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Anechoic chambers simulate open air test conditions and are advantageous for testing avionics systems in a secure, quiet electromagnetic environment. The 412th Electronic Warfare Group’s Benefield Anechoic Facility (BAF), located at Edwards AFB, California was designed for testing systems in the radio frequency (RF) range from 500 MHz to 18 GHz. For frequencies below 500 MHz, the installed radar absorbent material (RAM) does not effectively absorb incident RF energy, thereby allowing undesired RF scattering off the chamber’s floor, ceiling, and walls. This leads directly to measurement inaccuracies and uncertainty in test data, which must be quantified for error analysis purposes. In order to meet the desired measurement accuracy goals of antenna pattern and isolation measurement tests below 500 MHz, RF scattering must be mitigated. BAF personnel developed a test methodology based on hardware gating, range tuning and improved RAM setup, to improve chamber measurement performance down to 100 MHz. Characterizing the chamber using this methodology is essential to understanding test zone performance and thus increases confidence in the data. This paper describes the test methodology used and how the test zone was characterized with resulting data.
This is the second of a two-part paper discussing the NASA Debris Radar (NDR) system developed to characterize debris liberated by the space shuttle during its ascent into space. While initial NDR missions proved the extent of the debris detection and tracking challenge, improvements in NDR hardware, software, and mission operations resulted in very successful debris detection and tracking. These successes lead to a new challenge of processing and analyzing the large amount of radar data collected by the NDR systems and extracting information useful to the NASA debris community. Analysis tools and software codes were developed to visualize the shuttle metric data in real-time, visualize metric and signature data during post-mission analysis, automatically detect and characterize debris tracks in signature data, determine ballistic numbers for detected debris objects, and assess material type, size, release location and threat to the orbiter based on radar scattering and ballistic properties of the debris.
During the STS-107 accident investigation, radar data collected during ascent indicated a debris event that was initially theorized to be the root cause of the accident. This theory was investigated and subsequently disproved by the Columbia Accident Investigation Board (CAIB). However, the data itself and the lack of understanding of what debris data in radar meant to the shuttle program, required further analysis and understanding. The Space Shuttle Program Systems Engineering and Integration (SE&I) Office commissioned the Ascent Debris Radar Working Group (ADRWG) to characterize the debris environment during a Space Shuttle launch and to identify/define the return signals as seen by radar. Once the capabilities and limitations of the existing radars for debris tracking were understood, the team researched proposed upgrades to the location, characteristics, and post-processing techniques needed to provide improved radar imaging of Shuttle debris. The research phase involved in assessing the threat ultimately evolved into an inter-agency cooperation between NASA and the Navy for shared use of radar assets to the benefit of both agencies. Additional cooperative agreements were made with the Air Force and Army regarding various support aspects to the debris radar efforts. An aggressive schedule of field testing preceded the initial operations of the system during the STS-114 Return to Flight (RTF) mission in July of 2005.
Reflections in antenna test ranges can often be the largest source of measurement error within the error budget of a given facility [1]. Previously, a technique named Mathematical Absorber Reflection Suppression (MARS) has been used with considerable success in reducing range multi-path effects in spherical near-field antenna measurements [2, 3, 4, 5]. Whilst the technique presented herein is also a general purpose measurement and post-processing technique; uniquely, this technique is applicable to cylindrical near-field antenna test ranges. Here, the post-processing involves the analysis of the cylindrical mode spectrum of the measured field data which is then combined with a filtering process to suppress undesirable scattered signals.
This paper examines the inter-relatedness between the polarization vector of the linearly polarized feed/probe, the analysis coordinate system used for the DL-to-CP transformation, and how the test antenna generates its circular polarization response. The measurement of the performance characteristics of circularly polarized antennas is often accomplished using a linearly polarized feed/probe to measure horizontal and vertical polarization components. The measured orthogonal linearly polarized components are combined using a mathematical technique to transform them to the orthogonal right-hand and left-hand circular polarization components of the test antenna. One of the difficulties in using this technique is insuring the proper orientation of the feed/probe for the measurement, the analysis coordinate systems used for the DL-to-CP transformation algorithm. Another is understanding the manner in which the circular polarization is being generated by the antenna under test. These factors are inherently related, and without proper care the wrong answer can easily be calculated.
The primary purpose of a chamber for Far–Field (FF) antenna measurements is to create a test zone surrounding the AUT, where the electric field is to be as uniform as possible, and multiple reflections are kept to a minimum. It is well known, that typical rectangular anechoic chambers for Far–Field (FF) antenna measurements are subject to increased reflectivity from specular regions on the side walls, floor and ceiling. The reflectivity further increases if a larger test zone and, consequently, longer source antenna/ AUT separation is required. The alternative to a rectangular chamber, which can be implemented to reduce the reflectivity, could be a chamber with a shaped interior, where the side walls are to be shaped based on GTD/GO principles so that the reflections are diverted out of the test zone. Even more reflectivity suppression is expected, if, in addition, wedge absorbers are used throughout the specular region or entire wall with a smoothly varied wedge orientation chosen according to GTD principles. The combination of two approaches constitutes a chamber design method termed a “Two – Level GTD”. The chamber shape and wedge orientation for delivering reduced reflectivity in the test zone are not unique. According to a “Two -Level GTD” a plurality of solutions exists and can be practically implemented. Freedom in choosing these parameters can be utilized to satisfy the additional requirements for the chamber design to reduce RCS clutter and/or secondary reflections in the chamber. In this paper the method validity is confirmed based on comparison of various chamber designs performed using 3D EM analysis tools.
The NIST 18 Term Error Analysis Technique uses a combination of mathematical analysis, computer simulation and near-field measurements to estimate the uncertainty for near-field range results on a given antenna and frequency range. A subset of these error terms is considered for alignment accuracy of an antenna’s RF main beam. Of the 18 terms, several have no applicable influence on determining the beam pointing or the terms have a minor effect and when an RSS estimate is performed they are rendered inconsequential. The remainder become the dominant terms for identifying the alignment accuracy. There are six terms that can be evaluated to determine the main beam pointing uncertainty of an antenna with respect to dual band performance. Analysis of the near-field measurements is performed to identify the alignment uncertainty of the main beam with respect to a specified mechanical position as well as to the main beam of the second band.
RF guided missile developers require flight simulation of their target engagements to develop their RF seeker. This usually involves the seeker mounted on a Flight Motion Simulator (FMS) as well as an RF target simulator that simulates the signature and motion of the target. Missile intercept engagements are unique in that they involve highly dynamic relative motion in a short period of time. This puts demanding requirements on the RF target simulator to adequately present the desired phase slope, amplitude, and polarization to the seeker antenna and electronics under test. This paper describes a newly installed RF Target Simulator that addresses these requirements in a unique fashion. The design utilizes a compact range reflector, dynamically rotated in two axes as commanded by the flight simulation computer, to produce the desired changing phase slope and an RF feed network dynamically controlled to produce the desired changing polarization and amplitude. Physical optics analysis establishes an accurate correlation between reflector physical rotation and resulting angle-of-arrival of the wave front in the quiet zone. The RF Target Simulator is self contained in a two-man portable anechoic chamber that can be disengaged from the FMS and rolled to and from the FMS as needed. Measurements are presented showing the performance of the RF Target Simulator.
This paper reports the recent investigation of data-link strings as supporting structures for antennas used in a bistatic radar cross section (RCS) measurement system. Although several candidate strings exist, analysis of the strings’ scattering contribution needs a generic string model to make comprehensive comparisons. A simple theoretical two-dimensional (2D) dielectric coated cylindrical wire model is initially utilized to predict and compare scattering characteristics of various data-link string structures. In addition to the simple model, a non-destructive measurement method is proposed for extracting the material properties of the string material. Using the analytic 2D model of a dielectric clad wire as the generic string model, the unknown permittivity is computed from reflectivity measurements taken with a focused beam system. Extracted permittivity values are then used in a full-wave electromagnetic solver to validate the model. Measured and simulated results are shown to have excellent agreement for the 2D RCS, radar echo width, of different strings and polarization configurations.
In dynamic RCS measurements, uncompensated motion induces artifacts and distortions, from moderate to severe, into any Fourier-based analysis. A quadratic term in the kernel of the underlying spectra (length L) will stretch the spectra into one of L2 configurations. Whether radial velocity for stepped-chirp HRR measurements, or acceleration for fixed-frequency Doppler measurements, spectra often become buried in noise – the quadratic term spreading the bandwidth. Even an approximation to the quadratic term in the spectral kernel allows a variety of signal processing techniques to further refine and remove residual uncompensated motion within the stepped-chirp or Doppler vector. This paper presents blind and semi-blind techniques making use of Fourier-based processing, entropy calculation, and bandlimited resampling to compensate for motion. Doing so restores the SNR available to the individual underlying spectra.
A compact antenna test range has been analysed for stray signals. The analysis is based on GTD ray trac-ing, i.e. obeying the reflection law in the chamber walls and assuming straight edges of reflectors and walls. Comparisons to an RCS as well as a time-domain measurement of the quiet-zone performance show good agreements with respect to identification of the ray paths of the stray signals. Rough estimates of the power loss at reflections and diffractions show acceptable agreements with the measured levels.
The electromagnetic field within a test volume can be determined by use of spherical scanning techniques. Characterization of the field within the sphere requires compensation for probe-pattern effects. We provide a simple analysis to estimate uncertainties associated with this deconvolution.
The extension of the frequency range for commercial applications of mm-waves to 80 GHz and beyond often requires extended antenna characterization capabilities both at manufacturer and end-user facilities. Presently, most measurements are based on direct measurements using vector network analyzers (VNAs). VNAs that cover a continuous frequency range up to 67 GHz are commercially available. Above 50 GHz, extensions based on external mixers in waveguide technology are typically utilized. They require a tunable local oscillator (LO) that is usually provided by the two additional ports of a 4-port VNA. However, these extensions not only are restricted in bandwidth but also require a significant financial investment especially considering the fact that the expensive 4-port instrumentation is needed. As most laboratories already have conventional 2-port VNAs usable up to 10 GHz or higher and most antenna characterizations are based on transmission measurements, we present a simple extension scheme based on external mixers and a fixed frequency LO that allows for transmission factor measurements. We demonstrate the feasibility of such an extension scheme for transmissions between a pair of horn antennas ranging up to 60 GHz. The measurements include variation of antenna spacing and steering angle and are verified with a computational analysis based on the finite differences time-domain (FDTD) method.
We used a set of dihedrals to perform polarimetric calibrations on an indoor RCS measurement range. We obtain simultaneously hh, hv, vh, and vv polarimetric data as the calibration dihedrals rotate about the line-of-sight to the radar. We applied Fourier analysis to the data to determine the polarimetric system parameters, which are expected to be very small. We also obtained polarimetric measurements on two cylinders to verify the accuracy of the system parameters. We developed simple criteria to assess the data consistency over the very large dynamic range demanded by the dihedrals. We examined data contamination by system drift, dynamic range nonlinearities, and the presence of background and noise. We propose improved measurement procedures to enhance consistency between the dihedral and cylinder measurements and to minimize the uncertainty in the polarimetric system parameters. The final recommened procedure can be used to calibrate polarimetrically both indoor and outdoor ranges.
The probe correction of near-field measured data can be considered as being composed of two parts. The first part is a pattern correction that corrects for the effects of the aperture size and shape of the probe and can be analyzed in terms of the far-field main component pattern of the probe. The second part is due to the non-ideal polarization properties of the probe. If the probe responded to only one vector component of the incident field in all directions, this correction would be unnecessary. But since all probes have some response to each of two orthogonal components, the polarization correction must be included. The polarization correction will be the focus of the following discussion. Previous studies have derived and tested general equations to analyze polarization uncertainty12. This paper simplifies these equations for easier application. The results of analysis and measurements for Planar, Cylindrical and Spherical near-field measurements will be summarized in a form that is general, easily applied and useful. Equations and graphs will be presented that can be used to estimate the uncertainty in the polarization correction for different AUT/Probe polarization combinations and measurement geometries. The planar case will be considered first where the concepts are derived from the probe correction theory and computer simulation and then extended to the other measurement geometries.
Dipoles are a typical reference antenna in measurements. Because its performance is calculable even in the near field it is commonly used as a reference. But while the ideal dipole is a calculable device, the actual reference dipole used in the lab can be far from the ideal. In this paper end fed sleeve dipoles commonly used as references in wireless measurements and traditional quarter wavelength dipoles used in a wide variety of applications including RFID testing are study. Misalignment, manufacturing tolerances, variations on dielectric, and messy solder points will be analyzed numerically and in some cases compared with measured data to see the effects of these problems on the final performance of a reference dipole unit.
This paper describes or deals with a quality analysis and comparison of three radar reflectivity information or data types. The information or data types include radar cross section (RCS) as defined by IEEE Standard 100, the bowtie sector average, and the gross estimate radar return (commonly known as the fuzzball). The paper discusses the uncertainty analysis of measured RCS, and the paper provides analysis on the uncertainty of bowtie sector averages and “fuzzballs” (gross estimate radar returns). The comparison of the information or data types, their quality, uncertainties, and usefulness represents a significant part and focus of the study.
The slot line, a transmission line suitable for application to microwave integrated circuits, may be used in place of or in association with microstrip. This paper presents an alternative method based on the adaptive neuro-fuzzy inference system (ANFIS) for computing the characteristic impedances of slot lines. The ANFIS is a class of adaptive networks which are functionally equivalent to fuzzy inference systems. The ANFIS has the advantages of the expert knowledge of the fuzzy inference system and the learning capability of neural networks. Different optimization algorithms, hybrid learning, genetic, simulated annealing, and least-squares, are used to determine optimally the design parameters of the ANFIS. The algorithm performances for the optimization of the ANFIS model parameters are compared with each other. The results of ANFIS are compared with the results of a commercial electromagnetic simulator IE3D and closed form expressions (CFE) obtained by curve fitting technique to the numerical results.
A novel technique for estimating the spatial electromagnetic field distribution and its covariance error is presented based on variogram analysis and the statistical interpolation technique known as Kriging. The spatial structure of some field measurements are characterized by variogram analyses and their propagation properties identified. The physical implications of the Kriging interpolator functional fit to measured data is considered and illustrated. It is concluded that with specialist interpretation this new technique can be used as a valuable checking tool, or to reduce the number of field measurement, in a measurement programme, particularly when the costs of the latter are considered.
A large single reflector corner fed rolled edge compact range system, featuring an elliptical cylinder 12’ (H) x 16’ (W) x 16’ (L) quiet zone has been recently installed in a large anechoic chamber [1]. The Compact Range System parameters, such as reflector surface tolerance of better than 0.001” over the Quiet Zone section of the reflector and superior Quiet Zone field performance at frequencies down to 1.0 GHz were verified and validated. As a part of further studies of potential advantages delivered by the compact range system, the study of the compact range application to Antenna and RCS measurements at VHF/UHF frequencies was initiated. Though the reflector surface tolerance is not an issue at the VHF/UHF bands, successful compact range operation at these frequencies would be a significant expansion of the capabilities of the existing compact range system. In order to evaluate the system performance at VHF/UHF frequencies a number of challenging technical issues had to be resolved and performed. They include: Compact Range Quiet Zone Performance Analysis at the VHF/UHF bands Choice of a concept for a broadband feed suitable for the application and installation within the existing feed carousel Feed Design and Performance Validation Feed Installation in the existing feed carousel Quiet Zone Field Probing and Performance Verification All these issues were addressed in the development of a suitable low frequency feed, and are described in more detail below.