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Enhanced high resolution radar imaging
J.W. Odendaal,E. Walton, I.J. Gupta, November 1994

Radar with the 2-D Fourier trans- form of the scattered field data in frequency and/or have poor resolution. A modified brid method and a modified 2-D AR technique are proposed to high radar images us- limited backscattered field data. The final image presents the scattering properties of the target in a quantitative way. The peaks in the image represents the positions of centers contributing to the backscattered field. Furthermore, the amplitudes of the peaks correspond to the intensities of the scattering centers.

Radar cross section calibration measurements using helicopter suspended spheres
M.J. Prickett, November 1994

The Naval Command, Control and Ocean Surveillance Center, Research, Development, Test and Evaluation Division (NRaD) is tasked by the Navy to collect and evaluate full-scale radar cross section (RCS) measurements on ships and aircraft. The Radar Branch at NRaD, operates a radar range west of Pt Loma, San Diego, CA. This radar range has been used to collect X-band and Ku-band calibrated data on Naval ships for the past seven years. The NRaD radar calibration helicopter procedures are the focus of this paper. Using helicopters to suspend and measure "isolated" spheres in space as the primary reference is a major calibration element. A 1700-ft Kevlar line is used to suspend the sphere from the helicopter. This length of line is sufficient to isolate the helicopter from the sphere; thus, the helicopter is not in the significant antenna sidelobes.

RCS doppler measurements at millimeter wave frequencies
K. Schmitt,G. Wanielik, R. Schneider, S. Bhagavathula, W. Wiesbeck, November 1994

A versatile millimeter wave imaging radar is presented to conduct polarimetric doppler as well as wide band RCS measurements. The aim of the system is not only to acquire doppler measurements of determine the distance of an object but also to generate image-like information for classification purposes. A hardware gate controller is incorporated in the system to perform pulsed measurements. This controller can drive three different frequency extension modules covering frequency ranges from 8 to 18 GHz, 70 to 80 GHz and 75 to 77 GHz respectively. In all bands, dual polarized horns are used to allow fully polarimetric measurements. A network analyzer and a FFT analyzer are used as receivers. For both concepts the advantages and disadvantages are discussed. The transmit and the receive antenna are mounted on a positioner. Thus, a radar image using the real aperture of the antennas can be generated by mechanical scanning in azimuth and elevation.

Experimental results of strategic target identification by resonant radar cross section measurements
S. Kordella,J. Skelton, November 1994

RCS measurements of representative strategic targets in the resonant scattering regime are presented in this paper. The frequency and aspect dependent RCS signatures of various targets are shown to have close agreement to method-of-moment calculations which are based upon the known target shape and composition. Using the resonant scattering signatures, non-cooperative target recognition can be performed with high confidence using a discrete frequency sampling approach. The target set included cones, spheres, and canonical shapes which have been characterized in the VHF and UHF bands. Measurements made at the Lockheed Space Missile Company Rye Canyon facility have recorded calibrated RCS of representative hardware as a function of both frequency and aspect in the resonant region. These data compare well with prediction, and their use for non-cooperative target recognition will be explained. This effort is being conducted to develop signature models, laboratory measurements and useful discrimination algorithms which exploit the frequency variation of the resonant scattering RCS.

Analysis of wedge radar cross section
Y.J. Stoyanov,Y.J. Stoyanov, November 1994

The need for practical solutions to radar scattering in high-frequency regime have led to the development of a number of approximation methods. The high-frequency asymptotic methods use approximations based on physical optics (PO), geometrical theory of diffraction (GTD) or physical theory of diffraction (PTD) and their variations. Radar scattering from electrically large conducting surface includes traveling surface wave contributions which are not accounted by the high-frequency asymptotic methods. A hybrid method integrating GTD and traveling wave theory (TW) is used for verification and to illustrate important scattering mechanisms that influence radar cross section (RCS) of a wedge. Analysis of the wedge RCS signature identifies significant contributions of the traveling surface waves to the total RCS. Both measured and predicted RCS of the wedge are considered. Using hybrid GTD-TW method very good agreement between the predicted and measured RCS patterns is observed for all angles.

Radar absorbing material thermal characteristics
R.M. Taylor,H.D. Reynolds, M. Matteson, November 1994

The Benefield Anechoic Facility, Edwards AFB, California contains a large anechoic chamber for avionic integration test and evaluation. Because of the large chamber size, operational tests can require high-power aircraft radar emissions. To define the range of energy safely accommodated by currently installed radar absorbing material (RAM), a detailed analysis was performed and the results presented. The incident radar energy generates a heat transfer to the RAM. The RAM boundaries dissipate heat through convection, conduction, and radiation. A finite-difference solution demonstrates the temperature distribution in the material varies with the angle and polarization of the incident electric field. Discussions include the use of the RAM thermal characteristic's pretest evaluation to improve operating capability determinations and to facilitate assessment of customer requirements.

Application of genetic algorithms to the optimisation of wideband Jaumann radar absorbers for normal and oblique incidence
B. Chambers,A. Tennant, November 1994

The design of wide-band, multi-layer radar absorbing materials involves the solution of what is essentially an N-dimensional optimization problem. Genetic algorithms appear to offer significant advantages over conventional optimization techniques for this type of problem due to their robustness and independence of performance function derivatives. To illustrate their use, the paper considers the optimum design of wideband, multi-layer, Jaumann radar absorbers for normal and oblique incidence.

High resolution one dimensional radome characterization
A. van der Merwe,C.W.I. Pistorius, D.J. Janse van Rensburg, November 1994

In this paper radome evaluation based on high resolution imaging techniques is described. It allows anomalies on a large radome to be detected very accurately. It required scanning of the radome through only a small angular section using an inverse synthetic aperture radar approach. The one dimensional image formed from field data provides a linear distribution of scattering source locations. The calibration necessary to compensate for the translation and rotation of the antenna is discussed. The technique is demonstrated through measurements performed on a large fibre glass radome.

Combined pulsed/CW and pulsed-IF instrumentation radar system, A
D. Fleisch,B. Kent, H. Chizever, P. Swetnam, November 1994

In response to evolving USAF RCS measurement requirements, Lintex has developed a combined Pulsed/CW and Pulsed-IF instrumentation system for use at the Advanced Radar Cross Section Measurement Range. This instrumentation system, one of Lintek's Model 500 Series, couples the simplicity and high signal-to-noise ration of Pulsed/CW measurements with the flexibility and precise clutter rejection of Pulsed-IF systems. In this paper, a direct comparison of the Pulsed/CR and Pulsed-IF performance is presented. The theoretical sensitivity and throughput of the system as a function of duty cycle in each mode is calculated and compared to the measured results. The Pulsed-IF system is found to have better sensitivity and stability for short-range measurements due to the high PRF capability of this receiver. The Pulsed-IF mode of operation also offers much better sensitivity for measurements made at longer ranges, for which the duty-cycle losses of the Pulsed/CW mode become excessive. The wideband Pulsed-IF mode is also preferred in high-background environments, since precise time-gating may be used to reduce the clutter return. In areas of high RFI, the Pulsed/CW radar system has provided better results due to the narrow receiver bandwidth.

Compact modular instrumentation radar system, A
J. Paul,E. Lee, Y. Chu, Y.M. Woo, November 1994

A compact modular instrumentation radar system has been developed for antenna, RCS, and general RF measurements. The MMS-420 system consists of a single, rack mounted, programmable mainframe controller and display into which a wide range of RF, IF and signal processing modules can be installed. A family of external RF modules has also been developed to support measurements from VHF through millimeter-wave bands. It is designed to function as a stand-alone measurement system, or interface with network analyzers and other external processing equipment. The hardware and software are easy to customize for specialized measurement applications.

Band concatenation for higher resolution RCS imaging
D.P. Morgan, November 1994

Radar Cross Section (RCS) measurements are often performed in discrete frequency bands for a variety of reasons. Although some indoor ranges are capable of performing very wide-band measurements (with bandwidths up to or exceeding 9: 1), some are designed with very rigid illumination requirements on the coIIimating reflector(s) that can only be met over a narrow band. In addition, the bandwidth available on most outdoor ranges is limited by "ground plane" effects which make it impossible to maintain an adequate broadband field over the target. Often, RCS measurements are limited to half an octave at most. Since resolution in RCS imaging is directly proportional to bandwidth, there exists a need for concate nati ng several discrete bands of measurements into a single continuous band. This resulting band must be free of both amplitude and phase discontinuities that would affect the quality of the resultant image. This paper discusses the sources of discontinuities between measured bands on both indoor and outdoor ranges, and provides algorithms for removing them using linear filtering methods. Data is presented from an outdoor range illustrating the results on targets up to 70-feet in length.

Application of ML estimation to radar imaging
I.J. Gupta,E. Walton, M.W. Tu, November 1994

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.

ISAR image formation processing for airborne targets
R. Goodman,L. Joyce, L.. Cech, November 1994

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.

Amplitude and doppler modulation effects in ground to air ISAR data collected with a linear FM waveform
L. Cech,G. Fliss, W. Nagy, November 1994

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

Experimental RCS analysis of a communications antenna mounted on a large cylinder
E. Walton,H.W. Tseng, November 1994

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.

Application of common processing software as a tool for study of component interaction terms in RCS measurements, An
J. Matis,D. Yanke, November 1994

• 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

Vertical antenna array applications on a ground-bounce instrumentation radar range
B.E. Fischer, November 1994

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.

Waveform chirp speed effects on modulation artifacts in radar imagery
G. Fliss,D., Jr. Kletzli, W. Nagy, November 1994

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.

X-band linear frequency modulated radar for dynamic aircraft imaging, An
J. Trischman,E. Nelson, R. Bloomfield, R. Dinger, S. Jones, November 1994

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

Clutter mapping system
J.F. Aubin,C.T. Nadovich, November 1994

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.
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