AMTA Paper Archive
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Compact Range Phase Taper Effects Due to Phase Center Shift in Wide-Band Quad-Ridge Feeds
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.
VHF Capability and RCS Measurements from Long Cylinders
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.
Design and Analysis of a New Angularly Insensitive RCS Calibration Device
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.
NRTF's 14 Foot Pylon
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.
Phase-Dependent RCS Measurements
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.
The Design of Broadband Foam Columns
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.
Outdoor Low Frequency Bistatic Far Field Radar Cross Section Measurements
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.
The Effects of Target Motion on ISAR Imagery
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.
Revolutionary New Outdoor Testing with a Mobile Airborne Nearfield Test Facility (ANTF)
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).
AFRL Advanced Compact Range RCS Uncertainty Analysis for a General Target
A calibration uncertainty analysis was conducted for the Air Force Research Laboratory’s (AFRL) Advanced Compact Range (ACR) in 2000 . 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.
Field Control on a Ground Bounce Range Using Array Technology
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.
Outdoor Broadband RCS Measurements of Model Scale Aircraft
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.
Portable Dechirp-On-Receive Radar
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.
Statistical Analysis of Near Field-to-Far Field RCS Transformation Performance
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.
Simultaneous Axis Motion Applications in Antenna and Radome Measurements
This paper describes the use of simultaneous axis motion for various antenna, RCS, and radome applications, and the use of off the shelf hardware to support the corresponding measurement requirements. This is particularly relevant to polarization, low reflectivity target characterization, and radome measurements. Specific motion profiles required to accomplish various classes of tests are discussed, along with the implications on the mode of operation of the measurement system in order to achieve the most efficient collection of the required data. These simultaneous axis motion requirements may typically be user defined from the available set of axes composing the positioning system. Evaluation of the speed and real time tracking capability of the multiple axes are examined as they relate to the accuracy of the measurements that are required.
The RCC/SMSG Certification of Lockheed Martin Orlando Florida
The Range Commanders Council Signature Measurement Standards Group (RCC/SMSG) Performed a Demonstration program with three DOD Radar Cross Section Ranges to evaluate and improve their documentation and evaluation process and criteria documented in what is known as a "Range Book". After a successful Demonstration Program, The RCC/SMSG has embarked on the evaluation of Industry RCS Range Calibration and measurement processes and procedures and compliance with the RCC/SMSG ANSI-Z540 (ISO-25) evaluation criteria. The Lockheed Martin Helendale RCS Range was evaluated by a committee of industry volunteers appointed by the RCC/SMSG after a review of their experience and credentials. The Lockheed Martin Orlando RCS Range requested an evaluation of their "Range Book" shortly after the completion of the Helendale evaluation. Each review committee is made up of three RCC/SMSG approved reviewers, at least one of which has participated in a previous review either as a review requester or a review committee member. This paper will put forth the process used by this review committee and the lessons learned from this and previous reviews. This paper will also discuss the RCC/SMSG process for obtaining an RCC/SMSG review.
Chamber Design 101
This paper will deal with basic rectangular chamber design and the choices that most affect the performance characteristics of a typical Rectangular Anechoic Chamber. The first and foremost criterion that needs to be addressed is “What is the chamber for”. The answer to this question is the primary driving factor regulating the overall chamber design. Is the chamber to be used to evaluate low gain, low frequency antennas? Is the chamber going to be used for RCS measurements of unique test bodies? Is the chamber going to be used to test high gain high frequency antennas? Is the chamber going to be used for far field measurements? Is the chamber going to be used for near field measurements? On and on. The answers to these very basic questions have a dramatic effect on the overall design of the anechoic chamber. Since there are so many preliminary criteria that have to be decided before we can even attempt a design I will make the following assumptions: 1) The chamber is to be a far field antenna measurement facility 2) The chamber is to operate from 2.0 Ghz to 18.0 Ghz 3) The chamber is to be of a rectangular design 4) The quiet zone is to be a 4’ diameter sphere 5) The range length is to be 20’ 6) The desired Quiet Zone performance is a. –30 dB @ 2.0 Ghz b. –40 dB @ 4.0 Ghz c. –50 dB @ 10.0 Ghz d. –50 dB @ 18.0 Ghz With these parameters we will first look at the effect that source antenna selection has on the chamber deign. The first design example will be with a low gain broadband antenna chosen as the source and the second case will be with a high gain antenna chosen as the source. This paper will detail the different design approaches that this choice has on the overall size and absorber placement in the chamber. These will have a dramatic effect on overall chamber size and cost.
Design of Dielectric Rod Antenna for Near-Field Probe
A prototype design of the dielectric rod antenna is discussed. This novel design is suitable for nearfield probing application in that it provides broad bandwidth, dual-polarization and low RCS. The design details are provided in this document along with measurement data associated with important antenna characteristics such as VSWR and far-field radiation pattern
On the Estimation of Far-Field RCS From Monostatic Near-Field Data
Monostatic backscatter measurements made in the near-field have been used to generate high resolution images of complex targets; however, the appropriate use of this data for obtaining far-field RCS values needed further examination. In this paper we comment on some of the available methods, and discuss in some more detail the concept that Fourier Transform of monostatic backscatter data collected over a planar array indeed provides samples in Fourier Space directly.
Study of RCS Measurements From Small Spheres
New results are presented on using small spheres mounted on a foam tower for calibration. Subtraction of the foam tower response is found to be necessary and sufficient for the dual-calibration method to work.
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