AMTA Paper Archive
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New ground penetrating radar antenna design - the horn-fed bowtie (HFB), A
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
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
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
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
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
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 . 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  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 , and obtain the radar characteristics using the Fourier coefficients. Calibration integrity is checked by applying a variant of the dual cylinder calibration technique . Future directions of this measurement program are explored.
i4D: a new approach to RCS imaging analysis
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 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
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
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
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.
Feasibility of automated analysis of diagnostic radar images
This paper discusses the efforts of an on-going research program which has been exploring the use of expert systems (artificial intelligence) techniques to support automated analysis of wideband radar scattering data. The primary objective of this research is to explore and demonstrate the applicability of expert system techniques to the analysis of diagnostic radar images. There are two modes which are being explored. The first is an automated system that would allow lesser skilled (in radar imaging science) individuals to do the work of highly skilled engineers and analysts. The second mode would aid the highly skilled worker with the application and correct implementation of software tools, interpretation of phenomenology, and data quality assessment. In both cases, the expert system should allow for the increase through-put and accuracy of data being analyzed. A software prototype is being developed and tested with real data to demonstrate the feasibility and potential accuracy of such as system.
Quasi 3D imaging on a ground plane RCS range
A method is presented that gives a 3D ISAR image from a 2D measurement. An ordinary 2D image is created. An extra receive channel is used to give height information in every pixel. This channel gets its signal from two extra receive antennas with different elevation lobes. The antennas feed a hybrid which creates a difference signal that goes to the extra receive channel. Height information is derived like in an amplitude comparison monopulse radar. This way a height number is assigned to every pixel in the 2D image. Thus a 3D image is created. It is required that in a 2D resolution cell the reflexes come from only one height. If not, the height information given by the difference signal will be a weighted average of the heights of the reflexes. The method is applied to a ground plane RCS range. No measurements have yet been performed.
Study of a corner reflector of finite thickness
New measurements on the complete polarimetric responses from a 4" dihedral corner reflector from 4 to 18 GHz are presented. As a function of the azimuth, the vertically suspended object may present itself to the radar as a dihedral, a flat plate, an edge, a wedge, and combinations of these. A two dimensional method-of-moment (2-D MOM) code is used to model the perfectly electrical conducting (PEC) body, which allows us to closely simulate the radar responses and to provide insight for the data interpretation. Of particular interest are the frequency and angular dependences of the responses which yield information about the downrange separation of the dominant scattering centers, as well as their respective odd- or even-bounce nature. Use of the corner as a calibration target is discussed.
High resolution filtering of RCS measurements
This paper deals with High Resolution (HR) Filtering. Extracting the frequency dependence of radar scatterers is a common task in Radar Cross Section Analysis (RCS). This is usually achieved with signal processing tools like finite impulse response filters allowing filtering in the range domain. However, when range resolution is poor, it becomes impossible to extract the exact feature since it is not deconvolved in the range domain. We thus propose to use HR methods to overcome these difficulties. These methods are applied to estimate the frequency response of the creeping wave of a small sphere. The results show good agreement with the theoretical response.
Implementing an expert system to analyze phase-array antenna range and diagnostic data
The Antenna Repair Facility at McClellan AFB, which has been responsible for the repair and maintenance of Air Force antennas and radars over several decades, now faces a new challenge: transferring their many years of skill in antenna maintenance from their closing base to acquiring bases or contractors. This must be done while maintaining their in-house expertise and production levels, while manpower decreases. A possible solution to these problems is the implementation of expert systems. This is where human knowledge and expertise is transferred to computerized systems. Currently, the repair shop is testing such a system for analyzing diagnostic data on an electronically steered, phased-array satellite ground station antenna. This paper will examine the design options considered, such as using a commercial software package versus building in a higher order language. It will also discuss the design process for such an expert system, and cover the key issues of knowledge acquisition and selection. The paper will include details of the current design, such as structure and control, and discuss plans for future enhancements.
Analysis of radar measurement system stability factors, A
Instrumentation Radar systems evolution includes improved stability. Metrologists know frequency within Hertz. Amplitude and Phase variations are low. Ranges check drift with reference systems. Still, with increased capability, expectations of accuracy have increased. Todays instrumentation makes analysis of stability factors practical. This study analyzes Radar Cross Section (RCS) return of a stable target under controlled conditions. Methodology will be an analysis of a constant RCS target return. The target is a stable object at a typical measurement site. Data points are at several discrete frequencies in bands between S and Ku. This study sample is a set of data taken over a 87 hour span with several duty factors. Duty factors will range from minimal 0.1% to 1.5%, near the 2% maximum for the output amplifiers. Acquisition times for data sets are chosen for outdoor temperatures ranging from hot -- desert afternoon -- through cool in the early morning. This data will be analyzed statistically. If statistical correlations exist, analysis will quantify factor contributions with multiple linear regression. Hypothesis: Drift does not correlate to variables such as duty factor, & temperature.
Integrated antenna/RCS/EMI compact range based measurement facility, An
ORBIT/FR has recently delivered an integrated facility capable of being used for Antenna, Radar Cross Section (RCS), and EMI measurements to the Naval Underwater Warfare Center in Newport, RI. The facility includes a shielded anechoic chamber, a compact range system capable of producing a 6 foot diameter quiet zone, multi-axis positioning equipment, and a complete complement of Antenna, RCS, and EMI measurement instrumentation and data collection hardware/software. The facility is capable of operation over a frequency range of 100 MHz to 50 GHz, with compact range operation feasible above 2 GHz. The facility can be reconfigured to go between antenna and RCS measurements in any band using both frequency band and antenna/RCS mode switching. In addition, automatic positioning of the appropriate compact range feed to the reflector focal point is available. EMI measurements require minimal relocation of absorber in an isolated area of the chamber floor. Performance of the system is optimized by location of critical RF equipment on the compact range feed carousel or on the positioning system rail carriage. This system offers a unique combination of performance and convenience for making all three types of measurements.
Indoor RCS measurement capability at VHF in the Boeing 9-77 range
This paper discusses Radar Cross Section (RCS) measurement capability at Very High Frequencies (VHF) in the Boeing 9-77 Range in Seattle, Washington. This indoor facility provides a unique asset to the RCS measurement community. Initially operational in 1989, the 9-77 Range was upgraded in 1995 to include a VHF measurement capability. This was achieved using a 56 foot square array of 256 elements, for RCS measurements at frequencies from approximately 140 to 220 MHz, with a 40 foot quiet zone. In this paper, we discuss results from the characterization process used to verify the initial capability and ongoing operation of the RCS measurement system at VHF. We include data demonstrating the sensitivity, stability and dynamic range of the system. We also present samples of recent field probes, and background subtraction and stability measurements. A comparison is made between calculated and measured canonical target signatures.
Technique for collecting and procesing flight-line RCS data, A
Recently, several deployable, ground-to-ground col lection systems have been developed for the assessment of aircraft RCS on the flight-line. The majority of these systems require bulky rail or scanning hardware in order to collect diagnostic imaging data. The measurement technique described in this paper, while not a "cure-all", does eliminate the need for bulky hardware by allowing the collection system to move freely around the target while collecting radar backscattering data. In addition, a nearfield-to-farfield transformation (NFFFT) algorithm is incorporated in the process to allow the collection of scattering data collected in the near field to be processed and evaluated in the far field. The techniques described in this paper are a part of a data conditioning process which improves the data quality and utility for subsequent analysis by an automated diagnostic system described elsewhere in this proceedings . The techniques are described and demonstrated on numerically simulated and experimentally measured data.
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