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In recent years many of the problems making RCS measurements on a compact range have been addressed [1,2,3]. Factors such as ripple and taper in the target zone have been analyzed and existance of lower level effects such as stray radiation in the chamber. This paper discusses this problem and the way it was addressed in the design of the Harris Model 1606 Compact Range shown in Figure 1, 2 and 3. This range was designed to operate from 2 to 18 GHz with a six foot quiet zone with extension of the frequency range to 95 GHz possible.
This paper describes specially constructed instrumentation and positioning systems used in evaluating RF absorber, discusses measurement techniques, and presents data and conclusions from current programs. The selected absorbers which were evaluated are typical of those used in anechoic chambers and terminated ranges for antenna, radome and RCS testing.
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).
The Compact Range RCS Measurement System is comprised of the Harris Shaped Compact Range and the Hughes Short Pulse Coherent RCS Measurement System. The range offers a 10 foot spherical quiet zone with less than ±0.25 dB amplitude ripple, 0.2 dB amplitude taper, and ±2 degrees phase ripple. The short pulse system offers a pulse width as small as 5 nsec with range gate increments of 100 psec minimum. The system has a sensitivity of –70 dBsm without integration and –120 dBsm with 50 dB of coherent integration. System linearity is better that ±0.5 dB over the 70 dB instantaneous dynamic range. The Shaped Compact Range offers nearly 98 percent illumination efficiency with negligible spillover which minimizes the required anechoic chamber size and the amount of absorbing material necessary. The block diagram of the system is shown in Figure 1.
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.
There has been renewed interest in RCS measurements recently especially for the evaluation of low backscatter targets. In order to accurately measure such targets, one needs to evaluate the system performance for such applications. One such performance check is the field quality measured within the target test volume. The question is then asked, “What is a satisfactory amplitude and phase requirement?” The normal 1 dB amplitude specification is not satisfactory because it doesn’t indicate whether the error is due to a field taper or ripple. Taper indicates a uniform phase but variable amplitude; while, ripple indicates the presence of a stray signal. This paper indicates why one should require less than a 1/10th dD ripple error for low RCS target measurements; whereas, a dB taper is satisfactory.
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.
The two measurements PCAL / PMRC and PTARG / PMRT are ratioed and the PMRC / PMRT term accounts for changes in both power or phase since calibration, because the mid-range is of fixed RCS size and phase. Using this technique, Scientific-Atlanta has been able to hold calibrations to within 0.5 dB amplitude and 8 degrees phase for as long as 12 hours. This includes outdoor range effects.
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.
Versatile test bodies are extremely useful for RCS measurement facilities for many reasons, some of which are listed below: 1) evaluate the performance achievable for a given measurement facility 2) measure the RCS of components normally mounted on a ground plane, and 3) terminate a target pedestal in order to measure its cross-section since most pedestals are designed to attach directly to a target. In order to perform all of these functions a versatile test body should have flat sections to mount components efficiently, it should have a known smooth cross-section with angle of incidence from very low values to large ones, it should not use absorber that could attenuate the signal meant to illuminate the component pieces being tested, etc. Several such test bodies have been studied, some of which will be described.
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.
Pulsed systems have been used for many years to eliminate unwanted clutter in RCS measurements, but have not been used much for antenna measurements, even though similar clutter problems are common to both. There are many reasons for this, such as cost, increased bandwidth requirements, lack of necessary hardware, etc. However, with the development of modern pin diode switches, one can construct a low cost pulsed measurement system that simply adds to existing CW equipment. Using the system design presented in this paper, one can eliminate unwanted clutter from antenna measurements simply by adjusting the transmit and receive pulse widths and the delay between them. For example, it can be used to range gate out the ground bounce for outdoor measurements or the backwall for an indoor facility so that one can accurately measure the backlobe of a high gain antenna. The pulsed system is presented along with several measured examples of its use.
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.
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.
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.
Paper not available for presentation.
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.
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.