AMTA Paper Archive
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Interrogation Signal Optimization for Improved Classifier Performance when using “RF-DNA” for Non-Destructive Antenna Acceptance Testing
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
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.
A New Method for Millimeter-Wave Characterization of Thin Resistive Fabrics
As millimeter-wave applications become more widely available technologies, there is a demand to know material properties for design and application purposes. However, many mass produced materials are either not specified at these frequencies or the price materials can be costly. Therefore the easiest method for characterization is by measurement. Traditional methods of this measurement type involve the reflectivity of a fabric sample placed on a flat metallic reference plate. However, this method has some major difficulties at these high frequencies. For example, the surface of the reference plate must be very flat and smooth and must be carefully oriented such that their surface is precisely facing the transmitting and receive and antennas. Furthermore the electrically large size of the reference plate of this setup makes it difficult to measure in far-field and anechoic range time is expensive. Resistive and conductive fabrics have applications such as shielding, anti-static, and radio wave absorption. Radio wave absorption and radar cross section engineering is currently of high interest to the automotive industry for testing newly emerging automotive radar systems. Such fabric measurement has already been utilized to accurately characterize artificial skin for radar mannequins to recreate the backscattering of human targets at 77 GHz. This paper presents a new and convenient method for measuring the reflective properties of conductive and resistive materials at millimeter wave frequencies by wrapping fabrics around a metallic reference cylinder. This new approach to fabric characterization method is able to obtain higher accuracy and repeatability despite the difficulties of measuring at high frequency.
CAMELIA Quiet Zone Assessment using PEC Sphere RCS Measurements
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.
Narrowband 5 GHz Mobile Channel Characterization
Recently, the 5 GHz bands have become increasingly attractive for the wireless industry due to the large amount of unlicensed spectrum available and less congested than the 2.4 GHz band. For this reason, WiFi standards 802.11a/h/j/n/ac operate in the 5 GHz band and its use is also being proposed for LTE-U (LTE-Unlicensed), which will use the unlicensed spectrum in this band for increasing the capacity of LTE (Long-Term Evolution) networks. However, this band is allocated on a primary basis to aeronautical and satellite services, meteorological and military radars and other federal and non-federal uses. In this context, it is necessary to perform additional propagation and interference studies in order to characterize the 5 GHz mobile radio channel considering its current uses and future applications. This work contains measurements and characterization of 5 GHz mobile radio channel along indoor and indoor to outdoor paths using a novel narrowband propagation measurement system. This system, developed at the Institute for Telecommunications Sciences (NTIA/ITS), consists of a vector signal analyzer and highly stable oscillators. The stability, precision and flexibility of the system permit post-processing the baseband complex received signals in order to analyze the channel characteristics. We have performed narrowband measurements on the 5.41 GHz mobile radio channel in four different scenarios: indoor with line of sight, indoor through walls, indoor through different floors and indoor to outdoor. The data collected has been processed for estimating the path loss (including the attenuation factors and path loss exponents), fading statistics and Doppler power spectrum for each scenario. We also show that the processing permits the extraction of accurate information about the scattering environment. The results permit characterizing the 5 GHz mobile radio channel for future spectrum sharing designs, as well as for network planning and capacity planning for WiFi and cellular applications in this band.
Development of a FMCW Radar Sensor For Soil Humidity Estimation
To determine the proper moisture content in the soil is critical to get maximum grow in plants and crops and its estimation it is used to regulate the amount of irrigation that it is needed. For this reason, many sensors that measure water content have been developed to give the grower some feedback of the water content. Some methods such as the ones based in gravimetric properties are accurate but labor consuming, other such as the tension meters require periodic service, the neutron probe is also accurate but expensive. The more popular sensor is based in electrical resistance measurement that gives acceptable accuracy and it is not expensive. However, this sensor has the disadvantage that needs to be buried in the soil. Here, we are exploring the characteristics of electromagnetic propagation and its scattering properties as a tool to identify the physical soil composition. The presence of water changes drastically the dielectric properties of the soil affecting the reflected signal. In this research, we are assessing the viability of a sensor based in FMCW radar technology for water detection with the advantage of being portable and low cost. The research involves the fabrication of a directive antenna operating in a broadband regimen, transmitter/ receiver circuit and the signal processing of the return signal adjusted to the detection of moisture in soil. We present the calibration methods and graphic results of the intensity of the reflected signal of dry bare soil, wet soil, and soil covered by plants.
Monostatic RCS Calibration of Radar Target Using Extrapolation Method in Millimeter-wave Frequency Band
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
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.
Review of Cross-Eye Jamming
This paper gives a review of cross-eye (CE) jamming using the retro-directive channel implementation. CE jamming is an electronic warfare self-protection technique in which the phase-front of an electromagnetic wave, transmitted towards a threat radar, is distorted in a way similar to radar glint. A retro-directive channel is used in the implementing of the CE jammer to avoid prohibitive tolerance requirements on the electronic warfare (EW) system. In a practical implementation of the CE jammer in an EW system, active electronically scanned array antennas (AESA) can be used to fulfil effective radiated power requirements. The achievable reciprocity i.e. similarity between the transmission and reception radiation patterns in the AESA is central to the performance of the CE jammer system. Effects of the CE jammer on mono-pulse radar are presented and described. The effects include the mixing of a CE jammer signal and a target echo. The CE jammer can induce false target angles and prevent the radar from finding a stable settling angle. The origin of CE jamming is in the field of radar multipath phenomenon such as glint and reflections from water surfaces. The CE jamming technique has previously been described and analyzed in various literature. This paper summarizes the most recently published results and gives references to the publications.
Scattering Effects of Traveling Wave Currents on Linear Features
Backscattering responses of range-extended objects include those attributed to traveling-wave effects, typically caused by the termination of the object. Diagnosing the nature of scattering mechanisms contributing to the composite response is essential to modifying the object's radar signature. ISAR images reveal essential information on the location and nature of scattering features by decomposing the frequency/angle data into basis functions corresponding to independent point scatterers. When applied to responses of objects exhibiting other basis functions, such as those for traveling-wave scattering, ISAR images reveal unexpected results that can obscure proper interpretation of the scattering mechanisms. Because traveling-wave lobes are restricted to limited ranges of grazing angles and are frequency dependent, however, localizing their effects from high-resolution images can be elusive. Specifically, the traveling-wave responses are not readily distinguished from direct or diffracted responses. The paper deals with backscattering data collected on a slender cylindrical rod of 183cm length and 0.635cm diameter for aspect angles 0-180 degrees over a frequency range of 2-10GHz with polarization parallel to the plane of incidence, intended to emphasize effects of traveling waves at the rod's grazing angles. In spite of its relatively simiple geometry, the linear rod object presents complicated responses owing to the combined effects of traveling waves and multiple diffractions. Although ISAR images properly locate point scatterers, an understanding of the imaging process provides clues on the expected location of image elements corresponding to more complicated scattering features. The angle and frequency dependence of each scattering mechanism is illustrated in the paper by frequency responses, range responses, and ISAR images. The total scattering resonse of the rod for grazing incidence is characterized by at least 5 distinct scattering mechanisms, each interacting with the others as a function of viewing angle. Because images of traveling- and diffracted-wave components overlap for some grazing angles, their relative responses preclude separation. The results provide an example of the complex nature of scattering from a simple shape subject to traveling-wave effects.
A Radar Echo Emulator for the Evaluation of Automotive Radar Sensors
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
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 . Aside from the well-defined conditions for an indoor range, metal spheres may be dropped from the air free fall while being measured . A news article on January 5, 2016, reported that three metal spheres were picked up in three provinces in northern Vietnam . 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 . E. F. Knott, "Radar Cross Section Measurements," (Van Nostrand Reinhold, New York, 1993), pp. 176-180, (on spheres and the Mie series). . 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). . 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). . “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). . “3 mysterious spheres fall onto 3 Vietnam provinces,” Tuoi Tre, Tue, 05 Jan 2016. http://www.sott.net/article/309800-3-mysterious-spheres-fall-onto-3-Vietnam-provinces
BIANCHA: A spherical indoor facility for bistatic electromagnetic tests
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
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
The use of additive manufacturing (AM) techniques to manufacture microwave and mm-wave passive components has recently been demonstrated through various examples . 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  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 . 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.  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  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
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
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
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
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
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.
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