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This paper presents data from facility evaluation tasks on current projects. The data were obtained on outdoor free-space pattern test facilities, and in anechoic chamber RCS test facilities.
An investigation of the use of array mutual coupling measurements, to evaluate displaced phase center antenna (DPCA) performance, is made. The details of a subscale space based radar (SBR) DPCA phased array and the array mutual coupling technique are discussed. DPCA results are quantified experimentally under a number of test conditions. It is shown that the test array beam decorrelation computed from array mutual coupling data, is in good agreement with both theoretical predictions, planar near field measurements and direct far field measurements.
This paper presents some current measurement results obtained as part of a research program to investigate the theory, technique, apparatus and practicality of monostatic near-field radar cross-section measurement (MNFRCSM).
A novel technique for improved accuracy of sidelobe measurement by planar near field probing has been developed and tested on the modified near field scanner at the National Bureau of Standards. The new technique relies on a scanning probe which radiates an azimuth plane null along the test antenna’s mainbeam steering direction. In this way, the probe acts as a mainbeam filter during probe correction processing, and allows the sidelobe space wavenumbers to establish the dynamic range of the near field measurement. In this way, measurement errors which usually increase with decreasing near field signal strength are minimized. The probe also discriminates against error field which have propagation components in the direction of mainbeam steering, such errors may be due to multipath or scanner Z-position tolerances. Near field probing tests will be described which demonstrate measurement accuracies from tests with two slotted waveguide arrays—the Ultralow Sidelobe Array (ULSA) and the Airborne Warning and Control System (AWACS) array. Results show that induced near field measurement error will generate detectable far field sidelobe errors, within established bounds, at the –60dB level. The utility of te probe to detect low level radar target scattering will also be described.
There are several background return sources on the Ohio State University Compact Radar Range which affect the sensitivity, accuracy, and dynamic range of the measurement. This paper discusses the magnitude and time delay of the principal background “clutter” mechanisms. Next, data on the time drift properties will be presented, and the relation to system temperature and other physical variations will be discussed. Finally, the impact of system design and operation concepts on these performance factors will be discussed.
The Ohio State University (OSU) ElectroScience Laboratory (ESL) utilizes a parabolic reflector as part of the compact range system [1]. It is necessary to probe the plane wave zone of this reflector in order to measure the purity of the plane wave that is generated. Variations in the amplitude or the phase of the signal received by a probe antenna as the probe is moved linearly across the plane wave region indicate deviations from a pure plane wave in the test zone.
The accurate measurement of radar target scattering properties is becoming increasingly important in the development of stealth technology. This paper describes a low cost imaging Radar Cross Section (RCS) instrumentation radar capable of measuring both the amplitude and phase response of low RCS targets. The RCS instrumentation radar uses wideband FM wave-forms to achieve fine range resolution providing RCS data as a function of range, frequency and aspect. With additional data processing the radar can produce fully focused Inverse Synthetic Aperture Radar (ISAR) images and perform near field transformations of the data to correct the phase curvature across the target region. The radar achieves a range resolution of 4 inches at S-band and a sensitivity of –70 dBsm at a 30 ft range.
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.