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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.
Accurate RCS measurements of joints and cracks on a large vehicle have been difficult to to the limited quiet-zone size of indoor RCS measurement ranges and the high cost of the test model. An approach proposed here uses both simple RCS measurements of a planar test model and efficient analysis to evaluate the three-dimensions (3-D) RCS contribution of joints and cracks on an entire vehicle. Several test models with steps and grooves on planar and curved surfaces were constructed and the results of this approach were compared with actual 3-D RCS measurements. All comparisons showed very good agreement. The main advances of the measurement technique are simplicity, cost effectiveness, and its vast application to many complex discontinuity scattering problems.
The need for practical solutions to radar scattering in high-frequency regime have led to the development of a number of approximation methods. The high-frequency asymptotic methods use approximations based on physical optics (PO), geometrical theory of diffraction (GTD) or physical theory of diffraction (PTD) and their variations. Radar scattering from electrically large conducting surface includes traveling surface wave contributions which are not accounted by the high-frequency asymptotic methods. A hybrid method integrating GTD and traveling wave theory (TW) is used for verification and to illustrate important scattering mechanisms that influence radar cross section (RCS) of a wedge. Analysis of the wedge RCS signature identifies significant contributions of the traveling surface waves to the total RCS. Both measured and predicted RCS of the wedge are considered. Using hybrid GTD-TW method very good agreement between the predicted and measured RCS patterns is observed for all angles.
The Benefield Anechoic Facility, Edwards AFB, California contains a large anechoic chamber for avionic integration test and evaluation. Because of the large chamber size, operational tests can require high-power aircraft radar emissions. To define the range of energy safely accommodated by currently installed radar absorbing material (RAM), a detailed analysis was performed and the results presented. The incident radar energy generates a heat transfer to the RAM. The RAM boundaries dissipate heat through convection, conduction, and radiation. A finite-difference solution demonstrates the temperature distribution in the material varies with the angle and polarization of the incident electric field. Discussions include the use of the RAM thermal characteristic's pretest evaluation to improve operating capability determinations and to facilitate assessment of customer requirements.
This paper will discuss the system level design of a radome test system implemented in a compact range. The system includes a tracking pedestal controlled by an autotrack controller, a measurement receiver, a unique five-feed arrangement for the compact range which accommodates both tracking and measurement functions, and a laser autocollimator for coordinate system referencing. Key elements of system design include the required coordinate system transformations, the mechanical design of the positioning system and its contribution to the system error budget, the dynamics of the tracking system, and the synchronization of the autotrack controller, the measurement receiver, and the system controller. These aspects of system design will be discussed and measurement and analysis results will be presented.
A PTD (Physical Theory of Diffraction) code has been developed to analyze the quiet zone performance of serrated edge compact range reflectors. This PTD code utilizes a modified Ludwig's method to achieve fast convergence for the PO (Physical Optics) integration and the ILDC (Incremental Length Diffraction Coefficients) method to calculate the diffraction correction due to the edge current. In this paper, key formulations for this approach will be derived. Calculated results for an example range will be shown and compared with other published data.
Due to the new EMC regulations of the European Union (EU) the number of anechoic chambers recommended as EMC test facilities is rapidly growing. Very often, the size and shape of the chamber are dictated by the surrounding building, which may cause asymmetries in the chamber geometry. Sometimes, however the walls and/or the ceiling of the chamber are intentionally sloped to prevent modes from becoming dominant in the cross section. In this contribution, an accurate numerical analysis method, presented and verified at the AMTA '92 and '93 meetings, us applied to evaluate systematically the influence of these geometrical asymmetries on a specific example. It is shown, that the influences of the asymmetries are strongly dependent on the frequency and the actual shape of the chamber. Both parameters wok together in such a complex manner, that ordinary trial and error methods, often used for this purpose are unsuitable for an accurate analysis.
One critical issue in designing absorber for an anechoic chamber is the bistatic scattering performance of the absorber and its effect to the quiet zone field quality. The bistatic scattered fields from the absorber side walls, floor and ceiling of the range result in undesired stray signals which can cause significant measurement errors. Consequently, it is very important to analyze the performance of the absorber from the overall system point of view; i.e., the performance of the absorber in the range environment. This paper will address this issue and present calculated results of absorber wall performance for a compact range with a blended rolled edge reflector.
The Three-Antenna gain method is commonly used on far-field ranges to determine an antenna's absolute gain. This is especially true when no other calibrated antenna is available. This method has been used for years by calibration laboratories such as NIST to calibrate probes and gain standards for far and near-field ranges. In some cases, the calibration is too costly or does not meet the schedule requirements of the near-field test range. An alternative is to calibrate the probe or gain standards directly on the near-field range. In this paper we present the results of a study done to show the accuracy of the Three-Antenna gain method when used on a near-field range. An extensive error analysis is presented validating the utility of this method.
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.
• 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
Typical high-resolution dynamic target imaging radars have frequency scan rates that do not properly sample the modulation from rotating structures such as aircraft propellers, engine turbines and helicopter blades. This results in the scatterer modulation energy being aliased. Moreover, if the chirp rate is too slow blurring and of the scatterer can occur in the image. Often the utility of this data for RCS signature analysis is questioned. This paper addresses the utility of images generated from undersampled data of modu lating scatterers. Experimental results using various combinations of chirp scan, modulation, and target-body rotation rates are presented. Fast scan rates, typical of the Linear-FM waveform, are compared to the slower scan rates commensurate with step frequency wave forms. Images are shown illustrating how the different chirp speeds alter the two-dimensional image of a mod ulating target.
A proper knowledge of clutter characteristics is critical to the design, development, and test of military seeker and radar hardware. The Clutter Mapping System under construction at Flam & Russell, Inc. is simple yet powerful tool for the evaluation of potential radar sites or the analysis of current sites. It provides a maximum 40 foot synthetic aperture that can image a 60 degree sector of terrain out to a 20 mile range and beyond. Aside from this primary mission, it has the capability to perform RCS measurement of non-cooperative ground targets or to serve as a tactical, quickly deployed imaging system. Totally self contained, and transportable, this system can fulfill a wide variety of RCS measurement needs.
Martin Marietta has developed a new, automated facility for high-volume production testing of the Longbow millimeter wave missile. Two dedicated far field anechoic chambers were designed, both automated to support component test and analysis in the production environment. One standard far field chamber is used to perform the complete characterization of the antenna and rac1orne; it allows very accurate measurements of power sidelobes, monopulse errors, and cross polarization isolation. The completed radar missile sensor group is evaluated in the second far field chamber, which can reach higher-level parameters of the antenna, transceiver, and gimbal. This paper describes chamber and test station capabilities; time reduction benefits; and the novel, new assembly technique which allows for future portability of these chambers with limited downtime.
Test sequencing flexibility and high throughput are essential ingredients to a state-of-the-art near-field test range. This paper will discuss methods used by NSI to aid the operator through the near-field measurement process. The paper will describe NSI's expert system and customer applications of a unique test and processing sequencer developed by NSI for optimizing range measurement activities. The sequencer provides powerful control of software functions including multiplexed measurements, data processing and unattended test operations.
A Superresolution technique for estimation of the time-delays, and amplitude/phase dispersion of the electromagnetic scattering is presented. In order to estimate the frequency dependence of individual scattering features, a pre-processing technique for the MUSIC (Multiple SIgnal Classification) algorithm is developed and applied to the theoretical and experimental scattering data.
Effectiveness of data extrapolation to generate high resolution radar images is studied. It is shown that polar formatted scattered field data can be extrapolated more effectively than (f, 0) domain scattered field data. The reason for this is that the forward backward linear prediction is not suitable for extrapolating the scattered field data with respect to aspect angle (0). Also, when the scattered field data is extrapolated with respect to frequency to increase the down range resolution, there can be some degradation in the cross range resolution.
This paper demonstrates vertical bistatic Radar-Cross-Section measurement capability in the McDonnell Douglas Technologies, Inc. (MDTI) Radar Measurement Center )RMC). Data will be presented showing the system configuration, system specifications, and predicted and measured vertical bistatic RCF data on a variety of generic targets.
A Superresolution technique for estimation of the time-delays, and amplitude/phase dispersion of the electromagnetic scattering is presented. In order to estimate the frequency dependence of individual scattering features, a pre-processing technique for the MUSIC (Multiple SIgnal Classification) algorithm is developed and applied to the theoretical and experimental scattering data.
The feed support struts often cause noticeable strut lobes in the patterns of reflector antennas. For example, strut lobes are apparent in the measured and calculated patterns presented in Ref. [1] for the 8-foot diameter reflector with a prime focus feed. As pointed out in [1], the calculated strut lobes are higher than the measured ones. The reason for the difference is secondary scattering by the oppositely located strut, which was not modeled in the calculated pattern in [1]. Detailed examination showed a difference of about 2 1/2 dB caused by the secondary scattering for this reflector antenna design. The purpose of this paper is to present measured and calculated patterns which explicitly demonstrate the quantitative effect of the secondary strut scattering. This effort is shown by comparing the measured strut lobe levels with the oppositely located strut removed, i.e., by using 3 struts instead of 4 struts. Calculated patterns are also given in which the secondary scattering is modeled.