AMTA Paper Archive
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Detection of conductivity gaps and material imperfections using surface radar diagnostics
Low RCS signatures require verification of test body conductivity and material performance. A miniaturized radar system with a unique horn antenna was designed for the detection of conductivity gaps and material imperfections in radar absorbing material. The antenna system has a small aperture and low VSWR permitting direct placement against a surface for localization of electromagnetic phenomena. Test results indicate that test body construction gaps and material imperfections are readily detectable using the test system in either a handheld or robotic-type configuration. Preliminary results also indicate delaminations, conductive panel penetration, and structural component steps will be detectable.
VHF/UHF RCS measurements in indoor microwave facility
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.
Radar cross section measurements in a reflecting chamber
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.
Establishing data rate requirements for high performance imaging systems
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.
Advantages of the dual shaped reflector collimator for bistatic measurements
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.
Use of clustered feeds in a compact range for RCS measurements
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.
Diagnostic evaluation of wedge absorbers for RCS chambers
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.
Productivity improvements for a gated-CW radar
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.
Monostatic and bistatic polarimetric radar cross section measurements on canonical targets
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.
Correction/calibration of wide-band RCS radar data containing I/Q error
In-phase and quadrature (I/Q) aberrations in radar receiver data create problems in radars used for radar cross section (RCS) measurements. I/Q errors cause incorrect representations of the target under test. A method for correcting I/Q error and calibrating the measured amplitude to a scattering standard provides a means of obtaining a more accurate representation of the target under test. The RCS measurement instrumentation addressed here uses a wide band receiver with a single quadrature mixer for conversion of radio frequency (RF) to base band (also referred to as video) frequency. In the one-step down conversion, distortions in the I/Q constellation occur, causing I/Q errors. This method quantifies the extent of the I/Q problem by estimating the actual I/Q error from a series of calibration measurements. An algorithm is presented which quantifies parameters of the I/Q distortion, then uses the distortion parameters to remove the I/Q aberrations from the target measurement.
Short term stability performance of pulsed instrumentation radars using TWTAS
Pulse-to-pulse amplitude and phase noise can affect the overall measurement accuracy of RCS instrumentation radars. Depending upon the measurement requirements, such noise can limit the overall performance whenever pulse-to-pulse repeatability is required in the signal processing. Radar systems using pulsed TWTAs are subject to high noise due to limitations in the performance of the TWTA modulators and power supplies. A characterization of this additive noise is important to understand the limitations in system performance. Measurements have been made on kilowatt power TWTAs at L and X band as well as 20 watt pulsed TWTAs at S, C, and X/Ku band at various duty cycles and PRFs.
GO taper and cross-polarization error corrections for RCS measurements in compact range
Compact range systems have been widely used for high quality RCS measurements. However the taper and cross-polarization effects can lead to significant measurement errors especially as the target approaches the border of the target zone. The taper error is mainly caused by the feed’s finite beamwidth, and the cross-polarization error by the feed’s cross-polarized radiation and the offset configuration of the reflector. A method to correct these errors is presented. In order to perform taper and cross-polarization error corrections, one has to be able to predict the target zone fields and determine the locations and complex strengths of the various scattering centers associated with the target. The correction can then be done by compensating for the taper and cross-polarization effects for each localized scattering center. Several measurements have been taken, corrected and then compared with the theoretically expected results to validate this technique.
Amplitude taper removing in RCS measurement
Amplitude taper removing by software implementation has been made beyond the quiet zone region of a compact range reflector where the phase variation is still small. To remove amplitude taper effect in RCS measurement, actual amplitude taper of the range s first obtained by theoretically calculating the field distribution from the given range geometry and confirming with field measurement result. The processed target RCS contour is later implemented with the actual amplitude distribution around the region where the target is located. It is found that with the software implementation of amplitude taper removing the effective quiet zone of the compact range has been able to extend up to the size of the reflector diameter.
Global and local features of wideband RCS signatures
The utility of wideband RCS data for characterizing scattering mechanisms of complex objects has been established by wide-spread applications. The fundamental data from which the final products are derived consist of calibrated scattered fields measured coherently as a function of frequency and aspect angle. By processing these data, one-dimensional range or cross-range reflectivity profiles can be derived; by further processing, two-dimensional images can be derived. Modern RCS instrumentation systems capable of rapidly measuring and processing wideband data provide more object information than is conveyed by the RCS pattern, which has been the traditional descriptor of scattering behavior. The procedures of one- or two-dimensional imaging inherently involve integration processes, constituting many-to-one mappings in which data from a large set are collapsed to produce an individual pixel of the image. For example, a particular pixel of a range response is derived from the total object response “integrated” over a band of frequencies; similarly, a pixel of a two-dimensional image is derived from the object response “integrated” over frequency and angle. The exposure of a local feature of the object signature, obtained by collapsing the fundamental data, comes at the cost of obscuring the global descriptor. This paper explores techniques for presenting large amounts of information on single displays which retain both global and local features of the scattering process. These tools provide to the RCS analyst options for extracting and interpreting significant information from the measured data without arbitrary degrees of integration which can mask essential details represented in the data. The display methods utilize color coding to increase the amount of information conveyed by a single plot. Because color reproduction is not available for the proceedings, the paper is to be distributed at the conference.
ICCE: Interactive coherent clean editing
We discuss recent advances in signal-to-noise and signal-to-clutter enhancement technology applied to RCS measurements, with an emphasis on post-processing techniques. Then, we outline a technique we refer to as Interactive Coherent CLEAN Editing (ICCE). ICCE permits the analyst to segregate scattering features of the model under test into various groups. Clutter sources, such as the target support pylon, can be subtracted with potentially less error and more flexibility than other techniques. Limitations and the current status of ICCE are discussed.
High speed control of instrumentation for antenna and RCS measurements
Today’s measurement systems are placing ever increasing demands upon the computer systems which control instrumentation and collect data. This paper investigates high speed control of instrumentation for RCS and antenna measurements. Off-loading of I/O from control and data acquisition computers is examined with a view toward improving measurement throughput and simplifying I/O control tasks. These methods are particularly important for multi-tasking systems and networked resources where high speed real time control is burdensome. Attributes of I/O enhancement architectures are examined and tradeoffs between performance and flexibility are reviewed.
Gregorian compact range analysis and design
This paper deals with design and evaluation of Compact Range Antenna and RCS measurement systems. Reflector subsystem and feeders design as well as quiet zone evaluation and system performance qualification are considered. Acquisition, process and presentation software to control the whole system has been developed and successfully implemented. Two systems have been designed and are now at implementation stage. A Gregorian concept Compact Range is now been constructed at RYMSA (Spain). This facility has been fully designed by IRSA and will be operative by the end of 1990. Compact Payload Test Range (CPTR) at ESTEC (ESA) is now been tested. System Instrumentation and PAMAS (Payload and Antenna Measurement and Analysis Software) have been developed.
Wideband polarimetric determination of antenna radiation and scattering characteristics by RCS-measurements
A novel and very powerful measurement technique is presented which allows the determination of antenna radiation and scattering by radar-cross-section (RCS-_ measurements. The antenna under test is treated as a loaded scatterer using a polarization dependent network model that allows a complete antenna description in terms of scattered, radiated and absorbed waves. A load variation principle is used to determine the network model parameters and all commonly used antenna parameters like gain, antenna polarization, axial ratio, polarization decoupling, input impedance and also structural scattering can be derived from the backscatter measurement without using any additional standard antenna. With the antenna network description it is furthermore possible to examine the antenna behavior for arbitrary excitation or loading on their waveguide or radiation port.
An Overview of parameters determining productivity and sensitivity in RCS measurement facilities
A major objective in the design of an RCS measurement facility is to obtain the greatest possible productivity (overall measurement efficiency) while maintaining the accuracy and sensitivity necessary for low radar cross section targets. This paper will present parameters affecting the total throughput rates of an indoor facility including instrumentation, target handling, and band changes-one of the most time consuming activities in the measurement process. Sensitivity and accuracy issues to be discussed include radar capabilities, feeds and feed clustering, compact range, background levels, and diffraction control.
Design of an inflatable support for outdoor RCS measurements: mechanical and environmental considerations
Mechanical and environmental considerations for outdoor operation of an inflatable column are discussed in the context of a 30-ft-high column. The column is designed to support a 900-lb load in a 30-knot wind. Column RCS is less than -40 dBsm below 1 GHz for both horizontally and vertically polarized illumination. Designs using Mylar and Teflon-coated Kevlar as skin materials are compared. The primary concerns are wind loading, pressure regulation, and solar heating. Wind effects include static loading, gusting, and vortex shedding. In addition, wind-driven particulates, such as sand or stones propelled by passing vehicles can puncture the column. A pneumatic control system maintains a constant internal support pressure in the presence of leaks or pressure fluctuations due to changes in solar illumination.
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