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The ability to perform radar cross section (RCS) measurements, where background subtraction is applied, requires a measurement system that is very stable throughout the measurement time span. Background subtraction allows the measurement of low RCS components mounted in high RCS test bodies by permitting the scattering from the test body to be removed by coherently subtracting the test body (background) RCS from the target RCS measurement. Amplitude and phase variation of the illumination signal between the time that the target and background measurements are performed will limit the quality of subtraction achievable. Modern instrumentation radars can maintain extraordinary stability when exposed to controlled temperature environments, but controlling the temperature of a large compact range can be difficult. Other components of the measurement system, such as the reflector, can also be influenced by temperature fluctuations. Methods of controlling the thermal environment can have significant consequences. Lessons learned in the Advanced Compact Range at the Air Force Research Laboratory will be described.
The RFDM (Radio Frequency Development Model) of the PLANCK satellite has been tested in the Alcatel Space CATR (Compact Antenna Test Range) in 2002. The antenna was constituted by a telescope designed by the University of Minnesota for the Archeops balloon borne payload, and corrugated horns manufactured by electroforming process. At the beginning, characterization of the quiet zone of the Compact Range with a planar scanner is presented. A full amplitude/phase/co-pol and amplitude/cross-pol discrimination probing of a 5m x 5m quiet zone at 100 GHz is displayed. Then, we focus on measurements of the antenna response at 100 GHz performed in 4ð steradian with a dynamic range better than 100 dB. We cross-validate the measurement results with the RF predictions of the numerical model using the GRASP8 software developed by TICRA.
Undesirable antenna to antenna coupling has caused EMI problems between the WSC-6 SATCOM system and various systems in many shipboard installations. Long term solutions are currently being explored to resolve this EMI problem, which include adaptive interference cancellers and redesign of the WSC-6 feed and subreflector. However, these solutions are expensive and require several years to develop. An intermediate solution using RAM shrouds around the main reflector and subreflector edges of the WSC- 6 antenna has been proposed. The RAM shrouds were designed to reduce the spillover and diffraction of the antenna while having minimal impact on the antenna performances. A lightweight RAM was chosen to minimize the weight increase of the antenna. A prototype unit with the proposed modifications has been fabricated, assembled and tested in a tapered anechoic chamber, a near-field range, and a compact range. Significant reductions in the WSC-6 antenna sidelobes and backlobe have been verified via these measurements. Highlights of these modifications are described. Measured data (near field, compact range, tapered chamber, and shipboard) are presented.
In 2001, the Boeing 9-77 Indoor Compact Range successfully passed the range certification process. In preparation and during the On-Site Review in October 2001, RCS data on a pair of ultraspheres for the dualcalibration were collected. In this paper, we analyzed the data with regard to uncertainty analysis. An empirical approach for compensating the systematic error is presented.
Originally installed in 1992, the Advanced Compact Range (ACR) at Wright-Patterson Air Force Base was completely aligned using a Leica multi-theodolite measurement system. The Coherent Laser Radar (CLR) System provides an automated precision measurement capability which can gather significantly more data permitting a more complete characterization of the range in a relatively unobtrusive manner. This paper presents the process and results of applying Laser Radar Metrology as an optical range re-qualification tool within the Air Force Research Laboratory’s ACR.
A compact range was recently constructed at the Applied Physics Laboratory to measure broad-beam, fan-beam, and pencil-beam antennas (max aperture: 1 meters). Chebyshev absorber treatments, lightweight composite reflector, foam column mount for light-weight antennas, automated measurement software, and a novel feed spillover rejection algorithm are the technology elements implemented in this compact range measurement facility. This paper will describe a trade study that APL performed before the compact range antenna facility was built. Solutions to some of problems that were encountered during the construction will be discussed as well as the overall performance of the facility. The measurement of a broad-beam antenna will be compared to calculated pattern. This measurement will highlight the advantages of using a software range gate that was recently developed.
This paper presents a method for correcting RCS data obtained from objects extending beyond the boundaries of the test zone into the transition region of a large compact range collimator. The technique, exploiting the non-zero irradiation in the transition regions, uses results of calculated or measured field probes in conjunction with an image-based decomposition of the target angular response to correct for the field taper. The taper correction is developed as a weighting function applied to the spatial distribution with frequency-dependent coefficients derived from the field probes; the corrected RCS response is then obtained by an inverse operation. The paper addresses the conceptual notions of the approach and the limitations inherent to the underlying assumptions. Results of tests on canonical and actual targets are shown to demonstrate the applicability of the technique.
The possibility of launching satellites with increasing volume and weight leads to a higher economy and costefficiency for the service of future communication satellites, which are equipped with platforms up to 12 m in width for a variety of different antennas. For testing the radiation characteristics of the antennas of such large antenna farms, new test facilities are required to be designed and built up. Besides near-field test facilities, compact ranges exist, which provide additionally short test campaigns according to its real time measurement capability. Usually, for communication satellite testing, the highly accurate CCR 75/60 of Astrium GmbH, Germany, was used until now. For the future large satellites, Astrium newly designed the CCR 120/100, which provides a test zone of more than 8 m in diameter. The paper shows the requirements for testing of the large satellite antennas. Further, the design criteria, the range geometry and first simulation results of the CCR 120/100 are shown.
Wide frequency bandwidth feeds are used in compact ranges when multi-octave bandwidth operation of the range is desired. Dual-ridge or quad-ridge horns have been widely used in RCS applications as well as in antenna measurement applications to achieve wide band operation. This selection is made to take advantage of the lower cost of quad-ridge horns vs. other options. In designing a compact range, one primary concern is the beamwidth of the feed over the operating band. This affects the amplitude taper across the quiet zone of the range. Another primary concern is the movement of the phase center vs. frequency of the feed. This directly affects the phase taper across the quiet zone as a result of de-focusing of the reflector. Here we present measured data of the beamwidth and phase center movement vs. frequency of a wide-band quad-ridge feed designed to operate from 2.0-18.0 GHz. Measured and predicted quiet zone performance data over this bandwidth are presented with the feed installed in a Model 5751 compact antenna test range having a 4-foot quiet zone.
A filter-based approach to software range gating is presented. Conventional approaches to range gating are widely used and include hard gates applied in the time domain and running average filters applied in the frequency domain. The potential problems with those methods are well understood and involve (1) sideloberelated distortion of the frequency-domain data caused by hard clipping in time and (2) the dual problems that arise from finite-duration smoothing kernels in the frequency domain. Herein, range gating is formulated as a digital filter design problem. We employ a type-II Chebyshev design, which has a maximally flat pass-band and a specified stop-band attenuation. User parameters include constraints on the smoothness of the passband and the width of the gate transition. Edge effects are minimized by filtering symmetrically extended copies of the measured data. The results are illustrated on data acquired by the JHU-APL compact range.
Contoured multi-beams achieved by multi-feed reflector antennas, realized in modern communication satellites, like Intelsat VIII and IX generations satellite, require an economic measurement of their antenna characteristic. Further, highly accurate, but also fast and therefore real-time measurements are assumed to be applied for the testing of the antenna performance. For that aim, the Compensated Compact Range CCR 75/60, applied at e.g. Space Systems Loral (SSL) in Palo Alto (USA), at ALCATEL in Cannes (France), at the MISTRAL facility in Toulouse (France) and at Astrium GmbH (Germany) was developed and installed by Astrium GmbH. In order to optimize the measurement accuracy of the CCR, detailed error analyses and investigations for improvement measures were performed. Within this paper, the accuracy analyses and improvement steps will be presented in order to establish accuracy values, which can be realized in state-of-the-art compact range test facilities.
A calibration uncertainty analysis was conducted for the Air Force Research Laboratory’s (AFRL) Advanced Compact Range (ACR) in 2000 [1]. This analysis was a key component of the Radar Cross Section (RCS) ISO-25 (ANSI-Z- 540) Range Certification Demonstration Project. The scope of the RCS uncertainty analysis for the demonstration project was limited to calibration targets. Since that time we have initiated a detailed RCS uncertainty analysis for a more typical target measured in the ACR. A “more typical” target is one that is much larger with respect to wavelength than the calibration targets and characterized by a wide dynamic range of RCS scattering levels. We choose a 10’ ogive as the target due to the fact it is a large target, exhibits a wide dynamic range of scattering, and the scattering levels can be predicted using readily available CEM codes. We will present the methodology for the uncertainty analysis and detailed analyses of selected component uncertainties. The aspects of the uncertainty analysis that are unique to the “typical target” (i.e., a non calibration target) will be emphasized.
In a conventional manner, a majority of compact ranges are currently used between 2 GHz and 200 GHz. Mechanical stiffness limits compact ranges at high frequency and diffraction effects are dominant at low frequency. However, CNES has installed a single reflector with dedicated serrations to perform accurate measurements between 800 MHz and 2 GHz. These serrations are 2 meters long and minimize the ripple in both amplitude and phase within the quiet zone. In order to further improve its measurement capabilities at lower frequencies, CNES has installed, in co-operation with SATIMO, a spherical near field measurement system directly inside of its compact range building. The goal is to measure antennas within the frequency range 80 MHz – 400 MHz with a relatively good accuracy. The spherical near field measurement facility has been tested and validated with four antennas that had been previously measured in the compact range of CNES and other external ranges. This paper focuses in this smart approach, which allows to extend the lower frequency domain of compact ranges. This paper describes in details the measurement facility, the test and the validation of the system.
Test results of the compact range facility in the National University of Singapore are presented in this paper. The tests were performed for antenna and RCS measurements from L-band to Ka-band. Errors of experimental measurements are compared to errors in measurements calculated by results of field measuring in the quiet zone.
This paper presents the results of NCTR research performed at the POSTECH compact range. The radar cross section data of five scaled aircraft models, such as F4, F14, F16, F117 and Mig29, have been measured over a frequency region of X-band and an angular sector of 29.6o. Afterwards, one-dimensional radar signatures at several aspects of each target are obtained by modern spectral estimation techniques, including MUSIC, Fast Root-MUSIC, TLS-Prony, matrix pencil, TLS-ESPRIT. The proposed features are based on the central moments of a given radar signature distribution, and they can provide scale and translation invariance, which are essential for the improvement of NCTR performance. After the appropriate post-processing, the proposed features are classified by the Bayes classifier. Results show that our proposed technique has a significant potential for use in NCTR or ATR areas.
In order to better estimate the uncertainties in measured RCS for the Boeing 9-77 Compact Range, we study the responses from three high-quality objects, i.e., two ultraspheres of 14” and 8” in dia., plus the 4.5" squat-cylinder, each supported by strings. When calibrated against each other in pairs, the differences between measured RCS and predicted values are taken as the uncertainties for either object. Two standard-deviations from the target, reference, and background, as computed from repetitive sweeps, are taken as the respective uncertainties for the signals. Using the root-sum-squares (RSS) method, the error bars are found to be between + 0.1 to 0.2 dB for most of the frequency F, from 2 to 17.5 GHz. We also analyze the responses from a thin steel wire (dia. 0.020"), supported by fine fishing strings (dia. 0.012"), at broadside to the radar. When the ‘wire and string’ assembly is oriented vertically, the HH echo from the 3-ft metal wire alone happens to be comparable to the HH from the 30-ft dielectric strings. Varying with F4, the combined RCS in HH for the assembly spans a wide range of 38 dB from 2 to 18 GHz. The error bounds are found to bracket the measured traces even when the signals are barely above the noise floor.
Compact RCS measurement ranges all suffer from some level of non-ideal field illumination. Stray fields from interactions with the chamber wall and diffraction effects are major contributors to the non-uniformity of the incident field at the target. This non-uniformity gives rise to unavoidable errors in RCS measurements. We present a detailed analysis of how non-uniform illumination manifests itself into RCS measurement errors. The analysis approach is based on the plane wave spectral decomposition of the illumination. We compute the energy scattered by the planar components of the illumination and determine how much of this energy is coupled backi nto the radar antenna. We model the target as a diffuse scatterer by using a collection of point scatterers distributed within a specified volume. We present uncertainty results based on a simulation as well as field probe data collected from AFRL’s Advanced Compact Range (ACR).
Compensated Compact Ranges (CCR) represent a high standard of state-of-the-art test facilities with a fast and real time measurement capability up to the submm wave range. Future scientific and earth observation instruments of ESA/ESTEC such as MASTER, PLANCK and HERSCHEL are working within this frequency ranges and require a high measurement accuracy for large antenna apertures. Within the ADMIRALS study for ESA/ESTEC, transmit and receive modules up to 500 GHz and an appropriate large offset reflector antenna with precise surface accuracy in form of a Representative Test Object (RTO) were applied. Related tests in the CCR 75/60 of Astrium were performed in order to qualify the test facility and verify the antenna measurements with theoretical pattern calculations. The present paper shows measurement results with the highly accurate Plane Wave Scanner (PWS) of Astrium GmbH and the RTO. Through the measurements performed, the accuracy of the plane wave field as well as pattern accuracy in the quiet zone of the CCR 75/60 have been qualified up to 500 GHz.
Two ways of modelling a compact range design are presented, and the coupling to a given antenna under test (AUT) is determined and compared to the AUT far field. The compact range models are both based on physical optics (PO). The first model applies a simple presentation of the serrations of the range reflector while the second model is based on a new feature of GRASP8, which allows a detailed description of the triangles of the range serrations. The AUT measurement is modelled by an accurate coupling analysis between the current elements on the compact range reflector and the antenna under test. This coupling pattern is compared to the real far-field pattern and the differences are discussed. By including known range imperfections in the AUT-torange coupling a better agreement to the measured patterns may be obtained. All computations are carried out by GRASP8.
Translating pylon terminations are often used in narrowband RCS background measurements as means of separating the returns of the termination from those of the pylon itself. Typically, this is done by measuring the pylon while the fixture continuously translates in the range direction through a distance of at least half a wavelength. This paper describes a translated target processing (TTP) algorithmw hich is an extension of this technique to RCS measurements of rotating targets. The technique is applicable to both narrowband and wideband measurements. The algorithm is applied to the problemof mitigating multipath and ground interaction contamination in indoor and outdoor RCS measurements, respectively. Its performance was evaluated as a function of signal-to-noise ratio, target-tocontamination ratio, and translation distance and accuracy using point target simulations. We conclude with a demonstration of the TTP algorithm using actual measurements from the Boeing 9-77 compact range.