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Antennas that are used aboard next generation airborne, maritime and ground vehicles are increasingly required to satisfy both conventional radiation pattern and gain requirements as well as new radar cross section (RCS) requirements. In response to these requirements, Nurad and ORBIT/FR recently completed design, installation, and verification of a high performance, multi-purpose antenna and RCS measurement facility at the Nurad site in Baltimore, Maryland. This compact range facility features a 60x36x26 foot shielded anechoic chamber and a precision machined, serrated edge, offset-fed reflector system that produces a 5.3’H x 8’W x 8’L quiet zone over the 2-50 GHz frequency range. The facility includes a unique feed room structure that positions the primary radar components close to the feed mount for RCS measurements, and allows for easy change of compact range feed antennas. A removable pylon assembly is used for test body support during RCS testing, and a unique add on section to the pylon rotator allows for inclusion of a roll axis that enables measurement of small and medium size antenna assemblies without removing the pylon. Measurements performed on low RCS standard targets and antennas made in the chamber demonstrate that the chamber provides a high performance measurement environment while providing ease of use and rapid configuration and target changeover.
The incident .eld in the quiet zone on a compact range has been probed in a two dimensional grid. From the probe data, error sources have been identi.ed and a model have been created for the quiet zone. The model can be used for antenna measurement simulations. This could be used to calculate the expected interference levels in a real measurement. The measurement range analyzed here has a single, diagonally fed, serrated re.ector. The room measures 11×11×21 m3 and the quiet zone is a cylinder with 3 m diameter and length. The analysis have been done with two dimensional FFT and MUSIC for identi.cation of the directions to the error sources. Then the complex amplitudes for the major directions have been solved. The identi.ed error sources models the quiet zone ripple, in addition to that, the model includes a taper which models the feed.
The Geometrical Optics characteristics of single parabolic reflector compact range systems are presented in rules of thumb for amplitude taper, phase taper and cross polarization. This is illustrated on four different range configurations (two different focal lengths and two different offset angles). Also the influence of the feed system in regard to far field diagram and alignment is discussed for typical low and medium gain corrugated feeds. No diffraction effects are discussed in this paper. With the use of the rules of thumb, a fast and yet precise qualitative and quantitative analysis, optimization and trade off can be made for a compact range optimized for the available space as well as the application.
It is vital for many future scientific remote sensing satellite missions to develop accurate measurement techniques for high-gain sub-mm wave antennas. At microwaves and longer millimeter wavelengths, the measurement techniques are well established and several error compensation methods have been introduced. This paper proposes a novel error compensation technique suitable for compact antenna test ranges (CATRs) at sub-mm wavelengths. The method is based on antenna pattern comparison (APC). In the APC-technique, several antenna patterns are recorded at different positions in the quiet-zone field and the corrected pattern is obtained by averaging the measured patterns. In the proposed technique, the relatively small feed antenna of the CATR is moved instead of moving the heavy combination of the antenna under test (AUT) and the rotation stage. This is much easier to accomplish. The applicability of the proposed method is studied and the method is demonstrated by a combination of quiet-zone measurements and simulations of the antenna measurements in a hologram based compact antenna test range at 310 GHz. For verification purposes the results with this method is compared to the results with the conventional APC-technique.
We have constructed a hologram based compact antenna test range (CATR) and tested its performance at 650 GHz. A hologram of 0.93 meter in diameter was used as the focusing element of CATR. The test was done to demonstrate the feasibility of the hologram based CATR at high submillimeter wave frequencies. A suitable substrate material was found for the hologram. Direct laser writing of the hologram pattern combined to chemical wet-etching was used as the manufacturing method. The quiet-zone field was probed using a planar scanner. For an adequate dynamic range, a backward-wave oscillator (BWO) was used as the transmitter and a Schottky diode harmonic mixer as the receiver. The results from the quiet-zone testing are good. The applicability of the hologram based CATR for high sub-millimeter wave frequencies is considered on the basis of the results of this work.
Computer generated holograms can be used as collimating elements in compact antenna test ranges (CATRs). Recently, a 1.5 m parabolic antenna, the ADMIRALS representative test object (RTO), was tested at 322 GHz using a hologram based CATR that was built specifically for these tests. In this paper, the construction of the compact range is discussed. A 3meter hologram was used to realize a 1.8 meter diameter quiet-zone. The measured quiet-zone field amplitude and phase and the measured H-plane radiation pattern cut of the RTO are presented. The measured -3 dB beam width of the antenna was 0.050º in the H-plane.
In this paper, we present an analysis of the impact of positioning errors on the performance of the GDAIS circular image-based near field-to-far field RCS transformation (CNFFFT). The analysis is part of our continuing investigation into the application of near fieldto-far field transformations to ground-based signature diagnostics. In particular, the analysis focuses on the errors associated with ground-to-ground, near-field, whole-body measurements where the radar moves on a nominally circular path around the target. Two types of positioning errors are considered: slowly-varying, long term drift and rapidly-varying, random perturbations about the nominal circular path. The analyses are conducted using simulated data from a target comprised of an array of generalized point scatterers which model both single and multiple interactions on the target. The performance of the CNFFFT was evaluated in terms of the angle sector cumulative RCS statistics. The analyses were performed as a function of frequency for varying amounts of position error, both with and without (approximate) motion compensation. As expected, the results show that the CNFFFT is significantly more sensitive to rapidly-varying position errors, but that acceptable performance can be achieved with motion compensation provided an accurate estimate of the errors is available.
The time-dependent spectrum of rotating structures presents many significant challenges to radar cross section (RCS) test design, instrumentation parameter selection, signal processing methodology, data analysis, and data interpretation. This paper presents a multi-dimensional signal processing tool and a suite of associated data products, based on an efficiently scripted test design and execution strategy, that are responsive to the high throughput, high data volume requirements and real time data analysis demands associated with rotorcraft testing. We specifically address the NRTF’s realization of a suite of spectral, cepstral and statistical signal processing tools supported by animation that facilitate near-real time parametric data analysis and interpretation.
This paper presents a Radar Crossed Section (RCS) time-domain near-field measurement and its Inverse Synthetic Aperture Radar (ISAR) imaging. The target includes a pyramidal horn and a metallic aircraft scale model. A pulse generator excites the transmit antenna and a digital sampling unit collects the data at the receiving side. A time gating window is subsequently applied to reject the multiple reflections. An efficient 3-D algorithm for ISAR based on time-domain near-field data is presented. The test results for six cases demonstrate excellent ISAR images. In particular the geometry of 3-D reconstructed target can be displayed in perspective manner. The advantage of using time-domain near-field measurements is three-fold. First, it reduces measurement time in the order of one-tenth compared to frequency-domain measurements. Second, it mitigates the multiple reflection effects via time gating. Third, near-field measurements require relatively little real estate which reduces the cost tremendously since a compact range is not needed.
Radar cross section analysis essentially rely on classical spectral analysis methods. By inverse Fourier transforming the scattering coefficients, one can deduce the amplitudes and localizations of the scatterers. Unfortunately, such methods suffer from a lack of resolution since it is tied to the inverse of the extent of the data domain of interest. The use of high resolution spectral analysis can help to overcome these difficulties. Nevertheless, the expected gain of resolution is due to the enrichment of the model that is fit to the data (usually a sum of complex exponentials). One of the key point is then the order of the model, which can usually be found with appropriate criteria (MDL, AIC,…). The amplitudes and positions of the scatterers are finally estimated. The algorithm proposed here performs the detection and estimation tasks at the same time, which turns out to be more robust than conventional sequential algorithms.
This paper is designed to illustrate the technical advances in Network Analyzers and how they can be effectively utilized in an RCS test range. The Hewlett-Packard 8530A [1 - 4] has been utilized in antenna test ranges since the 1980’s and will be used as a reference comparison. Advances in network analyzer hardware and software provide increased functionality, speed and accuracy for RCS measurements. A typical RCS full polarization matrix imaging measurement will be used to illustrate these advances in technology. Range gating, digital and down-range resolution and alias-free range topics will be discussed illustrating the technical advances that can be utilized in an RCS test range. Flexibility of network analyzer hardware will also illustrate the effectiveness of reducing measurement hardware complexity resulting in an increase in measurement speed and accuracy.
Co-polarized and cross-polarized radar cross sections (RCS) are required to completely characterize a complex target. However, it is common for a RCS range to measure only the co-polarized RCS. This practice is primarily due to the inability to produce accurate cross-polarization analysis data for the calibration targets. The most commonly used calibration targets, spheres and cylinders, cannot be used to calibrate cross-polarized RCS due to lack of cross-polarized returns. In this paper, we consider objects that can potentially be used as calibration targets for cross-polarization measurements. Specifically, we numerically study the cross-polarized responses of the Tungsten rod, the grooved cylinder, and triangular dihedrals. Co-polarized measurement data are also included in this initial assessment. From this initial study, we find the counter-balanced dihedral to be a suitable calibration target for cross-polarized measurements.
This paper describes an evaluation of RCS measurements using a free-space focused beam system. Issues including effects from the Gaussian beam width and uncertainties associated with the system have been considered. Measurements and predictions of a generic embedded structure show close correlation over the frequency range of interest and indicates that this technique is ideal for rapid, accurate RCS measurements of physically small features.
RCS data from 8 to 18 GHz on an ensemble of aluminum spheres (dia. 14", 8", 6". and 3.22x) and stainless steel ball bearings (dia. 1.25", 1.0", and 0.75"), as supported by strings in the 9-77 Range, have been collected. For inter-range comparison, the same spheres as supported separately by strings and by a foam tower have been measured in the Millimeter Wave Range (MMWR). By taking selected dual calibration pairs, the uncertainty analyses on the three sets of data show general consistency between the two Ranges, as well as between the two methods of support. In addition, the results allow us to sort out the good spheres for calibration from the bad ones.
There is a trend within the RCS community to use squatty cylinders in place of spheres for calibration. A higher degree of accuracy can be achieved; however, cylinder calibrations require much more precision in the alignment procedures. This effort is doubled when the dual calibration target is also a cylinder. The dual calibration test article could be a sphere thus reducing calibration efforts as long as good correlation exists between theory and measurement sphere data. A series of measurements were collected at the NASA Langley Research Center Compact Range Pilot Facility to study measurement errors of spheres atop foam columns to determine their feasibility for dual calibration use.
In recent years the need for higher quality RCS calibrations has lead to several different calibration technique investigations, such as squat cylinders, bi-cones and hybrids of both. A desirable calibration technique requires: easy implementation, a known theoretical or calculable solution and minimal interaction. The sphere as a calibration target satisfies two of the three requirements. It has no alignment issues and can be easily calculated, but the sphere-holder interaction introduces several dB of error. To reduce this interaction error, a 3D string-reel support system has been developed and demonstrated that significantly improves sphere calibration accuracy. The string-reel sphere positioning system utilizes low dielectric and highly swept strings to achieve minimal calibration error. An additional benefit of this technique allows for field probing and quick quiet zone evaluations.
The paper deals with the Range Book review process, and describes the evaluation of the National RCS Test Facility (NRTF) RATSCAT Advanced Measurement System (RAMS) Central Measurement System (RCMS) Range Book against the criteria approved by the Range Commander’s Council Signatures Measurement Standards Group (RCC/SMSG). Three RCC/SMSG approved reviewers and one observer were charged with reviewing the processes and procedures documented in the RCMS Range Book against published criteria based on the ANSI-Z540 standard. The paper will concentrate on the processed used by the evaluators to perform their task, the training opportunities afforded the observer, the lessons learned by the evaluation team, and the benefits of the process to both the RCMS site and measurement community at large.
An internal research and development project at the Georgia Tech Research Institute (GTRI) focused on cohering multiple apertures into a single distributed aperture. Cohered distributed aperture antenna patterns were collected on the GTRI far-field range for a 1.5 GHz bandwidth at X-band frequencies. Both 1-way and 2-way antenna patterns were measured, with the 1-way antenna pattern measurement requiring coherence on receive only and the 2-way antenna pattern measurement requiring coherence on transmit and receive. The resulting data were compared with the ideal angular resolution and power-aperture gain product improvements from a perfectly cohered distributed aperture, and the results are presented. As measurement techniques were developed for collecting 1-way and 2-way antenna pattern data, sources of potential errors in measurement collection and aperture coherence were identified, with potential methods of error mitigation outlined.
In this paper the electrical displacement of phased array elements along the axis of a linear array, and in the direction normal to the array are examined. A closed-form solution is presented for determining the location of phased array elements from the first and second derivatives of the phase measured on a near-field antenna range. The technique is applied to swept CW measurement patterns of a 20-element, S-band array of open-ended waveguides. It is shown that the electrical location of edge elements differs significantly from the physical location in both x-dimension and z-dimension. The effects of wide array bandwidth on the phase center displacement are illustrated.