AMTA Paper Archive
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Two-Sphere Interference in RCS Measurements and Imaging
New RCS data on two-sphere in rotation are presented. From the simple geometry, the results allow us to verify both the cross-range and down-range distance scales in imaging. With the known RCS of the individual spheres, we find that it is feasible to calibrate the image RCS scale to dBsm, provided when care is taken to mitigate the shadowing and sidelobe effects.
Update on the Air Force Research Laboratory Advance Compact Range Calibration Uncertainty Analysis
A calibration uncertainty analysis was conducted for the Air Force Research Laboratory’s (AFRL) Advanced Compact Range (ACR) in 2000. This analysis was a key component of the Radar Cross Section (RCS) ISO-25 (ANSI-Z-540) Range Certification Demonstration Project. In this analysis many of the uncertainty components were argued to be small or negligible. These arguments were accepted as being reasonable based on engineering experience. Since 2000 the ACR radar has been replaced with an Aeroflex Lintek Elan radar system. A new measurement uncertainty analysis was conducted for the ACR using the Elan radar and for a general (non-calibration) target. We present results comparing the previous results to the current analysis results.
Uncertainty Analysis of RCS Calibrations at the Etcheron Valley Range
We have been developing an uncertainty analysis of RCS calibrations and measurements in the 2 – 18 GHz range at the Etcheron Valley RCS outdoor ground-bounce facility. In this study we report on the results of the uncertainty analysis primarily at 11.3 GHz, but results at some other frequencies are also discussed. We plan to address all components of uncertainty, and present here in some detail the procedures used to determine the uncertainties due to nonplanar illumination, drift, noise-background and nonlinearity. We use a measurement-based approach to obtain upper-bound estimates for the component uncertainties, which are combined using root-sumsquares (RSS) to obtain the overall uncertainty. The uncertainties at any frequency can be determined using these measurement procedures.
SOLANGE, An Enhanced RCS Measurement Facility of Full Size Aircraft
This paper describes the RCS measurement test facilities, CHEOPS, STRADI and SOLANGE which are operated in the Technical Center for Information Warfare (CELAR) in France, with a particular focus on SOLANGE. CHEOPS is an anechoïc chamber convenient for the measurement of small missiles as well as antennas measurement. STRADI is an outdoor facility, which is convenient for measurement of land vehicles, helicopters and large antennas. SOLANGE is an indoor RCS measurement facility used to measure long missiles and aircraft. Originally built in 1985, SOLANGE has been continuously upgraded to fulfill all customers requirements in the field of RCS measurement. Thanks to the in house radar instrumentation and data processing software, SOLANGE can reach a very good performance on small or big RCS targets from 200 MHz to 18 GHz. The UHF/VHF capacity has been recently enhanced thanks to the upgrade of the positioning system and the cooperation between CELAR and CEA.
Accurate Determination of a Compact Antenna Test Range Reference Axis and Plane Wave Quality
Highly accurate antenna and payload measurements in antenna test facilities require highly accurate alignment and boresight determination. The Angle of Arrival (AoA) of the plane wave field in the quiet zone of the CCR Compensated Compact Range CCR 75/60 of EADS Astrium GmbH, installed at Alcatel Space in Cannes . France, has been measured using three different methods (optical geometrical determination using theodolites, Radar Cross Section (RCS) maximization, planar scanner phase plane alignment). The proposed paper describes the three methods and the performed measurement campaign and provides the correlation between the resulting angles via a comparison of the results. The achieved absolute worst case values of lower than 0.005° demonstrates the high level of accuracy reached during the campaigns.
A Phase Hologram Based Compact RCS Range for Scale Models
A compact radar cross section (RCS) test range for scale model measurements is being developed. The test range is based on a phase hologram that converts the feed horn radiation to a plane wave needed for RCS determination. The measurements are performed at 310 GHz using continuous wave operation. A monostatic configuration is realized using a dielectric slab as a directional coupler. The main advantage of a scale model RCS range is that the dimensions of radar targets are scaled down in proportion to the wavelength. Therefore, RCS data of originally large objects can be measured indoors in a controlled environment. So far simple test objects such as metal spheres have been measured. The feasibility of the phase hologram RCS range has been verified. The basic operation and first measurement results of the monostatic measurement range are reported here.
Antenna Pattern Correction for Near Field-to-Far Field RCS Transformation of 1-D Linear SAR Measurements
In a previous AMTA paper , we presented a firstprinciples algorithm called wavenumber migration (WM) for estimating a target’s far-field RCS and/or far-field images from extreme near-field linear (1-D) or planar (2-D) SAR measurements, such as those collected for flight-line diagnostics of aircraft signatures. However, the algorithm assumes the radar antenna has a uniform, isotropic pattern on both transmit and receive. In this paper, we describe a modification to the (1-D) linear SAR WM algorithm that compensates for nonuniform antenna pattern effects. We also introduce two variants to the algorithm that eliminate certain computational steps and lead to more efficient implementations. The effectiveness of the pattern compensation is demonstrated for all three versions of the algorithm in both the RCS and the image domains using simulated data from arrays of simple point scatterers.
A Low-Cost Compact Measurement System for Diagnostic Imaging and RCS Estimation
The task of performing reliable RCS measurements in complex environments under near-field conditions is gaining more and more interest, mainly for a rapid assessment of RADAR performance of constructive details. This paper describes a low-cost compact measurement system fully developed by IDS, that allows fast and effective acquisition of diagnostic images under nearfield conditions and far-field RCS estimation in a nonanechoic environment. The hardware of the system is composed of a planar scanner, two horn antennas, a Vector Network Analyzer and a computer. The two axes scanner allows 2D scanning of antennas in a vertical plane. For each point of a predefined grid along the scanned area, the Analyzer performs a frequency scan. The acquisition software synchronizes scanner movements with data acquisition, transfer and storage on the computer’s HDD. The software has post-processing capabilities as well. A number of focusing algorithms permit to produce 2D and 3D diagnostic images of the target as well as 2D backprojection. It is moreover possible to reconstruct the RCS starting from near-field images. Along with system features, a summary of performances and some simple targets images are presented.
Far-Field Bistatic RCS From Near-Field Measurements
Bistatic radar cross sections of targets are computed from field measurements on a cylindrical scan surface placed in the near field of the target. The measurements are carried out in a radio anechoic chamber with an incident plane-wave field generated by a compact-range reflector. The accuracy of the computed target far field is significantly improved by applying asymptotic edge-correction techniques that compensate for the effect of truncation at the top and bottom edges of the scan cylinder. The measured field on the scan cylinder is a “total” near field that includes the incident field, the field of the support structure, and the scattered field of the target. The background subtraction method determines an approximation for the scattered near field on the scan cylinder from two measurements of total near fields. The far fields of metallic sphere and rod targets are computed from experimental near-field data and the results are verified with reference solutions.
An Effective and Practical Polarimetric Calibration Technique
The National RCS Test Facility (NRTF) has designed, fabricated, and implemented an efficient and robust calibration procedure and test body applicable to pylon based monostatic RCS measurements. Our unique calibration test body provides physical separation between the calibration device and pylon allowing the pylon to be outside the range gate of the calibration device. This separation reduces the calibration device uncertainty due to target support contamination and interaction. Spectral analysis and feature extraction of rotational dihedral/dipole data allows further rejection of background noise and clutter that possess different angular dependencies from those of the dihedral/dipole. Due to the significant reduction in the achievable crosspolarization isolation that occurs with a small degree of positioning error in dihedral/dipole roll angle, a data driven search algorithm has been developed to select the two dihedral/dipole angles used by the polarimetric distortion compensation algorithm.
Cam RCS Dual-Cal Standard, The
We introduce a new calibration standard geometry for use in a static RCS measurement system that can simultaneously offer multiple “exact” RCS values based on a simple azimuth rotation of the object. Called the “cam,” the new calibration device eliminates the problem of frequency nulls exhibited by other resonantsized cal devices by shifting the nulls through azimuthal rotation. Furthermore, the “cam” facilitates the use of dual-calibration RCS measurements without the need to mount a second cal standard. The “cam” is practical to fabricate and deploy; it is conducting, composed of flat and constant-radius singly-curved surfaces, and is compatible with standard pylon rotator mounts. High-accuracy computational results from moment-method modeling are presented to show the efficacy of the new standard.
Phase-Dependent RCS Measurements in the Presence of Outliers
Coherent radar cross section measurements on a target moving along the line-of-sight in free space will trace a circle centered on the origin of the complex (I,Q) plane. The presence of additional complex signals (such as background, clutter, target-mount interactions, etc.), which do not depend on target position, will translate the origin of the circle to some complex point (I0,Q0). This type of phase-dependent I-Q data has been successfully analyzed. However, the presence of outliers can introduce significant errors in the determination of the radius and center of the IQ circle. Hence, we implement a combination of a robust and efficient Least-Median Square (LMS) and an Orthogonal Distance Regression (ODR) algorithm is used (1) to eliminate or to reduce the influence of outliers, and then (2) to separate the target and background signals. This technique is especially useful at sub-wavelength translations at VHF, where spectral techniques are not applicable since only a limited arc of data is available. We analyze data obtained as an Arrow III target moves relative to its supporting pylon. To demonstrate the effectiveness of the technique, we introduce rf interference signals into S band data and show that the uncontaminated parameters can be recovered with acceptable uncertainties.
Development of a MATLAB Toolbox to Assist in the Process of RCS Range Calibration
Over the past few years, range certification activities have become more commonplace, as industry, government and academia have embraced the process and acted to implement documented procedures at their facilities. There is now a significant amount of documentation laying out the process, as well as templates to assist ranges in developing their range books. To date, however, there have been fewer examples of useful tools to assist the ranges in better understanding how the process will affect their specific range. The authors have developed a first generation MATLAB toolbox designed to provide ranges a “what-if” capability to see the impact of specific range errors on the range’s operations. Included within the toolbox are several types of additive and multiplicative errors, as well as means of modeling various aspects of radar operation.
Applications of Multilayer Resistive Strips (R-Card) in EM Measurements
A single tapered resistive strip (R-Card) has been used in the past in several applications related to antenna designs and ground bounce reduction for far-field ranges. Several antenna designs use single tapered R-Card to significantly reduce the diffracted fields from the antenna to achieve low side lobe performance and also maintain stable phase center location across wide frequency bandwidth. Single layer R-Card fences have also been successfully designed and used to reduce the ground bounce stray signal in far field ranges. Recently, a multilayer tapered R-Card concept has been investigated and implemented in two different applications for interaction reduction due to performance requirements. One of the applications is to use multilayer R-Card fences to reduce the groundbounce effect between two antennas for GPS applications. The second application is to embed the multilayer R-Card with the Styrofoam target support column used in RCS measurements to reduce the interaction between the target-under-test and the metallic azimuth rotator underneath the Styrofoam column. In both applications, the multilayer R-Card concept, with different resistance distributions and proper spacing, has been designed and evaluated such that it behaves as an absorber to reduce the interference/interaction between two antennas or two scattering objects. The design and evaluation of this new multilayer R-Card concept will be presented in this paper.
Cheetah PNA RCS and Antenna Measurement System
System Planning Corporation (SPC) is pleased to announce our new instrumentation radar measurement system denoted the Cheetah radar line. This radar system is based around the new Agilent PNA series of network analyzers. The PNA operates from 0.1 to 67 GHz and is utilized for making gated CW or CW RCS and Antenna measurements. The PNA has a built in synthesizer that allows the unit to be used without costly external synthesizers and external mixers. The PNA also has four identical receiving channels, two signal and two reference, that permit simultaneous co and cross pol measurements to be made. PNA IF bandwidth is selectable from 1 Hz to 40 kHz to optimize measurement sensitivity, dynamic range and speed. Using the segmented sweep feature of the PNA a single frequency sweep can be broken into segments, to further optimize the sensitivity, dynamic range, and speed. Each segment can have its own start and stop frequency, frequency step size, IF BW and power level. SPC has developed the high speed RF gating, low noise RF preamplifiers and high speed digital timing system, which allow maximum sensitivity, full up gated CW or CW radar measurements using the PNA. SPC has coupled the system to the CompuQuest 1541 RCS and Antenna Data Acquisition and Data Analysis Processing Software. This exciting new product line offers reduced cost and improved performance over current network analyzer based systems using the HP 8530, 8510, etc. Performance improvements are in the reduced noise figure, sensitivity, dynamic range and measurement speed. Measurement speeds are increased by at least a magnitude of order over the older systems and in some cases a couple of orders of magnitude.
A New Gated-CW Radar Implementation
This paper describes the new ORBIT/FR StingRay Gated-CW radar implementation that provides both performance and speed improvements over those previously utilized and fielded in RCS measurement systems. The radar is implemented using one or multiple pulse modulators used to provide gating of the transmit and receive signals, in conjunction with the new class of Performance Network Analyzer recently introduced by Agilent Technologies. The radar features an order of magnitude improvement in speed over that previously offered using implementations with the Agilent 8510 or 8530 network analyzer/receiver. In addition, base sensitivity improvements are realized, and the radar is more flexible with user selection among many IF bandwidth settings now available. The physical profile of the radar is also improved, meaning that additional performance gains may be realized by creating a more efficient packaging scheme where the radar may be located closer to the radar antennas, either in a direct illumination configuration or in a compact range implementation. These factors, when considered in aggregate, result in the new ORBIT/FR StingRay Gated-CW radar offering that provides a higher performance-to-cost value trade-off than was previously available to the RCS measurement community.
Reduction of Vertical Field Taper at a Ground-Bounce RCS Range
We have investigated a method that reduces the vertical field taper at a ground-bounce radar crosssection range using a vertical antenna array. An experiment was designed were the coherent data from two measurement channels were independently recorded and stored for post processing. The two datasets were weighted and added in the postprocessing to form the extended zone with improved vertical field taper. Vertically distributed point scatterers on a special test object were used to aid in optimizing the method using imaging techniques. The method is evaluated using simulations and measurements. The usefulness of this method for RCS measurements of full-scale objects such as vehicles and aircraft is discussed. We find that the method can be used to reduce the vertical field taper over a wide frequency band in the way that theory predicts.
An Approach to the Evaluation of Uncertainties for Complex RCS Measurement Data
The Radar Cross Section (RCS) measurement facility operated by the Stealth Materials Department of BAE SYSTEMS Advanced Technology Centre in the UK is an invaluable tool for the development of low observable (LO) materials and designs. Specifically, it permits the effect of signature control measures, when applied to a design, to be demonstrated empirically in terms of the impact on the RCS. The facility is operated within a 3m by 3m by 12m anechoic chamber where pseudo-monostatic, co-polar, stepped frequency data for a target can be collected in a single measurement run over a frequency range of 2- 18GHz, and for a range of azimuth and elevation angles using a Vector Network Analyser (VNA). The data recorded consists of the complex voltage reflection coefficients (VRC) for the chosen range of aspect angles. This includes data for the target, mount, calibration object, and the associated calibration object mounting where significant. All data processing is conducted offline using a bespoke post processing software routine which implements software time domain gating of the raw data transformed into the time domain prior to calibration. The significant sources of type A (random) and B (systematic) uncertainties for the range are identified, grouped, and an approach to the determination of an uncertainty budget for the complex S21 data is presented. The method is based upon the UKAS M3003 guidelines for the treatment of uncertainties that may be expressed by the use of real, rather than complex numbers. However, a method of assessment of the uncertainties in both real and imaginary parts of the complex data is presented. Finally, the uncertainties estimated for the raw VRC data collected are propagated through the calibration and the uncertainty associated with the complex RCS of a simple target is presented.
Antenna Beam Shape - Effects on RCS Measurement
This work is part of the UK Ministry of Defence initiative to examine causes of uncertainties in RCS measurements and to establish a network of certified facilities. Having developed a ‘best practice’ guide where causes of uncertainty were listed, the effect of polar diagram was selected as a priority topic. Correction algorithms for RCS measurements require knowledge of the beam shape and resolution in crossrange of the significant scatterers. Accordingly, the accuracy of polar diagram measurement, the effect of amplitude ripple and the applicability of the correction algorithms to near-field data were addressed. Measurements were made on two targets, a long cylinder and a small aircraft. Two antennas and two ranges were used to achieve 1dB, 3dB and 6dB illumination tapers across the cylinder. The 6dB taper situation was modelled for three different numbers of points. The work demonstrated that polar diagram effects are significant for point scatterers or simple targets, like the cylinder; however, for the small aircraft with a large number of distributed scatterers, the overall effect is less significant.
Analysis of Range Ambiguity Effects in a Gated Linear FM Homodyne Receiver
Radar systems that use pulsed waveforms for detection can be adversely affected by target returns whose round-trip time of flight is longer than the radar’s interpulse period. Unless techniques such as pulse repetition frequency (PRF) jitter or pulse phase encoding are employed, the receiver has no way of determining whether a target’s range is accurate. If this radar system is being used to collect radar cross section (RCS) data, the range ambiguities may exhibit themselves as clutter and cause unacceptable levels of data contamination. A Gated Linear FM Homodyne (gated LFMH) radar modulates its transmitted signal during the time of an individual chirp, or frequency sweep, which leads to two distinct PRFs; the chirp PRF and the interchirp pulse PRF. The chirp PRF is typically very low, on the order of tens to hundreds of chirps per second, and therefore insignificant with respect to range ambiguities. It is the interchirp pulse PRF that is typically of sufficient rate to factor significantly in the processing of data collected with range ambiguities present. This paper provides analysis of the effects of range ambiguities in a typical gated LFMH radar that occur during wideband RCS data collections. In addition, a method for optimizing the radar system parameters through the prediction of the range ambiguities will be shown.
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