AMTA Paper Archive
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Inverse displaced phase center antenna for aircraft motion emulation
In an airborne radar, the aircraft motion causes the returns from stationary ground clutter to be spread over a significant part of the Doppler space. Without flight testing, it is difficult to develop or verify the clutter suppression techniques required by future airborne radars. In this paper a technique is presented for emulating the angle and Doppler characteristics of airborne radar clutter from a fixed site, for the purpose of ground-based testing. An inverse displaced phase center antenna (IDPCA) is used to simulate the aircraft motion. The inverse displaced phase center antenna described is an 18-element linear UHF array whose phase center can be electronically shifted by means of a switching matrix. The motion emulation capability is demonstrated through the use of this antenna as an auxiliary array in conjunction with a stationary UHF surveillance radar. Examples of the clutter returns received by this system are given.
Measurements and comparisons of RCS under planar and cylindrical illumination
Mathematical techniques (calibration, background subtraction, software range gating, imaging, etc.) have become integral to the process of generating precision radar cross section measurements. The "reference target method" is a powerful RCS correction algorithm which yields plane wave illumination results from data acquired under an arbitrary but known illumination. This method is analogous to a two dimensional RCS calibration. Measurements of long bars (at X- and Ku-bands) and of a scale model aircraft (at C-band) were performed under the cylindrical wave illumination produced by March Microwave's Single-Plane Collimating Range (SPCR) at Arizona State University. The targets were also measured under the quasi-plane wave illumination produced by a March Microwave dual parabolic-cylinder CATR. The SPCR measurements were corrected using the reference target method. The corrected SPCR measurements are in good agreement with the CATR measurements.
A High resolution imaging radar for ground-based diagnostic applications
Lincoln Laboratory has developed a high resolution imaging radar in conjunction with Flam and Russell, Inc. or Horsham, PA. The radar is a highly mobile, ground based system that is capable of two and three-dimensional imaging at very close ranges to a synthetic aperture. The radar is fully coherent from 0.1 to 18 GHz and transmits CW pulses that are stepped in frequency across a preselected bandwidth. High range resolution is achieved by coherently processing the returned signals. The radar is being used for target imaging and for foliage penetration measurements.
An Instrumentation radar system for use in dynamic signature measurements
The dynamic, polarization/frequency diverse, Instrumentation Radar System (IRS) described herein combines the features of an X-band radar tracker with a wideband, fully polarimetric coherent data collection system. Mounted in a transportable trailer, the system can be towed to virtually any site to acquire radar signature measurements on moving aircraft. Specifically, this system can collect the complete, polarimetric target scattering matrix as a function of frequency in real time from all three traditional monopulse channels, as well as from the usually terminated diagonal difference channel. The acquired data can be used for multidimensional images, or for studying the characteristics and performance of monopulse trackers following real targets.
ISAR imaging of aircraft-in-flight using a ground-based radar
ISAR images and RCS signatures of aircract-in-flight using a ground based and an airborne radar system are presented. The ground-based measurements were at X-band and were of a Mooney 231 aircraft, which flew in a controlled path in both clockwise and counterclockwise orbits, and successiely with gear down, flaps in the take-off position and with the speed brakes up. The air-to-air measurements were made by a radar installed in the nose of the TA-3B aircraft which followed a KC 135 airplane at a range of approximately 450 ft. and traversed a cross-range angle component of (plus or minus) 30(degrees). The data indicates that these systems are useful tools for RCS signature diagnostics of aircraft in flight.
Super resolution radar target imaging of realistic targets
Recently, super resolution algorithm have been used in radar target imaging to increase the down range and/or the cross range resolution. In the open literature, however, the super resolution algorithms have been applied to simulated targets or very simple targets measured in a test range. In this paper, the super resolution algorithms, namely the hybrid algorithm and the 2-D linear prediction, are applied to more realistic targets. One of the targets is a flat plate model of the F-117 aircraft. The back-scattered fields of the flat plate model were measured in a compact range. The other target is a Mooney 231 aircraft. The aircraft was flown in a circular pattern approximately 10 miles from the radar. It is shown that the super resolution algorithm can be successfully applied to these targets.
High resolution radar imagery using parametric modeling and data extrapolation
A microwave diversity imagery based on parametric modeling of back scattered signal versus frequency and aspect is presented. Forward-backward linear prediction is used to compute the model parameters. After stabilizing the corresponding transfer function, data are extrapolated to adjacent frequencies or aspects. Superior range- and/or cross-range resolution can be obtained by using frequency- and/or aspect-extrapolated data. Cross-range resolution can also be enhanced by extrapolating the frequency data and using data at a higher center frequency. For severly (sic) restricted viewing angles, or for small radar bandwidth, the new imagery can significantly improve the image resolution.
Radar cross section matching of various conic base closures using high resolution ISAR imagery
This paper describes an effort to evaluate the effect on RCS of base closures on a metallic frustrum at various depths with conducting and electrically isolated plugs. The tests were conducted at Sandia National Laboratories using System Planning Corporation's (SPC) Mark IV radar from 8 to 18 GHz, in the step chirped Inverse Synthetic Aperture Radar (ISAR) mode. Data reduction was performed on Information Systems and Research's workstation using the KNOWBELL software package. The workstation allowed the study of the imagery data in many different modes, which assisted in determining ways to evaluate RCS matching.
Target positioning error effects on RCS magnitude and phase responses in ISAR data
Coherent subtraction algorithms, such as specular subtraction, require precision target alignment with the imaging radar. A few degrees of phase change could significantly degrade the performance of coherent subtraction algorithms. This paper provides an analysis of target position measurement errors have on ISAR data. The paper addresses how traditional position errors impact phase and image focusing. Target rotational positioning errors are also evaluated for their impact on magnitude errors from specular misalignment and polarization sensitive scattering and image phase errors from height-of-focus limitations. Several tables of data provide a useful reference to ISAR data experimenters and users.
Application of planar near field measurement techniques for large phased array testing
Despite their high cost, phased array antennas are becoming popular for radar applications because of their ability to provide reliable information even in a hostile environment. Evaluation of these antennas requires parameters like gain, radiation pattern, beam width, sidelobe (both near and far off) azimuth and elevation null depth, etc. to be tested over the entire range of frequency spots and scan angles. Typically, if the number of frequency spots are 24 and the number of beam positions for which the measurement has to be done are about 100, then the total number of measurements needed to generate the required data are 7200. In addition, phased arrays with a space feed have to be initially collimated at all the spot frequencies. The outdoor testing of these many parameters may not be convenient, and at times it may even be impossible. The planar near field measurement technique provides a systematic and accurate method of measuring large array antennas for all the required parameters.
Stereo optical tracker for compact range models
A Precision Optical Measurement System (POMS) has been designed, constructed and tested for tracking the position (x,y,z) and orientation (roll, pitch, yaw) of models in Boeing's 9-77 Compact Radar Range. A stereo triangulation technique is implemented using two remote sensor units separated by a known baseline. Each unit measures pointing angles (azimuth and elevation) to optical targets on a model. Four different reference systems are used for calibration and alignment of the system's components and two platforms. Pointing angle data and calibration corrections are processed at high rates to give near real-time feedback to the mechanical positioning system of the model. The positional accuracy of the system is (plus minus) .010 inches at a distance of 85 feet while using low RCS reflective tape targets. The precision measurement capabilities and applications of the system are discussed.
String support system for RCS measurements
The target support system at Boeing Defense & Space Group's 9-77 Compact Range includes a new string support system. The string support system consists of twelve string reels, six each of the High Capacity String Reels (HCSR). The string reel system is used to suspend and manipulate a target for radar cross section (RCS) measurements, primarily at frequencies below 1.5 GHz. The string reel system is capable of supporting targets up to 10,000 pounds and 40' in length and width. The manipulation and handling of targets, is a major consideration in a RCS measurement test plan. The following paper discusses the newly installed string reel system, enhancements to the 9-77 Range equipment which directly affect the use of the string support system, and future developments planned for the system.
Measurement of Constitutive Parameters at Microwave Frequencies
To characterize a material's electrical performance or to understand a material's affects (sic) on electromagnetic systems, the constitutive parameters (e, u) of the material must be determined accurately. Materials with high dielectric constants, high loss tangents, or which are layered or complex (e.g., frequency selective surfaces, radomes, radar absorbing material, etc.) are difficult to measure and analyze. For example, germanium is an infrared window substrate in high performance aircraft. The germanium is doped to 1 - 4 ()-cm to raise the maximum operating temperature and to provide electromagnetic shielding. The material is very brittle. The standard methods (coaxial, waveguide, and cavity) are difficult to use. The brittle germanium pieces cannot be made thin enough or have a center conductor hole inserted making coaxial donuts are nearly impossible to fabricate and use. Usable waveguide samples absorb the transmitted energy needed in standard waveguide tests. The brittle sample cannot be made thin enough for X-band measurements or above. The sample, having a high dielectric constant, and having a high conductivity, reduces the Q of resonance techniques difficult and not repeatable. This paper discusses our methodology and shows comparisons with calculations. The technique is based on reflection measurements against a ground plane standard. This technique requires more measurements than other techniques, but the results are numerically more stable.
Free space characterization of materials
A simple change to the HP8510C or HP8720C vector network analyzer block diagram coupled with the TRM (Thru Reflect Match) calibration leads to accurate measurements of the material properties of flat samples. Algorithms developed for transmission line measurements can also be used in free space measurements. A description of recent improvements in the transmission/reflection algorithms is reviewed. Free space measurement results based on the transmission/reflection algorithms found in the HP85071B materials measurement software package are presented.
Automated test sequencer for high volume near-field measurements, An
Test sequencing flexibility and high throughput are essential ingredients to a state-of-the-art near-field test range. This paper will discuss methods used by NSI to aid the operator through the near-field measurement process. The paper will describe NSI's expert system and customer applications of a unique test and processing sequencer developed by NSI for optimizing range measurement activities. The sequencer provides powerful control of software functions including multiplexed measurements, data processing and unattended test operations.
Portable RCS diagnostic system
This paper describes the most recent version of the Model 200 portable RCS diagnostic radar. The Model 200 was designed to provide high-resolution RCS measurements in unprepared rooms indoors as well as on outdoor ranges. The system can provide real aperture measurements, ISAR measurements, or SAR measurements without changing system configuration.
High duty instrumentation radar transmitters
Today's requirements for dynamic Radar Cross Section (RCS) test data set new demands upon instrumentation Radar systems. Transmitters must deliver high power and operate at high data rates. Additionally, noise floor reduction of coherent spurious signals improves raw data and minimizes the need for manipulation of data.
Implementation of a 22' x 22' planar near-field system for satellite antenna measurements
Design and implementation of a large horizontal planar near-field system delivered to Space Systems/Loral for satellite antenna testing will be discussed. The 22' x 22' scan plane is 25' above the ground and employs real-time optical compensation for the X, Y, Z corrections to the probe position. The system provides high speed multiplexed near-field measurements using NSI's software and the HP-8530A microwave receiver. System throughput is enhanced through the use of a powerful and flexible test sequencer software module.
Demonstration of bistatic electromagnetic scattering measurements by spherical near-field scanning, A
The far-field radar cross section (RCS) of a conducting sphere is obtained by transforming scattered near-fields measured on a spherical surface. A simple and convenient calibration procedure is described that involves measuring the incident field directly at the target location. Although a non probe-corrected transmission formula was used in this study the importance of prove correction in practice is demonstrated.
Measurement speed and accuracy in switched signal measurements
The interdependence of accuracy and speed should be considered when analyzing measurement requirements. Tradeoffs can be made to optimize the measurement when accuracy is of primary importance, or where speed is critical. Several different measurement modes of the HP 8530A Microwave Receiver are presented, each with different measurement speed and accuracy tradeoffs. Examples are given that illustrate which acquisition modes would be appropriate to optimize the acquisition speed and accuracy in a variety of applications
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