Experimental Study of Shadow Region Imaging Algorithm with Multiple Scattered Waves for UWB Radars

Abstract

Ultra-wide band (UWB) radar is capable of high range resolution near fleld sensing, and is thus applicable to security systems designed to identify a human body even when invisible. Although Synthetic Aperture Radar (SAR) creates a stable and accurate target image for such applications, it often sufiers from increased shadow regions in the case of complex or multiple targets. On the contrary, a multiple scattered wave has the potential to enlarge a visible range on target surfaces because it propagates a path which difiers from that of a single scattered wave. While various algorithms based on time-reversal processing with multiple scattered waves have been developed, these require a priori information of the surroundings or a target model. This paper proposes a shadow region imaging algorithm based on the aperture synthesis of multiple scattered waves. While the proposed algorithm only synthesizes a double scattered wave according to its propagation path, it can directly increase the visible area and is applicable to arbitrary target shapes. The results from the numerical simulation and experiment verify that the proposed algorithm directly makes a shadow region visible without a preliminary observation. 1. INTRODUCTION UWB pulse radar is promising as a near fleld sensing technique with high range resolution, and is applicable to non-contact measurement of re∞ector antennae or aircraft bodies that have precise and specular surfaces. It is also applicable to a collision avoidance for automobile under the low visibility. For such applications, various imaging algorithms have been published, as the SEABED accomplishes real-time imaging by using a reversible transform BST (1), and the Envelope+SOC reconstructs the target surface to an accuracy on the order of 1/100 wavelength (2). The SAR algorithm remains promising, in providing a stable and accurate image by using the full information of the received signals (3). However, in the case of complex or multiple targets, any of the algorithms sufiers from increased shadow regions because they use only the single scattered wave for imaging. On the contrary, except for an edge difiraction wave, a multiple scattered wave passes through a path, difierent from that of a single scattered wave, which means that the multiple scattered echo has the potential to enhance the visible range. Although the time reversal algorithms with multiple scattered waves have been proposed when focusing on reliable target detection or accurate positioning in cluttered situations (4{6), they require target modeling or a priori information of the surrounding environment like the walls. To expand the applicability of these methods, this paper proposes a non-parametric imaging algorithm based on the aperture synthesis of multiple scattered signals. The results obtained from numerical simulations and an experiment verify the efiectiveness of the proposed method which is applicable to arbitrary target shapes, and directly enlarges the visible range of the target surface. 2. CONVENTIONAL ALGORITHM Figure 1 shows the system model. It assumes that the target has an arbitrary shape with a clear boundary, and high conductivity like metallic objects. The propagation speed of the radio wave c is assumed to be a known constant. A mono-cycle pulse is used as the transmitting current, and the space is normalized by ‚ as the center wavelength of the pulse. The omni-directional antenna is scanned in the plane z = 0. The real space in which the target and antenna are located is expressed by the parameter r = (x;y;z). z > 0 is assumed for simplicity. s(X;Y;Z) is deflned as the output of the Wiener fllter at the antenna location at (x;y;z) = (X;Y;0), where Z = ct=(2‚) is expressed as a function of the time t. As a spatial measurement in the near fleld, the SAR algorithm has an ability to create a stable and accurate image by using the UWB signal. The distribution image I1(r) obtained by the SAR is formulated as