Detection and Imaging of Buried Objects

K.M. Scheff and J.P. Hansen
Radar Division

Introduction: A state-of-the-art ultrawideband (UWB) radar, designed and implemented by the Radar Division, is being used to develop techniques for the detection, imaging, and ultimate identification of objects buried near the Earth's surface.¹ This dual-frequency, dual-polarization, UWB radar system has performed ultra-high resolution measurements (2-cm range resolution) of land mines and other objects in a laboratory setting. Initial free-space measurements are currently being used to develop and test imaging algorithms, as well as to explore the feasibility and limitations of various synthetic aperture radar (SAR) configurations.

Hardware and Measurement Geometry: This laboratory version of the UWB system has video impulse-excited X-band and S-band traveling wave tubes (TWTs) that produce transmit pulses with 2 W of peak power and durations of 0.15 and 0.25 ns (3 dB power envelope). Separated, dual-polarized, UWB (2-18 GHz) horn antennas are used for transmit and receive to maintain receiver isolation without the use of a duplexer. The receive system detects signals in coherent baseband quadrature form with a unique direct-sampling detector based on a multiple-sampling-head, 8-bit, digital sampling oscilloscope. The heads simultaneously sample the in-phase and 90° phase-shifted components of a received signal.

For this experiment, synthetic aperture is achieved by positioning and directing the transmit and receive antennas at fixed locations of a 1-m² data-collection grid via a rail system above the ground. With respect to the center of the grid, each target is centered in the y-dimension and is offset in the x-direction (Fig. 3). The measurement system is configured so that the distance between the transmit-antenna and receive-antenna rails in the x-direction is constant. Both antennas are pointed at the target such that polarization direction is preserved (that is, vertical polarization is in the x-direction and horizontal polarization is in the y-direction). Data were collected every 2.5 cm along the receive-antenna rail at fixed antenna positions at X-band (0.15 ns) for three targets (a metallic land mine, a plastic land mine, and a calibration sphere) in free space.

Analysis: The amplitude images are formed in the time domain using

for all points in the image volume. After the time delays for the transmit (x_t, y_t, z_t) and receive (x_r, y_r, z_r) positions are calculated, voltages (V_I, V_Q) with the proper time delay are algebraically summed for a given position in the image volume to form the in-phase (I) and quadrature (Q) components of the image. Signals from objects outside the volume of interest are range gated out to avoid degradation of the image. The beauty and the power of using an UWB short-pulse system over conventional radars is in the ability to form images with this simple algorithm.

Experimental Observations: Figure 4 is a side view of a 15-cm diameter calibration sphere above its radar image. The extent of the measurement geometry can be inferred from the extent of the crescent shape, which indicates that the measurement covers a small fraction of the sphere. Because the positioning in the x-direction was off-center, the sphere was not illuminated on the right side, and the image of its top is somewhat asymmetric. This asymmetrical artifact is present in the following images of metallic (Fig. 5) and plastic (Fig. 6) mines and will be removed in subsequent measurements. For example, Fig. 5 provides a cross-sectional image in the xz-plane of an inert, cylindrical (33-cm diameter, 11.5-cm high), metallic, anti-tank mine. Observe that the image realistically includes corners, where the top portion of the mine joins the main body, and the rim around the base of the mine. However, because the positioning in the x-direction was off-center, the rim about the base was not illuminated on the right side, and the image of the top of the mine is somewhat asymmetric.

FIGURE 5 Side view of metallic mine.

FIGURE 6 Side view of plastic mine.