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Advanced Positioner Control Techniques in Antenna Measurements
Jacob Kunz, November 2014
Antenna, Radome, and RCS testing systems rely on high-fidelity positioner systems to provide high-precision positioning of articles for RF testing. Historically, the industry has relied on linear PID control techniques in torque, velocity, and position control loops on individual axes to drive the positioners. Recently, advancements have been made in the use of advanced control hardware including multiple-DOF laser and optical feedback devices, brushless DC motors, VFD AC motors, and multi-drive torque-biased actuation. Advanced control techniques including single-axis error correction, multi-axis global error compensation, and multi-axis coordinated motion have been implemented to improve positioner accuracy. Here, a survey is conducted of control technologies in other industries such as machine tools and industrial robotics. An assessment is conducted on the viability of other advanced techniques to provide insight into the potential future control and capabilities of positioning systems in the RF testing industry. Candidate advanced techniques include gain scheduling and sliding-mode control which could provide improved accuracy over a wider range of conditions including varying loads and operating points caused by differing movement speeds or large variations in static loading. Dynamic input-shaping and feed-forward techniques could help suppress dynamic vibrations and improve dynamic tracking behavior for improved continuous-measurement scanning accuracy. Adaptive and non-linear control techniques might improve disturbance and error rejection for improved accuracy while managing dynamic-behavior drift allowing for adaptation to long-term positioner changes without re-tuning.
A Dual-Linearly-Polarized Horn with Low Sidelobes for the upper VHF range: The Quasi-Open-Boundary Quad-Ridged Antenna
Vince Rodriguez, November 2014
RCS Measurements at the upper half of the VHF range of the spectrum have become increasingly important. This type of measurement is usually performed in an outdoor RCS range. The present paper shows a design for an antenna that can be used to illuminate a reflector or as the illuminating structure in a RCS measurement. The antenna is fairly compact given the wavelength and exhibits a low VSWR and a good time domain performance for use with pulses. The new antenna has low side-lobes that otherwise could illuminate adjacent structures to the outdoor range and reduce the dynamic range of the measurements, this is an improvement over the Open-Boundary Quad-Ridged Horns Introduced over the past 9 years. The new Feed is a Quasi-Open-Boundary Horn, in which RF absorber material is used to create the Open Boundary behavior, but an enclosed structure is  used to block the potential side-lobe radiation.
Radar Echoes from Dielectric Strings and Ropes
Pax Samuel Wei (retired), November 2015
At the Boeing 9-77 Range, we often encountered the need to support test objects of light to heavy weights with dielectric strings and fishing ropes of varying sizes from small to large.  Unlike a metallic material, which reflects the waves from its surface, the dielectric material is a volume scatterer [1].  Usually, the radar echoes from the strings or ropes at broadside to the wave-front are the highest, then they fall off quickly with angles away from normal.  In this paper we discuss several interesting cases learned, namely:   a). To deduce the dielectric constant of a rope by the ratio of co-pol to x-pol echoes.   b). To estimate the effective radius of a rope after being stretched under a heavy load.   c). Observation of interference between two or more scatterers in the same scene.   d). To process the angular dependent radar data of a tightly stretched rope as a         field-probe along that rope.   This paper is prepared in memory of and dedicated to a great teacher and friend on RCS [2]. ---------------------------------------------- ** Sam Wei  is at:  4123 - 205th Ave. SE, Sammamish, WA  98075-9600.     Email:,  Tel. (425) 392-0175   [1].  E. F. Knott, "Radar Cross Section Measurements," (Van Nostrand Reinhold, New York, 1993), Chapter 3, Target Support Structures, Section 3.2, String Supports, pp. 85-98.  [2].  In Memoriam: Eugene Knott, IEEE Antennas and Propagation Magazine, vol. 56,    No. 3, June 2014, pp. 132-133.
Dual-calibration Processing Based on Minimum Weighted Mean Squared Error (MWMSE) in RCS Measurement
Xiaojian Xu,Yongze Liu, November 2015
Dual-calibration was first proposed by Chizever et al. in 1996 [AMTA'1996] and had get wide applications in evaluation of the uncertainty in radar cross section (RCS) measurement and calibration. In 2013, LaHaie proposed a new technique based on jointly minimizing the mean squared error (MMSE) [AMTA'2013] among the calibrated RCS of multiple calibration artifacts, which estimates both the calibration function and the calibration uncertainty for each artifact. MMSE greatly improves the estimation accuracy for the radar calibration function as well as results in lower residual and RCS calibration errors. This paper presents a modified version of LaHaie's MMSE by minimizing the weighted mean squared error (MWMSE) for RCS calibration processing from  multiple calibrator measurements, which is related to the following functions and parameters: the calibration function; the theoretical and measured RCS; the number of calibration artifacts the number of frequency samples and the weight for ith calibration artifacts which may be defined in terms of the theoretical RCS of all the calibration artifacts. For example, if the weight is defined as the inverse of the total theoretical RCS of the ith calibration artifacts for all frequency samples, the error then represents the total relative calibration error instead of an absolute error as in MMSE. MWMSE then means that an optimal calibration function is found in terms of minimum total relative calibration error, which is expected for most applications. Numerical simulation results are presented to demonstrate the usefulness of the proposed technique.
Scattering Scenarios Exceeding the Description with Radar Cross Section – New Concepts and Measurement Approaches
Robert Geise,Georg Zimmer, Bjoern Neubauer, November 2015
The radar cross section is the standardized measure for describing scattering of objects. It is however always associated with the idealized propagation model of the Friis transmission equation with several constraints such as plane wave illumination. This contribution discusses the limited applicability of the RCS in some relevant scattering scenarios, e.g. objects like aircraft on ground or induced Doppler shifts from moving objects. In particular, the latter is a current research topic for radar and rotating wind turbines with strong impact on air traffic management. A new and more general description of scattering phenomena is proposed the standard RCS is just a subset of which for static objects under ideal illumination. It actually defines deviations from the ideal plane wave propagation allowing also to include amplitude and frequency modulation of a scattering propagation channel. In analogy to abstract concepts of communication engineering this quantity can be considered and understood as a wave response of a scattering object that can be applied to time-variant propagation channels.  A corresponding setup is presented on how to measure this wave response of scattering objects. Measurement examples are shown in a scaled measurement environment for moving, respectively rotating objects, especially for bistatic scattering configurations. Additionally, the illumination issue of objects is discussed reviewing scattering scenarios related to the instrument landing system.
CAMELIA Quiet Zone Assessment using PEC Sphere RCS Measurements
Pierre Massaloux,Philippe Bérisset, November 2015
An uncertainty budget for Indoor Radar Cross Section (RCS) measurements contains many contributors. Typically, one of the largest contributors comes from the Quiet Zone quality. To quantify the ripple and the tapper in the Compact Range Quiet Zone of the CEA’s indoor facility CAMELIA, a diagnosis method has been implemented, exploiting the radar response of a moving sphere located on a polystyrene mast. This polystyrene mast is fixed on the top of a linear-translating table over an azimuth positioner. The combination of the two axis capabilities allows to locate the PEC sphere in a horizontal plane cut of the quiet test zone volume. The other cuts at different altitudes are performed by changing the height of the polystyrene mast. This method samples the magnitude of the illuminating field at fixed spatial points (controlled by a laser tracking) in the Test Zone to determine the magnitude of the ripple and thus the Quiet Zone. These experimental data are then statistically processed to determine the measurement uncertainty at a given frequency. This paper introduces and analyses the results of a measurement campaign dedicated to the characterization of the Quiet Zone of the CEA’s indoor facility CAMELIA.
Monostatic RCS Calibration of Radar Target Using Extrapolation Method in Millimeter-wave Frequency Band
Michitaka Ameya,Satoru Kurokawa, Masanobu Hirose, November 2015
In this paper, we propose a calibration method for monostatic radar cross section (RCS) of simple radar targets (e.g. trihedral corner reflectors and square flat-plate reflectors) using extrapolation method. By the proposed method, we can calibrate the monostatic RCS of radar targets from 1-port S-parameter measurements. In our system, the applicable size of radar targets are 75 mm to 125 mm for corner reflectors and 40 mm to 75 mm for square flat-plate reflectors, respectively. The nominal RCS of reflector targets calculated by physical optics ranges from +3 dBsm to +15 dBsm in W-band.  The measured results are agree well with simulation results calculated by method of moment (MoM).
SAR-ISAR Blending Using Compressed Sensing Methods
Christer Larsson,Johan Jersblad, November 2015
Inverse Synthetic Aperture Radar (ISAR) measurements are used in this study to obtain images of full scale targets placed on a turntable. The images of the targets are extracted using compressed sensing methods. The extracted target images are edited and merged into measured Synthetic Aperture Radar (SAR) images. Airborne SAR field trials are complicated and expensive. This means that it is important to use the acquired data efficiently when areas with different background characteristics are imaged.  One would also like to evaluate the signature of targets in these background scenes. Ideally, each target should then be measured for many orientations as well as illumination angles which would result in a large number of measurement cases. A more efficient solution is to use ground based ISAR measurements of the desired targets and then blend these images into the SAR scene. We propose a SAR blending method where a noise free image of the target is extracted from the RCS measurement by using the compressed sensing method Basis pursuit denoise (BPDN) and then solving for a model consisting of point scatterers. The target signature point scatterers are then merged into a point scatterer representation of the SAR background scene. The total point scatterer RCS is evaluated in the frequency-angle domain followed by using that RCS for back projection to form a seamless SAR image containing the target with the desired orientation and aspect angle. A geometrically correct shadow, constructed from a CAD-model of the target, is edited into the background. The process is completed by adding noise to the image consistent with the estimated SNR of the SAR-system. The method is demonstrated with turntable measurements of a full scale target, with and without camouflage, signature extraction and blending into a SAR background. We find that the method provides an efficient way of evaluating measured target signatures in measured backgrounds.
A Radar Echo Emulator for the Evaluation of Automotive Radar Sensors
Domenic Belgiovane, Chi-Chih Chen, J. Landon Garry, November 2016
Automatic emergency braking (AEB) and collision imminent braking are beginning to be implemented by major automotive manufactures. AEB systems utilize automotive radar sensors operating in the 77 GHz frequency band for target detection. These said systems are capable of providing warning directly to the vehicle driver and when necessary apply automatic emergency braking. The effectiveness of such systems need to be accurately tested using standards and test procedures that are yet to be agreed upon among international automobile industry and government agencies. The Euro NCAP vehicle target (EVT) is the current European standard for AEB testing scenarios. The main goal of this research effort was developing a compact W-band radar echo emulator (REE) to be used for evaluating automotive pre-collision systems (PCS) operating in the 77 GHz frequency band. The proposed REE is capable of receiving radar signals from the PCS radar mounted on the vehicle under test (VUT) and then transmits modified radar signals back to PCS radar bearing the similar signatures (temporal, spectral, and pattern) as the Euro NCAP Vehicle Target (EVT). REE eliminates the need for the front vehicle target to produce radar responses which is currently accomplished with complicated arrangement of RF absorbers and reflectors as in the EVT and other vehicle surrogates. The adoption of REE means that the vehicle target only needs to bear optical signatures similar to an actual vehicle, and thus can be made with a much simpler balloon structure. Measurements present for the characterization of the Euro NCAP EVT over distance as well as the calibrated radar cross section (RCS). From this simply target model the REE echo power is empirically determined. The REE solution to PCS testing scenarios offers an easily adaptable return power various targets can be emulated with a single module.
Radar Echoes from Metal Spheres Large and Small
Pax Wei, November 2016
Wave scattering from a perfectly conducting sphere provides an important example for theoretical studies as well as RCS calibrations [1, 2].  At the Boeing 9-77 Range and the Millimeter Wave Range in Seattle, we measured spheres of large and small diameters, supported by strings or a foam tower, and through a wide range of frequencies.  In addition to co-polarized calibration, the emphasis was also on uncertainty analysis in order to verify that the experiments carried out under different conditions were mutually consistent [3].  Aside from the well-defined conditions for an indoor range, metal spheres may be dropped from the air free fall while being measured [4].  A news article on January 5, 2016, reported that three metal spheres were picked up in three provinces in northern Vietnam [5].  Though details of the experiments were obscure, from the pictures they happened to correspond to spheres of sizes from large to small.  Based on our experiences, some speculation will be discussed.  References [1]. E. F. Knott, "Radar Cross Section Measurements," (Van Nostrand Reinhold,  New York, 1993), pp. 176-180, (on spheres and the Mie series).   [2]. E. F. Knott, E. F. Shaeffer, and M. T. Tuley, "Radar Cross Section," (Artech House,      2nd ed, 1993), pp. 86 & 234-235, (on creeping waves).  [3]. P. S. P. Wei, A. W. Reed, C. N. Ericksen, and J. P. Rupp, “Uncertainty Analysis and      Inter-Range Comparison on RCS Measurements from Spheres,” Proc. 26th AMTA,      pp. 294-299 (2004).   [4]. “Mysterious silver balls fall down on town; can the black helicopters be far behind?”   By Steve Vogel, The Seattle Times, August 7, 2000, (from the Washington Post).  [5]. “3 mysterious spheres fall onto 3 Vietnam provinces,”  Tuoi Tre,  Tue, 05 Jan 2016.
Improved clutter removal for measuring wall reflectivity using the RCS technique
Marc Dirix, Amin Enayati, Joachim van Wesemael, Pawel Bajurko, November 2016
Absorber lining is an important part of an indoor antenna measurement chamber design. During the design phase different absorber types are selected for minimizing the expected reflection from given locations in the chamber. By the time of installation, these absorbers have already been measured as part of the production quality control. The question however arises if after installation, these absorbers still meet the requirements of the design.  The free-space-VSWR [1] measurement technique is a method to assess the overall reflectivity of the chamber at a certain location, i.e. quiet-zone reflectivity, but cannot be easily limited to measure the reflectivity of a single wall. In this work the RCS technique [2] is revised. The reflection of the wall is measured using a quasi-monostatic RCS setup which is mounted on a linear sliding system. The linear sliding system is positioned perpendicular to the wall. After measuring at several positions the measurement results are shifted in distance such that the reference target or wall add coherently and clutter or other walls destructively. Using this technique it will be shown that the reflectivity of an absorber-lined wall can be determined during installation where not all walls or floor have been covered yet.  [1]         J. Appel-Hansen, “Reflectivity level of radio anechoic chambers,” IEEE Trans. Antennas Propag., vol. 21, no. 4, pp. 490–498, Jul. 1973. [2]         G. Cottard and Y. Arien, “Anechoic Chamber Measurement Improvement,” Microw. J., no. March, 2006.
Nonredundant NF-FF Transformation with Bi-Polar Scanning: Experimental Testing
Francesco D'Agostino, Flaminio Ferrara, Claudio Gennarelli, Rocco Guerriero, Massimo Migliozzi, November 2016
The bi-polar scanning proposed by Rahmat-Samii et al. in [1, 2] is particularly attractive for its mechanical characteristics. The antenna under test (AUT) rotates axially, whereas the probe is at­tached to the end of an arm which rotates around an axis parallel to the AUT one. This allows the collection of the NF data on a grid of concentric rings and radial arcs. Such a scanning maintains all the advantages of the plane-polar one while providing a compact, simple and cost-effective mem. In fact, only rotational motions are required and this is convenient since rotating tables are more accurate than linear positioners. Moreover, being the arm fixed at one point and the probe attached at its end, the bending is constant and this allows one to hold the planarity. An efficient probe compensated NF–FF transformation with bi-polar scanning requiring a minimum number of NF data has been developed in [3] by applying the nonredundant sampling representations of electromagnetic (EM) fields [4, 5] to the voltage measured by the scanning probe and assuming the AUT as enclosed in an oblate ellipsoid. Thus, the plane-rectangular data needed by the classical NF–FF transformation [6] can be efficiently recovered from the nonredundant bi-polar samples by means of an optimal sampling interpolation algorithm. It is so possible to significantly reduce the number of required NF data and related measurement time without losing the efficiency of the previous approaches [1, 2]. Goal of this work is just the experimental validation of the nonredundant NF–FF transformation with bi-polar scanning [3], which will be carried out at the Antenna Characterization Lab of the University of Salerno. [1] L.I. Williams, Y. Rahmat-Samii, and R.G. Yaccarino, “The bi-polar planar near-field measurement technique, Part I: implementation and meas­urement comparisons,” IEEE Trans. Antennas Prop., vol. 42, pp. 184-195, Feb. 1994. [2] R.G. Yaccarino, Y. Rahmat-Samii, and L.I. Williams, “The bi-polar near-field measurement technique, Part II: NF to FF transformation and holographic methods,” IEEE Trans. Antennas Prop., vol. 42, pp. 196-204, Feb. 1994. [3] F. D’Agostino, C. Gennarelli, G. Riccio, and C. Savarese, “Data reduction in the NF-FF transformation with bi-polar scanning,” Microw. Optic. Technol. Lett., vol. 36, pp. 32-36, Jan. 2003. [4] O.M. Bucci, C. Gennarelli, and C. Savarese, “Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples,” IEEE Trans. Antennas Prop., vol. 46, pp. 351-359, March 1998. [5] O.M. Bucci and C. Gennarelli, “Application of nonredundant sampling representations of electromagnetic fields to NF-FF transformation techniques,” Int. Jour. Antennas Prop., vol. 2012, ID 319856, 14 pages. [6] E.B. Joy, W.M. Leach, G.P. Rodrigue, and D.T. Paris, “Applications of probe-compensated near-field measurements,” IEEE Trans. Antennas Prop., vol. AP-26, pp. 379-389, May 1978.
BIANCHA: A spherical indoor facility for bistatic electromagnetic tests
Patricia López-Rodríguez, Olga Hernán-Vega, David Poyatos-Martínez, David Escot-Bocanegra, November 2016
BIANCHA (BIstatic ANechoic CHAmber) is a singular facility located at the premises of the National Institute for Aerospace Technology (INTA), Spain, and was devised to perform a wide variety of electromagnetic tests and to research into innovative measurement techniques that may need high positioning accuracy. With this facility, both monostatic and bistatic tests can be performed, providing capability for a variety of electromagnetic measurements, such as the electromagnetic characterization of a material, the extraction of the bistatic radar cross section (RCS) of a target, near-field antenna measurements or material absorption measurements by replicating the NRL arch system. BIANCHA consists of two elevated scanning arms holding two antenna probes. While one scanning arm sweeps from one horizon to the other, the second scanning arm is mounted on the azimuth turntable. As a result, BIANCHA provides capability to perform measurements at any combination of angles, establishing a bistatic, spherical field scanner. In this regard, it is worth noting that in the last years, a renewed interest has arisen in bistatic radar. Some of the main reasons behind this renaissance are the recent advances in passive radar systems added to the advantages that bistatic radar can offer to detect stealth platforms. On the other hand, with the aim of developing new aeronautic materials with desired specifications, research on the electromagnetic properties of materials have also attracted much attention, demanding engineers and scientists to assess how these materials may affect the radar response of a target. Consequently, this paper introduces BIANCHA and demonstrates its applicability for these purposes by presenting results of different tests for different applications: a bistatic scattering analysis of scaled aircraft targets and the extraction of the electromagnetic properties of composite materials utilized in an actual aeronautical platform.
Efficient Full-Wave Algorithms for Monostatic RCS of Electrically Large Structures
Oscar Borries, Erik Jørgensen, Peter Meincke, November 2016
Finding the monostatic radar cross section (RCS) of a structure using computational electromagnetics (CEM) is a challenging task, particularly when the structure is large in terms of wavelengths. Such structures are challenging due to the large computational requirements, often combined with high accuracy demands and/or complicated geometry. Previously, these challenges have resulted in algorithms that either relax the accuracy requirements by using asymptotic methods or, if full-wave methods are used, require extreme runtimes even on very large computing clusters. For full-wave methods based on an integral equation formulation, such as Method of Moments (MoM), the reason for the large computational requirements can be found in the O(f^6) computational time scaling of monostatic RCS, where f is the frequency. Acceleration algorithms such as the Multi-Level Fast Multipole Method (MLFMM) reduce this to O(C(f,v) f^2 log f), where C(f,v) is the number of iterations required for convergence of an iterative solver, and v is the number of incident angles. Unfortunately, in most state-of-the-art implementations of monostatic RCS, C(f,v) is very large, meaning that in practice MoM is preferred to avoid an iterative solver. In this paper, we describe a range of efforts towards developing an efficient algorithm for large-scale monostatic RCS, in particular for structures that are too large to handle for MoM. These efforts include an efficient discretization based on higher-order basis functions and quadrilateral meshing of the structure, an MLFMM implementation focused on keeping memory requirements low, and a highly efficient block Krylov solver. The efficient higher-order discretization has already proven its worth for scattering problems, and the paper will demonstrate how its advantages over traditional RWG discretizations make it perfectly suited for RCS computation. In particular, combining the low amount of unknowns with a strong preconditioner allows rapid convergence of the iterative solver. The use of a low-memory MLFMM implementation, tailored for higher-order basis functions, means that problems of unprecedented size can be handled even on ordinary workstations, i.e., without resorting to expensive computing clusters. Finally, recent work on block Krylov solvers, along with interpolation algorithms for linear systems with a large amount of right-hand sides and efficient stopping criteria, allows a short computing time by significantly reducing the number of iterations.
Transfer Function Characterization for a Dual Reflector, Indoor Compact Range
Thomas Cowles, Lonny Walker, November 2016
Raytheon, El Segundo, CA chamber #2 is a dual reflector, indoor compact range that is the largest facility of its kind within the company.  A series of tests were performed to characterize the measured transfer function of the chamber because of a recent capital upgrade of the range measurement system. The purpose of this paper is to document and discuss the results of the characterization testing, review how the measured transfer function of the range was determined, and compare the current results with both past data and analytical predictions, and demonstrate how this transfer function is used for antenna and radar cross section (RCS) measurement characterization. The measured transfer function of the range is used for both antenna and RCS measurement characterization. For antenna measurements, the transfer function is used in the Friis transmission equation to determine, for example, the expected power at the receiver given the transmit power and gain of both the transmit antenna and the antenna under test. Appropriate amplification and/or attenuation can determined as part of the test planning process saving time during test setup and test execution. For RCS measurements, the transfer function was recently utilized to study the benefits and challenges of relocating our instrumentation radar from a smaller compact range to this large compact range. The motivation for the study was enhanced measurement capability for larger targets and lower frequencies. This study utilized noise equivalent RCS (NERCS) as the metric and transmit power, pulse width, and pulse integration as the study parameters to find a practical solution for optimizing NERCS.
Indoor 3D Spherical Near Field RCS Measurement Facility: 3D RADAR Images From Simulated And Measured Data
Pierre Massaloux, Pierre Minvielle, November 2016
Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern.  In order to perform more complete characterizations, a spherical experimental layout has been developed at CEA for indoor Near Field monostatic RCS assessment. This experimental layout is composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target is located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allows full 3D near field monostatic RCS characterization. This paper details a RCS measurement technique and the associated-post processing of raw data dedicated to the localization of the scatterers of a target under test. A specific 3D radar imaging method was developed and applied to the fast 3D spherical near field scans. Compared to classical radar images, the main issue is linked with the variation of polarization induced by the near-field 3D RCS facility. This method is based on a fast and efficient regularized inversion that reconstructs simultaneously HH, VV and HV 3-D scatterer maps. The approach stands on a simple but original extension of the standard multiple scatterer point model, closely related to HR polarimetric characterization. This algorithm is tested on simulated and measured data from a metallic target. Results are analyzed and compared in order to study the 3D radar imaging technique performances.
Near to Far Field Transformation of RCS Using a Compressive Sensing Method
Christer Larsson, November 2016
Near field Inverse Synthetic Aperture Radar (ISAR) Radar Cross Section (RCS) measurements are used in this study to obtain geometrically correct images of full scale objects placed on a turntable. The images of the targets are processed using a method common in the compressive sensing field, Basis Pursuit Denoise (BPDN). A near field model based on isotropic point scatterers is set up. This target model is naturally sparse and the L1-minimization method BPDN works well to solve the inverse problem.  The point scatterer solution is then used to obtain far field RCS data. The methods and the developed algorithms required for the imaging and the RCS extraction are described and evaluated in terms of performance in this paper.  A comparison to image based near to far field methods utilizing conventional back projection is also made. The main advantage of the method presented in this paper is the absence of noise and side lobes in the solution of the inverse problem. Most of the RCS measurements on full scale objects that are performed at our measurement ranges are set up at distances shorter than those given by the far field criterion. The reasons for this are, to mention some examples, constraints in terms of available equipment and considerations such as maximizing the signal to noise in the measurements. The calibrated near-field data can often be used as recorded for diagnostic measurements but in many cases the far field RCS is also required. Data processing is then needed to transform the near field data to far field RCS in those cases.   Separate features in the images containing the point scatterers can be selected using the method presented here and a processing step can be performed to obtain the far field RCS of the full target or selected parts of the target, as a function of angle and frequency. Examples of images and far field RCS extracted from measurements on full scale targets using the method described in this paper will be given.
Roughness Impact on the RCS of Simple Canonical Objects in the Terahertz Regime
Wei Gao, Xiao-Lin Mi, Yi Liao, Xiao-Bing Wang, November 2016
The higher the frequency is, the greater the influence of the precision and the realism of the CAD models on electromagnetic (EM) scattering characteristics are. In the terahertz (THz) regime, surfaces of most objects can’t be taken as smooth according to Rayleigh criterion. The interaction of EM waves and the surface presents a coherent part in the specular direction and a scattering part in the other directions. Unfortunately, the roughness of surface can’t be represented by the CAD geometry. Based on statistics theory, the rough surface height profile is fully determined by the height probability density function (pdf) and its autocorrelation functions. Without loss of generality, the height pdf of surface is assumed to be Gaussian. Under the assumption, the random Gaussian rough surface is correspondingly generated. The original CAD geometry and the random Gaussian rough surface are superposed as the input of EM computation. To demonstrate the roughness impact on RCS, EM scattering characteristics of simple canonical objects such as plate, dihedral and trihedral in the THz regime are investigated. Taking into account the statistical surface roughness, the ray-based high-frequency EM method, shooting and bouncing rays (SBR), is utilized to compute the RCS of the above objects in the THz regime. Furthermore, the inverse synthetic aperture radar (ISAR) images are also carried out via filtered back projection (FBP) method. The EM scattering characteristics of the above objects in the THz regime are analyzed. Great differences of the objects EM scattering characteristics between the smooth and rough ones are observed and discussed.
Utilization Of An Octocopter As A Two-Way Field Probe For Electro-Magnetic Field Measurements At An Outdoor Radar Cross Section Range
Andrew J. Knisely, Peter J. Collins, November 2016
RCS and Antenna measurement accuracy critically depends on the quality of the incident field.  Both compact and far field ranges can suffer from a variety of contaminating factors including phenomena such as atmospheric perturbation, clutter, multi-path, as well as Radio Frequency Interference (RFI).  Each of these can play a role in distorting the incident field from the ideal plane wave necessary for an accurate measurement.  Methods exist to mitigate or at least estimate the measurement uncertainty caused by these effects.  However, many of these methods rely on knowledge of the incident field amplitude and phase over the test region. Traditionally the incident field quality is measured directly using an electromagnetic probe antenna which is scanned through the test region.  Alternately, a scattering object such as a sphere or corner reflector is used and the scattered field measured as the object is moved through the field.  In both cases the probe/scatterer must be mounted on a structure to move and report the position in the field.  This support structure itself acts as a moving clutter source that perturbs the incident field being measured.  Researchers at the Air Force Institute of Technology (AFIT) have recently investigated a concept that aims to eliminate this clutter source entirely.  The idea is to leverage the advances in drone technology to create a free flying field probe that doesn’t require any support structure. We explore this concept in our paper, detailing the design, hardware, and software developments required to perform a concept demonstration measurement in AFIT’s RCS measurement facility.  Measured data from several characterization tests will be presented to validate the method.  The analysis will include an estimate of the applicability of the technique to a large outdoor RCS measurement facility.
Highly accurate fully-polarimetric radar cross section facility for mono- and bistatic measurements at W-band frequencies
Andreas Olk, Kais-Ben Khadhra, Thiemo Spielmann, October 2017
New requirements in the field of autonomous driving and large bandwidth telecommunication are currently driving the research in millimeter-wave technologies, which resulted in many novel applications such as automotive radar sensing, vital signs monitoring and security scanners. Experimental data on scattering phenomena is however only scarcely available in this frequency domain. In this work, a new mono- and bistatic radar cross section (RCS) measurement facility is detailed, addressing in particular angular dependent reflection and transmission characterization of special RF material, e.g. radome or absorbing material and complex functional material (frequency selective surfaces, metamaterials), RCS measurements for the system design of novel radar devices and functions or for the benchmark of novel computational electromagnetics methods. This versatile measurement system is fully polarimetric and operates at W-band frequencies (75 to 110 GHz) in an anechoic chamber. Moreover, the mechanical assembly is capable of 360° target rotation and a large variation of the bistatic angle (25° to 335°). The system uses two identical horn lens antennas with an opening angle of 3° placed at a distance of 1 m from the target. The static transceiver is fed through an orthomode transducer (OMT) combining horizontal and vertical polarized waves from standard VNA frequency extenders. A compact and lightweight receiving unit rotating around the target was built from an equal OMT and a pair of frequency down-converters connected to low noise amplifiers increasing the dynamic range. The cross-polarization isolation of the OMTs is better than 23 dB and the signal to noise ratio in the anechoic chamber is 60 dB. In this paper, the facility including the mm-wave system is deeply studied along with exemplary measurements such as the permittivity determination of a thin polyester film through Brewster angle determination. A polarimetric calibration is adapted, relying on canonical targets complemented by a novel highly cross-polarizing wire mesh fabricated in screen printing with highly conductive inks. Using a double slit experiment, the accuracy of the mechanical positioning system was determined to be better than 0.1°. The presented RCS measurements are in good agreement with analytical and numerical simulation.

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