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The mechanical rotator must be correctly aligned and the probe placed in the proper location when performing spherical near-field measurements. This alignment is usually accomplished using optical instruments such as theodolites and autocollimators and ideally should be done with the antenna under test mounted on the rotator. In some cases it may be impractical to place the alignment mirrors on the AUT or optical instruments may not be available. In these and other cases, it is desirable to check alignment with electrical measurements on the actual AUT and probe. Such tests have recently been developed and verified. Appropriate comparison and analysis of two near-field measurements that should be identical or have a known difference yields precise measures of some rotator and probe alignment errors. While these tests are independent of the AUT pattern, judicious choice or placement of the antenna can increase the sensitivity of the test. Typical measurements will be presented using analysis recently included in NSI software.
A new edge treatment approach using resistive cards (R-cards) for compact range reflectors is introduced in this paper. This new treatment has proven to be successful in achieving the design goal of reducing the diffracted fields from the reflector edges in the quiet zone. The design key of this new treatment relies on the proper choice of both the resistance distribution and placement of the R-cards in front of the reflector edge. Preliminary analysis and design studies show the potential of this new treatment in reducing the ripple level in the target zone over a wide range of frequencies. The simplicity, flexibility and the low cost of this new approach provide a viable alternative approach to the other edge treatments, such as serrated or blended rolled edges.
The new EMC-standards in Europe have strengthened the requirements for test facilities. In this paper examinations are concentrated on anechoic chambers, which are mostly used for measuring radionoise emissions. To become accredited a chamber have to own excellent performance, which is only possible by excellent absorbers and a careful choice of the measurement axis. A program for the evaluation of anechoic chambers has been developed and recently extended to permeable materials. This allows the calculation of chambers equipped with ferrite tiles or even a combination of ferrite and foam absorbers. Furthermore the numerical code is a very helpful tool during the planning phase of a chamber and offers the possibility to find the best way to improve the performance of older chambers. To estimate the performance the results are compared to the field distribution in an ideal Open Area Test Site (OATS).
Mode stirred chambers are used to perform radiated susceptibility tests on equipment that is expected to operate normally when exposed to electromagnetic (EM) fields. These tests are useful in identifying failure events in airborne equipment, medical equipment, and other electronic equipment that are exposed to EM fields. The two major methods of modal excitation are mechanical mode stirring and frequency stirring. The majority of reverberation chamber tests are done using the mechanical method. Mechanical mode stirring is the process of varying boundary conditions in a complex cavity to ensure that devices in the cavity are exposed to an isotropic and randomly varying electromagnetic field. This is analogous to an amplitude modulated signal, which would consist of a carrier with random amplitude and phase. An analysis of the frequency content of the energy around the carrier is performed, and the effects of this frequency spread on the use of mode stirred chambers for testing is investigated. The understanding of this frequency spread is important in quantifying upsets of equipment in the chamber. Examples of different rotation rates and the corresponding frequency spectra are examined.
Recently, a new method of wide band radar imaging has been developped within the framework of the two dimensional (2-D) continuous wavelet theory. Based on a model of localized colored and non isotropic reflectors, this method allows to obtain simultaneously information about the location, the frequency and the directi vity of the scatterers which contribute to the RCS of a target. We obtain a 4-D data set that we call hyperimage namely a series of images which depend on the frequency and orientation of illumination. In order to exploit efficiently hyperimages an interactive visual display software called i4D has been specifically designed. The purpose of this paper is to present the capabilities of i4D through the analysis of hyperimages constructed from monostatic and bistatic scattering data. The results show that the interactive and dynamic analysis that i4D procures allow to better understand the mechanisms that contribute to the RCS of targets.
A technology for target identification has been developed that is directly applicable to the analysis of the backscattering behavior of targets. For the latter purpose the target is placed on a turntable, and amplitude/phase data are collected over the aspect angle sector of interest, using a radar with sufficient bandwidth to resolve the target in range. For ground vehicles and small aircraft a range resolution of about 1 ft is sufficient. Standard processing is used to form an ISAR image over the appropriate aspect angle sector. The difference relative to the more conventional procedures is that the complex ISAR image, intensity and phase, is analyzed rather than only the intensity. This allows us to identify spurious responses that are generated by certain features on the target, but appear in locations other than those of the features. The analysis of the complex image permits us to associate the genuine image responses with the features responsible for the responses, so that the strength and type of backscattering can be determined for the target features. With respect to the type of backscattering, we can determine whether the effective location of the feature is stable, or whether it drifts with aspect angle or frequency. We. can also determine the effective crossrange and range widths of the various features. The features that can be analyzed are those with responses sufficiently strong to exceed the general background. This is typically a fairly large number.
For frequencies above 30 GHz, RCS reference target method is, in general, more accurate than scanning the field by a probe. Application of mechanically calibrated targets with a surface accuracy of 0.01 mm means that the phase distribution can be reconstructed accurately within approximately 1.2 degrees across the entire test zone at 100 GHz. Furthermore, since the same result can be obtained for both azimuth and elevation patterns, all data is available for the characterization of the entire test zone. In fact, due to the fact that the reference target has a well known radar cross-section, important indication of errors in positioning can be obtained directly from angular data as well. In the first place the data can be used in order to recognize the first order effects (+/- 5 degrees in all directions). Applying this data, defocussing of the system reflector or transverse and longitudinal CATR feed alignment can be recognized directly. Furthermore, mutual coupling can be measured and all other unwanted stray radiation incident from larger angles can be recognized and localized directly (using timedomain transformation techniques). Inmost cases even a limited rotation of +/- 25 degrees in azimuth and +/- 10 degrees in elevation will provide sufficient data for analysis of the range characteristics. Finally, it will be shown that sufficient accuracy can be realized for frequencies above 100 GHz with this method.
This paper discusses the efforts of an on-going research program which has been exploring the use of expert systems (artificial intelligence) techniques to support automated analysis of wideband radar scattering data. The primary objective of this research is to explore and demonstrate the applicability of expert system techniques to the analysis of diagnostic radar images. There are two modes which are being explored. The first is an automated system that would allow lesser skilled (in radar imaging science) individuals to do the work of highly skilled engineers and analysts. The second mode would aid the highly skilled worker with the application and correct implementation of software tools, interpretation of phenomenology, and data quality assessment. In both cases, the expert system should allow for the increase through-put and accuracy of data being analyzed. A software prototype is being developed and tested with real data to demonstrate the feasibility and potential accuracy of such as system.
This paper deals with High Resolution (HR) Filtering. Extracting the frequency dependence of radar scatterers is a common task in Radar Cross Section Analysis (RCS). This is usually achieved with signal processing tools like finite impulse response filters allowing filtering in the range domain. However, when range resolution is poor, it becomes impossible to extract the exact feature since it is not deconvolved in the range domain. We thus propose to use HR methods to overcome these difficulties. These methods are applied to estimate the frequency response of the creeping wave of a small sphere. The results show good agreement with the theoretical response.
Instrumentation Radar systems evolution includes improved stability. Metrologists know frequency within Hertz. Amplitude and Phase variations are low. Ranges check drift with reference systems. Still, with increased capability, expectations of accuracy have increased. Todays instrumentation makes analysis of stability factors practical. This study analyzes Radar Cross Section (RCS) return of a stable target under controlled conditions. Methodology will be an analysis of a constant RCS target return. The target is a stable object at a typical measurement site. Data points are at several discrete frequencies in bands between S and Ku. This study sample is a set of data taken over a 87 hour span with several duty factors. Duty factors will range from minimal 0.1% to 1.5%, near the 2% maximum for the output amplifiers. Acquisition times for data sets are chosen for outdoor temperatures ranging from hot -- desert afternoon -- through cool in the early morning. This data will be analyzed statistically. If statistical correlations exist, analysis will quantify factor contributions with multiple linear regression. Hypothesis: Drift does not correlate to variables such as duty factor, & temperature.
Recently, several deployable, ground-to-ground col lection systems have been developed for the assessment of aircraft RCS on the flight-line. The majority of these systems require bulky rail or scanning hardware in order to collect diagnostic imaging data. The measurement technique described in this paper, while not a "cure-all", does eliminate the need for bulky hardware by allowing the collection system to move freely around the target while collecting radar backscattering data. In addition, a nearfield-to-farfield transformation (NFFFT) algorithm is incorporated in the process to allow the collection of scattering data collected in the near field to be processed and evaluated in the far field. The techniques described in this paper are a part of a data conditioning process which improves the data quality and utility for subsequent analysis by an automated diagnostic system described elsewhere in this proceedings [1]. The techniques are described and demonstrated on numerically simulated and experimentally measured data.
The antenna repair shop of McClellan Air Force Base near Sacramento, California has been involved in the repair of military high frequency antennas for many years. With the acquisition of precision microwave measurement equipment, in the last five years, the antenna shop has developed innovative methods of gauging its antenna material repair processes. This paper will focus on the work done on a LS-band phased-array, satellite ground station antenna. Specific processed examined will be radome-point selection, phased-array receive element cleaning, and radome bonding. The history of the problems that required the repairs will be discussed. Several antenna/radome repair processes and RF test methods will be described. Manufacturer specifications will be examined where available. Included in this paper will be RF test data and data analysis.
At last year’s conference we presented the discrete implementation of an image-based near field to far field transform (IB-NFFFT) for predicting far field radar cross-section (RCS) from spherically-scanned near field measurements, along with some preliminary transform results using numerically-simulated data. This paper quantifies this expected performance in terms of the RCS prediction error (RMS dB difference) using numerically-simulated data for two ten wavelength-long canonical bodies, a thin wire and a conesphere. It will be shown that for the highly-resonant wire target, the NFFFT’s algorithm performance is limited by the multiple interactions resulting from the travelling wave reflections between the end of the wire, except at near broadside aspect angles. Conversely, very good performance is obtained for the conesphere at nearly all aspect angles, except very close to nose and tail-on. We will also shown that the IB-NFFFT algorithm performance is robust with respect to clutter and scan angle coverage.
Millimeter Wave (MMW) Radar Cross Section (RCS) measurements of full scale ground vehicles are used to develop and validate scattering models for smart weapons applications (target detection, discrimination and classification algorithms) and Hardware-in-the-Loop (HITL) missile simulations. This paper describes a series of MMW RCS measurements performed at Range C-52, Eglin AFB FL on a T-72M in a field environment using an exiting instrumentation radar (with slight modifications to allow for accurate height adjustment) and in-scene phase reference. The test methodology, instrumentation systems, 3-D Imaging Algorithm and sample data sets at 35 and 95 GHz will be presented as well as a detailed sensitivity analysis and discussion of error effects.
Radar cross section (RCS) range characterization and certification are essential to improve the quality and accuracy of RCS measurements by establishing consistent standards and practices throughout the RCS industry. Comprehensive characterization and certification programs (to be recommended as standards) are being developed at the National Institute of Standards and Technology (NIST) together with the Government Radar Cross Section Measurement Working Group (RCSMWG). We discuss in detail the long term technical program and the well-defined technical criteria intended to ensure RCS measurement integrity. The determination of significant sources of errors, and a quantitative assessment of their impact on measurement uncertainty is emphasized. We briefly describe ongoing technical work and present some results in the areas of system integrity checks, dynamic and static sphere calibrations, noise and clutter reduction in polarimetric calibrations, quiet-zone evaluation and overall uncertainty analysis of RCS measurement systems.
In 1993, we presented the newly completed compact range and tapered chamber facility [1]. As part of this presentation, the issue of “range certification” was presented. This paper will discuss the work that we have done with the compact range for radar cross section (RCS) measurement acceptance. For customer acceptance, we had to “prove” that the compact range made acceptable measurements for the fixtures and apertures involved. Schedule and funding did not permit the full exploitation of the uncertainty analysis of the chambers, not was it felt to be necessary [2]. The determination of our range capabilities and accuracy was based on system parameters and target measurements. Targets that were calculable either in closed form solutions (spheres) or by numerical methods (cylinders and rods) were used. Finally, range to range comparisons with the Rye Canyon Facility [3] of a standard target was used. The range to range comparison proved especially difficult due to customer exceptions, feed differences, and target mounting. This paper will discuss the “success” criteria applied, the procedures used, and the results. The paper will close with a discuss of RCS standards and the range certification process.
Compact Payload Test Ranges (CPTR) for test zones of 5 meters or larger can be used for both payload and advanced antenna testing. In both cases accurate calibration, including amplitude and phase characteristics across the test zone, is required. Accurate data analysis is needed in order to establish corresponding error budgets. In addition, boresight determination will be required in both measurement types for most applications. Since it may be difficult or even impossible to scan the test zone field using a (planar) scanner, application of a large reference target (a rectangular or circular flat plate) can be seen as in interesting alternative. RCS measurements are then performed and test-zone field characteristics are determined in both amplitude and phase. Time- and spectral domain techniques can provide valuable information as to the location of possible disturbances. The evaluations is complemented with the measurement of a VAlidation STandard (VAST) antenna in combinations with an advanced APC technique. These techniques have been demonstrated at the CPTR at ESTEC, Noordwijk, the Netherlands. Results and practical considerations are presented in this paper.
The calibration of nonreciprocal radars has been studied extensively. A brief review of known calibration techniques points to the desirability of a simplified calibration procedure. Fourier analysis of scattering data from a rotating dihedral allows rejection of noise and background contributions. Here we derive a simple set of nonlinear equations in terms of the Fourier coefficients of the data that can be solved analytically without approximations or simplifying assumptions. We find that independent scattering data from an additional target such as a sphere is needed to accomplish this. We also derive mathematical conditions that allow us to check calibration data integrity and the correctness of the mathematical model of the scattering matrix of the target.
A cost-effective, versatile instrumentation system for measuring antennas and radomes is described. The system features the use of high load capacity, high accuracy stepper motor based positioners as the primary system axes. The system is capable of being easily reconfigured to perform tests on antenna/radome systems with antennas fixed relative to the radome, or with the antenna and radome capable of movement relative to one another. Measurements may be performed at RF, IF or baseband, depending on the portions of the seeker or monopulse assembly to be included in the test. The system also contains analysis capabilities that simulated mode forming and beam forming functions to isolate antenna effects.
Modem antenna measurements require not only fast measurement but also quick transport of data to a multitude of computer for post-processing and analysis. Issues in the of a networked computer architecture for an antenna measurement laboratory are discussed. A case study is presented to highlight some of the considerations. Description and performance of a facility employing the concepts are included.