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This paper considers the radar cross-section (RCS) simulation, measurement, and analysis of rotating structures found in today’s modern airframes. Addressed will be scattering characteristics from helicopter main and tail rotor systems; how these characteristics can be simulated, measured, and reduced to identify the individual scatterers withing the helicopter. The effect of radar system parameters on the scattered signal will also be discussed. Finally, actual RCS measurements from helicopters in flight wil be resented and analyzed using the above discussed techniques.
Recently, the theory and computer programs on the Periodic Moment Method (PMM) for scattering from both singly and doubly periodic arrays of lossy dielectric bodies have been developed. The purpose is to design microwave wedge and pyramid absorber for low reflectivity so that one can improve measurements and/or reduce the size of the anechoic chamber. With PMM, the reflection and transmission coefficients of periodically distributed bodies illuminated by a plane wave have been accurately calculated on the Cray Y-MP supercomputer at the Ohio Supercomputer Center. Through these studies, some wedge and pyramid absorber configurations have been designed, fabricated and tested in the OSU/ESL Anechoic Chamber. Very good agreement between calculations and measurements has been obtained. In the 1990 AMTA meeting, several wedge absorber designs and results for the TM case and normal incidence were presented. In this paper, the measured and calculated frequency responses of some experimental wedge designs, as well as an 8” and 18” commercial wedge and pyramid absorber panels will be reported for both TM and TE polarizations. Time domain responses will also be shown for both measurements and calculations.
A three-layer sandwich structure consisting of a plastic film-to-foam lamination is presented as a low-RCS alternative to structural foam. Structural foam with 1-2 lb/ft density is commonly used as a low-RCS material. However, its RCS per unit load per unit volume is not as low as that of a composite foam structure. Equations relating mechanical strength and RCS are simultaneously solved for maximum mechanical strength and minimum RCS in the limit of Rayleigh and resonance region material thicknesses. A result is that a three-layer foam sandwich beam can have superior mechanical strength compared with an identical all-foam beam and a reduced RCS. Specific results for an optimized sandwich with mechanical strength equal to that of a homogenous beam and minimum RCS are presented. Experimental data quantify mechanical strength and RCS for several foam-mylar sandwiches.
This paper will discuss the need for performance verification, or calibration, of the transmitter and receiver systems used in an antenna or RCS range. Errors introduced by the range and positioning system means the instrumentation’s performance must be measured independently of the range and positioner. The performance verification should insure that the measurement system exceeds the manufactures’ specifications by a reasonable margin. The verification must be performed with the equipment installed on the range to insure adequate performance on the range. The system must als be verified as a system, rather than individual instruments. This guarantees that measurement errors in each instrument will not add together to exceed the system’s specifications. Testing of the system should be easy and repeatable to insure accuracy of the verification by the test technician. The tests should also be documented for later reference. The measurements should be traceable to a local standard such as NIST to certify the accuracy and stability of the measurement. The verification should be repeated on a regular basis to insure continued accuracy of the measurement system.
The target designer using a compact range to verify the predicted RCS of his target needs to know what measurement errors are introduced by the range. The underlying definition of RCS assumes that the target is in the far-field, in free-space, and illuminated by a plane wave. This condition is approximated in a compact range. However, to the extent that these conditions are not met, the RCS measurement is in error. This paper, using the results of the preceding companion paper1, formulates an error budget which shows the typical sources that contribute to the RCS measurement error in a compact range. The error sources are separated into two categories, according to whether they depend on the target or not. Receiver noise is an example of a target independent error source, as are calibration errors, feed reverberation (“ringdown”), target support scattering and chamber clutter which arrives within the target range gate. The target dependent error sources include quiet zone ripple, cross polarization components, and multipath which correspond to reflections of stray non-collimated energy from the target which arrives at the receiver at the same time as the desired target return. These error contributors depend on the manner in which the target interacts with the total quiet zone-field, and the bistatic RCS which the target may present to any off-axis illumination. Results presented in this paper are based on the design of a small compact range which is under construction at RRI. The results include a comprehensive error budget and an assessment of the range performance.
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 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.
The prototype of the March Microwave Single-Plane Collimating Range (SPCR) has been in operation at Arizona State University’s ElectroMagnetic Anechoic Chamber (EMAC) facility for approximately three years. The unique SPCR produces a cylindrical-wave test region by bouncing spherical wavefronts off a parabolic cylindrical reflector. Consequently, a simplified algorithm can be applied to determine antenna far-field patterns. Both computation and acquisition times can be reduced considerably when compared to classical NF/FF cylindrical scanning techniques. To date, this is the only SPCR in operation. Some of the fundamental quantities which characterize an antenna/RCS measurement range are the size and quality of the “quiet zone”, usually expressed in terms of ripple and taper of the illuminating fields relative to an ideal planar wavefront. Direct one-way probing of the quiet zone fields in the vertical and horizontal planes has been recently completed at ASU. An overview of the range geometry, the field probing methodology, and the data processing will be presented. The results of the quiet zone scan will be presented as amplitude ripple, amplitude taper, and phase ripple versus frequency from 4 GHz to 18 GHz in four bands. The vertical-scan phase deviations are relative to an ideal planar wavefront, while those of the horizontal scan are relative to an ideal cylindrical wavefront.
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.
In the present work, different configurations of reflector systems for indoor antenna and RCS measurements have been studied and compared. These include the Single Offset reflector, Dual Parabolic Cylinder configuration, Shaped Cassegrain, Front-fed Cassegrain and Dual Chamber Gregorian. The above comparison between the different systems is made in terms of: Configuration efficiency; Cross Polar level introduced by the reflector configuration; Scanning capability; ratio of the configuration equivalent focal length to main reflector aperture diameter and ratio of subreflector area to main reflector area; RCS background levels; phase errors due to reflectors surface roughness as a function of the frequency. In order to illustrate the above discussion, several examples of commercially available compact ranges (S.A., March, Harris) are examined, as well as some recently developed European facilities (MBB, ESTEC, RYMSA). As it will be shown, each configuration is best suited to satisfy different user requirements. For example Shaped Cassegrain/Gregorian configurations seem to be the most efficient for RCS measurements whereas the Front-fed Cassegrain quiet zone can be scanned with low degradation.
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.
The new Compact Test Range at Dornier GmbH, operational since early 1990, is presented. The system is designed for both antenna and RCS measurements, for support of in-house projects as well as for third party measurement needs. Great emphasis has been on improving measurement through put to reduce effective measurement costs. The major system components are evaluated (anechoic chamber, compact range reflector system, RF instrumentation, positioner system, computer system and measurement software). System specifications, and where possible measured performance data are presented. Finally a typical antenna and RCS measurement are described to get an idea of possibilities together with required range time.
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.
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.
Errors due to the interaction between test body and the Device Under Test are often overlooked in test body design. Interactions which cannot be gated or subtracted can be present even in low RCS test bodies. This paper presents an approach to evaluate the edge interaction errors of a component RCS test body. In order to quantify the interactions, small cylinders were attached to the face of the test body and measured from grazing to 50 degrees. The scattering of the cylinders illuminated the edges so that the interactions could be measured. This data is presented along with the results of several computer models which were used to determine the interactions involved. A method of moments model of the cylinders on an infinite ground plane gave the theoretical level of the cylinders. A pattern of a monopole antenna on a test body shaped ground plane was used to determine the contribution of each edge; and a point source model was used to locate the points on the edge where the diffraction occurred. This technique allows the dominant source of error signals to be identified.
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.
Two-dimensional RCS imaging systems utilize wide-band, ISAR processing to spatially isolate scattering sources on complex objects. Although the measured data consist of the frequency and angle responses of the entire object, the image process allows the possibility of extracting the responses of scattering components which comprise the total signature. These methods of image editing generally involve the application of spatial filters to the image, followed by a reconstruction of the angle and frequencies responses associated with the filtered image. The objective of these procedures is to determine the responses of localized scattering sources or to delete the contributions of scattering sources on the overall signature of a complex object.
A number of spectral analysis techniques which offer significantly higher resolution than the FFT technique have been developed in recent years. The application of these super-resolution techniques to scattering analysis is of interest. With these techniques it is possible to identify the closely spaced scattering centres even with RCS data over relatively small bandwidths. This can be of significant importance in applications where data over large bandwidths are not available. The use of Autoregressive and Eigen analysis based super-resolution techniques in the scattering analysis of two basic targets, a sphere and a cube, is investigated and the results of the study are presented in this paper.
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.
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.