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Radar
Compact range performance
M. Arm (Riverside Research Institute),L. Wolk (Riverside Research Institute), M. Rochwarger (Riverside Research Institute), N. Erlbach (Riverside Research Institute), R. Reichmeider (Riverside Research Institute), November 1991
A performance simulation for analyzing the measurements of target RCS in a compact radar range has been applied to a small indoor range which will be installed at RRI. A dual reflector collimator has been examined with respect to both quiet-zone quality and the amount of stray energy in the chamber which eventually end up as clutter or multipath interference. The complicated ray geometries, beyond the reach of hand calculation, are discovered by complete tracing of all the rays from the feed source. The ray pats which interfere with target measurements are shown convincingly by graphical display. Vector clutter subtraction is widely used in compact ranges in order to reduce the background clutter to an acceptable level. Some of the effects which limit the effectiveness of clutter subtraction are also addressed in the paper. The sources of measurement errors which are obtained by this simulation are used in the measurement-error budget analysis, which is the subject of the follow-on paper.
A Novel, bistatic, fully polarimetric radar cross-section measurement facility
A.J. Blanchard (Space Technology and Research Center),B.A. Williams (Space Technology and Research Center), B.D. Jersak (Space Technology and Research Center), B.D. Krenek (Space Technology and Research Center), J.K. Glazner (Space Technology and Research Center), R.F. Schindel (Space Technology and Research Center), W.N. Colquitt (Space Technology and Research Center), November 1991
A new radar cross-section (RCS) measurement facility has been designed and built at the Houston Advanced Research Center in Houston, Texas. This facility is capable of performing fully polarimetric RCS measurements over a frequency bandwidth of 2-40 GHz ad nearly an entire hemisphere of bistatic angles. What makes this facility unique is the fact that both the transmit and receive antennas are mounted on moveable platforms. The transmit antenna is fixed at 0º azimuth, but can be positioned anywhere from 10º to approximately 165º in elevation. The receive antenna can be positioned anywhere from 0º to 180º in azimuth and the same range in elevation as the transmit antenna. Monostatic measurements can be approximated by moving the transmit and receive antennas close together. The radar equipment is built around the HP 8510 vector network analyzer, and the measurement process is controlled and automated by an HP UNIX workstation running HP’s Visual Engineering Environment software.
An Advanced on-line RCS data analysis sytem using a Tektronix XD-88 superworkstation
D. Yanke (McDonnell Douglas Technologies Incorporated), November 1991
Advanced Radar Cross Section (RCS) Data Analysis, consisting of comparisons of measured RCS data to predictions, multiple plot overlays, imaging, etc., it is most often performed off-line. This causes a lag in data acquisition time by as much as several days. McDonnell Douglas Technologies Incorporated’s (MDTI) Radar Measurement Center, a large target (40 feet) indoor RCS measurement facility, used an advanced RCS data analysis system, based on a Tektronix XD-88 superworkstation, for on-line data processing. This system connects over a Local Area Network to the data acquisition computer. This allows the workstation access to each data file immediately after each measurement for processing, without affecting the data acquisition capabilities of the radar system. The hardware used for connections, capabilities of the MDTI-written software, and the capability to store plotted data on VHS videotape directly from the workstation, is described herein.
Application of RCS antenna measurements to multiport antennas
E. Heidrich (Institut fur Hochstfrequenztechnik und Elektronik),W. Wiesbeck (Institut fur Hochstfrequenztechnik und Elektronik), November 1991
New results of wideband polarimetric radar-cross-section-(RCS-) antenna measurements are presented. A special antenna network description including polarization information and multiport feeding offers new insight in antenna behavior. The procedure omits the utilization of a standard gain antenna for absolute gain determination and no RF-feedline is necessary to the antenna under test. Antenna radiation, scattering and feed characteristics are all obtained with one measurement setup. Theory as well as measurements on different dual-polarized antenna types demonstrate the efficiency and uniqueness of this technique.
Performance measurements of an active aperture phased array antenna
L.D. Poles (Rome Laboratory),E. Martin (Rome Laboratory), J. Kenney (Rome Laboratory), November 1991
Transmit – receive modules (T/R) utilizing GaAs monolithic microwave integrated circuit (MMIC) technology for amplifiers, attenuators, and phase shifters are becoming integral components for a new generation of radars. These components, when used in the aperture of a low sidelobe electronically steerable antennas, require careful alignment and calibration at multiple stages along the RF signal path. This paper describes the calibration technique used to measure the performance of an active aperture 64 element S-band phased array antenna that employs T/R modules at every element. RF component performance and phased array sidelobe characeristics are presented and discussed.
Radar-cross-section measurement errors caused by test objects interaction with low-dielectric-constant supports
B.C. Brock (Sandia National Laboratories),D.H. Zittel (Sandia National Laboratories), K.W. Sorensen (Sandia National Laboratories), W.E. Patitz (Sandia National Laboratories), November 1991
In the search for an ideal test-object support for simulate free-space radar-cross-section (RCS) measurements, low-density polystyrene foam has achieved considerable popularity. However, significant error can be introduced into a measurement by the use of an inappropriately designed support. Although low back-scatter radar cross section (RCS) can be obtained with this material, interactions can occur between the test object and the mount which will cause measurement errors in excess of several dB. We present results of measurements performed on a simple test object supported on a low-density foam column which demonstrate this effect. As we discuss, this error can be incorrectly interpreted to be caused by poor alignment of the test object with the radar-range coordinate system. Finally, we show that the errors can be explained by differential propagation effects. In addition, this simple theory provides the insight necessary to devise appropriate measures to minimize the errors cause by the presence of the support.
An Improved background subtraction technique
E.A. Urbanik (Westinghouse Electric Corporation),D.H. Wenzlich (Westinghouse Electric Corporation), November 1991
Clutter rejection is designed to remove range clutter and repeatable radar parameters from the measurement. The technique to be used is one that gains the customer acceptance, does not significantly increase range time, and produces good results. Techniques which require significant data processing have not been accepted by our customers. Fixture subtraction requires very accurate target positioning and is too slow. Only background subtraction met all the requirements. This paper will discuss a new background subtraction method. In this technique the pylon is effectively removed from the measurement area, but not from the chamber. This is done with a small pole termination target and the antenna measurement slide. Thus a true background is measured. The technique has been highly successful, gaining the acceptance of our customers and users alike. Range measurements will show how well the technique works.
RCS target support background determination using translating test body
D.P. Morgan (McDonnell Douglas Technologies Incorporated), November 1991
When attempting to make accurate Radar Cross Section (RCS) measurements, it is vital to understand the background levels of both the range and the target support fixture. Typically these support fixtures are either foam columns or metal pylons. Determining the RCS levels of the metal pylons requires the installation of a termination device to hide the rotator which has a significantly lower RCS than the pylon being measured. Quite often this is an impossible task, especially at lower frequencies. An algorithm that accurately determines the pylon background levels independent of the RCS contribution of the pylon terminator is presented. This algorithm requires translating the terminator linearly and isolating the background from the resulting interference pattern. Data is included that validates the implementing computer code.
Clutter supression with pseudo random phase coding
R. Richardson (System Planning Corporation),T. Thompson (System Planning Corporation), November 1991
Clutter returns can seriously limit the performance of high sensitivity Radar Cross-Section (RCS) measurement ranges. Within the direct sample space of the target, clutter is controlled by: minimizing the antenna response outside of the angle subtended by the target and by careful transmit pulse control. However, clutter returns are also produced from areas outside the sample space of the target. This paper discusses the application of pseudo random phase coding techniques to suppress this type of clutter. It defines the nature of this type of clutter, identifies a method to suppress it, describes the hardware used for online suppression, and presents experimental results to demonstrate the effectiveness of the technique. The technique is important for both outdoor and indoor ranges (particularly in unprepared, echoic, environments); experimental data is present for both cases.
Superresolution radar target imaging
E. Walton (The Ohio State University ElectroScience Laboratory),A. Moghaddar (The Ohio State University ElectroScience Laboratory), C. DeMattio (The Ohio State University ElectroScience Laboratory), November 1991
Classical radar target imaging uses an inverse synthetic aperture radar (ISAR) algorithm based on the two dimensional Fourier transform. The technique has resolution limitations in the time-domain (or down-range) dimension somewhat larger that the inverse of the band-width of the interrogating radar system (depending on the frequency domain windowing function utilized). The resolution in the cross-range domain (or doppler-domain) is related to the inverse of the aspect angle sector over which the target is observed. This paper will present radar target imaging techniques based on modern autoregressive (AR) spectral estimation algorithms (superresolution) which overcome these limitations. Techniques are shown for the generation of ISAR images even with severly [sic] limited frequency or angle domain data. Images will be shown where the quality of the image does not degrade even when the bandwidth of the original data is reduced by a factor of 16. Thus clear images are produced using these techniques with data where the classical Fourier-based techniques produce only “fuzzy blobs”
Superresolution signal processing for RCS measurement analysis
B.W. Deats (Flam & Russell, Inc.),D. Farina (Flam & Russell, Inc.), November 1991
Superresolution (SR) processing techniques have been used for many years in direction finding applications. These techniques have proved valuable in extracting more information from a limited data set than conventional Fourier analysis would yield. SR techniques have recently proven to be an extremely powerful radar cross section (RCS) analysis tool. Typical resolution improvements of 2 to 30 times may be achieved over conventional Fourier-based range domain data in both the one-dimensional and two-dimensional image domains. Typical measurement scenarios which can most benefit from SP processing are presented. These include: VHF/UHF RCS measurements, measurement of resonant targets, and performing detailed scattering analysis on complex bodies. Measurement examples are presented illustrating the use of SR processing in a variety of test conditions. When the advantages of SR processing are combined with the accuracy of Fourier techniques, a new window is opened through which target scattering characteristics can be seen more clearly than ever.
Dynamic helicopter radar signatures
A.D. Siegel (System Planning Corporation), November 1991
This paper addresses measurement and data processing techniques for dynamic helicopter radar signatures. Data products are presented and interpreted to highlight the utility of instrumentation radar systems as a means for determining radar scattering characteristics of objects with rotating components. Investigation of rotor-body multipath phenomena in helicopter imagery cannot sufficiently resolve ambiguities regarding ray traces that contribute to observed scattering events. The diagnostic insights gained from concurrent doppler spectral data aid in resolving these ambiguities. Unique spectral signatures resulting from rotor-body interactions are investigated, and a methodology is developed for diagnosis of the responsible scattering mechanisms. The results provide valuable insights into the radar spectral signatures o conventional helicopters.
Three dimensional radar imaging by using tomographic algebraic reconstruction algorithm
D-C. Chang (Chung Shan Institute of Science and Technology),I.J. Fu (Chung Shan Institute of Science and Technology), R.C. Liou (Chung Shan Institute of Science and Technology), T.Z. Chang (Chung Shan Institute of Science and Technology), Y.P. Wang (Chung Shan Institute of Science and Technology), November 1991
Range resolution of a radar image can be obtained by use of wide-band signal (linear FM or chirp waveform) and cross-range resolution by object rotation which synthesized a large antenna aperture (the so called ISAR method, refer [1]). Although both cross-range profiles can be resolved by rotation of the abject about two mutually orthogonal axes, however, the data manipulation would be quite cumbersome and the measurement implementation would require a mechanical support system by which the objet [sic] can be independently tilted and rotated relative to the radar axis. In this paper, the algebraic reconstruction technique (ART)[2] for tomography is used to resolve the vertical cross-range profile (along the axis normal to the ground) while the horizontal cross-range profile still resolved by ISAR method. Applications of the ART to a simple circular pattern and a complicated emblem pattern of the CSIST show that ART is a suitable approach and easier than ISAR method to obtain the second cross-range resolution.
High performance 2-18 GHz power amplifier provides increased power and reduced ring down time
F.A. Miller (Quarterwave Corp.), November 1991
This paper describes new developments in broadband Microwave power amplifiers for compact RADAR Cross Section (RCS) Ranges. The RF Power level of transmitters used in compact RCS ranges for the most part has been limited to a watt or two. This is due to the limitations of the power available from solid state RF amplifiers and the power handling capabilities of PIN diode switches, used to pulse modulate the RF amplifier output. Inherent impedance mismatches of the PIN diode switch, RF amplifier and RF output circuits produce reflections of RF energy. The reflected RF energy reverberates between the output circuits of the RF amplifier and the antenna. Reverberation of RF energy between mismatches continues until circuit losses reduce the energy to zero. These reverberations manifest as deterioration of the RF output pulse fall time waveshape. The radiated pulse fall time is extended and damped rather than abrupt. This deterioration of pulse waveshape, due to reverberations, is ring down time. RF pulse ring down deteriorates the resulting RCS measurements. New broadband microwave Traveling Wave Tube (TWT) technology, combined with extremely quiet power supplies and modulator, provide increased power, low noise floor and reduced ring down time resulting in improved RCS measurements.
Measurement of RCS in an operational environment
L.R. Burgess (Flam & Russell, Inc.),R. Flam (Flam & Russell, Inc.), November 1991
As new military aircraft with low radar signatures pass from the design stage to production and deployment, the techniques for measuring and confirming their low signatures must move from the laboratory to the flight line. Measuring the RCS characteristics of carrier-based aircraft is particularly difficult because it must be done either while the aircraft is in flight or while it is one a crowded flight deck or hanger deck. This paper describes an approach to Navy flight-line RCS measurements that minimizes space, yet still provides enough information to identify a degradation in low signature performance and to pinpoint the source of the problem. It uses a small reflector on a positioner combined with a stepped frequency gated CW radar at 8-12 GHz to sweep a spot illumination over the aircraft while producing downrange profiles at each spot. The primary advantage of this configuration is that it restricts the RF radiation in all three spatial dimensions, thereby minimizing the scattering from other objects in the crowded environment. A secondary advantage is that the data can be processed to yield resolution of scatterers on the aircraft under test to within two or three feet. Adding an automatic focusing ability to the reflector antenna can improve the resolution to about one foot.
Plane wave analysis and evaluation of an indoor far field conductive chamber
W.S. Arceneaux (Martin Marietta Company),C. Christodoulou (University of Central Florida), November 1991
Martin Marietta designed and brought on-line an indoor far-field chamber used for radar cross section (RCS) evaluation. The range has conductive walls on all sides except for the pyramidal absorber covered back wall. The chamber was designed such that wall/floor/ceiling interactions occur with a distance (time) delay allowing for their isolation from the test region. Software gating techniques are used to remove these unwanted signals. This paper presents an analysis of the conductive chamber using Geometrical Optics (GO). The objective was to analyze and evaluate the plane wave quality in the chamber test region. The evaluation of the plane wave was performed using the angle transform technique. The measured results were compared to analytical results and measured antenna patterns.
An Automatic system for measuring complex permittivity and permeability of solid materials at microwave frequencies
Y. Kantor (RAFAEL),A. Geva (RAFAEL), S. Bolker (RAFAEL), November 1991
A novel low-cost automatic system is described to measure both the complex permittivity and permeability of solid materials at 2 to 18 GHz. It is particularly useful for evaluating the frequency dependence of radar absorbing materials (RAM). The RF and the mechanical setups are described, including the computer algorithm and the measurement procedure. The results and the experimental errors of three materials are presented, which agree with results that were obtained by other methods, while the cost of putting up the system is considerably lower than any comparable alternative.
Techniques for RCS quality control measurements in unimproved environments
J. Stewart (System Planning Corporation),R. Richardson (System Planning Corporation), November 1991
Measuring the radar cross section of low-observable (LO) vehicles require an RCS quality control (QC) program that will last throughout the life cycle of the vehicle, from component production to operational deployment and depot maintenance. Testing must be done at regular intervals to ensure that surface or sub-surface damage has not degraded the RCS characteristics of the vehicle beyond acceptable limits. In the past, these measurements were complicated by the requirement for and expensive, well-prepared RF test environment. The test range—usually a fixed site—is often remotely controlled. System Planning Corporation (SPC) has developed an RCS QC measurement technique that requires little or no facility improvements while offering high sensitivity inverse synthetic aperture radar (ISAR) images. The instrumentation radar system can be located at the production, maintenance, or operational site of the vehicle or component. As a result, the QC program is both economical and reliable.
Design your measurement system for optimum throughput
G. McCarter (Hewlett-Packard Company), November 1991
To achieve optimum measurement accuracy and range throughput in antenna and radar cross-section (RCS) measurement applications requires a careful and thorough design of the measurement system. Measurement accuracy requirements, test time objectives, system flexibility, and system costs must all be balanced to achieve an optimum system design. Considering these issues independently will result in unwanted and/or unexpected system performance tradeoffs. This paper examines these issues in some detail and suggests a system design approach which balances microwave performance and measurement speed with system cost.
VHF/UHF indoor RCS measurements using a tapered or compact range
L. Pellett, November 1991
Lockheed’s Advanced Development Company (LADC), located in Burbank, California, has been evaluating the capability of indoor anechoic chambers to measure VHF/UHF RCS. Two chambers were available for evaluation. A 155 feet long, 50 feet high by 50 feet wide tapered horn chamber and a compact range having dimensions of 97 feet long, 64 feet high by 64 feet wide, featuring a 46 feet wide collimator. For comparison purposes, a common instrumentation radar was used in each chamber. This radar was based on a network analyzer using a Lockheed designed pulse-gate unit to increase transmit/receive isolation. Various antenna feed system were tried in both chambers to ascertain their characteristics. Theoretical and experimental data on system performance will be presented emphasizing practical implementation and inherent limitations.


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