Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
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.
The new HP 8530A microwave receiver has been designed specifically for antenna and radar cross section (RCS) measurement applications. With its capabilities and features, high-speed single parameter and multiple parameter measurements are possible. High-Speed measurements are a necessity for certain applications but oftentimes other factors will determine the actual test time. Measurement speed for various applications will be discussed and, more specifically, multiple parameter measurements using the HP 8530A’s internal multiplexer or external PIN switching.
The purpose of this paper is to present an overview of a turnkey mobile dynamic R.C.S. system, presently under design and development. The test system includes no less than 16 antennas, installed on two heavy duty tracking positioners, trailer mounted. The RF instrumentation is split over racks located on the positioners and in the mobile shelter housing the control equipment and operators and includes 14 receivers and 7 high power transmitters. The paper describes the antenna system, RF instrumentation, control and processing software as wek as operational and modularity aspects of this dynamic RCS facility.
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.
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.
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.
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.
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.
Radar cross section (RCS) measurements were performed in the 0.1-1 GHz band in an anechoic chamber optimized for microwave frequencies. Selection of proper instrumentation, antennas, measurement techniques and processing software are discussed. Experimental results, showing the accuracy and sensibility of the system are presented.
This paper analyzes the data rate requirements for RCS imaging systems as a function of measurement parameters and identifies the measurement conditions most likely to tax a system’s capability. Data rate estimates can assist in determining hardware and software design requirements and guide the selection of data storage devices to maintain high throughput rates.
The use of shaped reflector compact range collimators for application to indoor bistatic RCS measurements is discussed including electromagnetic performance and structural design issues. Room sizing and layout are presented for an assumed measurement system configuration. Coupling paths between the two collimators and associated time delays are reviewed for the assumed configuration and a range of bistatic angles. Collimator/chamber interaction issues are discussed. The mechanical design of the moveable collimator in a bistatic range is similar to the design of large steerable antenna structures. The same analytical tools and techniques are applied directly to the panelized reflector system, resulting in a design that will accommodate small deflections between the individual panels without permanent deformation. These conditions are not unlike the requirements for the Harris 40 foot quiet zone compact ranges to withstand Zone 4 earthquakes. The forces resulting from moving the collimator and the unevenness of the track are the input conditions to the finite element model. A real-time characterization of the collimator is provided by a laser measurement system similar to that used on the Harris compact range field probe.
Increased productivity and higher resolution imaging capabilities are becoming of greater concern for RCS ranges. The ideal measurement scenario involves taking data on all desired frequencies for a target combination in a single rotation. This could involve one or more frequencies in several bands, imaging data on more than one band or very high resolution imaging data covering several bands. Placing several feeds in a cluster at the focal point of an offset fed com-pact range can provide these capabilities. The effects of feed clustering such as beam tilt are discussed along with cluster sizes that provide little if any degradation in compact range performance. Experimental data is shown that gives an indication of the quality of data that may be obtained. The concepts are also applicable for outdoor ranges that have an array of antennas offset from range boresight.
The Hughes Aircraft Company conducted a study to characterize the backscattering performance of wedge shaped anechoic absorbers for use in treating the sidewall regions of RCS chambers. ISAR imaging techniques were utilized to obtain a diagnostic results at near-grazing incidence angles which were not possible with conventional testing methods. These techniques allowed for separation and identification of individual scattering sources from each of the evaluated samples. As a result, the backscattering from an entire wall of absorber can be simulated by evaluating only a few samples. Absorber performance data was collected over frequencies from 2 to 40 GHz. Results from this study clearly show that differences in absorber fabrication methods have a significant impact on the performance of the materials. Various approaches for impregnating, loading, and cutting the absorber have also been evaluated. Gaps, formed during installation, at the joint between two pieces of material are shown to significantly degrade performance, whereas, offsets and glue lines are shown to have less of an effect, provided the absorbers are uniformly loaded.
This paper will describe new developments in a gated-CW radar that has been designed to improve the productivity and sensitivity of RCS measurements. Improvements in data acquisition speeds result from the design of a fast synthesizer, a data acquisition co-processor and a pulse modulator. Each of these new products have been specifically designed to take advantage of the high speed capabilities of Scientific-Atlanta’s Model 1795 Microwave Receiver. The RF sub-system has also been designed to permit continuous 2-18 GHz, full polarization data acquisitions. Critical RF components are now mounted at the feed in the chamber, improving the sensitivity and ringdown of the system. Productivity in analysis activities has been improved by the use of a multi-tasking system controller which permits simultaneous use of the system for acquisitions, analysis and plotting.
This paper describes results of extensive polarimetric Radar Cross Section (RCS) measurements on canonical targets. Amplitude and phase of both co- and cross- polar returns are measured for horizontally and vertically polarized transmit signals in order to determine the complete complex scattering matrix. Measurements have been carried out on a variety of targets. Results presented with this summary show data for a metallic and a dielectric disk. Details of measurement and calibration procedure, hardware, and software are also presented.
This paper is concerned with the measurement of RCS in a room with conducting walls. The experimental measurement system uses a moving antenna to produce a scan of the target and the clutter. The scattered signal as a function of frequency and position is recorded. New field crossrange processing is then used to map the target zone. Example images will be shown for both two-dimensional and one-dimensional scans. Images from point targets and distributed targets will be presented.