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Inverse synthetic aperture radar (ISAR) imaging is used to produce high cross-range and down-range resolution on objects undergoing a change of aspect angle relative to the radar. In this application, the ISAR technique was used on an outdoor ground-bounce radar cross-section (RCS) measurement range. The objective is to locate, identify, and quantify the scattering properties of the target-under-test (TUT). The TUT is mounted well above ground on a target pole and can rotate in azimuth and elevation. The TUT’s rotational motion about an axis perpendicular to the radar line of sight is used to produce the cross-range resolution. For range resolution, a high-bandwidth frequency stepped waveform is used. The data are processed entirely in the digital domain with an algorithm that consists of a procedure to remove the dispersive properties and amplitude variations of the complete end-to-end range response, followed by a two-dimensional, polar-to-rectangular resampling filter and a two-dimensional fast Fourier transform (FFT). The processor has achieved images with amplitude and distortion products that are below the system’s noise floor with up to 48 dB of processing gain. The radar imagery is presented to the RCS engineer on a high-resolution color graphics terminal with true-perspective color-coded RCS displays in logarithmic amplitude or linear phase scales. The design of the ISAR processing algorithm is described in this paper as are the results for both simulated and actual radar data.
In any antenna or RCS measurement range, the walls, floor, and ceiling are covered with radar absorbing material (RAM) so that spurious scattering will be reduced. The bistatic scattering characteristics of these walls etc. are often not accurately known, however. This situation is exacerbated by the techniques often used to measure the scattering characteristics of the RAM used on the walls etc. The measurement techniques are typically “arch type” measurements, where the scattering from a section of absorber (often 3x3 feet) is compared to that scattered by a conducting plate of the same size. These type measurements are often corrupted by edge and corner diffraction terms and the results are often not very accurate.
This paper briefly discusses the calibration techniques used in the Sandia National Laboratories Radar Cross-Section Test Range (SCATTER). We begin with a discussion of RCS calibration in general and progress to a description of how the range, electronics, and design requirements impacted and were impacted by system calibration. Discussions of calibration of the electronic signal path, the range reference used in the system, and target calibration in parallel and cross-polarization modes follow. We conclude with a discussion of ongoing efforts to improve calibration quality and operational efficiency. For an overview description of the SCATTER facility, the reader is referred to the article Sandia SCATTER Facility, also in this publication.
The design of a multipurpose Antenna/RCS range at GTRI is described. A novel approach to design of the far-field antenna range utilizes the bottom 40-foot section of a 130-foot windmill tower. The top 90-foot section is used as the main support for a slant RCS measurement range offering a maximum depression angle of 32º. A 100-tom capacity turntable, capable of rotating an M1 Tank, is located 150 feet from the 90-foot tower. The rigidity and stability of the tower should allow accurate phase measurement at 95 GHz for wind speeds up to 10 mph. In addition, a 500-foot scale-model range uses the ground plane effect to enhance target signal-to-noise and is designed to be useful at frequencies up to 18 GHz. Initially, the radar instrumentation to be utilized with the ranges includes several modular instrumentation systems and associated digital data acquisition equipment at frequency bands including C, X, Ku, Ka, and 95 GHz. The properties of these systems, which include coherence, frequency agility, and dual polarization, are discussed.
The receiver discussed in this paper was developed for Rome Air Development Center (RADC) under Contract F30602-81-C-0261 for testing Electronic Counter Measure (ECM) antenna systems at the Stockbridge Test Annex. This receiver, under computer control, can record ECM responses to threat radar stimuli. The ECM testing required the receiver to have a 400 kHz frequency multiplex rate in the 2-18 GHz frequency band and a 20 MHz amplitude sampling rate capability. An 80 dB interference rejection provides an accurate recording of low level signals in a multiple emitter environment. Although designed for ECM antenna testing, this receiver can have multiple uses for general antenna tests.
This paper describes a digitally controlled receiver-recorder capable of time division multiplexing in the frequency domain at a 400 KHz rate and in the amplitude domain at a 20 MHz rate. Good sensitivity and interference rejection are other features of this receiver which operates over the 2-18 GHz band. It is utilized to obtain a measure of antennas performance as impacted by air frames upon which the antenna(s) are mounted.
The design concept for outdoor antenna ranges operated at frequency 50 MHz is discussed. The antenna range is designed for test of VHF antennas mounted on a full-scale satellite mockup. Due to the large size of test objects, a tradeoff between cost and test accuracy among carious range configurations is addressed. Due to near-omni directional characteristics of test antennas, the multipath interference may be severe. The interference ground reflection, surface wave and multiple scattering are quantified and evaluated.
The fast fourier transform capabilities of the Hewlett-Packard 8510 Network Analyzer provide the basis for an RCS measurement system covering the 50 MHz to 26 GHz frequency range. When used in the broadband mode, fine range resolution is achieved. Vector subtraction and gating capabilities permit the acquisition of accurate data in the presence of strong range reflections. Combining this instrument with a high speed data collection and analysis system yields a powerful RCS measurement capability.
This paper describes the relationship of probe polarization correction to probe-pattern corrected and non-probe-pattern-corrected spherical near-field measurements. A method for reducing three-antenna polarization data to a form useful for polarization correction is presented. The results of three-antenna measurements and the effects of polarization correction on spherical near-field measurements are presented.
This paper describes the equipment, mechanics and methods of one of the outdoor ranges at Teledyne Micronetics. A computer controlled microwave transceiver uses pulsed CW over a frequency range of 2-18 GHz to measure the amplitude, phase and polarization of the signal reflected off the target. The range geometry, calibration and analysis techniques are used to optimize measurement accuracy and characterize the target as a set of subscatterers.
Rapid advances in digital and micro-computer technology have revolutionized automated control of most measurement processes and the techniques for analysis, storage and presentation of the resulting data. Present-day computer capabilities offer many “user-friendly” options for antenna instrumentation, some of which have yet to be exploited to their full potential. These range from vendor-integrated turnkey systems to innovative designs employing a multitude of subsystem components in custom-interfaced configurations. This paper reviews system and component choices keeping in mind their relative merits and trade-offs. Key design considerations are outlined with particular emphasis on: a) Integration and interfacing of different instrumentation, hardware and software subsystems. b) Upgrading and/or designing of completely new facilities. Various other problems, such as vendor package compatability, and those associated with the analysis and application of measured antenna data are discussed. In addition, suggestions are offered to promote the establishment of a mechanism to facilitate the interchange of data between different antenna measurement laboratories and analysis centres.
One of the variables to be quantified when making antenna measurements is position. Without accurate and timely position information, the spatially dependent data cannot be correctly interpreted. Scientific-Atlanta’s 1885 Positioner Indicator and 1886 Position Data Processor offer several improvements in providing position information which can enhance an antenna measurement system. New position indicating techniques have been implemented to allow a higher degree of accuracy and speed than previously attainable. These have been combined with advanced features for automatic system flexibility to create a high performance instrument for many applications. This paper describes the capabilities of these two instruments and how they can be used to improve system performance.
The complete sequence of RF tests required to evaluate the electrical performance of a broad band UHF helix antenna to be used in the zero gravity environment of space is described. The development of an adequate structure which would support the antenna and yet cause no pattern perturbation is mentioned. The test range configuration used, with the UHF antenna inside and anechoic chamber and the source antenna illuminating it through a polyfoam window in one side, is discussed. The problems encountered in taking radiation pattern plots and in making gain measurements using a gain standard near the low frequency limit, 250 MHz, of the antenna test range and the methods utilized to minimize their effect are given in some detail.
RCA-Astro Electronics in Princeton, N.J. designs, develops and tests multiple-beam offset reflector antenna systems in the C and Ku frequency bands for satellite communications. Antenna measurements are performed at the antenna subsystem and the system level and on the complete spacecraft to demonstrate that alignment and performance meet their specification. This paper discussed the antenna range designs and test techniques involved in data acquisitions for contour patterns, cross-polarization isolation and antenna gain characterization. A description of the software required to obtain, analyze and present the data will be included in addition to typical test results.
The purpose of an instrumentation radar is to characterize the Radar Cross Section (RCS) of a target as a function of target aspect and radar frequency. In addition, an instrumentation radar may be used to produce a high resolution radar image of a target which is useful in target identification work and as a diagnostic tool in radar cross section reduction. These purposes differ from those of a conventional radar, in which the objective is to detect the presence of a target and to measure the range to the target. Several different radars are currently used to perform radar cross section measurements. Common instrumentation radars may be classified as CW, Pulsed CW (Low-Bandwidth IF), Linear FM (FM-CW), Pulsed (High-Bandwidth IF) and Short Pulse (Very High-Bandwidth IF). These radars accomplish the measurement task in distinct manners, and it is sometimes difficult to determine where the strength or weakness of each radar lies. In this paper, a set of performance criteria is proposed for RCS measurements. The proposed criteria can be applied uniformly to any instrumentation radar independent of the type of radar design employed. The criteria are chosen to emphasize those performance characteristics that relate directly to RCS measurements and thus are most important to the user. Two instrumentation radars which have been designed at Scientific Atlanta, namely the Series 2084 (Linear FM) and the Series 1790 (Pulse), are used to illustrate the application of the performance criteria.
Wideband RCS instrumentation systems can provide a high degree of range resolution. By combining wideband RCS data with a synthetic-aperture or Doppler processing, the spatial distribution of radar reflectivity can be determined. These systems provide diagnostic capabilities which are useful for locating scattering sources on complex objects and for assessing the effectiveness of modifications. The Proceedings of the 1983 meeting included a paper which described a linear-FM system operating over a 3 GHz bandwidth capable of measuring RCS vs range, cross range, and frequency using a single measurement set-up. This paper analytically demonstrates a procedure for extracting CW RCS patterns from the wideband data obtained using the linear-FM system. By combining the latter and the former processing, it is possible to obtain from a single data array both wideband responses showing the spatial distribution of scatterers and narrowband responses which are the traditional CW RCS patterns. The paper includes experimental verifications of these assertions by comparing results of CW measured data with data extracted from wideband RCS measurements.
In order to accurately determine the far-field of an antenna from near-field measurements the receiving pattern of the probe must be known so that the probe correction can be performed. When the antenna to be tested is circularly polarized, the measurements are more accurate and efficient if circularly polarized probes are used. Further efficiency is obtained if one probe is dual polarized to allow for simultaneous measurements of both components. A procedure used by the National Bureau of Standards for determining the plane-wave receiving parameters of a dual-mode, circularly polarized probe is described herein. First, the on-axis gain of the probe is determined using the three antenna extrapolation technique. Second, the on-axis axial ratios and port-to-port comparison ratios are determined for both the probe and source antenna using a rotating linear horn. Far-field pattern measurements of both amplitude and phase are then made for both the main and cross components. In the computer processing of the data, the on-axis results are used to correct for the non-ideal source antenna polarization, scale the receiving coefficients, and correct for some measurement errors. The plane wave receiving parameters are determined at equally spaced intervals in k-space by interpolation of the corrected pattern data.
This paper presents a three-antenna plane-wave scattering-matrix (PWSM) formulation and a formal solution. An example will be demonstrated in which two of the three antennas are electromagnetically identical (the transmitter and the receiver) and the third (the scatterer) has arbitrary electromagnetic properties. A reduced reflection integral-matrix will be discussed which describes the transmit, scatter, receive (TSR) interaction. An antenna scatterer spectral tensor Greens function is identified. In this formulation the transmit spectrum will be scattered by the third arbitrary antenna (target) and this scattered spectrum may be considered to have originated from a transmitting antenna. Near-field antenna measurement techniques are applicable with determine the electric (scattered) field spectral density function.1, 2 If a second deconvolution is applied, a transmit probe corrected spectral density function or scattering tensor can be determined in principle. In either case, a near- or far-electric field can be calculated and a radar cross section determined.
The purpose of an instrumentation radar is to characterize the Radar Cross Section (RCS) of a target as a function of target aspect and radar frequency. In addition, an instrumentation radar may be used to produce a high resolution radar image of a target which is useful in target identification work and as a diagnostic tool in radar cross section reduction. These purposes differ from those of a conventional radar, in which the objective is to detect the presence of a target and to measure the range to the target. Several different radars are currently used to perform radar cross section measurements. Common instrumentation radars may be classified as CW, Pulsed CW (Low-Bandwidth IF), Linear FM (FM-CW), Pulsed (High-Bandwidth IF) and Short Pulse (Very High-Bandwidth IF). These radars accomplish the measurement task in distinct manners, and it is sometimes difficult to determine where the strength or weakness of each radar lies. In this paper, a set of performance criteria is proposed for RCS measurements. The proposed criteria can be applied uniformly to any instrumentation radar independent of the type of radar design employed. The criteria are chosen to emphasize those performance characteristics that relate directly to RCS measurements and thus are most important to the user. Two instrumentation radars which have been designed at Scientific Atlanta, namely the Series 2084 (Linear FM) and the Series 1790 (Pulse), are used to illustrate the application of the performance criteria.
A monostatic radar measurement system at the U.S. Navy Pacific Missile Test Center (PACMISTESTCEN) located at Pt. Mugu, California was utilized to obtain incidence angle performance of radar absorbing structure (RAS) panels. The traditional methods of obtaining reflectivity data for absorptive materials over a range of incidence angles is a technique known as the NRL arch. Developed over 30 years ago by the U.S. Naval Research Laboratory, the technique utilizes moveable bistatic antennas on an arch equidistant from the test material panel in order to obtain incidence angle data.