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This paper reports on a study conducted by the Georgia Institute of Technology for the U.S. Army Electronic Proving Ground, Fort Huachuca, Arizona to determine the feasibility of a large (50-foot quiet zone) outdoor compact range located at Fort Huachuca. The range is to be operated over the frequency range from 5 to 100 GHz. The main function of the range would be to measure patterns of low gain antennas mounted on military vehicles and aircraft, to determine whether antenna/vehicle interactions were degrading system performance. The paper presents both the electromagnetic and mechanical rational used as a basis for feasibility. The feasibility study considered many possible compact range configurations including the center fed paraboloidal reflector, the offset fed paraboloidal reflector (both prime feed and subreflector feed) and the dual crossed parabolic cylinder (DCPC) reflectors.
The Compact Range RCS Measurement System is comprised of the Harris Shaped Compact Range and the Hughes Short Pulse Coherent RCS Measurement System. The range offers a 10 foot spherical quiet zone with less than ±0.25 dB amplitude ripple, 0.2 dB amplitude taper, and ±2 degrees phase ripple. The short pulse system offers a pulse width as small as 5 nsec with range gate increments of 100 psec minimum. The system has a sensitivity of –70 dBsm without integration and –120 dBsm with 50 dB of coherent integration. System linearity is better that ±0.5 dB over the 70 dB instantaneous dynamic range. The Shaped Compact Range offers nearly 98 percent illumination efficiency with negligible spillover which minimizes the required anechoic chamber size and the amount of absorbing material necessary. The block diagram of the system is shown in Figure 1.
Compact range reflector systems have been previously used for far zone measurements in which case the feed is located at the reflector focus. It has been determined that near zone antenna pattern and backscatter measurements are feasible if the feed is appropriately located. Feed location information has been determined as a function of the radius of curvature of the near zone incident wavefront at the center of the measurement volume. Furthermore, numerous field quality data have been calculated. Field quality is defined as the closeness of the near zone field distribution in the measurement volume to the desired uniform spherical wavefront. The capability to measure near zone backscatter data was demonstrated with a 4-inch diameter cylinder, 4 feet in length. These measurements were made at 10 GHz, for a near zone range radius of 50 feet in the Ohio State University compact range facility. The near zone backscatter response for this cylinder was also calculated using a GTD analysis. A comparison of the calculations and measurements demonstrate the feasibility of the compact range for near zone backscatter measurements. This development leads to the consideration of compact range reflector systems for more general electromagnetic field simulations. For example, by employing an array feed, instead of a single feed element, the incident field in the measurement volume can be controlled in a rather flexible way. It is the purpose of this paper to explore some possible simulations.
When performing far-field testing on large-aperture antennas, the range length 2D2/? (that is needed to achieve a ‘flat’ phase front at the test plane) is sometimes inconviniently long. In these instances, the compact range of Figure 1 may be used as an alternate. In this range, the spherical wave radiated by the range source antenna is converted to an approximately plane wave by a large parabolic reflector. The antenna to be tested is immersed in this plane wave, at a location that is well within the near-field of the reflector. Also, for many antennas of interest, the reflector is likewise in the near-field of the test antenna, although this is not a requirement. (For those cases where the reflector is in the far field of the test antenna, there is little motivation to use a compact range, since a conventional far-field range of the same length would suffice.)
There are several background return sources on the Ohio State University Compact Radar Range which affect the sensitivity, accuracy, and dynamic range of the measurement. This paper discusses the magnitude and time delay of the principal background “clutter” mechanisms. Next, data on the time drift properties will be presented, and the relation to system temperature and other physical variations will be discussed. Finally, the impact of system design and operation concepts on these performance factors will be discussed.
The Ohio State University (OSU) ElectroScience Laboratory (ESL) utilizes a parabolic reflector as part of the compact range system [1]. It is necessary to probe the plane wave zone of this reflector in order to measure the purity of the plane wave that is generated. Variations in the amplitude or the phase of the signal received by a probe antenna as the probe is moved linearly across the plane wave region indicate deviations from a pure plane wave in the test zone.
System-2000 instruments were created for pattern range applications. The SD-2000 Synchro Monitor was developed in 1983, the MC-2000 Motor Controller in 1984, and System-2000 Host Processor in 1985. Dual black and white video monitors are being used both for graphics and closed circuit television. A rigid body motion application written in FORTH includes graphic primitives to simulate range components. In this paper, a simple aircraft model is installed on a model tower. A square hole in the vertical stabilizer simulates where a probe or antenna is to be located. The hole is offset from the inter-section of model and tower rotation axes, for discussion. Raster and spiral scanning are examined. Spiral scanning required simultaneous control o two drive motors. Emphasis is placed on using System-2000 dual axis features for motor control and graphic imaging of successive model positions.
In this paper we describe the implementation of an antenna pattern recorder using a desktop digital computer to replace the conventional analog electro-mechanical element. This means that all pattern recorder front-panel controls and charts are displayed on and accessed via the computer’s CRT, keyboard and peripherals. It has all the regular features, e.g. choice of scales, pen up/pen down etc., plus a multitude of additional features, obtained owing to the use of a digital computer, which will later be outlined in detail. In spite of the numerous options available, the instrument is very easy to master, requires no preliminary knowledge of computer operation and programming. It is entirely menu-driven and designed to trap most operator errors while maintaining a user-friendly environment suitable for technician-level operation.
This paper is a discussion of the upgrade of an automated antenna pattern data acquisition and analysis system located at the U.S. Army Electronic Proving Ground (USAEPG), Ft. Huachuca, Arizona. The upgrade was necessary as the existing facility was inadequate with respect to frequency coverage, data processing, and measurement speed and accuracy. The upgrade was also necessary in view of USAEPG long range plans to automate a proposed large compact range.
The accurate measurement of radar target scattering properties is becoming increasingly important in the development of stealth technology. This paper describes a low cost imaging Radar Cross Section (RCS) instrumentation radar capable of measuring both the amplitude and phase response of low RCS targets. The RCS instrumentation radar uses wideband FM wave-forms to achieve fine range resolution providing RCS data as a function of range, frequency and aspect. With additional data processing the radar can produce fully focused Inverse Synthetic Aperture Radar (ISAR) images and perform near field transformations of the data to correct the phase curvature across the target region. The radar achieves a range resolution of 4 inches at S-band and a sensitivity of –70 dBsm at a 30 ft range.
There has been renewed interest in RCS measurements recently especially for the evaluation of low backscatter targets. In order to accurately measure such targets, one needs to evaluate the system performance for such applications. One such performance check is the field quality measured within the target test volume. The question is then asked, “What is a satisfactory amplitude and phase requirement?” The normal 1 dB amplitude specification is not satisfactory because it doesn’t indicate whether the error is due to a field taper or ripple. Taper indicates a uniform phase but variable amplitude; while, ripple indicates the presence of a stray signal. This paper indicates why one should require less than a 1/10th dD ripple error for low RCS target measurements; whereas, a dB taper is satisfactory.
Inverse synthetic aperture radar (ISAR) imaging is used to produce high cross-range and down-range resolution on objects undergoing a change of aspect angle relative to the radar. In this application, the ISAR technique was used on an outdoor ground-bounce radar cross-section (RCS) measurement range. The objective is to locate, identify, and quantify the scattering properties of the target-under-test (TUT). The TUT is mounted well above ground on a target pole and can rotate in azimuth and elevation. The TUT’s rotational motion about an axis perpendicular to the radar line of sight is used to produce the cross-range resolution. For range resolution, a high-bandwidth frequency stepped waveform is used. The data are processed entirely in the digital domain with an algorithm that consists of a procedure to remove the dispersive properties and amplitude variations of the complete end-to-end range response, followed by a two-dimensional, polar-to-rectangular resampling filter and a two-dimensional fast Fourier transform (FFT). The processor has achieved images with amplitude and distortion products that are below the system’s noise floor with up to 48 dB of processing gain. The radar imagery is presented to the RCS engineer on a high-resolution color graphics terminal with true-perspective color-coded RCS displays in logarithmic amplitude or linear phase scales. The design of the ISAR processing algorithm is described in this paper as are the results for both simulated and actual radar data.
In any antenna or RCS measurement range, the walls, floor, and ceiling are covered with radar absorbing material (RAM) so that spurious scattering will be reduced. The bistatic scattering characteristics of these walls etc. are often not accurately known, however. This situation is exacerbated by the techniques often used to measure the scattering characteristics of the RAM used on the walls etc. The measurement techniques are typically “arch type” measurements, where the scattering from a section of absorber (often 3x3 feet) is compared to that scattered by a conducting plate of the same size. These type measurements are often corrupted by edge and corner diffraction terms and the results are often not very accurate.
The two measurements PCAL / PMRC and PTARG / PMRT are ratioed and the PMRC / PMRT term accounts for changes in both power or phase since calibration, because the mid-range is of fixed RCS size and phase. Using this technique, Scientific-Atlanta has been able to hold calibrations to within 0.5 dB amplitude and 8 degrees phase for as long as 12 hours. This includes outdoor range effects.
This paper briefly discusses the calibration techniques used in the Sandia National Laboratories Radar Cross-Section Test Range (SCATTER). We begin with a discussion of RCS calibration in general and progress to a description of how the range, electronics, and design requirements impacted and were impacted by system calibration. Discussions of calibration of the electronic signal path, the range reference used in the system, and target calibration in parallel and cross-polarization modes follow. We conclude with a discussion of ongoing efforts to improve calibration quality and operational efficiency. For an overview description of the SCATTER facility, the reader is referred to the article Sandia SCATTER Facility, also in this publication.
Versatile test bodies are extremely useful for RCS measurement facilities for many reasons, some of which are listed below: 1) evaluate the performance achievable for a given measurement facility 2) measure the RCS of components normally mounted on a ground plane, and 3) terminate a target pedestal in order to measure its cross-section since most pedestals are designed to attach directly to a target. In order to perform all of these functions a versatile test body should have flat sections to mount components efficiently, it should have a known smooth cross-section with angle of incidence from very low values to large ones, it should not use absorber that could attenuate the signal meant to illuminate the component pieces being tested, etc. Several such test bodies have been studied, some of which will be described.
The RF characteristics of a four-element diagonal array configured to yield low sidelobes, dual circular polarization with low axial ratio and high front-to-back ratio are described. The array was designed for use as source antenna in a VHF test range, where the test antenna is nearly omnidirectional and ground multipath effects are a major problem. To achieve broadband performance, crossed open-sleeve dipoles were used as array elements. The array is capable of operation over a 1.66:1 band with a VSWR of <2:1. Experimental studies were made by means of scale model antennas in the 240 to 400 MHz band. The axial ratio is <1 dB, and the sidelobe/backlobe levels vary from –25 dB to –30 dB over the measurement frequency range.
This paper describes the new antenna test facility now in operation at General Electric Space Systems Division in Valley Forge, PA. The antenna test facility is located in a new building 155’ x 74’ x 53’ high. It consists of a shielded anechoic room 60’ x 56’ x 35’ high which contains both a Compact Range and a Spherical Near Field Range, instrumented over the Frequency Range 1-100 GHz to perform automatic and manual measurements of antenna characteristics. In addition it provides for a 700’ boresight range accessible through large doors with an RF trans-parent window. A 3-axis positioner can accommodate antenna apertures up to 20’. The facility is used for both, testing of antenna systems and testing of entire spacecraft for electromagnetic compatibility and interference.
Antennas are designed to operate with planar phase fronts, but are usually tested on finite length ranges that produce curved phase fronts. The result is a pattern error near the main beam. For conventional antennas the accepted range length requirement is R>2D2/? which produced a spherical phase error of 22.5 at the perimeter of a diameter D at wavelength ?. This, in turn, causes a 35 dB shoulder. For ultra low sidelobe antennas (ULSA) even longer ranges have been suggested. Such range sizes may be unavailable as well as undesirable, since the larger the range the more difficult it is to eliminate reflections.
The design of a multipurpose Antenna/RCS range at GTRI is described. A novel approach to design of the far-field antenna range utilizes the bottom 40-foot section of a 130-foot windmill tower. The top 90-foot section is used as the main support for a slant RCS measurement range offering a maximum depression angle of 32º. A 100-tom capacity turntable, capable of rotating an M1 Tank, is located 150 feet from the 90-foot tower. The rigidity and stability of the tower should allow accurate phase measurement at 95 GHz for wind speeds up to 10 mph. In addition, a 500-foot scale-model range uses the ground plane effect to enhance target signal-to-noise and is designed to be useful at frequencies up to 18 GHz. Initially, the radar instrumentation to be utilized with the ranges includes several modular instrumentation systems and associated digital data acquisition equipment at frequency bands including C, X, Ku, Ka, and 95 GHz. The properties of these systems, which include coherence, frequency agility, and dual polarization, are discussed.