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SOLANGE is a large RCS indoor measurement facility operated at SHF and V/UHF frequencies. In the V/UHF band, couplings between the target and the walls can be exhibited. These perturbations due to non-directive transmitting/receiving antenna, and non-absorbing walls must be eliminated to derive the intrinsic response of the target. To reduce their levels CELAR introduced smart methods («SAV »: Site Altitude Variable and « EAV »: Environnement Altitude Variable): the transmitting/receiving antenna (and also the target in the EAV method) is translated along the elevation axis, and the acquired data are averaged. CELAR and CEA collaborated to qualify the chamber in the U/VHF band. The aim of the study is to identify and quantify the error sources, and to suggest some improvements. The analysis, based on RCS measurements of canonical targets, includes data processing (clutter reduction) and evaluation of the effects of SAV and EAV on the couplings. A theoretical algorithm is used to assess the performances of the processing, and to optimize measurement altitudes. It introduces an analytical model for the antenna and its images with respect to the walls, and calculates the scattered near field. This study enabled us to suggest improvements in the parameters of the processing, as well as in the RCS facility configuration.
In 2001, the Boeing 9-77 Indoor Compact Range successfully passed the range certification process. In preparation and during the On-Site Review in October 2001, RCS data on a pair of ultraspheres for the dualcalibration were collected. In this paper, we analyzed the data with regard to uncertainty analysis. An empirical approach for compensating the systematic error is presented.
When making large scale RCS measurements on a ground bounce range, EPS foam columns are frequently used as target support structures for test bodies and air vehicles. Thus, the design of foam columns is a key part in preparing for a large-scale outdoor test. Range engineers require foam column design methods and tools that are both efficient and reliable. This paper describes effective foam column design methods and shows comparisons of predicted column RCS to column measurements performed at NRTF. These comparisons give credibility to the concept of foam column modeling and ground bounce range scattering simulations, and give range engineers confidence in their foam column design process.
Tapered chambers have long been used for far-field antenna and RCS measurements. Conventional taper chambers used commercial antennas such as horns or log-period dipoles as wave launchers. One problem of this approach is the movement of the phase center associated with the antenna design. The positioning of the antenna inside the chamber is also critical. Undesired target-zone amplitude and phase distortion are caused by the scattering from the absorber walls. A novel feed antenna design for a tapered chamber is proposed here to provide broadband and dual polarization capabilities. This design integrates the absorber and the conducting walls behind the absorbers into to ensure a stationary phase center over a wider frequency range. In such a design, the dielectric constant of the absorber is utilized to maintain a clean phase front and a single incident wave at high frequencies. The conductivity of the absorber is also utilized to shape the field distribution at low frequencies. As a result, a wider frequency range can achievable for a given chamber size. One trade-off of this design is its reduced efficiency could be associated with the absorber absorption. Some simulation results from a 3-D FDTD model of a prototype design will be presented.
Target support pylons and foam columns have been in use since the late 1970’s to provide target support for RCS measurements. Pylons currently limit our low frequency measurement capability due to the moderately high scattering from the pylon edges. Additionally both foam column and pylon support structures interact with the target scattering which can limit our ability to completely subtract the target support scattering from the target signature data. Target suspension using a string support system has the potential to eliminate these limitations. MRC has recently completed a string support technology demonstration program to identify the critical components for implementing an indoor string support system for RCS measurements. Critical components identified and demonstrated under this program included a survey of string materials for RCS measurements, development of low coefficient of friction swivel bearings, structural target to string interfaces, and three different techniques for providing target rotation. This presentation will highlight the results from the demonstration program showing viability of string support systems to provide an enhanced RCS measurement capability for indoor RCS measurement ranges
This paper presents a method for correcting RCS data obtained from objects extending beyond the boundaries of the test zone into the transition region of a large compact range collimator. The technique, exploiting the non-zero irradiation in the transition regions, uses results of calculated or measured field probes in conjunction with an image-based decomposition of the target angular response to correct for the field taper. The taper correction is developed as a weighting function applied to the spatial distribution with frequency-dependent coefficients derived from the field probes; the corrected RCS response is then obtained by an inverse operation. The paper addresses the conceptual notions of the approach and the limitations inherent to the underlying assumptions. Results of tests on canonical and actual targets are shown to demonstrate the applicability of the technique.
Wide frequency bandwidth feeds are used in compact ranges when multi-octave bandwidth operation of the range is desired. Dual-ridge or quad-ridge horns have been widely used in RCS applications as well as in antenna measurement applications to achieve wide band operation. This selection is made to take advantage of the lower cost of quad-ridge horns vs. other options. In designing a compact range, one primary concern is the beamwidth of the feed over the operating band. This affects the amplitude taper across the quiet zone of the range. Another primary concern is the movement of the phase center vs. frequency of the feed. This directly affects the phase taper across the quiet zone as a result of de-focusing of the reflector. Here we present measured data of the beamwidth and phase center movement vs. frequency of a wide-band quad-ridge feed designed to operate from 2.0-18.0 GHz. Measured and predicted quiet zone performance data over this bandwidth are presented with the feed installed in a Model 5751 compact antenna test range having a 4-foot quiet zone.
In order to better understand the capability and limitation of the radar in the VHF band, we present the results from RCS measurements on simple calibration objects of sizes from small to large. Though the uncertainty for measuring a small object is usually well behaved to within +0.2 dB, the greatest difficulty for a large object is the lack of knowledge on the distribution of the incident field. Some qualitative ideas may be obtained from fieldprobes along a few directions. Yet, a thorough investigation of the field in 3-D as a function of the frequency and polarization is generally beyond time and budget constraints. For the special cases of long and thin cylinders at broadside, we find that the difference in HH-VV is very sensitive to ka, which allows us to distinguish them apart.
The accurate measurement of static Radar Cross Section (RCS) requires precise calibration. Conventional RCS calibration objects like plates and cylinders are subject to errors associated with their angular alignment. Although cylinders work well under controlled alignment conditions, and have very low targetsupport interaction, these devices may not always suitable for routine outdoor ground-plane RCS measurements. We seek a design which captures the low interaction mechanisms of a cylinder, yet can be easily aligned in the field due to its excellent angular insensitivity. In a sense, this target has the best characteristics of both the cylinder and the sphere. This paper will describe the design of a "hypergeoid", a new calibration device based on a unique body of revolution. Calculations and measurements of some elementary hypergeoids are presented.
The National RCS Test Facility (NRTF) has a variety of unique test capabilities. Looking to further expand our testing options at the Mainsite test facility, the NRTF began developing a pit/pylon and rotator shroud test bed capability that would allow for radar cross section (RCS) measurement of test articles that are physically too small to accept a rotator. To reach the desired background RCS levels, the use of an expanded polystyrene foam column was not a viable option. In order to maintain the integrity of the calibrated system and enable the measurements of test articles with and without rotator bays on the same pit/pylon, a pit/pylon and shroud combination was required. Other important considerations that influenced the viability of a pylon system include cost effective mounting/dismounting of test articles, safety of the test articles and personnel, and the effective determination of backgrounds due to a stable and low observable pylon system. Our primary goal was to design and fabricate an inhouse system that met the needs of potential customers while satisfying our own clutter and background criteria. This paper documents the fabrication of the pylon and rotator shroud test bed. The results of an RCS characterization are also presented demonstrating the system’s ability to meet the desired RCS background goals.
Free space, coherent radar cross section measurements on a moving target trace a circle centered on the origin of the complex (I,Q) plane. Noise introduces only small random variations in the radius of the circle. In real measurement configurations, additional signals are present due to background, clutter, targetmount interaction, instrumentation and the average of the time-dependent system drift. Such signals are important contributors to the uncertainty in radar cross section measurements. These time-independent complex signals will translate the origin of the circle to a complex point (I0,Q0). Such data are then defined by the three parameters (I0,Q0), the center of the circle, and st, the radar cross section of the target. Data obtained when a target is moved relative to its support pylon can be separated into phasedependent and phase-independent components using the techniques of (1) three-parameter numerical optimization, (2) least-median-squares fit, (3) adaptive forward-backward finite-impulse response procedure, and (4) orthogonal distance regression applied to a circle fit. We determine three parameters with known and acceptable uncertainties. However, the contribution of systematic errors due to unwanted in-phase electric signals must still be carefully evaluated.
We present a methodology for the design of foam columns useful for the support of targets during static outdoor radar cross section (RCS) measurements. The methodology uses modal solutions along with genetic algorithms to optimize the design of a homogeneous column with resistive layers that provides minimal scattering over the design bandwidth. The methodology widens the design space, allowing for better design trades between electromagnetic and structural column performance. Results are presented for two representative design cases (broadband and spot-frequency narrowband), and the performance of the optimized column design is shown to be significantly better than that of the baseline foam column. Further design improvements are also suggested, including the use of the Born approximation for non-axisymmetric columns.
The bistatic radar signature of military systems is of interest for various applications including performance evaluation of semi-active missile systems, surveillance systems, and survivability assessment. While bistatic radar cross section (RCS) measurements have been made for high frequencies at several U.S facilities, there has been little reported work in low frequency bistatic RCS measurements. This paper presents the results of recent low frequency coherent bistatic RCS measurements from 210 MHz to 1.99 GHz at bistatic receiver angles of 0°, 35°, 70°, 120° and 145°. These measurements were successfully completed at the Naval Air Systems Command Weapons Division Etcheron Valley Range (EVR), formerly known as Junction Ranch (JR), China Lake, California This paper describes the process and provides results of low frequency bistatic RCS measurements on a hemisphere-capped cylinder target. Comparisons are presented of measured data to predicted results from moment method models of the calibration object and the cylinder target. Methodologies used in optimizing RCS data quality are also provided.
There is a conflict between the requirement of a very low RCS target support system, and the need for high stability and accurate target setting. To meet the ideal of measuring targets in free space, multiple string suspension systems from overhead gantries have been devised. Despite measures to the contrary, it was found air turbulence and mechanical vibration could produce complex perturbations of the target during ISAR imaging. Over the frequency range of interest (1-100GHz), even sub-millimetre disturbances can produce significant and unwanted image artefacts. Model code was written to provide representative parametric dynamic models for the oscillatory motion of the targets. Modelling results over a wide range of motion patterns, acquisition configurations, and radar parameters allows a quantitative assessment of the limitations and validity of ISAR imagery. Image degradation is affected not only by the amplitude of the target’s motion, but also by its direction, and relationships between the radar frequency sweep rate and characteristic period of oscillation. The benefits to image recovery of data averaging and frequency sweep randomisation are examined. A motion-correction system is discussed, based around a video photogrammetry system that provides a record of a target’s 3-dimensional motion during data acquisition. This work was carried out under the UK Ministry of Defence’s Corporate Research Programme.
This paper will draw the attention to a revolutionary new, extremely mobile and flexible approach on a nearfield test facility concept for outdoor measurements. After addressing the current measurement dilemma, the potential measurement objects are indicated, covering application areas in telecommunication, defense, air traffic management, research and verification of outdoor antenna & RCS test facilities. Further an outlook will be given on the future and urgent necessity on measurements of the radiated performances of outdoor antenna installations. The presented antenna test facility is based on a remotecontrolled and floating platform, enabling probing of electromagnetic fields within relatively large air volumes of up to 100 x 100 x 100 meters. In combination with precise position techniques, accurate measurements of up to 20 GHz are considered to be achievable. The design philosophy and system concept will be explained. The paper concludes with a prediction on the system performance and with a brief realization schedule. The proposed ANTF concept will allow detailed radiation analyses in unprecedented depth and quality, representing a real breakthrough in characterizing electromagnetic fields in open air test sites (OATS).
A calibration uncertainty analysis was conducted for the Air Force Research Laboratory’s (AFRL) Advanced Compact Range (ACR) in 2000 [1]. This analysis was a key component of the Radar Cross Section (RCS) ISO-25 (ANSI-Z- 540) Range Certification Demonstration Project. The scope of the RCS uncertainty analysis for the demonstration project was limited to calibration targets. Since that time we have initiated a detailed RCS uncertainty analysis for a more typical target measured in the ACR. A “more typical” target is one that is much larger with respect to wavelength than the calibration targets and characterized by a wide dynamic range of RCS scattering levels. We choose a 10’ ogive as the target due to the fact it is a large target, exhibits a wide dynamic range of scattering, and the scattering levels can be predicted using readily available CEM codes. We will present the methodology for the uncertainty analysis and detailed analyses of selected component uncertainties. The aspects of the uncertainty analysis that are unique to the “typical target” (i.e., a non calibration target) will be emphasized.
When making large scale RCS measurements at outdoor ground bounce ranges, vector background subtraction is often not performed. To get a clean measurement, range engineers must control the backscatter from the target supports that mainly dictate the background level. Presently, nearly all ranges use single high gain antennas to concentrate incident field energy onto the target, but single antennas have physical limitations for controlling the incident field energy in the target support region. To improve the incident field distribution, an array of transmitting elements can be used instead of a single radiator. With an array, engineers can control the illumination in both vertical and cross range dimensions, making it possible to concentrate the incident field energy on the target while reducing the field level over the target supports. This paper describes ground bounce range and incident field modeling, shows beamforming applied to foam column scattering, and demonstrates that a 2-dimensional array can improve the cross range phase taper. It also discusses design sensitivity issues.
In real life, most radar targets are located outdoors. Here we present results from outdoor broadband RCS measurements at the X-, Ka- and W-band of “Holger”, a metallized model-scale aircraft with cavities. RCS vs. angle data in the wing plane (0° elevation) were recorded at discrete frequencies (9, 35 and 94 GHz) in both horizontal (HH) and vertical (VV) polarizations. ISAR data at 7-13, 32-38 and 92-97 GHz were acquired. Results from a 104.1 m ground range and a 162.7 m free space range will be compared.
Sensor Concepts Inc. has prototyped a fast, lightweight, dechirp-on-receive radar called the SCI-Lr to provide the capability of a range instrumentation radar in a highly portable package. The small weight, size and power requirements of the SCI-Lr allow a variety of new deployment options for the user including in a small general aviation aircraft or on a mountaintop that is accessible only by four wheel drive. Pulse rates up to 20 KHz enables investigation of high Doppler bandwidth phenomenon such as ground vehicle microdoppler features. The dual integration from dechirp-on-receive matched filtering in fast time and Doppler processing in slow time provides high sensitivity with low output power. Planned enhancements of waveform bandwidth up to 2 GHz , frequency operation between .2 and 18 GHz and pulseto- pulse polarization switching will provide high information content for target discrimination. The flexibility provided by the hardware is augmented by software tools to examine data in near real time to monitor data quality and sufficiency. A variety of applications are being investigated including RCS measurement, SAR and ISAR imaging, Ground Moving Target Indication, and signature collection for ATC.
In previous AMTA presentations, we developed and evaluated an image-based near field-to-far field transformation (IB NFFFT) algorithm for monostatic RCS measurements. We showed that the algorithm’s far field RCS pattern prediction performance was quite good for a variety of frequencies, near field measurement distances, and target geometries. In this paper, we quantify the statistical RCS prediction performance of the IB NFFFT using simulated data from a generalized point scatterer model and method of moments (MoM) code, both of which allow modeling of targets with single and multiple interactions. It is shown that the predicted RCS statistics remain quite accurate under conditions where the predicted far field patterns have significantly degraded due to multiple interactions and other effects.