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The need for accurate measurements of antenna parameters particularly for satellite antennas considering their relationship to satellite system budgets has become increasingly important. As a multinational agency, the Space Agency has, over the years, introduced the concept of inter-range comparisons of measurements made on "standard" antennas in order to ensure a of measurement quality from ranges throughout This process has evolved into the concept and application of test range validation and accreditation where the antenna being measured by the range is used not to calibrate the range, but rather as a means of assessing the ability of the range to perform accurate measurements. To this end, ESA has developed a Validation Standard (VAST) antenna which will shortly form the basis of accreditation for european test ranges used for ESA satellite antennas. This paper wilt describe the evolution, objectives, and the proposed application of the ESA antenna range accredition programme and its relevance to the activities of the European Union.
This paper describes the of the National Association of Authorities (NATA) which is for the accreditation of calibration and aboratories in Australia. The development of accreditation criteria in the area of antenna measurements is with the roles of NATA's Electrical Registration Advisory Committee and the National Measurement Laboratory being defined. The evolving EMC regulatory environment which has driven the demand for calibration and testing facilities is outlined. Specific issues addressed include the acceptability of calibrations in terms of traceability from either within Australia or overseas, the validation of test sites, the determination of uncertainties of measurement and the relationship of uncertainties to test limits and specifications. Some of the specific problems encountered at laboratory assessments are highlighted. Finally, NATA's international linkages, which have been established via mutual recognition agreements, are discussed together with their significance for accreditation and the acceptability of measurement results.
This paper describes the implementation of the National Voluntary Laboratory Accreditation Program (NVLAP) for calibration laboratories at the National Institute of Standards and Technology (NIST) NVLAP Office in Gaithersburg Maryland. It chronicles the efforts of the National Conference of Standards Laboratories (NCSL) Total Quality Management (TQM) Committee for Calibration Systems Requirements in demonstrating the need for the program, and NIST's efforts in reaching the decision to implement the program in response to a petition from the NCSL. The development of Calibration Laboratories Program Handbook (NIST Handbook 150), and a companion Calibration Laboratories Technical Guide (NIST Handbook 150-2) for the eight fields of calibration covered by the program are discussed. The recruitment and training of Technical Experts (TEs) who are used in the assessment of laboratory competence are outlined. The issue of proficiency testing as it relates to determination of laboratory competence, and the importance of the program as it relates to international markets via recognition by international accreditation community, are discussed.
Radar Cross Section (RCS) measurements are often performed in discrete frequency bands for a variety of reasons. Although some indoor ranges are capable of performing very wide-band measurements (with bandwidths up to or exceeding 9: 1), some are designed with very rigid illumination requirements on the coIIimating reflector(s) that can only be met over a narrow band. In addition, the bandwidth available on most outdoor ranges is limited by "ground plane" effects which make it impossible to maintain an adequate broadband field over the target. Often, RCS measurements are limited to half an octave at most. Since resolution in RCS imaging is directly proportional to bandwidth, there exists a need for concate nati ng several discrete bands of measurements into a single continuous band. This resulting band must be free of both amplitude and phase discontinuities that would affect the quality of the resultant image. This paper discusses the sources of discontinuities between measured bands on both indoor and outdoor ranges, and provides algorithms for removing them using linear filtering methods. Data is presented from an outdoor range illustrating the results on targets up to 70-feet in length.
Radar scattering can be modeled as a sum of contributions from a finite number of canonical scattering centers. These canonical scattering centers (edges, corners, specular points) all have different amplitude behavior as a function of frequency. We completely characterize this behavior with a single parameter in a parametric model of the scattering data. The estimation of this amplitude dispersion parameter along with down range location and rela tive amplitudes is presented.
ISAR images are formed by Fourier processing coherent wideband responses collected with angle diversity. Unfortunately, physical and practical considerations limit the frequency and angle diversities achievable. The finite diversities induce sidelobes, which are usually mitigated by application of tapered windows in the spectral domain. This procedure reduces image sidelobes at the cost of increased mainlobe width, thus degrading resolution. Spatially-Variant Apodiz.ation (SVA), a new non linear method developed at ERIM to improve the quality of SAR imagery, reduces sidelobe levels while preserving the mainlobe width corresponding to unwindowed data. In contrast to conventional window techniques which simply apply the same window function to every image element, SVA operates on the image by adaptively applying a window optimized for each spatial element. The algorithm uses phase information available from the coherent RCS data to distinguish processing sidelobes from correct responses. Mainlobes are passed using rectangular weighting, while sidelobes are reduced or eliminated entirely. This paper discusses the concept, theory, and implementation of SVA for ISAR imaging, and summarizes capabilities and limitations of the method. Results using SVA are presented and compared to conventionally windowed one- and two-dimensional images. The sensitivity of the procedure to additive noise and phase errors is investigated
A new approach to microwave tomography only requiring monotonic RCS data is presented. The amplitude and phase variation of signals backscattered from the target are measured in uniform angular increment and then analyzed by using wavelet transform. The wavelet transform with multiresolution property is suited for transforming the measured data into aspect vs. Doppler frequency (due to the phase variation of the rotating scatterers) domain. The scatterers location can then be derived by extracting the Doppler frequency variation and peaks occurence delay from the resultant 2-D representation, hence it makes the microwave tomography possible. Two discrete points targets are considered and the resultant microwave tomograms are shown. In our works, the entire processing can be completed in less then ten minutes for a 41 x 41 pixels tomogram nmning on 80486 DX-33 PC and only with single frequency illuminating signal. Furtherrnore, the scattering mechanisms are clearly identifiable in the resultant 2-D representation which can not be achieved by any other microwave tomography methods.
An efficient maximum likelihood (ML) estimator to obtain the scattering center locations of a target and the relative scattering level of these scattering cen ters from the scattered field data is described. In the proposed method, ML estimation is carried out in the image domain rather than in the frequency-aspect do main. Inverse Fourier transform is used to transfer the scattered field data from frequency-aspect domain to the image domain (down range-cross range). As ex pected, the scattered field data in the image domain have some major lobes. The location and shape of the major lobes are used to obtain the initial guess for the ML estimator. The scattered field data samples in the major lobe regions are then used for ML estimations. It is shown that by carrying out the ML estimation in the image domain one can increase the computation efficiency by an order of magnitude.
A digital processing technique capable of forming fine resolution ISAR imagery of air vehicles in dynamic flight is presented. The interactive algorithm is predi cated on the ability to isolate one or two point-like scat terers in the target signature. Phase information extracted from these prominent point scatterers is pro cessed to yield high fidelity estimates of target motion over the image formation interval. Motion estimates are subsequently used to perform conventional ISAR motion compensation and to achieve equi-angular spa tial sampling between radar pulses. Existence of promi nent points obviates the need for any auxiliary information, such as on-board inertial navigation data, and permits focusing of images from non-cooperative targets. The processing procedure is illustrated with X band measurements of a Convair CV580 aircraft taken by the Ground to Air Imaging Radar (GAIR) system.
The utility of high resolution ISAR data in the devel opment and maintenance of low observable (LO) and conventional aircraft and the identification and charac terization of threat aircraft is well established. However, the task of ISAR image RCS interpretation is difficult. Often imaging effects introduced by rotating blades and jet engine modulation (JEM) can compound the already difficult interpretation task. It is easy for these effects to be obscured, ignored, or erroneously misinterpreted in ISAR down-range versus cross-range (Doppler) imag ery and range compressed versus time domain data. This paper presents cases of amplitude and phase modulated ISAR data collected from two airborne targets; a propel ler driven airplane and a helicopter, using a linear FM waveform radar. This will be supplemented with mathe matical models describing the modulation phenomenon and the resultant imaging effects
As mobile and satellite phased array antennas move from to concept production the demands on test station throughput increases dramatically. Completely characterizing a Transmit/Receive (TIR) module may require thousands of S-parameter measurements under CW and high-power pulsed conditions, as well as, harmonics, spurious, and noise figure measurements. The measurement throughput of instrumentation used in characterizing the prototype TIR modules simply may not be capable of handling the added volume of a production environment. The volume of measurements, the multiport nature of the device, and the integrated TIR module control make it necessary to reexamine the traditional approaches of separate network analyzers, noise figure meters, and spectrum analyzers. The result is a high-speed modular test ystem that completely characterizes the device in a single connection. The system contains a single receiver and a dedicated controller that utilizes the instrumentation in the most efficient method while maintaining or increasing the accuracy of traditional approaches. This paper describes the high-speed test stations that have been designed and built and are currently in use in several production facilities. Test system architecture is discussed and measurement throughput numbers are given and compared to conventional approaches.
The radiation characteristics for an active phased array receive antenna operating at K Band were measured at the Ipswich Research Facility. On-pole and cross-pole radiation patterns were measured for several scan angles. In this paper we'll discuss the general design of the antenna and the instrumentation ensemble used to perform the far field and near field characterization of this antenna. Measurements taken on a 2600 foot far field range vs. a near field planer scanner are compared.
Norden Inc. has developed and instrumented its JSTARS 1000' Outdoor Antenna Range with a multi-port antenna measurement system designed to acquire antenna data (patterns and other related signals) at the antenna's respective radar system's intermediate frequency (IF). The measurement system utilizes the JSTARS RF microwave receivers attached to the multiple channels of the JSTARS antenna. These receivers obtain the RF signal from these multiple channels and provide the IF signals to the measurement system.
The radar scattering from a small communications antenna mounted on a large cylinder was measured at the Ohio State University ElectroScience Laboratory compact range. This paper will describe the experimental measurement techniques and the details of the analysis of the experimental. The small (5 cm) blade/slot/cavity antenna was mounted on a 1.82 meter long cylinder of 0.61 meter diameter. The cylinder was treated with RAM on the ends to reduce the direct and interactive end scattering effects, and was mounted in the OSU compact RCS measurement range. Measurements over the 2 to 18 GHz band both with and without the antenna were made and the results subtracted during the calibration effects to further remove the end effects. We will demonstrate these techniques and evaluate their effectiveness. ISAR imaging of both the antenna and the scattering term associated with the load on the end of the antenna transmission line will be shown. This will demonstrate that the transmission line and loan can be separately evaluated using such techniques. A time frequency distribution (TFD) analysis technique will also be demonstrated as a means of extracting various antenna resonance terms from the data. A description of the theoretical computation of the scattering will also be given and the special aspects of this problem outlined. The theoretical RCS data will be compared to the experimental measurements of the RCS.
This paper describes final results on the of non destructive measurement methods of missiles in terms of stealthiness. Measurements performed on full scale missiles allow to determine the reflectivity of the material and give estimation of its real RCS with to its nominal RCS. Different measurement techniques are reviewed, based on the use of coaxial transmission line, circular waveguide and spot-focusing horn lens antenna. Modeling, and characterization of spot-focusing corrugated horn lens antennas operating in the frequency range 2 - 18 GHz are presented. Finally, system configuration of full scale missile RCS measurements currently being utilized for production control is presented.
• Some Radar Cross Section (RCS) measurements contain significant contributions from the interaction of test article components. Usually the direct measurement of these terms is difficult. When these terms are not major factors, they need little attention. In other circumstances they should at least be quantified. There terms are often studied with special models, and/or Doppler measurements, and analysis. These relatively expensive methods yield the required information. For some purposes a more economical, limited method would be useful. RCS measurement and analysis facilities use software designed to present data in usable formats, with appropriate processing. This software is often run on a powerful workstation, or mainframe. McDonnell Douglas Technologies Inc. (MDTI) processing software "runs" on an .HP730 series workstation. The speed and capacity of such a system makes processing data a convenient option. MDTI demonstrated the ability to extract interaction terms from an easily acquired data set. This extraction required only the use of standard data software. Results with generic shapes demonstrate the ability to extract terms > 30 dB below the return of the test article specular return
A vertical array of antennas is used to beamform the farfield used in the measurement of Radar Cross Section (RCS) on a ground-bounce radar range. By properly weighting (attenuating) and phasing (through line length adjustments) each antenna, a desired far-field pattern can be obtained. This paper discusses some benefits of the technique and outlines a basic mathematical approach. Implementation is considered, and wide band ramifications of a practical design are discussed. At RATSCAT, this basic understanding was used to examine a simple two element array. This paper preceded that study and was originally written just for that purpose.
A technique is presented for obtaining the radiation patterns and the antenna gain of elliptically polarized antennas from two vector measurements of the far-field. The two measurements correspond to different polarizations which can be obtained by rotating one of the antennas around its boresight axis. The discussion emphasizes a particularly interesting case, for which accurate radiation patterns and gain of the antenna under test (AUT) can be obtained without prior knowledge of the polarization of the second antenna. The radiation pattern of a nearly circularly polarized (CP) antenna is conveniently represented by the CP co-polarized and cross-polarized components. The axial ratio and any other quantities commonly used to specify the antenna polarization can also be obtained since the pair of initial vector measurements completely characterize the polarization of the AUT. The technique is illustrated by measurements of a CP patch antenna.
This paper demonstrates a high speed antenna measurement capability recently developed in the MDTI Radar Measurement Center. Originally constructed as a Radar Cross Section facility, the RMC has added the capability to measure antenna patterns on apertures up to 40-feet in length in the far field. Data will be presented to demonstrate system performance through the use of modern output formats, such as global plots and videotape presentations.
For an anechoic chamber design, one normally spec ifies the field quality throughout the quiet zone in terms of the ripple level requirement. The ripple in the quiet zone field is caused by the interfer ence of various stray signals with the desired plane wave. The stray signals in an anechoic chamber can come from absorber or other parts of cham ber. However, from a range performance point of view, it is more important to know the ef fects of stray signals on the measurement accu racy of an antenna radiation or target scattering pattern. Consequently, it is very critical to eval uate how the chamber stray signals will affect a given measurement. This paper addresses this is sue by simulating pattern measurements of a phase scanned array in a compact range and discuss the effects of various stray signals associated with the scattering from absorber walls and feed spillover.