AMTA Paper Archive
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Inflatable target support for RCS measurements
A stressed-skin inflatable target support provides an improvement over a foam column for radar cross section (RCS) measurements in an anechoic chamber. Theoretical analysis indicates that backscatter from the support is minimized because its mass is reduced below that of a foam column and is distributed to favor incoherent scattering. Compared with a foam column, a pressurized thin shell has superior mechanical stability under both axial and transverse loads. Experimental observations using Mylar -- a low dielectric constant, high tensile strength film -- confirm these results. Spurious reflections from rotational machinery located below an inflatable column are reduced by a layer of absorber within the base of the inflatable support.
Target alignment techniques for the compact range
Many targets today exhibit radar cross sections sensitive to the angular orientation of the target. While some of these targets have prominent scattering centers which can be exploited to obtain a relative positional reference, many targets unfortunately do not. In addition, many complex targets have a highly directional scattering behavior requiring careful alignment to the incident planar field. This need for accurate positioning has prompted the development of laser alignment techniques for the compact range. One such system has been under development at the ElectroScience Laboratory, and the designs and results of the first prototype are presented here. Performance goals and design criteria are discussed, and future improvements are considered. In addition, similar systems for feed and pedestal location reference systems are presented.
Characterizing the bistatic performance of anechoic absorbers
The requirement to measure lower radar cross-section (RCS) levels within anechoic chambers has demonstrated the need to further analyze the performance of microwave absorbers. The interactions of the feed system, compact range reflector, target mount, and target/test body with the microwave absorber greatly effect both the measurement accuracy and ambient noise level within the anechoic chamber. Better absorber characterization and understanding leads to improved chamber performance analysis and chamber design modeling. Past absorber studies have evaluated the backscatter performance of most absorber types, however, bistatic performance characterizations have been limited. This paper will discuss a method of obtaining bistatic absorber data which offers the advantages of time gating and synthetic aperture imaging to improve measurement isolation and accuracy. The approach involves illuminating a large absorber test wall about several incidence angles with the plane wave generated by a compact range. A receive antenna is then moved about the test wall and bistatic scattering is observed. The technique provides improved measurement results over methods utilizing NRL arch type systems. Bistatic absorber data has been collected and analyzed over angles from normal to near grazing incidence. Test results will be demonstrated with different absorber shapes, sizes, orientations, and material transitions from wedge to pyramidal. Various bistatic conditions will be analyzed for both polarizations over a number of frequencies.
A Synthetic aperture imaging method for evaluating anechoic chamber performance
Evaluation methods for analyzing the performance of anechoic chambers have typically been limited to field probing, free space VSWR and pattern comparison techniques. These methods usually allow the users of such chambers to qualify or determine the amount of measurement accuracy achievable for a given test configuration. However, these methods in general do not allow the user to easily identify the reasons for limited or degraded performance. This paper presents a method based on synthetic aperture imagery which has been found usable for finding and identifying anechoic chamber performance problems. Photographs and illustrations of a working SAR imaging/mapping system are shown. Discussions are also given regarding the method's advantages and disadvantages, system requirements and limitations, focusing processing requirements, calibration techniques, and hardware setups. Both monostatic and bistatic configurations are considered and both RCS and antenna applications are discussed. The SAR system constructed to date makes use of a portable HP-8510 based radar placed on a hydraulic manlift for easy system maneuverability and flexibility. The radar antenna is mounted on an 8 foot mechanical scanner directed toward the area to be mapped. An image is processed after each scan of the receive antenna. Measured data and example results obtained using the mapping system are presented which demonstrate the system's capabilities.
Circularly polarized RCS measurements
Circularly polarized radar cross-section (RCS) measurements place stringent requirements on an RCS range. Indoor compact ranges without the problems of ground reflections have the potential of making accurate circular polarization (CP) measurements. A simple method for CP RCS measurements is described using broadband meander-line polarizers over the compact range feed horns. Axial ratio and differential phase measurements were performed to evaluate the polarizer fabrication accuracy. Basic scattering shapes were measured to test the performance of the CP measurement system. Comparison of CP measurements with analytical predictions demonstrated the success and limitations of the technique.
Automated millimeter wave evaluation system for advanced materials and frequency selective surfaces
An automated instrumentation system has been configured for the purpose of evaluating advanced composites, radar absorbing materials, and frequency selective surfaces (FSS) in free space. Electrical test frequencies are divided into three bands that range from 18 to 60 GHz for any linear polarization. Software has been incorporated to calculate dielectric properties from the measured transmission and reflection characteristics. Using the HP9836 computer, software was written to automate and integrate the Anorad 3253 positioner with the HP8510 network analyzer. This system allows for the input of up to five incident angles at vertical, horizontal, and cross polarization. The measured transmission loss (amplitude and phase) at multiple incident angles is then plotted for comparison. This paper gives a complete description of the system configuration, calibration techniques, and samples of output data. Material properties are computed and compared to specified and theoretical values. Measured results of an FSS structure are compared to its predicted response.
Electromagnetic surface roughness for composite materials
Present day manufacturing techniques often employ composite materials in the fabrication of many structures. Graphite is one common material used to form structurally strong fibers for use in a resin binder. The material characteristics of graphite composites naturally differ from those of metallic materials. An interesting characteristic is the smoothness or roughness of composite materials as examined from an electromagnetic viewpoint. Radar backscatter measurements of several different planar panels were performed near grazing incidence to compare their scattering characteristics against a smooth metallic surface. These results show the "electrical" smoothness of the surfaces in terms of fabrication and material dependencies.
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.
Performance of gated CW RCS and antenna measurement
Conventional receivers for pulsed radar systems employ a wideband final filter that is matched to the pulse width and risetime. However for pulsed RCS measurements on small test ranges, instrumentation receivers with narrow IF bandwidth have proven useful. This paper analytically examines the differences between narrowband and matched filter instrumentation receivers and describes typical conditions under which gated CW measurements are made. Useful relationships between PRF and IF bandwidth are derived.
ISAR image quality analysis
In practical ISAR applications the quality of the image obtained depends upon the distortions in the wavefront illuminating the target, effects introduced by the radar-target path, the accuracy of the angle and frequency steps used in obtaining the data, vibration, and multiple reflections from neighboring objects. Results of analysis, simulation and data obtained in an RCS compact range are presented to quantify the relationships of the image degradation introduced by these effects.
Interpretation of two-dimensional RCS images
The objectives of RCS imaging are to spatially isolate and quantitatively measure the strength of scattering mechanisms on complex objects. Although some isolation can be provided directly by using radars with high spatial resolution, most current RCS systems achieve the required resolution by synthesizing the image from measurements of the object response to variations in frequency and rotation angle.
Measurement techniques for the RADARSAT SAR antenna
A study of RF testing methods was conducted for the Radarsat SAR antenna. The implementation tolerances of a planar and a cylindrical near-field facility were computed, by simulation of the effects of different types of measurement errors on the reconstructed far field. The results are presented and the two types of near-field facility are compared.
Quiet zone RCS errors
A unique RCS field probe system is described which determines: 1) the two way phase and amplitude field taper, and 2) the RCS measurement error within the quiet zone. The RCS of a suspended target is measured by the radar at selected locations or while moving in the quiet zone. The field taper is obtained from a time gated target return. The quiet zone RCS error for a target is obtained by comparing RCS measurements from anywhere in the quiet zone with the target RCS measured at the center of the quiet zone. A quiet zone containing a high quality illumination field was measured and found to have more than a 5 dB quiet zone RCS error. The RCS error magnitude is dependent upon the radar variables which are determined by the target size. There is a significant difference between the implied RCS error based on the illumination field quality and RCS measurement error caused by the additional contributions of multipath and target dependent clutter that are peculiar to each facility. Accurate RCS measurements require detailed knowledge of the test facility's multipath, target dependent clutter characteristics, and the target's bistatic signature.
Error analysis in RCS imaging
In the last few years, the interest in Radar Cross Section (RCS) measurements has increased rapidly. The development of high-performance Compact Ranges (CR) has made possible measurements on large targets down to very low RCS levels (below -70 dBsm). RCS imaging is a powerful tool to determine the location of scattering sources on a target. The response of the target is measured as a function of the frequency and aspect angle. A two-dimensional Fourier transform then gives the reflection density as a function of down-range and cross-range. If the response is measured vs. azimuth and elevation, even a complete 3-D image is possible. For high-resolution imaging (large bandwidth, wide aspect-angle span) a direct 2-dimensional Fourier transform gives rise to errors caused by the movement of the scatterers during the measurement. These errors can be corrected by applying a coordinate transformation to the measured data, prior to the Fourier transforms. This so called focused imaging allows further manipulation of measured data. However, the measurement accuracy can be a limiting factor in application of these techniques. It will be shown that the Compact Range performance as well as positioning accuracy can cause serious errors in high-resolution imaging and thus in interpretation of processed data.
A Novel approach for two- and three-dimensional imaging
Conventional radar imaging requires large amounts of data over large bandwidths and angular sectors to produce the location of the dominant scattering centers. A new approach is presented here which utilizes only two swept frequency scans at two different look angles for two-dimensional images or three swept frequency scans at three different look angles for three-dimensional images. Each swept frequency scan is the backscattered response of a target. A different plane wave illumination angle can be conveniently obtained by offsetting the feed horn from the focus of a compact range reflector without rotating the target. The two- and three-dimensional target information for the location of the dominant scattering centers is then obtained from the band limited impulse responses of these swept frequency scans.
Speeding up the HP8510B for antenna and RCS measurements
Antenna and Radar Cross Section measurements require a large amount of data collection. Network Analyzers are often used to characterize these systems, and although these data ideally are collected automatically by computer it is not unusual for a single characterization to require many hours or even days to perform. We describe a technique for speeding up these measurements by at least an order of magnitude. Clearly making measurements in an hour that formerly took a day or making measurements in a day that formerly took two weeks is extremely appealing. The method we describe may be used for applications which require a large number of automatically performed measurements with sequentially swept frequencies, but which find lack of speed in tuning the network analyzer to be a limiting factor. Antenna, and Radar Cross Section measurements benefit substantially since frequency response measurements must be repeated many times to provide spatial characterization.
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.
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 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.
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