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Theoretical and Experimental Studies of Hand-Held SAR Concepts
L. Cai,E. Walton, November 1998

It is known that electromagnetic signalls can penetrate through non-metallic barriers such as building walls. A hand-held Sythetic Aperture Radar (SAR) unit capable of transmitting and receiving such signals is desirable in various military and civilian applications. Theoretical and experimental issues associated with through-wall Ultra Wide Band (UWB) SAR imaging of buildings are studied here. It may be inconvenient and impractical for a hand-held unit to collect data at uniformly spaced positions. A back­projection algorithm is developed for the case where spatial sampling is not uniform. In addition, a spherical wavefront (as opposed to a uniformly planar wave­ front) is assumed in the algorithm to account for the proximity of a radar unit relative to a target scene. Images of simulations using point targets and measurements of canonical targets such as a corner reflector and a cylinder are generated. Images of a standing human in free-field and through-wall are compared.

Verification of Antenna Radiation Patterns and Scattering Returns (RCS) of Full Size Targets Using Missile Engagement Simulation Arena (MESA) Facility (NAWCWPNS, China Lake CA) Radar System, and a Hardware in the Loop Radar System
L.L. Mandeville,J.P. McQuire, November 1998

Most often when performing antenna and RCS measurements, integrating the results is performed with some type of computer generated simulation or model of the application scenario. In the case of Missile Engagements for Fuze Radars, there is an opportunity to engage full size targets in a near real engagement. The missile fuze antenna can be mounted on the test cart which is able to position the fuze antenna in azimuth, pitch and roll. For instrumentation the MESA Facility has available a PN coded BiPhase multi-range gate radar system. Various Full size targets are available for use in the arena. The target are positioned for a multitude of trajectories utilizing an overhead target positioning system. The Overhead Target Positioning System suspends and moves the targets using a multipoint string system that controls, Pitch, Roll, height, and azimuth positioning. The Overhead Target Positioning System (OTS) is also controlled in lateral movement. (across the range) This paper will show the verification of antenna patterns and RCS returns of full size targets using the MESA Radar system, and verification of these measurements using a hardware in loop fuze radar system simultaneously.

Overview of the Bistatic Coherent Measurement System (BICOMS)
T.L. Lane,C.A. Blevins, November 1998

The Georgia Tech Research Institute (GTRI), under contract to the U.S. Air Force 46 Test Group, Radar Target Scattering Division (RATSCAT), at Holloman AFB, NM, has designed and developed a fully polarimetric, bistatic coherent radar measurement system (BICOMS). It will be used to measure both the monostatic and bistatic radar cross section (RCS) of targets, as well as create two-dimensional, extremely high-resolution images of monostatic and bistatic signature data. BICOMS consists of a fixed radar unit (FRU) and a mobile radar unit (MRU), each of which is capable of independent monostatic operation as well as simultaneous coherent monostatic and bistatic operation. The two radar systems are coherently locked via a microwave fiber optic link (FOL). This paper discusses the key system features of the BICOMS.

BICOMS Antenna Positioner System (APS) and Automated Field Probe (AFP)
O.D. Asbell,M. Hudgens, November 1998

BICOMS (Bistatic Coherent Measurement System) is a RATSCAT radar cross section (RCS) range at Holloman AFB, NM. BICOMS includes a Mobile Radar Unit (MRU), Fixed Radar Unit (FRU), and an Automated Field Probe (AFP). The MRU's antenna positioner system moves eight antennas using single pivot elevation/azimuth positioners and screw jack and cable hoist height actuators. The Automated Field Probe (AFP) raster scans a 40 x 40-foot aperture in front of the target under test. A 4- wheel drive scissors lift provides mobility and vertical axis travel. A cable drive moves a carriage horizontally along a 48-foot truss boom, mounted on the lift platform. The system computer controls both axes, as well as microwave data acquisition. All structures and systems feature minimum weight and wind resistance.

Intelligent cruise control radar development
E. Walton,D. Farkas, F. Paynter, November 1997

As part of the 1997 Automated Highway System Demonstration, the Ohio State University ElectroScience Laboratory (OSU/ESL) developed and operated a pair of automobiles equipped with radar systems for steering and cruise control. In a national demonstration attended by six autonomous vehicle teams, the system was used to convoy three autonomous vehicles along a 7 mile stretch of closed highway lanes near San Diego. The goal of the look ahead radar system was to acquire and track the vehicle ahead.

Radar image normalization and interpretation
J.P. Skinner,B. Kent, D. Andersh, D. Mensa, R.C. Wittmann, November 1997

Calibrated radar images are often quantified as radar cross section (RCS). This interpretation, which is not strictly correct, can lead to misunderstanding of test target scattering properties. To avoid confusion, we recommend that a term such as "scattering brightness" (defined below) be adopted as a standard label for image-domain data.

Rocket motor plume measurement facility
W.W. Harrington, November 1997

The Plume Measurement Facility is a new outdoor facility providing the capability to characterize tactical rocket motor plumes. Radar cross section of the plume is measured by both a near field and a far field radar. Infrared/ultraviolet/visible (IR/UVNIS) charac­ teristics are measured by numerous instruments recording spacial, temporal, and spectral data. All instrumentation is calibrated and adjusted to realtime standard day meteorological data and all data is recorded on a common synchronized time base.

Compact range for radar system testing, A
V. Jory,B. Richardson, D. Oxford, D.M. Breiner, November 1997

This paper discusses the design, fabrication, installation, and testing of a Scientific-Atlanta Model 5702 Compact Range used for radar system testing. The unique feature of this compact range is that it provides a plane wave target source for automated closed loop radar system testing. Techniques employed for meeting and verifying stringent specifications such as phase and amplitude gradients over the quiet zone are discussed. Results from closed loop testing of radar systems in the compact range are also presented.

Wide band feed for a virtual vertex reflector, A
W.D. Burnside,A.J. Susanto, E.A. Urbanik, November 1997

Sanders, A Lockheed Martin Company, measures radar cross section (RCS) and antenna performance from 2 to 18 GHz at the Com­ pany's Compact Range. Twelve feed horns are used to maintain a constant beam width and stationary phase centers, with proper gain. However, calibration with each movement of the feed tower is required and the feed tower is a source of range clutter. To Improve data quality and quantity, Sanders and The Ohio State University ElectroScience Laboratory designed, fabricated, and tested a new wide band feed. The design requirement for the feed was to maintain a constant beam width and phase taper across the 2 - 18 GHz band. The approach taken was to modify the design of the Ohio State University's wide band feed [1]. This feed provides a much cleaner range which reduces the dependence on subtraction and other data manipulation techniques. The new feed allows for wide band images with increased resolution and a six fold increase in range productivity (or reduction in range costs). This paper discusses this new feed and design details with the unique fabrication techniques developed by Ohio State and its suppliers. Analysis and patterns measured from the feed characterization are presented as well. This paper closes with a discussion of options for further improvements in the feed.

New ground penetrating radar antenna design - the horn-fed bowtie (HFB), A
C-C. Chen, November 1997

Bowtie dipole antennas have been widely used for surface-based ground penetrating radar ( GPR) applications. This type of GPR antennas share common problems such as low directivity, antenna ringing, unstable characteristic impedance, RFI and large size. Special treatments have been used to improve their performance. Resistive terminations have been used to reduce the antenna ringing at t he price of efficiency. Some use reflectors to increase directivity at the price of bandwidth and the risk of cavity ringing excitation. Absorbing material is also used to shield RFI with increased size and weight. Some people use horn antennas because of bet ter gain. However, they are limited to high frequency applications where their size are still reasonable to handle. This means they can only do shallow target measurements. Horn antenna approach also faces the strong reflection arising at the air-ground interface. A new type of GPR antenna design presented in this paper has been developed to overcome the above difficulties.

Automated highway radar guidance antenna and system testing results
J.D. Young,D. Farkas, November 1997

The Ohio State University ElectroScience Laboratory participated along with the Center for Intelligent Transportation Research as one of six teams to demonstrate an automated highway concept at the National Automated Highway Demo at San Diego in August, 1997. The forward looking radar concept which was demonstrated used the FSS highway stripe which was presented at the 1995 AMTA Meeting. This paper describes the radar system as implemented for automated guidance, and presents measured results on the system antenna array and on the system itself. In addition, results of the demonstration in San Diego will be discussed. The radar used a monopulse guidance architecture, where the amplitude from left and right receive antennas are subtracted, and then divided by the sum of the left and right antennas in order to provide a normalized steering error signal. The antenna array used a single transmit horn, and a matched pair of receive horns, all vertically polarized. All three antennas were nestled into the composite front bum per beam, looking out through a foam radome panel about the size of a license plate. Performance data on the antennas and the steering sensing information will be presented. The radar system was a chirp radar covering a frequency spectrum of 10 to 11 GHz. The narrow frequency of the FSS radar stripe occu rred at 10.95 GHz, allowing its signature to be distinguished from the return of vehicles and other objects out in front of the vehicle. Radar system measured results in the highway situation will be presented, and its performance in San Diego will be discussed.

Establishment of a common RCS range documentation standard based on ANSI/NCSL Z-540-1994-1 and ISO Guide 25
B. Kent,L.A. Muth, November 1997

This paper presents a brief overview of ANSI/NCSL standard Z-540 (1). Z-540 offers a straightforward way to organize range documentation. We discuss the major points and sections of Z-540, and how to organize a format-universal "range book". Since Z-540 is the US equivalent of International Standard (ISO) 25, it is especially useful for two reasons; (1) it is applicable to Radar Cross Section (RCS) ranges and (2) its quality control requirements are consistent with the ISO 9002 series of quality standards. Properly applied, Z-540 may greatly improve the quality and consistency of RCS measurements produced, and reported to range customers.

Interlaboratory comparisons in radar cross section measurement assurance
L.A. Muth,B. Kent, R.C. Wittmann, November 1997

The National Institute of Standards and Technology (NIST) is coordinating a radar cross section (RCS) interlaboratory comparison study using a family of standard cylinders developed at Wright Laboratories. As an important component of measurement assurance and of the proposed RCS certification program, interlaboratory comparisons can be used to establish repeatability (within specified uncertainty limits) of RCS measurements within and between measurement ranges. We discuss the global importance of intercomparisons in standards metrology, examine recently conducted comparison studies at NIST, and give a status report on the first national RCS intercomparison study. We also consider future directions.

Bistatic cross-polarization calibration
R.J. Jost,R.F. Fahlsing, November 1997

Calibration of monostatic radar cross section (RCS) has been studied extensively over many years, leading to many approaches, with varying degrees of success. To this day, there is still significant debate over how it should be done. In the case of bistatic RCS measurements, the lack of information concerning calibration techniques is even greater. This paper will present the results of a preliminary investigation into calibration techniques and their suitability for use in the correction of cross-polarization errors when data is collected in a bistatic configuration. Such issues as calibration targets and techniques, system stability requirements, etc. will be discussed. Results will be presented for data collected in the C and X bands on potential calibration targets. Recommendations for future efforts will also be presented.

Interlaboratory comparisons in polarimetric radar cross section calibrations
L.A. Muth,B. Kent, D. Hilliard, M. Husar, W. Parnell, November 1997

The National Institute of Standards and Technology (NIST) is coordinating a radar cross section (RCS) interlaboratory comparison study using a rotating dihedral. As an important component of measurement assurance and of the proposed RCS certification program, interlaboratory comparisons can be used to establish repeatability (within specified uncertainty limits) of RCS measurements within and among measurement ranges. The global importance of intercomparison studies in standards metrology, recently conducted comparison studies at NIST, and the status of the first national RCS intercomparison study using a set of cylinders are discussed in [1]. In a companion program, we examine full polarimetric calibration data obtained using dihedrals and rods. Polarimetric data is essential for the complete description of scattering phenomena and for the understanding of RCS measurement uncertainty. Our intent is to refine and develop polarimetric calibration techniques and to estimate and minimize the correstponding measurement uncertainties. We apply theoretical results [2] to check on (1) data and (2) scattering model integrity. To reduce noise and clutter, we Fourier transform the scattering data as a function of rotation angle [2], and obtain the radar characteristics using the Fourier coefficients. Calibration integrity is checked by applying a variant of the dual cylinder calibration technique [3]. Future directions of this measurement program are explored.

i4D: a new approach to RCS imaging analysis
J.C. Castelli,G. Bobillot, November 1997

Recently, a new method of wide band radar imaging has been developped within the framework of the two dimensional (2-D) continuous wavelet theory. Based on a model of localized colored and non isotropic reflectors, this method allows to obtain simultaneously information about the location, the frequency and the directi­ vity of the scatterers which contribute to the RCS of a target. We obtain a 4-D data set that we call hyperimage namely a series of images which depend on the frequency and orientation of illumination. In order to exploit efficiently hyperimages an interactive visual display software called i4D has been specifically designed. The purpose of this paper is to present the capabilities of i4D through the analysis of hyperimages constructed from monostatic and bistatic scattering data. The results show that the interactive and dynamic analysis that i4D procures allow to better understand the mechanisms that contribute to the RCS of targets.

RCS measurements on target features
A.W. Rihaczek,S.J. Hershkowitz, November 1997

A technology for target identification has been developed that is directly applicable to the analysis of the backscattering behavior of targets. For the latter purpose the target is placed on a turntable, and amplitude/phase data are collected over the aspect angle sector of interest, using a radar with sufficient bandwidth to resolve the target in range. For ground vehicles and small aircraft a range resolution of about 1 ft is sufficient. Standard processing is used to form an ISAR image over the appropriate aspect angle sector. The difference relative to the more conventional procedures is that the complex ISAR image, intensity and phase, is analyzed rather than only the intensity. This allows us to identify spurious responses that are generated by certain features on the target, but appear in locations other than those of the features. The analysis of the complex image permits us to associate the genuine image responses with the features responsible for the responses, so that the strength and type of backscattering can be determined for the target features. With respect to the type of backscattering, we can determine whether the effective location of the feature is stable, or whether it drifts with aspect angle or frequency. We. can also determine the effective crossrange and range widths of the various features. The features that can be analyzed are those with responses sufficiently strong to exceed the general background. This is typically a fairly large number.

Compensation of unknown position-induced phase errors in a driveby imaging radar
P.N.R. Stoyle, November 1997

One approach to getting near-field ISAR measurement costs down is to dispense with track or turntable, and instead mount the SAR antenna on a vehicle and simply 'drive by' the target of interest, which might be a vehicle or aircraft standing on tarmac. In this situation the antenna path will depart from a perfect straight line or circular arc, and there will also be vibrational wobble at the antenna phase center. These effects can defocus the images obtained. One way to overcome the focus problem is to mount strategically placed corner reflector(s) in front of the target, each in a different range cell. These then act as phase references, used to refocus the image. However it is not strictly necessary to employ reflectors - a good focus can normally be obtained by suitably processing just the target return itself. This paper will describe autofocus procedures which have been sucessfully used in conjunction with chirp radar data, in the 'driveby' situation.

Application of RCS reference targets for frequencies above 30 GHz
V.J. Vokurka,J. Reddy, J.M. Canales, L.G.T. van de Coevering, S.C. van Someren Greve, November 1997

For frequencies above 30 GHz, RCS reference target method is, in general, more accurate than scanning the field by a probe. Application of mechanically calibrated targets with a surface accuracy of 0.01 mm means that the phase distribution can be reconstructed accurately within approximately 1.2 degrees across the entire test zone at 100 GHz. Furthermore, since the same result can be obtained for both azimuth and elevation patterns, all data is available for the characterization of the entire test zone. In fact, due to the fact that the reference target has a well known radar cross-section, important indication of errors in positioning can be obtained directly from angular data as well. In the first place the data can be used in order to recognize the first order effects (+/- 5 degrees in all directions). Applying this data, defocussing of the system reflector or transverse and longitudinal CATR feed alignment can be recognized directly. Furthermore, mutual coupling can be measured and all other unwanted stray radiation incident from larger angles can be recognized and localized directly (using time­domain transformation techniques). Inmost cases even a limited rotation of +/- 25 degrees in azimuth and +/- 10 degrees in elevation will provide sufficient data for analysis of the range characteristics. Finally, it will be shown that sufficient accuracy can be realized for frequencies above 100 GHz with this method.

Applications of the fractional Fourier transform in radar imaging
A. Blank,Z. Zalevsky, November 1997

The recently introduced Fractional Fourier Transform (FRT) operation was shown to be useful for various spatial filtering, optical and signal processing applications. In contrast to the Fourier Transform, the real part of the FRT of a delta function can be "tailored" (by the position of the delta and the transform order) to have many zeros in specific areas of the spectrum and fewer zeros at other areas. This property was exploited to design new spectral windows, which pass through zero many times in the spectral points we define. These windows can be used in case of loss of information or partial information collected in a usual ISAR stepped-frequency data process. Another, more valuable property of the FRT, used here for radar imaging applications, is it's being a projection of a rotated Wigner transform of a signal. This property was used to filter resonant structures in the radar image, which cause the image to be smeared in the range.

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