AMTA Paper Archive
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Measurements On Long And Rigid Objects For Radar Field Probe
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.
Wideband Measurements Of The Forward Rcs And The Extinction Cross Section
This paper describes the development of a method based on measurements of the radar cross section (RCS) in the forward direction to determine the extinction cross section for the 2.5-38GHz frequency range using the optical theorem. Forward RCS measurements are technically complicated due to that the direct signal has to be subtracted from the total signal at the receiving antenna in order to extract the forward RCS. The efficiency of this subtraction as a function of time is evaluated. A traditional calibration method using a calibration target and a second method that does not require a calibration target are investigated and compared. The accuracy of the forward RCS measurements is determined using small spheres of different sizes. The spheres have a forward RCS that is straightforward to calculate with good accuracy. The method is also extended to polarimetric measurements on a small helix that are compared to theoretical calculations.
A Model-Based Technique With l1 Minimization For Defect Detection And Rcs Interpolation From Limited Data
Method of moments (MoM) codes have become increasingly capable and accurate for predicting the radiation and scattering from structures with dimensions up to several tens of wavelengths. In an earlier AMTA paper , we presented a network model (NM) algorithm that uses a Gauss-Newton iterative nonlinear estimation method in conjunction with a MoM model to estimate the “as-built” materials parameters of a target from a set of backscatter measurements. In this paper, we demonstrate how the NM algorithm, combined with the basis pursuits (BP) l1 minimization technique, can be used to locate unknown defects (dents, cracks, etc.) on a target from a limited set of RCS pattern measurements. The advantage of l1 minimization techniques such as BP is that they are capable of finding sparse solutions to underdetermined problems. As such, they reduce the requirement for a priori information regarding the location of the defects and do not require Nyquist sampling of the input pattern measurements. We will also show how the BP solutions can be used to interpolate RCS pattern data that is undersampled or has gaps.
Dynamic Rcs Measurement With A Network Analyzer
IDS has developed a RCS measurement solution capable to operate both in indoor and outdoor test ranges. The solution is based on the Agilent PNA series of network analyzers, whose performance are enhanced by a dedicated RF front end (named "Pulser"), resulting in a low-cost, compact and flexible system covering the frequency band from 2GHz to 18GHz. At first, the capability of the measurement solution was verified in a near field test range, demonstrating sensitivity compliant with low observable platform requirements (typical values of Noise Equivalent RCS can be in the order of -50 dBsm indoor at 30 m). Recently the RF front end has been upgraded to be usable for outdoor dynamic RCS measurements as well, being the upgraded solution named "Pulser_EV". This paper describes the performance of the Pulser_EV, its application field and possible developments.
Pedestrian and Bicyclist Radar Scattering Signatures at 76-77GHz
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 Beamwidth Antenna Array using Phase Adjustment of Array Elements
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.
RCS Rotator/Pylon Architecture – Pushing Back the Boundaries of Structural and Operational Performance
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.
The Study on a New Type of Low-profile and Passive Radar Retro-reflector
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.
Implementation of a Burst-Mode Technique and Variable Coherent Integration to Minimize Radar Data Collection Time
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.
A 100 GHz Polarimetric Compact Radar Range for Scale-Model Radar Cross Section Measurements
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.
Transformation of Monostatic Near-Field Scattering Data By Fast Irregular Field Transformation Algorithms
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.
Development of a Precision Model Positioning System for a Multi-Use Electromagnetic Test Facility at NASA Langley Research Center
This paper describes the mechanical design, control instrumentation and software for a precision model positioning system developed for use in the Experimental Test Range (ETR) electromagnetic test facility at NASA Langley Research Center. ADC has a contract to design, build, and install major components for an updated indoor antenna characterization and scattering measurement range at NASA Langley Research Center. State-of-the-art electromagnetic systems are driving a demand to increase the precision and repeatability of electromagnetic test ranges. Sophisticated motion control systems can help meet these demands by providing electromagnetic test engineers with a level of positioning fidelity and testing speed not possible with previous generation technology. The positioning system designed for the Experimental Test Range at NASA Langley Reseach Center consists of a rail positioning system and four rail positioning carriages: an antenna measurement positioner, scattering and RCS measurement pylon, an azimuth rotator to support foam columns, and an electric personnel lift for test article access. A switching station allows for rail positioning carriages to be quickly moved on and off of the rail system. Within the test chamber there is also a string reel positioning system capable of positioning test articles within a 40’ x 40’ x 25’ volume. Total length of the rail system is 112’ with laser position encoding for the final section of the rail system. Linear guide rails are used to support the carriages and each carriage is position with a rack and pinion drive. Rails mount to steel weldments that are supported with 8” diameter feet. Capacity of the rail system is 7,300 lbs. A switching station allows for positioning components to be moved off of and onto the rail system independently and a place to dock positioning components when they are not in use. A curved linear guide rail supports the switching station so that the platform can be rotated manually. Hardened tapered pins are used to align the switching station with mating rail segments. The scattering and radar cross section (RCS) measurement pylon is a 4:1 ratio ogive shape and has a 3,000 lb load capacity. A pitch rotator tip or spline driven azimuth tip can be mounted to the pylon. The spline drive shaft can be removed to allow for the pitch tip to be mounted to the end of the pylon. Total height of the pylon is 18’ from the floor to the pitch positioner mounting plate. Keywords: RCS, Scattering, Pylon, Positioner, Antenna Design, Rotator, Instrumentation, electromagnetic, Radio Frequency, Radar
On the Use of Basis Pursuit and a Forward Operator Dictionary to Separate Specific Background Types from Target RCS Data
RCS measurements are often comprised of a combination of the coherent summation of many things in addition to the desired target. Those other things contribute to error in RCS measurements and include noise, clutter and background, which can be further characterized according to specific types. An approach has been developed that is capable of capturing and separating certain types of noise, clutter and background based on specific forward models to include RFI, target support (e.g., pylon), and many others, such that engineers can clearly see the separated components and selectively choose to include, exclude, or edit as the case may be. This approach affords far more flexibility than classic image edit reconstruct (IER), and offers more editing accuracy than Fourier-based approaches including entire phase history based approaches. This paper describes the basic approach and shows examples with measured data.
Nearfield RCS Measurements of Full ScaleTargets Using ISAR
Near field Radar Cross Section (RCS) measurements and Inverse Synthetic Aperture Radar (ISAR) are used in this study to obtain geometrically correct images and far field RCS. 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. 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 e.g., constraints in terms of budget, available equipment and ranges but also technical 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. However, 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. A straightforward way to image the RCS data recorded in the near field is to use the backprojection algorithm. The amplitudes and locations for the scatterers are then determined in a pixel by pixel imaging process. The most complicated part of the processing is due to the near field geometry of the measurement. This is the correction that is required to give the correct incidence angles in all parts of the imaged area. This correction has to be applied on a pixel by pixel basis taking care to weigh the samples correctly. The images obtained show the geometrically correct locations of the target scatterers with exceptions for some target features e.g., when there is multiple or resonance scattering. Separate features in the images can be gated and an inverse 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 ISAR processing techniques described in this paper will be given.
Measurement of Operational Orientations Using Coordinate Transforms and Polarization Rotations
Antenna and Radar Cross Section (RCS) measurements are often required for orientation sets (cuts) that are difficult or impossible to produce with the positioning instrumentation available in a given lab. This paper describes a general coordinate transform, combined with a general polarization rotation to correct for these orientation differences. The technique is general, and three specific examples from actual test programs are provided. The first is for an RCS measurement of a component mounted in a flat-top test fixture. The component is designed to be mounted in a platform at an orientation not feasible for the flat-top fixture, and the test matrix calls for conic angle cuts of the platform. The transforms result in a coordinated, simultaneous two-axis motion profile and corresponding polarization rotations yielding the same information as if the component had been mounted in the actual platform. The second example is for a pattern measurement of an antenna suite mounted on a cylindrical platform (such as a projectile). In this case, the test matrix calls for a roll-cut, but the range positioning system does not include a roll positioner. The transforms again result in a coordinated, simultaneous two-axis motion profile and corresponding polarization rotations to provide the same information as the required roll-cut but without the use of a roll positioner. Finally, the third example is for an antenna pattern measurement consisting of an extremely large number of cuts consisting of conic yaw cuts, roll cuts and pitch cuts. The chosen method involves the use of the Boeing string suspension system to produce great-circle cuts at various pitch angles combined with the use of the coordinate and polarization transforms to emulate, off-line, any arbitrary cut over any axis or even multiple axes. Keywords: Algorithm, Positioning, Polarization, Coordinates, RCS
Integral Equation Modelling of Reverberation Chambers using Higher-Order Basis Functions
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  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  can be overcome.  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 ARCHE 3D: Influence of the target supporting mast in RCS measurement
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.
Distinguishing Localized and Non-Localized Scattering for Improved Near-Field to Far-Field Transformations
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.
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