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RCS

RCS errors due to target support structure
W.T. Wollny (Quick Reaction Corporation), November 1988

The deleterious effect of tilting the pylon on the measured RCS of a low level target is shown. A two scatterer computer model is developed to demonstrate the harmful effect of the pylon on the target signature. Predicted RCS plots are provided for the pylon to target ratios of -20, -10, 0, and +10 dB. The familiar error curve for two interfering signals is shown as applicable to bound the RCS errors of two scatterers. A method for computing the pylon RCS from linear motion RCS measurements is described with sample data plots. A knowledge of the pylon RCS allows the inclusion of measurement confidence levels on all RCS plots which is very valuable to the analyst. All radar data that is below the known RCS of the target support structure can be blanked from the plotted data to prevent confusion since these RCS values are an artifact of the measurement system and are not a true representation of the target RCS.

Target mounting techniques for compact range measurements
H. Shamansky (The Ohio State University),A. Dominek (The Ohio State University), November 1988

The compact range provides a means to evaluate the radar cross section (RCS) of a wide variety of targets, but successful measurements are dependent on the type of target mounting used. This work is concerned with the mounting of targets to a metal ogival shaped pedestal, and in particular focuses on two forms of mounting techniques: the "soft" (non-metallic) and "hard" (metallic) mounting configurations. Each form is evaluated from both the mechanical and electromagnetic viewpoints, and the limitations associated with each type are examined. Additional concerns such as vector background subtraction and target-mount interactions are also examined, both analytically and through measurements performed in the ElectroScience Laboratory's Anechoic Chamber.

The Radar image modeling system
R. Renfro (David Taylor Research Center), November 1988

The characteristics of a unique indoor RCS modeling facility are described. The David Taylor Research Center (DTRC) has implemented an indoor, over-water radar cross section measurement facility. Major components of the facility are the DTRC Seakeeping Basin, an imaging radar, an underwater target mount and rotator, a calibration system, and video monitoring equipment. Initial operational capabilities include dynamic pulse-to-pulse polarization-agile measurements at X and Ku bands, elevation angles from grazing to 7 degrees, maximum target length of 50 feet, and simulated sea states adjustable between state 0 and state 3. Several data products are available, including high-resolution inverse synthetic aperture radar images. Eventual capabilities will include extended elevation angles up to 30 degrees, frequencies to beyond 100 GHz, and SAR imagery.

Transfer efficiency of the compact range
R.W. Kreutel (Scientific-Atlanta, Inc.), November 1988

Over the years formulations have been developed which provide an implicit measure of transfer efficiency of the compact range. Reasonable accuracy has been demonstrated for both antenna and RCS measurement applications. In general, however, these formulations require specific design details pertaining to the collimating reflector. In this note a more general formulation is examined in which efficiency is explicitly expressed in terms familiar to antenna engineers and which do not directly involve reflector parameters. Applications of this formulation are presented.

Precision compact range feed
K.R. Goudey (Harris Corporation GCSD),L.R. Young (Harris Corporation GCSD), November 1988

This paper describes how corrugated feed horns are designed for compact ranges with tight pattern control. Both the amplitude and phase of the horn pattern must be invariant over a wide frequency band. A horn synthesis computer program has been developed using the JPL HYBRIDHORN computer program as the analysis module which is driven by a Harris developed synthesis code (OPTDES). This paper also discusses launching techniques used to generate the HE(11) hybrid mode in the corrugated horn as well as design methods to eliminate ringing effects observed in both the input waveguide circuits and corrugated horns when used for RCS measurements.

A Modular positioner control system
W.L. Tuttle (Scientific-Atlanta, Inc.), November 1988

A variety of positioner control systems are available for making antenna and RCS measurements, but few can be upgraded economically as test facilities are expanded. Positioner control system components may include a controller, positioner motor drive unit, and a position indicator. Integration of these functional components into a single modular unit to operate the desired number of axes provides the basis for a positioner control system. Other desired features may include programmability, remotability, operation outdoors, and expansion capability. This paper will address the development of a modular positioner control system that can economically be upgraded as changing test requirements dictate. Functional capabilities such as remotability, expansion capability, and programmability will be highlighted. System configuration and integration will also be discussed.

Automated radome test and characterization systems
R. Flam (Flam & Russell, Inc.),J.P. MacGahan (Flam & Russell, Inc.), R.E. Hartman (Flam & Russell, Inc.), November 1988

This paper describes an automated radome test and evaluation system, which very accurately and quickly determines the shift in electrical boresight and loss in antenna gain caused by the presence of a radome in front of a monopulse antenna. The measurement system hardware, which is shown in Figure 1, is based on the Flam & Russell, Inc. ADAM 8003 Antenna and RCS Measurement System and consists of a DEC MicroVAX computer, Hewlett Packard 8510B network analyzer and a highly accurate two axis positioning system. The monopulse antenna and radome are attached to the same positioner. The monopulse antenna is electrically steerable, thus allowing different areas of the radome under test to be examined.

Methods for the calculation of errors due to wall effects in an RCS measurement compact range
T.P. Delfeld (Boeing Military Airplane Company), November 1987

A method for the calculation of the errors induced through target-wall-target interactions is presented. Both near-field and far-field situations are considered. Far-field calculations are performed both with Fraunhoffer diffraction theory and target antenna analogies. Absorber is considered as both a specular and a diffuse scatterer. The equations developed permit trade studies of chamber size versus performance to be made.

Effects of mechanical discontinuities on the performance of compact range reflectors, The
B.J.E. Taute (The Ohio State University),I.J. Gupta (The Ohio State University), W.D. Burnside (The Ohio State University), November 1987

Reducing ripple in the aperture field of the parabolic reflector is one of the main considerations in the design of a compact range, since it determines the "usable" target zone for RCS and antenna measurements. The usable target zone is typically defined as the aperture region where the ripple is less than 0.1 dB [1]. Studies [2,3] have shown that edge diffractions and therefore ripple can be significantly reduced by using blended rolled edges such as in Figure 1. For low aperture field ripple, it is assumed that the junction between the parabolic surface and the blended rolled edge is smooth. In practice, however, the rolled edges may be machined separately and then fitted to the main reflector. If this is done, small wedge angle errors (Figure 2) or step discontinuities (Figure 3) may be mechanically introduced at the junctions. Typically, angle deviations of plus-or-minus 0.5 degrees and steps of plus-or-minus 0.005 inches may be expected. If the parabola and part of the rolled edge is machined as a unit, diffraction due to discontinuities in the mechanical junction between this surface and the rest of the rolled edge can have less effect on ripple in the aperture field. Now, the questions to be answered are: * How much of the target zone is lost due to discontinuities at the edge of the parabola? * How much of the rolled edge need to be machined with the parabola to prevent mechanical discontinuities from decreasing the usable target zone? * What range of discontinuities can be tolerated? In this paper, these questions are answered for a 12 foot radius semi-circular compact range reflector with cosine-blended rolled edges.

Model 1603 compact range: a room sized measurement instrument
J.K. Conn (Harris Corporation), November 1987

Harris Corporation has developed and introduced a miniature version of its shaped compact range called the Model 1603. This model is actually a scaled version of its very large compact ranges. The range features a three foot quiet zone in a very compact configuration, allowing the range to be set up in an anechoic chamber as small as a normal conference room. Performance features are equivalent to those achieved in large compact ranges by Harris, such as the Model 1640 with a forty foot quiet zone. Key features are very low quiet zone ripple, extremely low noise floor, and low cross polarization. This range can be used for the full gamut of precision RCS testing of small models or precision testing of antennas. It should also find wide application in production testing of these items. Harris can also provide turnkey compact range test systems based on the Model 1603 that use available radar instrumentation. Several of these miniature compact ranges have been delivered and are in use.

A Modeling Technique for Predicting Anechoic Chamber RCS Background Levels
S. Brumley (Motorola Govt. Elect. Group), November 1987

Current demands for accurate low-level radar cross section (RCS) measurements require anechoic chambers and compact ranges to have extremely low background scattering levels. Such demands place difficult requirements on the entire chamber and warrant the need to predict and mathematically model chamber performance. Accurate modeling, prior to chamber construction, also aids in chamber performance optimization through improved chamber designs.

Rotated feed horns in a compact range for RCS measurements
C.M. Luke (Scientific-Atlanta, Inc.),B.C. Brock (Sandia National Laboratories), M.C. Baggett (Scientific-Atlanta, Inc.), November 1987

A way has been found to utilize the reflector return in a compact range as a source of continuous drift compensation. This is performed by translating receive polarizations 45 degrees with respect to the transmit polarizations to ensure returns in co- and cross-polarizations. An added benefit is the simplicity of alignment for the polarization calibration standard.

Two-dimensional RCS image focusing
D. Mensa (Pacific Missile Test Center),K. Vaccaro (Pacific Missile Test Center), November 1987

A wide variety of precise, automatic instrumentation systems is currently available for RCS testing. These systems, either commercially available as integrated units or assembled from laboratory test instruments, can automatically measure the RCS of a target over fine frequency increments spanning wide bandwidths. When the frequency responses are measured for discrete increments of target rotation, the resulting two-dimensional (frequency-angle) data arrays can be processed to obtain two-dimensional RCS images.

Evaluation of anechoic chamber absorbers for improved chamber designs and RCS performance
S. Brumley (Motorola Govt. Elect. Group),D. Droste (Motorola Govt. Elect. Group), November 1987

This paper discusses an anechoic chamber absorber evaluation which was conducted for the purpose of improving anechoic chamber and compact range performance through better absorber characterization. This study shows that performance of conventional absorber materials is dependent on selection of the material's shape, size and orientation with respect to the incident energy direction. This, demonstrates the importance of better characterization of the material. Nonhomogeneities in the material composition and physical structure were also found to significantly modify performance; in some cases even improving it. Also shown, is the need for improved evaluation techniques and procedures over conventionally used methods. An evaluation procedure using modern imaging techniques is presented. Several measured results for various absorber types and sizes are presented which show the usefulness of the evaluation technique and demonstrate relative performance characteristics for these materials. Measured reflectivity data on various absorber types, which consistently show better performance than levels specified by the vendors, are also presented.

Cost Effective, High Performance Anechoic Chamber Design
R.G. Immell (Motorola Government Electronics Group), November 1987

Motorola's Government Electronics Group (GEG) located in Scottsdale, Arizona has recently completed construction of an indoor Antenna/RCS Test Facility. Motorola achieved quality construction of this new facility by utilizing local building contractors working under Motorola supervision through concept study, design, and construction phases. Motorola achieved quality chambers without turn-key costs. Three anechoic chambers and one shielded computer room were fabricated. The chambers sizes vary from 20'W x 16'H x 41'L to 36'W x 36'H x 72'L. All chambers were evaluated using techniques described by MIL-STD-285 (Attenuation measurements for enclosures, electromagnetic shielding, for electronic test purposes, Method of) and indicated shielding effectiveness, before absorber installation of -60 to -70 db at 400 MHz and -80 db from 1-18 GHz. Shielding effectiveness increased to -80 dB at 400 MHz and to greater than -115 dB from 1-18 GHz after absorber installation. In addition, the building contains eight individual security areas meeting government standards for security as prescribed in the Defense Intelligence Agency Manual (DIAM 50-3).

Remodeling of the ESL-OSU Anechoic Chamber
H. Shamansky (The Ohio State University),A. Dominek (The Ohio State University), W.D. Burnside (The Ohio State University), November 1987

The indoor compact range has proven to be quite successful in measuring the radar cross section (RCS) of various targets. As the performance capabilities of the compact range have expanded, the use of larger, heavier, and more sophisticated targets has also expanded. Early target dimensions were limited by the size of the useful test area, as well as the capacities of the low RCS pedestal mount used. Today, our anechoic chamber has a large useful test area, thus the size and weight of targets dictate that a new method be employed in target handling and positioning, as well as target mounting to a low RCS pedestal. Work was recently completed here at the Ohio State University ElectroScience Laboratory to remodel our anechoic chamber to allow for the new generation of targets and the demands that they place on the anechoic chamber. This work included the addition of a one ton motorized underhung bridge crane to our anechoic chamber, the design and construction of an hydraulic assist to smoothly and precisely raise and lower the target for the final linkup of the support column and the receiving hole in the target, the design and installation of a one ton telescopic crane in the chamber annex to link with the main chamber crane, the design and installation of the necessary microwave treatments to minimize the impact of the remodeling on accurate RCS measurements, the development and installation of a sloping raised floor, the design and manufacture of a track guided rolling cart to shuttle operating personnel to and from the target area, the replacement of the existing radar absorbing material, the improvement of the ambient lighting in the chamber to facilitate film and video tape documentation, and the development of new target mounting schemes to ensure ease of handling as well as secure mounting for vector background subtraction.

Hardware Gating Improves HP8510 Based RCS Measurement Systems
M. Boumans (March Microwave Inc.),S. Brumley (Motorola Govt. Elect. Group), November 1987

An RCS measurement system based on the HP 8510 and a Compact Range reflector system has the following limitations: high clutter levels limit the maximum transmit power and therefore the system's sensitivity, the maximum number of frequency points limit the maximum resolution and/or range length, and the proper separation of clutter and test target data requires taking data describing the entire range, even for a desired CW measurement, thus increasing measurement times significantly.

Performance of the model 1606 compact range
G.M. Briand (Harris Corporation), November 1987

Characteristics of the Harris Model 1606 Compact Range are summarized and considered for applicability to RCS measurements. Measured characteristics of quiet zone performance (amplitude and phase distributions) and standard target RCS data are presented. Of particular interest is a comparison of predicted and measured radar cross section versus aspect angle of some familiar standard targets under various conditions.

Radar cross-section and scattering matrix measurements on microwave radar navigation targets
Y.M.M. Antar (National Research Council, Ottawa),L.E. Allan (National Research Council, Ottawa), S. Mishra (National Research Council, Ottawa), November 1987

This paper presents both radar cross section and polarization scattering matrix measurements on microwave radar navigation targets. The polarization measurements are performed using a unique two-channel facility which allows for measuring the circularly polarized scattering matrix elements at X-band. For the same targets conventional RCS measurements are performed using an automated system comprising a network analyzer (HP-8510) and a desk top computer system (HP-236 or 310). This system allows wide frequency range measurements. Details of these measurement techniques, and results will be presented.

A Pulsed/CW RCS measurement system using the HP8510 network analyzer
P.S. Kao (Massachusetts Institute of Technology),G.L. Sandy (Massachusetts Institute of Technology), J.A. Munoz (Massachusetts Institute of Technology), November 1987

This paper describes an automated, frequency-step, pulsed/CW Radar Cross Section (RCS) measurement system using the HP 8510 network analyzer. The system has been built using the concepts developed at Lincoln Laboratory (1) and is being utilized in an operational capacity. The unique features of this system are the use of (a) a dual-probe antenna for the transmission and reception of RF signals, and (b) a pulse system for separating the target-scattered signals from the incident and background signals. The single antenna configuration provides a true monostatic backscatter measurement. A polarization control circuit makes RCS measurements for all combinations of transmit/receive polarizations possible (linear and/or circular). The pulse system uses pin-diode switches capable of generating a 7-ns pulse width and a repetition rate up to 8 MHz. The pulse system effectively eliminates unwanted signals at ranges other than the target range. Therefore, the full dynamic range of the receiver can be used for the measurement of the target.







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