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In practical ISAR applications the quality of the image obtained depends upon the distortions in the wavefront illuminating the target, effects introduced by the radar-target path, the accuracy of the angle and frequency steps used in obtaining the data, vibration, and multiple reflections from neighboring objects. Results of analysis, simulation and data obtained in an RCS compact range are presented to quantify the relationships of the image degradation introduced by these effects.
In this presentation we consider the features and performance of a large serrated-edge compact range reflector. This is a straightforward innovation from earlier compact range reflectors. The virtual vertex reflector is a paraboloidal surface truncated to exclude the vertex. This layout provides the advantages of better use of reflector surface area, reduced feed blockage, and reduced feed backscatter. The design is made economical by the use of serrations.
A simple computer program which performs a direct calculation of the Discrete Fourier Transform (DFT) was written to generate the plane wave spectrum of a compact range from sampled field probe data. This program was used to analyze idealized quiet zone fields, computer predicted quiet zone fields and measured field probe data. Data from this analysis is presented along with suggestions for the correct interpretation of the results.
The use of serrated edge treatment in the design of a compact range collimating reflector is one method of mitigating the effects of edge diffraction on quiet zone performance. In this note a physical optics analysis is applied to the serrated reflector. The computational procedure is described and several results are presented. In particular, computed results are presented for the S-A Model 5755 compact range reflector and compared with experiment.
The design, fabrication, and testing of a high directivity, constant beamwidth feed horn is presented in this paper. The subject feed horn is designed to illuminate a shaped reflector compact range operating from 140 to 170 GHz. Design considerations related to pattern control and VSWR are discussed. Fabrication challenges are also considered. Primary pattern test results are presented and compared to predictions. Integration (into the reflector system) considerations are reviewed and quiet zone performance is discussed.
Currently many new compact range facilities are being constructed for making antenna pattern measurements indoors. Limited suppression of stray signals ~ due to range layout, confined surroundings and residual absorbing material reflectivity ~ represents a limitation on the accuracy of the measurements made in these facilities. Time-gating of the compact range signal appears to be a very attractive technique to reduce unwanted reflections. The authors have carried out an experimental investigation of time gating in a compact range. It is demonstrated that time-gating can improve the uniformity of the aperture field by removing the feed backlobe radiation; and, it is demonstrated that time-gating can remove the effects on a pattern of certain room reflections and of feed backlobes. When compared to conventional methods of reducing reflections based on placement of absorber, time gating appears equivalent. It does not appear however that time gating improves the conventional methods, except for measuring wide beamwidth antennas.
In the last few years, the interest in Radar Cross Section (RCS) measurements has increased rapidly. The development of high-performance Compact Ranges (CR) has made possible measurements on large targets down to very low RCS levels (below -70 dBsm). RCS imaging is a powerful tool to determine the location of scattering sources on a target. The response of the target is measured as a function of the frequency and aspect angle. A two-dimensional Fourier transform then gives the reflection density as a function of down-range and cross-range. If the response is measured vs. azimuth and elevation, even a complete 3-D image is possible. For high-resolution imaging (large bandwidth, wide aspect-angle span) a direct 2-dimensional Fourier transform gives rise to errors caused by the movement of the scatterers during the measurement. These errors can be corrected by applying a coordinate transformation to the measured data, prior to the Fourier transforms. This so called focused imaging allows further manipulation of measured data. However, the measurement accuracy can be a limiting factor in application of these techniques. It will be shown that the Compact Range performance as well as positioning accuracy can cause serious errors in high-resolution imaging and thus in interpretation of processed data.
Conventional radar imaging requires large amounts of data over large bandwidths and angular sectors to produce the location of the dominant scattering centers. A new approach is presented here which utilizes only two swept frequency scans at two different look angles for two-dimensional images or three swept frequency scans at three different look angles for three-dimensional images. Each swept frequency scan is the backscattered response of a target. A different plane wave illumination angle can be conveniently obtained by offsetting the feed horn from the focus of a compact range reflector without rotating the target. The two- and three-dimensional target information for the location of the dominant scattering centers is then obtained from the band limited impulse responses of these swept frequency scans.
Results of an experimental study of the interactions between a scattering target and the absorber-coated walls and ceiling of the OSU Compact Range Anechoic Room are reported. A 6 ft. square flat metal reflector was mounted in the quiet zone and oriented at selected angles non-orthogonal to the range symmetry axis. In theory, this target (when non-orthogonal) has a relatively low backscattering signature, and a strong planar bistatic scattering beam which can be pointed at several regions and absorber types in the room. By processing, the bistatic iteration terms can be separated form the plate backscatter, and frequency domain spectra and/or transient response signatures of the different mechanisms produced. Th paper will present calibration information on the actual performance of the bistatic scattering beam of the plate, and measurements of both backscattering and bistatic scattering of the absorber-coated walls in the ESL chamber. Suggested guidelines for use of this as a standard anechoic room diagnostic test will be discussed.
This paper describes recent advances in antenna measurement instrumentation for millimeter frequency applications. Application of a new, lightweight, programmable, ruggedized signal source at 40 and 60 GHz is outlined. An RF instrumentation system for millimeter frequency antenna range application is detailed. A millimeter-to-microwave converter is described which improves millimeter antenna range performance. System performance levels are predicted. Compact range configuration and operation at millimeter frequencies is detailed. Specific measurement examples are presented to demonstrate the measurement sensitivity which can be achieved.
This paper reports the quiet zone characteristics of the Harris family of compact ranges. Field probe measurements of systems having quiet zones of 3, 6, and 40 feet are presented. The quiet zones were characterized using a two way measurement with a trihedral corner reflector target. One way CW field probe measurements with an open ended waveguide are also presented for the Model 1606 range. A Discrete Fourier Transform (DFT) is imbedded in the test set software and provides an angle domain signature of extraneous signals illuminating the quiet zone. The two way range transfer functions of the Model 1603 and 1606 ranges are verified using calibrated spherical targets with the HP-8510 network analyzer operating as a time domain reflectometer.
Harris Corporation is in the final stages of implementing the Model 1640 Compact Range for The Boeing Corporation. This paper provides an overview of the development, fabrication, installation, and test activities on this very significant advance in the compact range state-of-the-art. This range represents a significant increase in quiet zone size over prior art. It features the wide dynamic range, low noise floor, and high quality quiet zone that is achievable using the Harris-proprietary shaped range technique. Another feature of this range is the completely panelized construction technique. This allows the production of very large, very precise reflector systems. Primary technical features of the Model 1640 are a 40 foot quiet zone, a -70 dBsm noise floor, and a frequency range extending from VHF to millimeter-wave frequencies.
A near field focus procedure for image processing using near zone backscattered fields obtained in a compact range is presented in this paper. An array of defocussed feeds is used to illuminate a target in a compact range. The backscattered field received at each feed antenna with the target being stationary is compensated by a phase factor. These compensated signals are then summed coherently to obtain a cross range image of the target which indicates the location of the various scattering centers associated with the target. Numerical examples are presented to validate this technique.
The compact range provides a means to evaluate the radar cross section (RCS) of a wide variety of targets, but successful measurements are dependent on the type of target mounting used. This work is concerned with the mounting of targets to a metal ogival shaped pedestal, and in particular focuses on two forms of mounting techniques: the "soft" (non-metallic) and "hard" (metallic) mounting configurations. Each form is evaluated from both the mechanical and electromagnetic viewpoints, and the limitations associated with each type are examined. Additional concerns such as vector background subtraction and target-mount interactions are also examined, both analytically and through measurements performed in the ElectroScience Laboratory's Anechoic Chamber.
A model is presented for the analysis of gaps in reflector panels. In this model, Butler's method of expanding fields in terms of a series of Chebyshev functions is used to determine the gap aperture fields. These aperture fields are then transformed into the quiet zone to obtain the final scattered field expression. Because of simplifying approximations, this method is only valid for gap widths that are less than both the panel dimensions and one-third the operating wavelength. Quiet zone fields are calculated for compact range antennas modeled as parabolic cylinders using this method. An RMS sum of the scattered fields is used to determine the worst case effects of frequency, gap width and differing number of panel gaps on peak-to-peak quiet zone amplitude ripple. Results are presented for a large range with a 75 foot diameter reflector and a smaller range with a 18.75 foot diameter reflector.
A procedure to design blended rolled edges for arbitrary rim shape compact range reflectors is presented. The reflector may be center-fed or offset-fed. The design procedure leads to continuous and smooth rolled edges and ensures small diffracted field from the junction between the paraboloid and the rolled edge. The performance of a compact range reflector designed using the prescribed procedure is also presented.
Over the years formulations have been developed which provide an implicit measure of transfer efficiency of the compact range. Reasonable accuracy has been demonstrated for both antenna and RCS measurement applications. In general, however, these formulations require specific design details pertaining to the collimating reflector. In this note a more general formulation is examined in which efficiency is explicitly expressed in terms familiar to antenna engineers and which do not directly involve reflector parameters. Applications of this formulation are presented.
The radar cross section of large targets has previously been measured on large outdoor far field ranges. Due to environmental and security limitations of outdoor ranges, low cost indoor compact ranges are preferred. To optimize compact range performance and to minimize size, careful attention must be paid to the design of feeds which are required for the proper illumination of the reflector. This paper describes a new polarization diversified parasitic multimode/corrugated (PMC) feed for a compact range reflector. The performance attributes of the PMC feed are presented. The PMC feed provides several advantages over other known commercially available compact range feeds.
This paper describes how corrugated feed horns are designed for compact ranges with tight pattern control. Both the amplitude and phase of the horn pattern must be invariant over a wide frequency band. A horn synthesis computer program has been developed using the JPL HYBRIDHORN computer program as the analysis module which is driven by a Harris developed synthesis code (OPTDES). This paper also discusses launching techniques used to generate the HE(11) hybrid mode in the corrugated horn as well as design methods to eliminate ringing effects observed in both the input waveguide circuits and corrugated horns when used for RCS measurements.
The development of a small compact range facility that has been integrated into an existing laboratory space is described. This facility uses a commercially available offset reflector with a 6 ft projected diameter and has sufficiently precise construction for operation at EHF frequencies. The edge diffraction degradation of the quiet zone is controlled by reducing the reflector edge illumination rather than using a complex edge treatment or a dual reflector design. Measured values of the quiet zone fields compare very well with calculated values. The facility can be used to measure antennas and radar targets whose dimensions do not exceed 20 in at high microwave and millimeter-wave frequencies. The low cost and simplicity of this compact range design are key features.