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.)
ISAR images are formed by Fourier processing coherent wideband responses collected with angle diversity. Unfortunately, physical and practical considerations limit the frequency and angle diversities achievable. The finite diversities induce sidelobes, which are usually mitigated by application of tapered windows in the spectral domain. This procedure reduces image sidelobes at the cost of increased mainlobe width, thus degrading resolution. Spatially-Variant Apodiz.ation (SVA), a new non linear method developed at ERIM to improve the quality of SAR imagery, reduces sidelobe levels while preserving the mainlobe width corresponding to unwindowed data. In contrast to conventional window techniques which simply apply the same window function to every image element, SVA operates on the image by adaptively applying a window optimized for each spatial element. The algorithm uses phase information available from the coherent RCS data to distinguish processing sidelobes from correct responses. Mainlobes are passed using rectangular weighting, while sidelobes are reduced or eliminated entirely. This paper discusses the concept, theory, and implementation of SVA for ISAR imaging, and summarizes capabilities and limitations of the method. Results using SVA are presented and compared to conventionally windowed one- and two-dimensional images. The sensitivity of the procedure to additive noise and phase errors is investigated
A new approach to microwave tomography only requiring monotonic RCS data is presented. The amplitude and phase variation of signals backscattered from the target are measured in uniform angular increment and then analyzed by using wavelet transform. The wavelet transform with multiresolution property is suited for transforming the measured data into aspect vs. Doppler frequency (due to the phase variation of the rotating scatterers) domain. The scatterers location can then be derived by extracting the Doppler frequency variation and peaks occurence delay from the resultant 2-D representation, hence it makes the microwave tomography possible. Two discrete points targets are considered and the resultant microwave tomograms are shown. In our works, the entire processing can be completed in less then ten minutes for a 41 x 41 pixels tomogram nmning on 80486 DX-33 PC and only with single frequency illuminating signal. Furtherrnore, the scattering mechanisms are clearly identifiable in the resultant 2-D representation which can not be achieved by any other microwave tomography methods.
An efficient maximum likelihood (ML) estimator to obtain the scattering center locations of a target and the relative scattering level of these scattering cen ters from the scattered field data is described. In the proposed method, ML estimation is carried out in the image domain rather than in the frequency-aspect do main. Inverse Fourier transform is used to transfer the scattered field data from frequency-aspect domain to the image domain (down range-cross range). As ex pected, the scattered field data in the image domain have some major lobes. The location and shape of the major lobes are used to obtain the initial guess for the ML estimator. The scattered field data samples in the major lobe regions are then used for ML estimations. It is shown that by carrying out the ML estimation in the image domain one can increase the computation efficiency by an order of magnitude.
A digital processing technique capable of forming fine resolution ISAR imagery of air vehicles in dynamic flight is presented. The interactive algorithm is predi cated on the ability to isolate one or two point-like scat terers in the target signature. Phase information extracted from these prominent point scatterers is pro cessed to yield high fidelity estimates of target motion over the image formation interval. Motion estimates are subsequently used to perform conventional ISAR motion compensation and to achieve equi-angular spa tial sampling between radar pulses. Existence of promi nent points obviates the need for any auxiliary information, such as on-board inertial navigation data, and permits focusing of images from non-cooperative targets. The processing procedure is illustrated with X band measurements of a Convair CV580 aircraft taken by the Ground to Air Imaging Radar (GAIR) system.
The utility of high resolution ISAR data in the devel opment and maintenance of low observable (LO) and conventional aircraft and the identification and charac terization of threat aircraft is well established. However, the task of ISAR image RCS interpretation is difficult. Often imaging effects introduced by rotating blades and jet engine modulation (JEM) can compound the already difficult interpretation task. It is easy for these effects to be obscured, ignored, or erroneously misinterpreted in ISAR down-range versus cross-range (Doppler) imag ery and range compressed versus time domain data. This paper presents cases of amplitude and phase modulated ISAR data collected from two airborne targets; a propel ler driven airplane and a helicopter, using a linear FM waveform radar. This will be supplemented with mathe matical models describing the modulation phenomenon and the resultant imaging effects
As mobile and satellite phased array antennas move from to concept production the demands on test station throughput increases dramatically. Completely characterizing a Transmit/Receive (TIR) module may require thousands of S-parameter measurements under CW and high-power pulsed conditions, as well as, harmonics, spurious, and noise figure measurements. The measurement throughput of instrumentation used in characterizing the prototype TIR modules simply may not be capable of handling the added volume of a production environment. The volume of measurements, the multiport nature of the device, and the integrated TIR module control make it necessary to reexamine the traditional approaches of separate network analyzers, noise figure meters, and spectrum analyzers. The result is a high-speed modular test ystem that completely characterizes the device in a single connection. The system contains a single receiver and a dedicated controller that utilizes the instrumentation in the most efficient method while maintaining or increasing the accuracy of traditional approaches. This paper describes the high-speed test stations that have been designed and built and are currently in use in several production facilities. Test system architecture is discussed and measurement throughput numbers are given and compared to conventional approaches.
The radar scattering from a small communications antenna mounted on a large cylinder was measured at the Ohio State University ElectroScience Laboratory compact range. This paper will describe the experimental measurement techniques and the details of the analysis of the experimental. The small (5 cm) blade/slot/cavity antenna was mounted on a 1.82 meter long cylinder of 0.61 meter diameter. The cylinder was treated with RAM on the ends to reduce the direct and interactive end scattering effects, and was mounted in the OSU compact RCS measurement range. Measurements over the 2 to 18 GHz band both with and without the antenna were made and the results subtracted during the calibration effects to further remove the end effects. We will demonstrate these techniques and evaluate their effectiveness. ISAR imaging of both the antenna and the scattering term associated with the load on the end of the antenna transmission line will be shown. This will demonstrate that the transmission line and loan can be separately evaluated using such techniques. A time frequency distribution (TFD) analysis technique will also be demonstrated as a means of extracting various antenna resonance terms from the data. A description of the theoretical computation of the scattering will also be given and the special aspects of this problem outlined. The theoretical RCS data will be compared to the experimental measurements of the RCS.
This paper describes final results on the of non destructive measurement methods of missiles in terms of stealthiness. Measurements performed on full scale missiles allow to determine the reflectivity of the material and give estimation of its real RCS with to its nominal RCS. Different measurement techniques are reviewed, based on the use of coaxial transmission line, circular waveguide and spot-focusing horn lens antenna. Modeling, and characterization of spot-focusing corrugated horn lens antennas operating in the frequency range 2 - 18 GHz are presented. Finally, system configuration of full scale missile RCS measurements currently being utilized for production control is presented.
• Some Radar Cross Section (RCS) measurements contain significant contributions from the interaction of test article components. Usually the direct measurement of these terms is difficult. When these terms are not major factors, they need little attention. In other circumstances they should at least be quantified. There terms are often studied with special models, and/or Doppler measurements, and analysis. These relatively expensive methods yield the required information. For some purposes a more economical, limited method would be useful. RCS measurement and analysis facilities use software designed to present data in usable formats, with appropriate processing. This software is often run on a powerful workstation, or mainframe. McDonnell Douglas Technologies Inc. (MDTI) processing software "runs" on an .HP730 series workstation. The speed and capacity of such a system makes processing data a convenient option. MDTI demonstrated the ability to extract interaction terms from an easily acquired data set. This extraction required only the use of standard data software. Results with generic shapes demonstrate the ability to extract terms > 30 dB below the return of the test article specular return
A vertical array of antennas is used to beamform the farfield used in the measurement of Radar Cross Section (RCS) on a ground-bounce radar range. By properly weighting (attenuating) and phasing (through line length adjustments) each antenna, a desired far-field pattern can be obtained. This paper discusses some benefits of the technique and outlines a basic mathematical approach. Implementation is considered, and wide band ramifications of a practical design are discussed. At RATSCAT, this basic understanding was used to examine a simple two element array. This paper preceded that study and was originally written just for that purpose.
A technique is presented for obtaining the radiation patterns and the antenna gain of elliptically polarized antennas from two vector measurements of the far-field. The two measurements correspond to different polarizations which can be obtained by rotating one of the antennas around its boresight axis. The discussion emphasizes a particularly interesting case, for which accurate radiation patterns and gain of the antenna under test (AUT) can be obtained without prior knowledge of the polarization of the second antenna. The radiation pattern of a nearly circularly polarized (CP) antenna is conveniently represented by the CP co-polarized and cross-polarized components. The axial ratio and any other quantities commonly used to specify the antenna polarization can also be obtained since the pair of initial vector measurements completely characterize the polarization of the AUT. The technique is illustrated by measurements of a CP patch antenna.
For an anechoic chamber design, one normally spec ifies the field quality throughout the quiet zone in terms of the ripple level requirement. The ripple in the quiet zone field is caused by the interfer ence of various stray signals with the desired plane wave. The stray signals in an anechoic chamber can come from absorber or other parts of cham ber. However, from a range performance point of view, it is more important to know the ef fects of stray signals on the measurement accu racy of an antenna radiation or target scattering pattern. Consequently, it is very critical to eval uate how the chamber stray signals will affect a given measurement. This paper addresses this is sue by simulating pattern measurements of a phase scanned array in a compact range and discuss the effects of various stray signals associated with the scattering from absorber walls and feed spillover.
Present day accuracy requirements on high-performance antenna measurements are difficult to meet on any type of compact range. Numerical correction techniques can offer a good solution. An easy and effective method is the Advanced APC-technique. This method requires patterns to be measured on different locations in the test zone so that disturbances of the plane wave can be distinguished. In case of suitable distances, the "true" pattern can be derived from measured amplitude and phase data. Usually, scanning is performed in longitudinal direction. The advantage is that mutual coupling can be distinguished well, but the field ripple in this direction due to extraneous fields varies much slower than in transversal direction. Consequently, first sidelobes can be corrected more efficiently when transversal scanning is performed. Therefore, in this paper a new and flexible way of positioning is proposed depending on the location of extraneous field sources.
This paper discusses the instrumentation techniques that can be used for the measurement and characterization of antennas that are to be tested in a pulsed-RF mode of operation. A pulse-parameter chart is presented that illustrates all possible ranges of pulse width and pulse repetition frequencies for antennas operating in a pulsed mode. An antenna operating in a pulse mode will have pulse parameters that lie somewhere on the pulse parameter chart. This paper defines five different measurement regions of the pulse-parameter chart, and presents the measurement techniques for measuring pulsed antennas that operate in each of these regions.
Typical high-resolution dynamic target imaging radars have frequency scan rates that do not properly sample the modulation from rotating structures such as aircraft propellers, engine turbines and helicopter blades. This results in the scatterer modulation energy being aliased. Moreover, if the chirp rate is too slow blurring and of the scatterer can occur in the image. Often the utility of this data for RCS signature analysis is questioned. This paper addresses the utility of images generated from undersampled data of modu lating scatterers. Experimental results using various combinations of chirp scan, modulation, and target-body rotation rates are presented. Fast scan rates, typical of the Linear-FM waveform, are compared to the slower scan rates commensurate with step frequency wave forms. Images are shown illustrating how the different chirp speeds alter the two-dimensional image of a mod ulating target.
Inverse synthetic aperture radar (ISAR) images of dynamic targets can be generated using stepped- frequency radars [1,2]. However, a stepped-frequency waveform requires many pulses transmitted over tens of milliseconds to achieve range resolution. This has the undesirable property that a target's rotating parts (such as propeller blades and jet engine compressor blades) can move significantly during this time. This results in of the Doppler sampling, and shifting and blurring in range, (range-Doppler coupling), which degrade the image quality. The Naval Command, Control and Ocean Surveillance Center, RDT&E Division (NRaD) has added a linear frequency modulated (LFM) waveform to its X-band imaging radar. This radar measures a 500 MHz bandwidth in 600 ns. The received signal is baseband converted, digitized, and stored. Data from this radar have been successfully processed into ISAR images that do not exhibit many of the undesirable properties of stepped frequency measurements
ISAR imaging has proved to be a _ significant diagnostic tool for the evaluation of RCS signatures because of its ability to resolve scatterers in both the cross range and down range dimensions. There is a growing desire to extend the imaging capability to include the vertical dimension of a target, or three dimensional (3D) imaging. Several techniques have been suggested with varying degrees of success and complexity. These techniques include triangulation from two or more ISAR images of the same target taken at different elevation angles, tomographic algebraic reconstruction, and true 3D ISAR imaging using the FFT. Each technique requires progressively more data and more complex algorithms, but results in more resolution. This paper examines these various techniques, and evaluates their advantages and disadvantages based on actual implementations using simulated data.
A proper knowledge of clutter characteristics is critical to the design, development, and test of military seeker and radar hardware. The Clutter Mapping System under construction at Flam & Russell, Inc. is simple yet powerful tool for the evaluation of potential radar sites or the analysis of current sites. It provides a maximum 40 foot synthetic aperture that can image a 60 degree sector of terrain out to a 20 mile range and beyond. Aside from this primary mission, it has the capability to perform RCS measurement of non-cooperative ground targets or to serve as a tactical, quickly deployed imaging system. Totally self contained, and transportable, this system can fulfill a wide variety of RCS measurement needs.
NATO and former Warsaw Pact nations have agreed to allow overflights of their countries in the interest of easing world tension. The United States has decided to implement two C-135 aircraft with a Synthetic Aperture Radar (SAR) that has a 3-meter resolution. This work is being sponsored by the Defense Nuclear Agency ( DNA) and will be operational in Fall 1995. Since the SAR equipment must be exportable to foreign nations, a 20-year-old UPD-8 analog SAR system was selected as the front-end and refurbished for this application by Loral Defense Systems. Data processing is being upgraded to a currently exportable digital design by Sandia National Laboratories. Amplitude and phase histories will be collected during these over flights and digitized on VHS cassettes. Ground stations will use reduction algorithms to process the data and convert it to magnitude-detected images for member nations. System Planning Corporation is presently developing a portable ground station for use on the demonstration flights. Aircraft integration into the C-135 aircraft is being done by the Air Force at Wright-Pafterson AFB, Ohio
Lincoln Laboratory has developed a high resolution imaging radar in conjunction with Flam & Russell, Inc., of Horsham, PA. This highly mobile, ground-based system is capable of 2-D and 3-D imaging of targets at very close ranges to a synthetic aperture. The radar is fully-polarimetric, and operates over two frequency bands (0.05-2 GHz and 2-18 GHz). The radar is currently being used for target imaging and for foliage and ground penetration experiments. In this paper, the radar system is described. In addition, data calibration and image formation are explained. Sample imagery, both 2-D and 3-D, are shown.