and environment sensing. Meanwhile, HFPBR's own particularity and complexity are just the research objectives of this paper.(3) Using the signal structure of digital shortwave broadcast, the key modules of reference signal extraction method based on "demodulation-and-remodulation" reconstruction. This impact of this reconstruction method on radar detection performance is quantitatively evaluated from the aspects including match filtering performance and clutter rejection ability. This reconstruction method also is compared with another reference signal acquirement method. The radar detection performance's demands for the reference signal's purity are defined, with different reference signal acquirement methods. The advantages, disadvantages and suitable application fields of different reference signal acquirement methods are pointed out.(4) Using the point that the digital shortwave broadcast signal exploits the multi-carrier modulation technique, the array signal model of HFPBR is established; the spatial, temporal and carrier-domain clutter rejection method based on the orthogonal-frequency-division-multiplexing (OFDM) signal structure is studied; and a temporal and carrier-domain clutter rejection algorithm appropriate for OFDM signal structure is proposed, which also can be applied to passive bistatic radar in other frequency bands and using signals with OFDM waveform. In addition, the influence of synchronization error on the performance of clutter suppression is quantitatively analyzed.(5) The radio frequency interference's features in passive radar based on OFDM waveform are qualitatively analyzed. Based on the waveform properties and signal processing procedure, the radio frequency interference's form in received signal and its impact on target information acquisition are studied. Meanwhile, the interference suppression methods available are considered.(6) The target localization methods of HFPBR in different propagation modes are preliminarily analyzed based on the new radar system's features, including the estimate method of target's direction of arrival and actual distance from the receiver. Especially for HFPBR involving ionosphere propagation path, the target's actual distance is studied; a distance estimate method based on the simplified plane model is presented; and more accurate target localization methods like ray tracing are discussed.(7) With the field experimental measured data, the application performance of HFPBR based on digital shortwave broadcast is confirmed from an experimental point of view. From the measurement data results, the rightness of the proposed theory and the feasibility of the key techniques in this paper are evaluated, which has provided the theory and experimental basis for a further development of HFPBR's detection performance.