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An Experimental and Computational Investigation of High-Accuracy Calibration Techniques for Gain Reference Antennas
Olav Breinbjerg, Kyriakos Kaslis, Jeppe Nielsen, October 2017

Gain is a principal property of antennas; it is essential in establishing the link budget for communication and sensing systems through its presence in Friis’ transmission formula and the radar range equation. The experimental determination of antenna gain is most often based on a gain-transfer technique involving a reference antenna for which the gain has been calibrated to high accuracy; this is typically a pyramidal horn antenna [1]. The required accuracy of antenna gain obviously depend on the application; in some cases it can very high, ±0.1 dB or less, and this implies an even higher accuracy, of the order of ±0.01dB, for the gain reference antenna. This work investigates the accuracy to which a gain reference antenna can be calibrated; the investigation is based on experimental spherical near-field antenna measurements [2] and computational integral equation / method of moments simulations [3]. While calibration of gain reference antennas has been studied in many previous works, even works from early 1950s [4]-[6], this work is novel in systematically supporting measurements with full-wave simulations. Such simulations facilitate the study of e.g. the effect of multiple reflections between antennas at short distances. We study two absolute calibration techniques for the gain of pyramidal horn antennas. The first technique determines gain as the product of directivity and radiation efficiency; this technique has been referred to as the pattern integration technique [7] (which is not an entirely adequate designation since gain cannot be determined from the radiation pattern). The second technique determines the gain from Friis’ transmission formula [8] for two identical antennas; this technique is generally referred to as the two-antenna technique [1]. These two calibration techniques involve very different steps and contain very different sources of error; for both techniques our investigation involves measurements as well as simulations. For the pattern integration technique we compare experimental and computational results for the directivity and demonstrate agreement within one-hundredth of a dB. The radiation efficiency is calculated by different techniques based on the surface impedance boundary condition for the metallic walls of the pyramidal horn. This technique is not influenced by proximity effects or by impedance mismatch between the measurement system and the gain reference antenna. For the two-antenna techniques we compare experimental and computational results for the gain and we compare the calculated distance-dependence with that of the extrapolation technique [9]. It is demonstrated how the use of the phase center distance in Friis’ transmission formula notably decreases the necessary separation between the antennas for a required accuracy, but that multiple reflections may then become a limiting factor. This technique is highly influenced by the impedance mismatch that must be accurately accounted for. We compare the gain values resulting from the pattern integration technique and the two-antenna technique, including their very different uncertainty estimates, for a C-band standard gain horn. The work is related to an on-going ESA project at the DTU-ESA Spherical Near-Field Antenna Test Facility for the on-ground calibration of the scatterometer antennas of the EUMETSAT MetOp Second Generation B-series satellites. IEEE Standard – Test Procedures for Antennas, Std 149-1979, IEEE & John Wiley & Sons, 1979. J.E. Hansen, “Spherical Near-Field Antenna Measurements”, Peter Perigrinus Ltd., London 1987. W.C. Jakes, “Gain of Electromagnetic Horns”, Proceedings of the IRE, pp. 160-162, February 1951. E.H. Braun, “Gain of Electromagnetic Horns”, Proceedings of the IRE, pp. 109-115, January 1953. W.T. Slayton, “Design and Calibration of Microwave Antenna Gain Standards”, Naval Research Laboratory, Washington D.C., November 1954. A. Ludwig, J. Hardy, and R. Norman, “Gain Calibration of a Horn Antenna Using Pattern Integration”, Technical Report 32-1572, Jet Propulsion Laboratory, California Institute of Technology, October 1972. H.T. Friis, “A Note on a Simple Transmission Formula”, Proceedings of the I.R.E. and Waves and Electrons, pp. 254-256, May 1946. A.C. Newell, R.C. Baird, P.F. Wacker, “Accurate Measurement of Antenna Gain and Polarization at Reduced Distances by an Extrapolation Technique”, IEEE Transactions on Antenna and Propagation, vol. 21, no. 4, pp. 418-431, July 1973.

The 7 Common Habits of Highly Effective RF Target Simulators
David Wayne, October 2017

The evaluation of RF Sensors often requires a test capability where various RF targets are presented to the Unit Under Test (UUT). These targets may need to be dynamic in time, represent multiple targets and/or decoys, emulate dynamic motion, and simulate real world RF environmental conditions. An RF Target Simulator can be employed to perform these functions and is the focus of this paper. The total test system is usually called Hardware in the Loop (HITL) involving the sensor mounted on a Flight Motion Simulator (FMS), the RF Target Simulator presenting the RF Scene, and a Simulation Computer that dynamically controls everything in real time. The realization of a highly effective target simulator, one that truly meets the user’s needs at an affordable cost, is the result of understanding the complex interrelationship of requirements, architecture and constraints. In this presentation, those relationships are examined in seven areas of discussion, employing examples of realized systems; Determining the necessary test zone volume Determining the necessary quality of RF target signal Sizing the field of view, range and facilities Creating each target’s RF signal Creating RF target motion Integration and real-time operation within the range Locating and minimizing the effects of error sources

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.

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.

Dual-polarized Monolithic Leaky Wave Antenna Enabled by Additive Manufacturing
Esteban Menargues, Maria Garcia-Vigueras, Emile de Rijk, Juan R. Mosig, November 2016

The use of additive manufacturing (AM) techniques to manufacture microwave and mm-wave passive components has recently been demonstrated through various examples [1]. The term AM comprises all techniques based on the successive building of thin layers of material one on top of each other to create a device. When properly implemented, AM offers the possibility to manufacture light-weight and highly complex devices without generating significant costs increase. Among all AM techniques, Stereo-Lithography (SLA) is the most interesting one for the production of mm-wave components. In SLA, the materials are non-metallic epoxy-based polymers, that require a metallic coating to allow them to become RF functional. In contrast to other AM techniques, SLA manufacturing tolerances and surface roughness permit the design of devices up to 300 GHz. SWISSto12 has recently reported the successful performance of metal plated SLA devices, based on a proprietary chemical plating technology enables the processing of monolithic devices. In this contribution, we aim at exploiting the previously described SWISSto12’s AM-SLA technique [1] to obtain a monolithic directional dual-polarized high-directive Leaky-Wave Antenna (LWA) operating at mm-wave frequencies. The LWA consists of a square cross section waveguide perforated with crossed slots in its top aperture [2]. Moreover, the antenna already includes a side-arm orthomode transducer (OMT) and a smooth waveguide  twist, specifically co-designed with the LWA. The squared waveguide supports the propagation of the two first orthogonal modes, which are radiated through the cross-shaped slots. Thus, the vertically (horizontally) polarized mode inside the waveguide produces theta-polarized (phi-polarized) radiation. The pointing angle is approximately 50°, the same for both beams. The simulated cross-polarization values are very low according to the simulations. Moreover, the directivity of each orthogonal beam is controlled by the dimensions of the cross-shaped slot. Weather observation radars are considered as a privileged potential application of this kind of systems. Two different prototypes of this LWA+OMT subsystem (one operating at 30 GHz and the other one at 60 GHz, both achieving gains above 15 dB) are currently being manufactured by SWISSto12. The prototypes and their performance will be included in the final paper. [1] de Rijk, E.; Silva, J.S.; Capdevila, S.; Favre, M.; Billod, M.; Macor, A.; von Bieren, A.; "Additive Manufactured RF components based of Stereo-Lithography", in Antenna and RF Systems for Space Science 36th ESA Antenna Workshop, 6-9 Oct 2015 [2] M. Garcia-Vigueras, M. Esquius-Morote and J.R.Mosig, "Dual-polarized one-dimensional leaky wave antenna," 9th European Conference on  Antennas and Propagation (EuCAP), Lisbon, Portugal, 13-17 April 2015, pp.1-2.

Compact First-Order Probe for Spherical Near-Field Antenna Measurements at P-band
Oleksiy Kim, November 2016

A number of European Space Agency's (ESA) initiatives planned for the current decade require metrology level accuracy antenna measurements at frequencies extending from L-band to as low as 400 MHz. The BIOMASS radar, the Galileo navigation and search and rescue services could be mentioned among others. To address the needs, the Technical University of Denmark (DTU), who operates ESA’s external reference laboratory “DTU-ESA Spherical Near-Field (SNF) Antenna Test Facility”, in years 2009-2011 developed a 0.4-1.2 GHz wide-band higher-order probe. Even though the probe was manufactured of light-weight materials -- aluminium and carbon-fibre-reinforced polymer (CFRP) -- it still weighs 22.5 kg and cannot be handled by a single person without proper lifting tools. Besides that, a higher-order probe correction technique necessary to process the measurement data obtained with such a probe is by far more demanding in terms of the computational complexity as well as in terms of calibration and post- processing time than the first-order probe correction. On the other hand, conventional first-order probes for SNF antenna measurements utilizing open-ended cylindrical waveguides or conical horns fed by cylindrical waveguides operating in the fundamental TE11-mode regime also become excessively bulky and heavy as frequency decreases, and already at 1 GHz an open-ended cylindrical waveguide probe is challengingly large. For example, the largest first-order probe at the DTU-ESA SNF Antenna Test Facility operates in the frequency band 1.4–1.65 GHz and weighs 12 kg. At 400 MHz, a classical first-order probe can easily exceed 1 cubic meter in size and reach 25-30 kg in weight. In this contribution, a compact P-band dual-polarized first-order probe is presented. The probe is based on a concept of a superdirective linear array of electrically small resonant magnetic dipole radiators. The height of the probe is just 365 mm over a 720-mm circular ground plane and it weighs less than 5 kg. The probe covers the bandwidth 421-444 MHz with more than 9 dBi directivity and |µ| ? 1 modes suppressed below -35 dB. The probe design, fabrication, and test results will be discussed.

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.

Changes In The DO-213 Standard For Commercial Nose-Radome Testing
Scott McBride, Steven Nichols, Mike Murphy, Vince Rodriguez, George Cawthon, November 2016

“RTCA DO-213 Minimal Operation Performance Standards For Nose-Mounted Radomes” is a document frequently referenced in nose-radome testing requirements for commercial aircraft.  This document was produced and is maintained by the Radio Technical Commission for Aeronautics (RTCA).  The specifications of weather-radar systems have recently changed within RTCA’s DO-220A, and as a result DO-213 was updated to DO-213A in March, 2016, to ensure that radome requirements are consistent with those of the weather radar.  In addition to the new requirements for radome evaluation, several existing requirements were clarified.  These clarifications addressed such things as suitability of near-field measurements, proper procedures and processing, and appropriate measurement geometries. RTCA coordinated the document revision, with the bulk of the technical inputs coming from a broad-based working group.  This working group had representatives from radar, aircraft, and radome manufacturers, government agencies, and providers and users of radome-testing systems.  When requirements were added or when common practice conflicted with existing requirements, there was considerable effort and analysis employed to ensure that each change or clarification was truly required.  Nevertheless, DO-213A has some significant impacts to many existing radome-testing facilities.  This paper discusses the significant changes in DO-213A and their implications for radome test facilities, concentrating on after-repair radome electrical testing.

Multiple Target, Dynamic RF Scene Generator
David Wayne, John McKenna, Scott McBride, November 2016

The evaluation of RF Sensors often requires a test capability where various RF scenes are presented to the Unit Under Test (UUT). These scenes may need to be dynamic, represent multiple targets and/or decoys, emulate dynamic motion, and simulate real world RF environmental conditions. An RF Scene Generator can be employed to perform these functions and is the focus of this paper. The total test system is usually called Hardware in the Loop (HITL) involving the sensor mounted on a Flight Motion Simulator (FMS), the RF Scene Generator presenting the RF Scene, and a Simulation Computer that dynamically controls everything in real time. This paper describes the system concept for an RF Scene Generator that simultaneously represents 4 targets, in highly dynamic motion, with no occlusion, over a wide range of power, frequency, and Field of View (FOV). It presents the test results from a prototype that was built and tested over a limited FOV, while being scalable to the total FOV and full system capability. The RF Scene Generator employs a wall populated with an array of emitters that enables virtually unlimited velocity and acceleration of targets and employs beam steering to provide high angular resolution and accuracy of the presented target positions across the FOV.   Key words: RF Target Simulator, RF Scene Generator, Multiple Targets, Beam Steering Wall of Emitters, Steering Array Calibration, Plane-Wave Generator, Radar Environment Simulator.

Automotive Antenna Evaluation
Garth D'Abreu, November 2016

The automotive industry is changing rapidly through the evolution of on board and embedded components and systems. Many of these systems rely on the over the air performance of a communication link and the evaluation of these links is a key requirement in understanding both the real world performance, and associated operating limits of a particular system. The operating frequency range of the installed communication systems now extends from the traditional AM bands from 540KHz to almost 6GHz for WiFi. There are several different antenna design options available to cover this range and in many cases, the performance of an antenna when installed on a vehicle differs from a measurement of the same antenna in isolation. There is also a growing use of high frequency RADAR systems operating at frequencies approaching 80GHz that also need to be included in the performance analysis. The behavior of the individual components using conducted methods for example, is an important step but the direct measurement of antenna pattern and data throughput under ideal steady state and also varying spatial and operating conditions is likely to be the most robust method of channel evaluation. There is a steady march toward vehicle autonomy that is pushing the development of increasingly complex and sophisticated sensors, receivers, transmitters and firmware, all installed on an already well populated platform. The interoperability and EMC performance of these embedded systems is an extension to the need for a fundamental understanding of performance. This paper will present some of the available measurement and evaluation options that could be used as part of an integrated test environment which takes advantages of a number of established techniques.

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.

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.

Interrogation Signal Optimization for Improved Classifier Performance when using “RF-DNA” for Non-Destructive Antenna Acceptance Testing
Mathew Lukacs,Peter Collins, Michael Temple, November 2015

The cost of quality is critical to all industrial processes including microwave device production. Microwave device production is often labor intensive and subject to many production defects. Early detection of these defects can markedly improve production quality and reduce cost. A novel approach to industrial defect detection has been demonstrated using a random noise radar (RNR), coupled with Radio Frequency Distinctive Native Attributes (RF-DNA) fingerprinting processing algorithms to non-destructively interrogate microwave devices. The RNR is uniquely suitable since it uses an Ultra Wideband (UWB) noise waveform as an active interrogation method that will not cause damage to sensitive microwave components and multiple RNRs can operate simultaneously in close proximity, allowing for significant parallelization of defect detection systems. The ability to classify defective microwave antennas and phased array elements (prior to RF system assembly) has been successfully demonstrated and presented at the 36th AMTA symposium. This paper expands on the prior research by focusing on the effects of altering interrogation signal characteristics to include operational bandwidth and signal frequency while actively interrogating similar antennas using an UWB noise signal. The focus of the experimental variation was to optimize classifier performance since unique device characteristics will be excited by various interrogation signal traits that can be exploited by the fingerprint generation and classifier algorithms. Experimentation with several typical UWB antennas and a phased array antenna is demonstrated. The effects of signal bandwidth on classifier performance on simulated fault conditions was performed using various antenna terminations and attenuators. Interrogation of the phased array was demonstrated using the array “backend” for signal down-conversion enabling a quick quality check control method with a simple back-end connection. The ability to wirelessly discriminate multiple fault conditions on individual phased array elements and discern phased array operational range of motion, both in pristine and heavy RFI environments is also shown. This method ensures that each produced phased array meets quality and operational requirements.

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.
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