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RCS analysis of targets from reconstructed images
The ISAR image is a domain that possesses many of the spatial physical characteristics of the target. Certain procedures can be performed in the image domain that are equivalent to physical operations on the target. These operations include the ability to modify the amplitude of the scatterers that are represented in the image and, after performing these modifications, subjecting the image to an inverse transformation that recovers RCS data of the whole body as a function of frequency and aspect angle. The RCS plots obtained by transforming the edited image are representative of similar modifications made to the physical body and are of value in eliminating the need for many model modifications and retests in low-observable model development. This paper describes, using simulated and actual target data, some of the procedures that can be fruitfully applied in this type of analysis.
New near field RCS--and antenna--measurement techniques
In this paper a new system consisting of a single parabolic reflector and a point source will be presented. Such a system is capable of producing a cylindrical wavefront over a wide frequency range. Moreover, physically large text-zone dimensions can be realized. The principle of operation is identical to that of the near-field/far-field cylindrical scanning, however, the far-field antenna pattern or RCS response can be computed more efficiently by performing a simplified transformation procedure in one dimension only. It will be shown that such a system is suitable for both antenna and RCS measurements. Finally, experimental RCS data will be presented.
Refractivity fluctuations on an RCS test range: comparative measurement, characterization, and implications for calibration procedures
The performance of an outdoor, ground-plane RCS measurement range can be degraded by fluctuations in the atmospheric reflectivity N. These fluctuations can introduce error into RCS measurements, particularly when they do not manifest in the radar return from the secondary calibration standard. A propagation anomaly study at the RATSCAT RCS range compares the N-fluctuations -- obtained from meteorological instruments and separately from RF receivers -- at several levels above the ground. The fluctuation mechanisms are discussed in terms of temperature lapse rates, "constant-N" cell sizes, wind velocity, and rough ground effects. The optimal RF sensor height for propagation anomaly indications is found to depend on the cell size. This has implications for the positioning of secondary calibration standards.
Calibration and normalization of windowed RCS images
It is common practice to window RCS data prior to inverse Fourier transformation into an image. Windowing reduces image sidelobes at the expense of some loss of resolution. When the window shape is adjusted to give the best resolution-sidelobe tradeoff for the given application, however, the apparent RCS of features in the image varies unless the correct calibration of normalization is applied. This paper discusses the proper calibration and normalization techniques to use with RCS imaging. These techniques permit efficient generation of images that accurately depict the RCS of significant target features, independent of the data window shape.
A Wide band instrumentation radar system for indoor RCS measurement chambers
An instrumentation radar system suitable for collection of backscatter characteristics of targets in an indoor chamber was built and installed in the Ohio State University ElectroScience Laboratory. The radar is a pulsed system with continuous coverage from 2 to 18 GHz, and spot coverage from 26 to 36 GHz. The system was designed to have maximum flexibility for various test configurations, including complete control of the transmit waveform, H or V transmit polarization, dual receive channels for simultaneous measurement of like and cross polarization, greater than 100 dB dynamic range, and convenient data storage and processing. A personal computer controls the operation of the radar and is capable of limited data reduction and display functions. A mini-computer is used for more widely sophisticated data reduction and display functions along with data storage. This paper will present details of the radar along with measured performance capabilities of the system.
Applications of autoregressive spectral analysis to high resolution time domain RCS transformations
Modern analysis techniques of radar scattering data or radar cross section (RCS) data often include transformation to the time domain for the purpose of understanding the specific scattering mechanisms involved or to isolate or identify specific scattering points. The classic technique is to transform from the frequency domain to the time domain using an inverse (Fast) Fourier Transform (IFFT). Often, however, the scattering centers are too close together to resolve or the requirement for accuracy in the measurement of the differential time delay is too high given the IFFT inverse bandwidth. This paper presents a technique for determining the time domain response of a radar target by processing the data using modern autoregressive (AR) spectral analysis. In this technique, the scattering from a radar target in the high frequency regime is shown to be autoregressive. This paper will show examples using the maximum entropy method (MEM) of Burg.
RCS calculations for the optimization of target pylons
The monostatic RCS of ogival tilted pylon was calculated in a two stage computational process. First, a two-dimensional model of an infinite cylinder of ogival cross-section was employed. At the second stage, the effects of finite length and inclination were incorporated. The RCS was predicted by two independent methods, namely, the method of equivalent currents and the method of moments. Excellent agreement between the results of the two methods was found in the overlap domain of their respective validity. The results indicate a weak dependence of the RCS on the ogive ratio. Similarly, it was found that in the case of an infinite straight ogival cylinder the effect of frequency variation is negligible. The main contribution to RCS reduction is derived from the finiteness of the pylon and its tilt. It also becomes evident that beyond a certain tilt angle the marginal decrease of RCS does not justify the increasing mechanical complexity.
RCS errors due to target support structure
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
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
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.
Precision compact range feed
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
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
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.
Performance of a shaped compact range with a 12 foot quiet zone
This paper summarizes the performance of the new Harris 1612 Compact Range Standard Product. The Harris 1612 is a dual shaped reflector collimator system. Measured data, both amplitude and phase from the 12-foot diameter quiet zone is presented. The quiet zone was characterized using an automated two-way HP8510B based measurement system. The inherent system benefits for both antenna and RCS measurements are also discussed.
Financially justifying an antenna/RCS measurement system
This paper examines the economic justification process for a new Antenna or Radar Cross-Section (RCS) measurement system, and presents the techniques that can be used to determine the financial feasibility of a new system. Specific examples are given that will allow engineers to customize calculations to fit their company's specific accounting methods and labor rates.
Dual domain RCS data diagnostics
In this paper, a general methodology for data reduction and analysis of wide-band RCS data is discussed. This methodology encompasses normal image processing, clutter removal, and noise filtering. Examples of the usefulness of the approach are presented.
Post processing corrections to indoor RCS VS aspect measurements
This paper describes two signal processing techniques that have been used to overcome specific problems in a Lockheed Aeronautical Systems Corporation (LASC) indoor compact RCS measurement range. Both techniques are post processing techniques used to enhance the accuracy of RCS vs. Aspect measurements. These two techniques can speed up measurement time, increase measurement accuracy, and increase target sizes on a compact range.
Millimeter wave compact range measurements
This paper discusses the configuration and performance of millimeter-wave measurement systems comprised of standard Harris Shaped Compact Ranges, Hewlett Packard (HP) 8510B Network Analyzer, and Millitech frequency extenders designed for use with the network analyzer. Millimeter-wave capabilities have been integrated into the Harris automated measurement system to allow computer controlled millimeter-wave compact range characterizations. This system offers a new measurement alternative for antenna and Radar Cross Section (RCS) measurements. Measured 35 GHz data from the Harris Model 1606 compact range, and 95 GHz data from the Model 1603 compact range are included.
A Measurement technique using gated ISAR imaging
Measured component RCS results are frequently dominated by the test body and target mounting structures. This paper will present a measurement technique that will improve measurement accuracy using a less complex and expensive test body. The design of the test body and measurement geometry allows isolation in both range and cross range from the static return of the room and mounting structure. This is accomplished by first creating an ISAR image of the target and test body, gating the image in two dimensions, then transforming back into the frequency and spatial angle domains to determine the scattering levels of the target by itself. Details of this technique, covering both its advantages and limitations, will be discussed. Data will be presented to verify the approach and illustrate the level of performance attainable using this technique.
Target diagnostics with high resolution multifrequency radar
This paper describes methodology for performing high resolution target radar cross section (RCS) diagnostic measurements with a new type of portable multifrequency radar. The Model 200 radar system is capable of operating at extremely short ranges, and does not require an anechoic chamber for performing highly sensitive radar cross section measurements. Measurements can be made in conventional low range resolution polar plot modes, in high-range-resolution (HRR) mode, in Inverse Synthetic Aperture (ISAR) mode, and in Synthetic Aperture (SAR) mode. The radar is described and the implications for present and future measurement technology are discussed.
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