Richard A. Marr

Bistatic RCS Calculations From Cylindrical Near-Field Measurements—Part II: Experiments

Bistatic radar cross section (RCS) is computed from cylindrical near-field measurements obtained in a radio anechoic chamber with the target illuminated by a compact-range reflector. Near-field measurement is convenient where the RCS of complex targets is not amenable to computation or where computational results require experimental confirmation. A companion paper addresses the theory of cylindrical near-field scanning with reference to our experimental system. RCS of canonical targets derived from near-field measurement are in good agreement with theory. This paper compares the far fields computed from the near-field measurements with numerical solutions. Separating the target-scattered fields from incident and background fields presents a major challenge in an indoor bistatic radar configuration. We discuss the errors introduced by a residue of the incident field that is not canceled by the background subtraction method currently in use. A slow drift in system parameters and probe column oscillations are among the contributing factors

Bistatic RCS Calculations From Cylindrical Near-Field Measurements—Part I: Theory

A theory is presented for computing scattered far fields of targets from cylindrical near-field measurements. The targets are illuminated by plane waves and measured in a radio anechoic chamber on a cylindrical scan surface. The scattered field on the scan cylinder is obtained by background subtraction. The near-field data is truncated at the top, bottom, and angular edges of the scan cylinder. These truncation edges can cause inaccuracies in the computed far fields. Correction techniques are developed for the top and bottom truncation edges. The cylindrical wave expansions automatically apply angular tapers to the near-field data that reduce the effect of the angular truncation edges. The taper functions depend on the angular sample spacing and are related to the currents induced on perfectly electrically conducting PEC cylinders in related scattering problems. The method of stationary phase is employed with asymptotic expressions for the taper functions to determine the area on the scan cylinder that is most important for computing the far field in a given direction. The theory is validated through numerical examples involving electrically large scatterers. The edge-correction techniques significantly increase the accuracy of the computed far field. A companion paper presents experimental results

Methods for Locating Stray-Signal Sources in Anechoic Chambers

Two complementary numerically efficient frequency-domain methods for locating stray-signal sources in anechoic chambers are investigated and applied in combination to actual measurement data. Both methods use single-frequency near-field data collected on a planar surface and process them to reconstruct field values (images) elsewhere. The first method, which is based on the fact that the probe output satisfies the Helmholtz equation, uses plane waves to backpropagate the scan-plane data and is well suited for fast-Fourier-transform (FFT)-based rapid reconstruction of images on planar surfaces parallel to the scan plane. The second method uses the simple spherical-wave focusing technique and is flexible, in that, it can be used to generate images on either planar or nonplanar surfaces from the data collected on either planar or nonplanar surfaces. When data and image points are both located on a regular grid, the method can be implemented using the FFT-based fast convolution technique. Both methods include a spatial filter for isolating selected plane-wave spectrum components. The two methods are used in combination to successfully locate the strong multiple-bounce stray signals that degrade the quiet zone of a near-field bistatic radar cross-section facility. Subsequent scan data confirm that the suppression of these stray signals indeed substantially improves the quality of the quiet zone. The spherical-focusing method is also used to evaluate the effectiveness of the various absorber configurations applied to selected edges of the reflector to control edge-diffracted fields. It is shown that the reduction of the edge-diffracted fields further improves the quiet zone.

Imaging from limited-angle backscattered data from strongly scattering targets

A measurement technique for collecting scattered field data around a small solid angle in the backscatter direction is described. Given these highpass filtered data, a number of methods that can recover an image of the target, depending on whether the scattering target is strongly scattering or not, are briefly reviewed. Due to the nature of these limited frequency data, a method is described which allows one to incorporate prior knowledge about the target, in order to estimate a reconstruction. The performance of the method is compared against traditional Fourier methods.