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Papa Ousmane Leye, Adamo Banelli, Shaikha Aldhaheri, Chaouki Kasmi, Felix Vega, Islem Yahi, October 2023
The purpose of radar cross-section (RCS)
measurement is to determine the amount of scattering that
occurs when the radar signal illuminates the target. It is
generally performed to prove a design concept. RCS
measurement chamber requires a good signal-to-noise ratio
during the measurement. When the measurement is
performed in a non-controlled environment, coherent
background subtraction associated with time gating is
commonly used to improve the quality of the RCS data.
Although these techniques are usually effective, residual
clutter and background level still need to be removed to
accurately characterize the target’s RCS in highly cluttered
environments, such as semi-anechoic chambers. In this
paper, a four-step post-processing technique is presented.
In addition to the vector background subtraction and timegating
techniques implemented in our previous work, a
statistical algorithm called Principal Component Analysis
(PCA) is applied to the ISAR image of the target. It is an
extension of the PCA technique to RCS measurement. It is
shown that residual background and clutter can be reduced
by the statistical filtering method through eigenvalue
decomposition of the RCS data. The technique is presented
and evaluated through measurement of the RCS of a
dihedral corner reflector at the X-band in the semi-anechoic
chamber of the Directed Energy Research Center.
Papa Ousmane Leye, Adamo Banelli, Shaikha Aldhaheri, Chaouki Kasmi, Felix Vega, Islem Yahi, October 2023
The purpose of radar cross-section (RCS)
measurement is to determine the amount of scattering that
occurs when the radar signal illuminates the target. It is
generally performed to prove a design concept. RCS
measurement chamber requires a good signal-to-noise ratio
during the measurement. When the measurement is
performed in a non-controlled environment, coherent
background subtraction associated with time gating is
commonly used to improve the quality of the RCS data.
Although these techniques are usually effective, residual
clutter and background level still need to be removed to
accurately characterize the target’s RCS in highly cluttered
environments, such as semi-anechoic chambers. In this
paper, a four-step post-processing technique is presented.
In addition to the vector background subtraction and timegating
techniques implemented in our previous work, a
statistical algorithm called Principal Component Analysis
(PCA) is applied to the ISAR image of the target. It is an
extension of the PCA technique to RCS measurement. It is
shown that residual background and clutter can be reduced
by the statistical filtering method through eigenvalue
decomposition of the RCS data. The technique is presented
and evaluated through measurement of the RCS of a
dihedral corner reflector at the X-band in the semi-anechoic
chamber of the Directed Energy Research Center.
The use of squat cylinders as both primary and
secondary calibration targets is commonplace within the radar
cross section (RCS) measurement community. Secondary
calibrations have become a best practice activity for ranges
seeking or maintaining certification. The calibration process, often
referred to by the measurement community as a “Dual-Cal,” uses
two squat cylinders of similar but unequal dimensions that provide
range operators with a broadband calibration vector and a
measurement uncertainty metric important to range certification.
Despite their popularity, the need to ensure resonance scattering
occurs below the desired measurement band results in physically
large cylinders at UHF. In addition, the need to access the test zone
for separate cylinder measurements may add substantial time to
the calibration process and require specialized equipment,
especially for large ranges.
In response to these issues, a 22.5-degree right dihedral has been
inserted into a squat cylinder form factor, creating a primary and
secondary calibration target within one body, each separated in
azimuth by 180 degrees. This two-target calibration device
removes the need to access the target zone twice and mitigates
errors associated with separate mounting schemes. The cylinder
aspect, now truncated by the imposition of a dihedral, has 50%
extended lower frequency coverage at UHF due to oblique edge
scattering at vertical polarization. At horizontal polarization, the
dihedral interruption of the cylinder creeping wave reduces its
contribution for ka<4. The dihedral aspect provides a full
polarimetric calibration, resulting in co-equal frequency responses
for each polarization in the high frequency limit. The design
parameters of the squat cylinder-dihedral device, its computed
full-wave frequency response, and relevant scattering features are
discussed.
In a compact range when the antenna is used
for both transmitting and receiving in a monostatic
fashion, the wave packet senses everything within its
view. An extended long object usually gives rise to a
bright reflection (glint) when viewed near its surface
normal. To take advantage of this phenomenon, a
discrete Fourier transform (DFT) on RCS measurements
would yield a spectrum of incident wave
distribution along that object, provided the scattering
property is uniform along its length. Compared with
traditional field-probes which translate a sphere across
the test zone in horizontal and vertical directions, this
new method extends out from the usual quiet zone, and
is faster and less interfering to the field being probed.
Inspired by this idea, the progression to practical
innovation is discussed.
Gil Yemini, Stefano Sensani, Andrea Giacomini, Lars Foged, Marcel Boumans, Matan Kahanov, Maria Baskin, Ilan Kaplon, October 2023
A new compact range for RCS measurements has
been installed and qualified by Orbit/FR Engineering Ltd. MVG.
It has a Quiet Zone of 3m diameter, 3m length and operates from
0.7 to 50 GHz, with a feed carousel that allows for fully
automated feed change. The RF design is not intended for
antenna measurements in its current configuration, but mainly
dedicated to RCS. The operational frequency band is split into
three sub-bands: each of the lower two bands have a monostatic
operated dual polarized feed, while the higher band has a quasimonostatic
operated feed configuration with two dual polarized
feeds. Pulsed Tx/Rx modules are directly integrated into the feed
assembly. Also, the RF band switching equipment, as well as the
network analyzer, are integrated in the feed carousel, so that
there are no flexing cables or any other relative movement of RF
components when the relevant feed is moved into the focus.
Together with tight temperature control, this leads to the best
possible RF stability. Since all measurements are time gated,
there is no need for an absorber baffle wall to prevent feed direct
leakage into the quiet zone. Thus, all feeds are mounted on a
clean absorber disk without any absorber blockage and
unwanted primary pattern distortion down to a conical angle of
90deg. This allows to obtain an exceptionally good QZ
performance even at the lowest frequencies, with an outstanding
comparison with the predictions based on Physical Optics.
The paper will describe the range design fundamentals, the feed
carousel concept and the relevant RF instrumentation. The Quiet
Zone performance evaluated by field probing with a Shorted
Antenna located in the Quiet Zone will be extensively presented,
demonstrating full compliance with the specifications.
Gil Yemini, Stefano Sensani, Andrea Giacomini, Lars Foged, Marcel Boumans, Matan Kahanov, Maria Baskin, Ilan Kaplon, October 2023
A new compact range for RCS measurements has been
qualified. It has a quiet zone of 3m diameter, 3m length and
operates from 0.7 to 50 GHz. The range is oriented for RCS
measurements, whereas antenna measurements are not foreseen.
All RF equipment is integrated close to the feeds with highly
integrated pulsed Tx/Rx-modules. Therefore, classical field
probing by moving a probe antenna along a linear slide would
require significant modification of the RF system. If one measures
the RCS of a target on the linear slide, it is difficult to distinguish
the target down range reflection from the reflection of the linear
slide structure. A long stand-off between target and slide is not
practical for mechanical reasons in regard to accuracy
requirements at 50 GHz. More important, simply measuring a
reflective plate will not give any cross-polarization information. A
more advanced target is created by using an antenna with a short
circuit after an RF cable to locate the reflection of the short well
behind the scanner in down range. In addition, the antenna
receives only nominal quiet zone co-polarization, consequently,
only reflects co-polarization from the short, and the feed receives
the compact range induced cross-polarization at the feed (oneway).
The method has shown to be extremely effective. More
important, it uses the RF instrumentation and RCS measurement
methods as designed for regular operation without any
modification, thus is the most realistic system level quality
representation of the quiet zone, can be repeated at any time
without elaborate range reconfiguration requirements and can
serve as part of the commissioned RF system performance
qualification.
The paper will present the quiet zone field probe test setup, a
calculation of antenna and RF cable requirements, an analysis of
the down range profile of scanner and reflective antenna and field
probing results.
Jon Kelley, Kurt Norris, Brian Mackie-Mason, Brody Barton, David Chamulak, Scott Schaeffer, Mark Martin, Kendall Crouch, Clifton Courtney, Ali Yilmaz, October 2023
—Cylindrical hubs with fan blades are inserted
into a pipe inside a modified camera box—a recently
introduced structure intended to host differently-shaped
ducts behind an aperture. The resulting structures increase
the reproducibility of commonly used simplified jet-engine
inlet models and are designed to serve as precisely-defined
radar cross section (RCS) benchmarks with reliable
reference results. The design, manufacturing, and assembly
of the measured structures are detailed; the RCS measurement setup, data collection, and post processing are
documented; and the uncertainty in measured RCS data is
quantified with the help of simulations. Results show that the
fields scattered by the structures, while highly sensitive to
geometric and material perturbations, can be both measured
and simulated accurately even at frequencies with many
propagating modes inside the pipe.
We propose a compact bistatic radar cross section
(RCS) measurement system using a new 2D plane-wave synthesis
(PWS) employing 2D propagating plane-wave expansion and a
single-cut near-field far-field transformation (SCNFFFT). Our
system has been successfully applied to the bistatic RCS
measurements of a metasurface (100 mm width, 50 mm height,
and 0.127 mm thickness) at 60 GHz where two horn antennas are
used for the PWS (Tx) and the SCNFFFT (Rx) and placed at the
circular distances of 1.735 m and 0.35 m respectively. The peak
and pattern errors of the RCS are 0.4 dB and below -25 dB
respectively. Using the proposed 2D PWS and SCNFFFT, the
compact 2D bistatic RCS measurement system is realized without
large equipment such as CATR.
Donald Hilliard, Michael Emire, Long To, October 2023
This paper presents research results conducted at the Naval Air Warfare Center Weapons Division (NAWCWD) Radar Reflectivity Laboratory (RRL) to characterize RCS measurement quality of a compact range anechoic chamber using a small resonant sphere as a test probe measured over a 3.17-octave bandwidth, which covers the first half of the resonance region. Specifically, tests were performed on 1-inch and 12-inch diameter spheres over 2-18 GHz, which is a very prevalent test spectrum for RRL customers. The spheres were tested at the quiet zone center and the 1-inch was rotationally scanned over a 1- meter radial arc within the test zone. Spectral and spatial analysis was performed using techniques developed by Dr. Dean L. Mensa [1].
Inverse Synthetic Aperture Radar (ISAR) image gating for RCS extraction using backprojection is compared with image gating using smoothed reweighted L1-optimization in this study.
The RCS of an object is measured by placing the object placed on a turntable which is rotated in an angular range while sweeping the frequency in the desired frequency range.
A common model with isotropic point scatterers fixed in the object coordinate system is used in the ISAR imaging process. This model is used to define a forward operator. The ISAR image can be formed by operating with the backpropagation operator (i.e. backprojection), the adjoint of the forward operator, on the measured RCS. This robust method to solve the inverse problem gives an image with a resolution limited by the frequency bandwidth and the angular range. The RCS for a scattering feature is commonly determined by using the forward operator on the point scatterers in the image that are determined to belong to the scattering feature in ISAR image gating.
L1-optimization is a method that can be used to get images with higher resolution and hence better separation of the different scattering features than backprojection. L1-optimization is well suited for naturally sparse ISAR images. One method to mitigate that the scatterers are restricted to a fixed grid is to use smoothed reweighting [1]. L1-optimizations are performed consecutively in a few steps where a smoothed version of the previous solution is used to determine a weighting matrix for the next step. Smoothed reweighted L1-optimization gives images with better separation of the scattering features in the ISAR image.
Simulated and measured RCS data are used to compare image gating using backprojection with gating using smoothed reweighted L1-optimization. The main conclusion of this study is that the RCS can be extracted for scattering features, not resolved in backprojection images, using the smoothed reweighted L1-optimization.
[1] D. Pinchera and M. D. Migliore, “Accurate reconstruction of the radiation of sparse sources from a small set of near-field measurements by means of a smooth-weighted norm for cluster-sparsity problems,” Electronics, vol. 10, no. 22, p. 2854, 2021.
Papa Ousmane Leye, David Martinez, Shaikha Aldhaheri, Chaouki Kasmi, Nicolas Mora, October 2022
The RCS of a target can be estimated using electromagnetic modeling if accurate geometries and material descriptions are available. An exact numerical calculation often requires prohibitive processing times. Moreover, numerical predictions with approximate techniques are difficult as it is challenging to consider all the physical phenomena. Therefore, a suitable RCS measurement facility adapted to the target size and specifications is required to estimate the RCS of a given target and to validate the numerical predictions. In general, the measurement of RCS takes place in anechoic chambers that simulate free-space and far-field conditions and where the unwanted reflections (walls, target mount, objects in the range, and the target interactions) are reduced.
This paper presents a broadband measurement and validation of the RCS of a metallic trihedral corner reflector of 30 cm sides when fully anechoic conditions are not available, and consequently, some undesirable echoes are present in the measurements. Firstly, the measurement facility calibration and the target calibration are outlined. A single target reference approach is performed using a sphere as a reference, and its scattering response is shortly described. Then, the measurement of the target is performed. After these steps, a processing procedure is applied to isolate the target response from the background and the close responses due to unwanted reflections. The post-processing technique and the acquisition system are presented and discussed. The measurements are performed at X band as a function of the viewing angle for vertical transmit and receive polarization.
To validate the technique, the RCS of the trihedral corner reflector is numerically simulated using the Integral Solver (I-Solver) of CST, with a Gaussian excitation, for vertical transmit and receive polarization. Measurements are compared with results obtained from CST software and show a good agreement with the numerical simulations. This setup will be used for RCS measurement of different complex targets and compared with measurements from other facilities to analyze and evaluate the RCS measurement uncertainty.
Shoaib Anwar, Evgueni Kaverine, Fabien Henry, Nicolas Gross, Francesco Scattone, Darko Sekuljica, Andrea Giacomini, Francesco Saccardi, Alessandro Scannavini, Per Iversen, Lars Foged, October 2022
Plane wave generator (PWG) for Over The Air (OTA) characterization of beamforming millimeter wave devices, provides an attractive solution comparing to conventional measurement techniques (Compact Antenna test Ranges (CATR) and Far-field chambers). MVG’s Plane wave generator for 5G NR FR2 applications ([1]-[4]) is an innovative tool which permits the user to measure the radiating elements with low to medium directivity radiation characteristics with excellent precision. Conventional CATR systems are not suited for stationary DUT (with / without person) measurement scenario.
In this paper, experimental results are presented for a dual-polarized PWG system, covering the 3GPP bands n257, n258 and n261 (24.25-29.5 GHz). System measurement results show good comparison with simulations and measurements of the PWG alone.
Another advantage of PWG presented here, is that we can modify the size of the QZ. Results from a pre-production unit for a 15cm QZ shows amplitude variation of less than ±1 dB and achieve more precision for smaller DUT.
Measurement results from the pre-production unit with a quiet zone of up to 38cm sphere diameter, show amplitude variations of less than ±2dB. This variation is compatible with the DUT + phantom or human measurement application.
Pattern results for Antenna Under Test (AUT) with low to medium directivity (6dBi up to 17dBi) compare well with simulations and measurements from other systems. For a given AUT, the impact of different positioning mast is also evaluated. Excellent stability of patterns, when the AUT is placed at different positions inside the QZ, is observed.
These results confirm that the dual-polarized PWG system presents an attractive solution for FR2 characterization of low to medium directivity radiating elements.
Joseph Friedel, David Oyediran, David Rohde, October 2022
The mission of the Naval Surface Warfare Center, Indian Head, Maryland, EOD Department, is to utilize the latest available technology in the advancement of Explosive Ordnance Disposal (EOD) equipment and techniques. This mission includes the test and evaluation of current and developmental systems, which will be discussed in this paper. EOD exploits multiple physical phenomena in its task of ordnance detection, including chemical and electromagnetic. Electromagnetics include RF fields, light (including laser, infrared and ultraviolet), and nuclear radiation. For each phenomena, there may be several different technologies used to provide multi-mode detection capability. This study focuses on the electromagnetic subset of detection RADAR, and specifically Ground Penetrating Radar (GPR), which is distinguished by its earth surface domain and generally downward field of view. The paper will give a very brief overview of GPR theory and equipment, its use in EOD, and then will focus on the RF test and measurement of electromagnetic fields generated by GPR systems and antennas. An RF antenna/system test plan will be detailed, along with the design and development of antenna gain and radiation pattern measurement techniques. The measured data from GPR technology will be graphically displayed, analyzed and compared in terms of the potential for GPR effectiveness.
In this paper, a 77 GHz microstrip comb-line antenna array for an automotive RADAR application with a low sidelobe level is proposed. The microstrip technology is used for the antenna due to its low fabrication cost, small size, and easy integration with other microwave circuitry. At very high frequencies such as millimeter waves, the gain of a single element patch antenna is not enough to withstand the RADAR application requirements, hence an array of antennae is beneficial. A The Phased array antenna configuration is needed to have a high gain and low sidelobe level of -20 dB and a beam steering mechanism.
The design procedure used here is the implementation of a single comb antenna, that is further realized into a 1 x 10 uniform linear array of a comb line array. It has a gain of 14.81 dB and a sidelobe level of -15 dB. The radiation in the comb antenna is primarily due to the open sides with the lengths of the comb serving as transmission lines. The adjacent combs are placed at the distance of λ in order to co-phase the antenna elements at the desired frequency. Additionally, with an aim of reducing the sidelobe level, Taylor amplitude distribution is used, and the tapered array is designed. This methodology helped to achieve a sidelobe level of -20 dB. The gain of an overall array is increased to 20 dB by realizing the array of 4 x 10. Another requirement of the Automotive Radar is beam steering to accurately detect the target. Butler matrix is a beamforming network chosen to feed the phased array antenna.
The proposed antenna array is simulated in Ansys HFSS with Rogers RO 3003 substrate of the thickness of 1.27 mm and has an overall dimension of 9 x 14.96 mm2. The goal of the design of this antenna is to acquire an appropriate radiation pattern with a low side lobe level better than -20dB and achieve beam steering using the Butler matrix to have a phased array configuration.
Index Terms— RADAR, Antenna array, Comb line array, Butler Matrix, Phased array.
Christopher Howard, Kenneth Allen, Bill Hunt, October 2022
The precise characterization of the complex permittivity, particularly loss tangent, in low-loss dielectric samples at microwave frequencies usually employs resonant cavity methods, where the quality factor of some resonance is determined by a precisely dimensioned sample of the material placed inside the cavity. In order to characterize materials over a broad set of frequencies, a separate measurement fixture and sample is required for each frequency, a tedious and expensive endeavor. In response, one may turn to a single broadband measurement system, such as the focused beam system, but simple transmission and reflection measurements suffer from poor loss tangent sensitivity. In this work, a hybrid approach is investigated whereby a highly resonant periodic metallic array adjoined to a dielectric sample is measured in a broadband focused beam system. A frequency selective surface (FSS) is designed to be placed against a planar dielectric sample to create a transmission or reflection response that is sensitive to the loss tangent of the material under test. This sandwiched structure is illuminated by the focused beam system to approximate plane-wave-like incidence, and scattering parameters measured. It is shown that the magnitude of response at the resonant frequency is linearly dependent on the loss tangent of the material under test for a certain range of loss tangents, and sources of error that limit sensitivity at lower loss tangents are explored. The effect of various FSS design parameters on loss tangent sensitivity is investigated, including sample thickness, FSS substrate thickness and complex permittivity, and FSS element pattern. Techniques for extracting complex permittivity from the scattering parameters of the focused beam measurement are presented, along with measured permittivity data from the FSS against a variety of well-known materials.
Marlow Coronado Rumreich, Sean Raffetto, October 2022
The Boeing 9-77 Compact Radar Range has utilized low-frequency solutions since the 1990s. However, compact radar ranges have innate challenges when it comes to low-frequency measurements, typically due to facility size limitations. Due to increasing demand for more reliable data across a broad set of frequencies, an upgrade to the existing Ultra High Frequency (UHF) antenna feeds was designed and implemented in July 2020. This antenna was developed with field quality improvements, reliability, repeatability, and maintainability in mind. Unlike the previous design, this antenna was designed as an array with weighted feeds to complement the characteristics of the pre-existing range Gregorian reflector system. This new UHF antenna array leveraged the Weighted Element Method (WEM) along with extensive electromagnetic modeling and trade studies to achieve an efficient design at a minimum size. As a result of these design choices, the new antenna has doubled the efficiency in the band of interest. In addition, the frequency bandwidth of the antenna was improved while also reducing calibration and background drift. Lastly, this array has significantly improved the field quality of the quiet zone compared to the previous antenna system and improved the signal-to-noise ratio. This paper describes the UHF Antenna Array design process and the compact range measurements results to demonstrate the benefit of the WEM for feed arrays in a compact range. Additionally, the authors present an evaluation of methods used to create a digital twin of the UHF Antenna Array and a summary of best practices for future development of weighted antenna arrays for compact radar ranges.
Andreas Schwind, Willi Hofmann, Ralf Stephan, and Matthias A. Hein, October 2021
One benefit of cooperative automated and connected driving lies in the fusion of multiple mobile wireless sensor and data transmission nodes, covering complementary technologies like radar, cellular and ad-hoc communications, and alike. Current developments indicate enormous potential to increase the environmental awareness through joint communication and radar sensing. In this respect, future channel models require knowledge of bi-static reflectivities of road users over a range of illumination and observation angles, both in the nearfield and in the far-field. To establish reference data and model such angle-dependent RCS variations, this paper deals with realistic pedal and wheel rotations of a bicycle based on electromagnetic simulations. In the simulation setup, idealized far-field conditions with plane-wave illumination and observation were assumed, while the angles covered the entire azimuth with 201 variations of the pedal and wheel positions. The fluctuation of the RCS is analyzed and discussed in terms of its probability density and cumulative distribution functions. Depending on the angular constellation, the range of the fluctuation varied between 1 dB and 14 dB, while the specular reflection and forward-scattering showed almost no fluctuation.
The radiation and scattering pattern characteristics of open-ended rectangular waveguide with a chamfered tip are examined. Despite common and widespread use as a probe antenna for planar near-field antenna measurements, a methodical investigation of the chamfered-tip design and resultant performance has not been published. A computational electromagnetics (CEM) model for an open-ended rectangular waveguide probe with a parameterized chamfered tip has been constructed and results for both radiation and scattering patterns are presented. A comparison of results includes a probe without a chamfer and a probe typical of that available from commercial suppliers. It is shown that, for a series of standard waveguide size probes sharing a common thickness for the waveguide wall and chamfered tip, the radiation pattern is relatively insensitive to the chamfer tip designs studied until frequency increases into W-band (WR-10). The scattering pattern characteristics for the same series of standard waveguide size probes show a reduction in on-axis (boresight) monostatic radar cross section (RCS) for chamfered tip waveguides compared to blunt-ended waveguides and that this reduction increases for increasing frequency.
Oscar Borries, Martin Haulund Gæde, Andreas Ericsson, Peter Meincke and Erik Jørgensen, Dennis Schobert and Erio Gandini, October 2021
We present a fast source reconstruction method suitable for antenna diagnostic applications of radiating structures on electrically large platforms. The method is based on a novel implementation of a recent reformulation of the inverse electromagnetic scattering problem, and is solved using a Higher Order Method of Moments (MoM) discretization. The novel implementation achieves asymptotically better scaling the previously possible, and in particular the memory use is substantially lower than was previously possible. Results from two example cases are presented where the new method is compared to the current commercial state-of-the-art solver in DIATOOL 1.1, and significant improvements are observed in terms of computation times and memory requirements.
Louis E. Sheffield and R. Jerry Jost, October 2021
Instrumentation radar metrology waveform techniques that simultaneously transmit two orthogonal sequences of orthogonal electromagnetic polarizations are explored for applicability toward both static and dynamic RCS signature and ultra-wideband imaging measurements using simultaneous H-pol and V-pol (SHV) waveforms. Static, pulsed measurements with independent transmit polarizations are modulated and radiated; reflections from a depolarizing target are measured where the return signals are coherently combined. Each transmit polarization is independently modulated using a diverse phase sequence, which leaves a unique “fingerprint” by which the orthogonal polarization separation is achieved. Using only the coherent combination and associated transmit and receive RF channel characterizations, the original measurements are reconstructed. Simulations serve as a baseline for measured results, from generating pure SHV waveforms and then providing simultaneous full scattering matrix (FSM) measurements, in order to achieve greater purity of FSM signatures, while reducing measurement times by a factor of two.
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