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Reverberation chambers (RCs) are important measurement tools, and thus it is often required to simulate their behaviour numerically. However, due to their special characteristics, especially for high Q factors, they are often considered too challenging for application of standard numerical software. In particular, a recent publication [1] listed the perceived state-of-the-art in integral equation modelling of RCs, and identified numerous unsolved problems. The present paper illustrates that using Higher-Order (HO) basis functions in the integral equation discretization can allow the numerical analysis of relatively large RCs to be performed with limited computer resources. Applying a dedicated HO Multi-level Fast Multipole Method scheme allows even larger problems to be solved. After a discussion and brief review of existing methods for RC modelling, we will turn to a description of the key features of HO basis functions and their related MLFMM implementation, focusing on how they allow surpassing some of the challenges faced by lower-order discretizations. Then, several RC test-cases are analyzed, drawing comparisons to other results from the relevant litterature. The conclusion is that with use of HO basis functions and a thorough MLFMM implementation, some of the challenges identified in [1] can be overcome. [1] H. Zhao, “MLFMM-Accelerated Integral-Equation Modeling of Reverberation Chambers,” IEEE Antennas and Propagation Magazine, vol. 55, no. 5, pp. 299–307, Oct. 2013.
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 mast (polystyrene or Plexiglas) 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 investigates the influence of the material of the mast supporting the target under test. Across several measurement steps, we compare different RCS measurement results of canonical targets in order to eliminate the unwanted RCS measurement contribution due to the mast. The aim is to find out the mast which disturbs the least the RCS of the target under test but still compatible with the measurement facility ARCHE 3D. All these measurements are also compared to Near Field and Far Field calculations taking into account the material of the supporting mast.
Historically, the inverse synthetic aperture radar (ISAR) reflectivity assumption has been used in the implementation of Image-Based Near Field-to-Far Field Transformations (IB-NFFFT) to estimate monostatic far field radar cross-sections (RCS) from monostatic near field radar measurements. The ISAR assumption states that all target scattering occurs at the location of the incident field excitations, i.e., the target is composed entirely of non-interacting localized scatters. Certain non-localized scattering phenomenon cannot be effectively handled by the IB-NFFFT approach with the ISAR assumption. Here we have used the adaptive Gaussian representation, which is a joint time-frequency decomposition technique, to coherently decompose near field measured data into two subsets of scattering features: one subset of localized scatterers and the other of non-localized scatterers. The localized scattering features are processed through the IB-NFFFT as typical, which includes compensating for the R4 fall-off present in the near field measured data. The non-localized scattering features, more appropriately scaled, are then coherently added back in to the post-NFFFT localized scattering phase history. Although this does not properly transform the non-localized scattering features into the far field, it does avoid the over-estimation error associated with improperly compensating distributed non-localized scattering features by a R4 power fall off based strictly on downrange position.
Antenna, Radome, and RCS testing systems rely on high-fidelity positioner systems to provide high-precision positioning of articles for RF testing. Historically, the industry has relied on linear PID control techniques in torque, velocity, and position control loops on individual axes to drive the positioners. Recently, advancements have been made in the use of advanced control hardware including multiple-DOF laser and optical feedback devices, brushless DC motors, VFD AC motors, and multi-drive torque-biased actuation. Advanced control techniques including single-axis error correction, multi-axis global error compensation, and multi-axis coordinated motion have been implemented to improve positioner accuracy. Here, a survey is conducted of control technologies in other industries such as machine tools and industrial robotics. An assessment is conducted on the viability of other advanced techniques to provide insight into the potential future control and capabilities of positioning systems in the RF testing industry. Candidate advanced techniques include gain scheduling and sliding-mode control which could provide improved accuracy over a wider range of conditions including varying loads and operating points caused by differing movement speeds or large variations in static loading. Dynamic input-shaping and feed-forward techniques could help suppress dynamic vibrations and improve dynamic tracking behavior for improved continuous-measurement scanning accuracy. Adaptive and non-linear control techniques might improve disturbance and error rejection for improved accuracy while managing dynamic-behavior drift allowing for adaptation to long-term positioner changes without re-tuning.
RCS Measurements at the upper half of the VHF range of the spectrum have become increasingly important. This type of measurement is usually performed in an outdoor RCS range. The present paper shows a design for an antenna that can be used to illuminate a reflector or as the illuminating structure in a RCS measurement. The antenna is fairly compact given the wavelength and exhibits a low VSWR and a good time domain performance for use with pulses. The new antenna has low side-lobes that otherwise could illuminate adjacent structures to the outdoor range and reduce the dynamic range of the measurements, this is an improvement over the Open-Boundary Quad-Ridged Horns Introduced over the past 9 years. The new Feed is a Quasi-Open-Boundary Horn, in which RF absorber material is used to create the Open Boundary behavior, but an enclosed structure is used to block the potential side-lobe radiation.
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The need to maintain very low observability, along with the need to manipulate the model through a large range of motion, result in a challenging set of problems. These have been effectively addressed over decades of RCS equipment design. In recent years however, RCS applications have become much more demanding. Models are ever larger and heavier, with length exceeding 150 feet, and with weight up to 50,000 lbs. Required accuracy with some applications has increased to ±0.01°, an increase of 67% as compared to legacy values. MI Technologies has developed products that significantly expand the structural and operational envelopes of rotator/pylon systems to meet the demand for higher performance. This paper presents the various challenges encountered in RCS Rotator and Pylon design, and the innovative solutions that have arisen from recent engineering efforts.
Abstract—Nowadays, radar retro-reflector has been widely applied as a decoy, to seduce an incoming assault away from the target, or towards a less vulnerable part of it to communication systems and remote identification as their characteristics of low-profile, low-cost and Radar Cross Section(RCS) enhancement. A passive retro-reflector is a device which can be used to be reflected most of the energy incident upon it in the direction of the in-going wave. The Luneberg lens and a sphere are widely used as their self characteristics. In this paper one of the retro-reflector, is paid more attention as time goes by, is introduced. The retroreflector is consist of patch antenna arrays and feeding system and can be defined as Retro-directive arrays (RDA). It has a very simple structure and can focus outgoing waves back at the direction of incident waves. The character of the re-radiation pattern affected by the size and type of patch and width and length of feeding network related are optimized by the HFSS. The final results are validated experimentally.
Abstract— Compact ranges are ideal settings for collecting low-RCS measurement data at high pulse rates. However, until recently, two operating constraints have limited the efficiency of instrumentation radar systems in this setting: (1) system delays limiting Pulse Repetition Frequency (PRF) and (2) fixed integration across frequency resulting in more time spent on certain frequencies than required. In this paper, we demonstrate the capability to significantly increase data throughput by using a Burst-Mode to increase the usable PRF and a frequency table editing mode to vary integration levels across the frequency bandwidth. A major factor in the choice of PRF for a specific application is system hardware delays. We describe the use of a Burst-Mode of operation in the MkVe Radar to reduce delays caused by physical layout of the instrumentation hardware. Burst-Mode essentially removes setup time in the system, reducing the time between pulses to the roundtrip time of flight from the antenna to the target. Most pulsed-IF instrumentation radar users fix the coherent integration level for the entire measurement waveform, even though the set level of integration may not be required at all frequencies to achieve the desired sensitivity. We describe the use of a frequency table Parameter Editor Mode in the MkVe that allows the integration level to vary for each step in the waveform. We demonstrate the use of both methods to reduce data collection time by a factor of seven using a MkVe Radar installed in a compact range.
Abstract— A fully polarimetric compact radar range operating at a center frequency of 100 GHz has been developed for obtaining radar cross section, inverse synthetic aperture radar imagery and high range resolution profiles on targets and structures of interest. The 100 GHz radar range provides scale-model RCS measurements for a variety of convenient scale factors including W-Band (1:1 scale), C-band (1:16 scale), and S-band (1:26 scale). An overview of the radar range is provided in this paper along with measurement examples of ISAR scale-model imaging, scale-model through-wall imaging, and preliminary kHz sweep-rate Doppler that demonstrate a few of the diverse and unique applications for this system. The 100 GHz transceiver consists of a fast-switching, stepped, CW microwave synthesizer driving dual-transmit and dual-receive frequency multiplier chains. The stepped resolution of the system’s frequency sweep is sufficient for unambiguous resolution of the entire chamber. The compact range reflector is a CNC machined aluminum reflector edge-treated with FIRAM™-160 absorber serrations and fed from the side to produce a clean quiet zone. This range is the latest addition to a suite of compact radar ranges developed by the Submillimeter-Wave Technology Laboratory providing scale-model radar measurements at nearly all of the common radar bands.
Abstract—It is well-known that a complete bistatic set of near-.eld scattering data is required to compute far-.eld radar cross section (RCS) quantities. In many practical applications, however, only monostatic scattering data is available. Almost all algorithms for the transformation of monostatic near-.eld data are based on the synthetic aperture radar (SAR) image representation.Since these algorithms are often acceleratedbythe fastFouriertransform(FFT),they usuallypose manylimitations on the measurement procedure such as regularly spaced grids and separate treatment of the different polarizations due to scalar processing. In this paper, a novel and .exible algorithm is presented which is not based on the FFT but on multi-level fast multipole method (MLFMM) principles. Therefore, it is similar to the fast irregular antenna .eld transformation algorithm (FIAFTA) which has been designed for the transformation of antenna .elds and measurements. Numerical results of different scenarios show that these principles can also be successfully applied to monostatic scattering data. In summary, this approach is superior to existing algorithms, because it provides more .exibility while it is still very ef.cient.
Radar sensor working at 76-77GHz band, because of its long detection range, high resolution and excellent performance in different weather and illumination conditions, has been used to develop on-road pedestrian collision avoidance system. Therefore, studying the pedestrian radar scattering features is important to develop reliable on-road pedestrian detection algorithm. In this paper, we first discuss the measurement setup requirement at 76-77GHz to obtain reliable radar cross section (RCS) data of human subjects. Then the RCS pattern of human subjects with different postures and different body features are measured and studied. The observed radar features could be further developed into stable radar signatures to improve the pedestrian identification algorithm.
Reconfigurable radar antennas with rapid, real-time control of the radiation pattern beamwidth provide expanded performance for many instrumentation radar applications, including RCS signature measurement and dynamic Time Space Position Information (TSPI) radar tracking applications. Adaptive adjustment of antenna radiation patterns was traditionally accomplished by electro-mechanically selecting predefined aperture dimensions that corresponded to desired beamwidths (e.g., ? ?/D). For an array antenna consisting of as few as 200 elements, beam shaping can be accomplished by adjusting the relative phase of individual array elements, a technique defined as beam spoiling or decollimation. This paper analyzes an operational radar antenna array incorporating reconfigurable beamwidth and beam shape through independent phase control of each subaperture. By adjusting the relative phase of radiating elements, the system can illuminate a programmable field of regard with full transmit power. For this array, the phase distributions across the elements map to a smaller "virtual aperture" displaced behind the physical array. Theoretical and measured results are presented to validate the reconfigurable array pattern control technique.
We present a method for measuring ultra-high frequency radio-frequency identification (UHF RFID) tag differential RCS that has the potential for being easier and more accurate than current and proposed methods [1-2]. Our method is based on accurately characterizing the reflection states of a modulated load, accounting for transmission losses between the load and an antenna, and using a well-known, low gain antenna. This has the benefit of using a well characterized “golden tag” reference (i.e., repeatability), while being more linear in power response, independent of reader signal, and independent of manufacturer or process changes. Characterizations of the losses in the reference scatterer allow for direct comparisons between tags on different test beds.
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 in 2007 at CEA for indoor near field monostatic RCS assessment. This experimental layout was composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target was located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allowed full 3D near field monostatic RCS characterization. A new study was conducted in 2011 in order to achieve a more accurate positioning of the measurement antenna. The main objective is to enhance the RCS measurement performances, especially the environment subtraction directly related to the positioning repeatability of the measurement antenna. This new mechanical design has therefore been optimized to allow a +/-100° azimuth range with an angular positioning repeatability of less than 1/1000°. To achieve this level of accuracy, several keys design elements were considered: robust mechanical design, position control system… This paper describes the new experimental layout and the results of a positioning accuracy assessment campaign conducted using a laser tracker.
Numerous applications for antenna, radome and RCS measurements require a very accurate positioning capability to properly characterize the product being tested. Testing of weapons (missiles), guidance systems, and satellites, among other applications, require multi-axis position accuracies of a few thousandths of an inch or degree. For global positioning, spherical error volumes can be extremely small having diameters of .002 inches to .005 inches. This paper addresses the issues that must be resolved when highly accurate mechanical positioning is required. Many factors such as thermal stability, axis configuration, bearing runout and mechanical alignment can adversely affect the overall system accuracy. Additionally, when examined from a global positioning system perspective, the accuracy of the entire system is further degraded as the number of axes increases. Successful system implementation requires carefully examining and addressing the most dominant error factors. The paper will cover current tools and techniques available to characterize and correct the contributing errors in order to achieve the highest possible system level accuracy. A recently delivered 4 ft radius SNF arch scanner, which achieved ± .0043° global positioning accuracy, will provide insight into these methods and show how the dominant factors were addressed.
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 in 2007 at CEA for indoor near field monostatic RCS assessment. This experimental layout was composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target was located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allowed full 3D near field monostatic RCS characterization. A new study was conducted in 2011 in order to achieve a more accurate positioning of the measurement antenna. The main objective is to enhance the RCS measurement performances, especially the environment subtraction directly related to the positioning repeatability of the measurement antenna. This new mechanical design has therefore been optimized to allow a +/-100° azimuth range with an angular positioning repeatability of less than 1/1000°. To achieve this level of accuracy, several keys design elements were considered: robust mechanical design, position control system… This paper describes the new experimental layout and the results of a positioning accuracy assessment campaign conducted using a laser tracker.
Radar Cross Section (RCS) and Antenna measurement ranges share many common features and are often used for both purposes. Calibration of these dual-purpose ranges is typically done using the substitution method for both RCS and antenna testing, but with separate RCS and antenna standards. RCS standards are typically based on a geometric shape having a well known theoretical value – and corresponding small uncertainty. By contrast, antenna standards typically must be “calibrated” in a separate antenna calibration system to be used as a gain standard, often yielding higher uncertainties. This paper presents an efficient method for transferring an RCS measurement calibration to an antenna measurement range configuration, allowing a range to be used for both purposes with a single calibration. Insight into the best ways to re-configure the instrumentation between RCS and antenna testing is included. Validation measurements from a compact range are included along with an uncertainty analysis of the method.
As a novel concept for field probes, RCS measurements on long rigid objects rotated within a small angular range about the broadside condition are reported. The rotation was maintained either in a horizontal (H) plane or in a vertical (V) plane containing the center of the quiet-zone (QZ). Processing the RCS data by DFT yields a spectrum which is recognized as the field distribution along that object. This spectrum compares extremely well to traditional field-probes taken earlier by translating a sphere across the QZ in H- or V-direction. Preliminary results at several S-band frequencies are presented and discussed.