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Automatic Detection of Radar Signature Defects
N. Cheadle,D. Tackett, R. de Lacaze, R. Pierce, November 1999

Field-level maintenance of radar signature treatment requires that non-specialist military personnel properly identify needed repairs. To simplify this task, an automated method is required that can compare radar signature data to baseline data, measure the differences, and identify the source of serious defects. Significant work has been done using artificial intelligence (AI) techniques to simplify this diagnostic task. A portable measurement radar was used to gather signature data on a small MQM-107D target drone. One set of data was collected of a baseline vehicle. Then data was collected after several anomalies were introduced, such as an uncovered pitot tube, wing joint untaped, or fastener screw not tightened. The data was processed as global downrange plots, and then baseline data was subtracted from anomaly data and the difference was compared to signature specifications as a function of angle. AI was used to identify signature defects that require repair. The results showed that an AI-aided diagnostic tool could help identify places where signature treatment repair was needed. This tool can be adapted to a variety of user and target needs.

Antenna Calibrations at NPL
D. Gentle,P. Miller, November 1999

NPL has been providing antenna gain standards since the late 1970's, initially to service internal needs for microwave field strength standards. To meet the increasing industrial demand for the calibration of microwave antennas in areas such as satellite communications and radar, NPL has developed an antenna extrapolation range. The current facility, which is due to be replaced by the end of the year, is used to measure the gain of microwave antennas in the frequency range 1 to 60 GHz, often with a gain uncertainty as low as ± 0.04 dB. Axial ratio, tilt, sense of polarisation and pattern measurements can also be made in the same facility, while for larger antennas a planar near-field scanner is used. Of the many measurement techniques for determining the gain of an antenna, the most accurate is the three antenna extrapolation technique [1,2] which was developed at the National Institute of Standards and Technology (NIST) at Boulder, Colorado, and is the method used at NPL. This is an absolute method as it does not require a prior knowledge of the gain of any of the antennas used. Since calibration data is often required across a wide frequency band, the measurement techniques and software have been developed to allow measurements to be performed at a large number of frequencies simultaneously. This reduces the turn round time, the cost and the need for interpolation between measurement points.

MMW Instrumentation Systems for RCS Measurements & Applications
W.C. Parnell, November 1999

A variety of unique instrumentation radars have been developed by the RF & MMW Systems Division at Eglin Air Force Base in order to support both static and dynam ic Radar Cross Section (RCS) measurements for Smart Weapons Applications. These systems include an airborne multispectral instrumentation suite that was used to collect target signatures in various terrain and environmental conditions (95 GHz Radar Mapping System - 95RMS), a look-down tower based radar designed to perform RCS measurements on ground vehicles (MMW Instrumentation, High Resolution Imaging Radar System MIHRIRS), two high power (35 & 95 GHz) systems capable of mapping/measuring both attenuation and backscatter properties of Obscurants and Chaff (MMW Radar Obscurant Characterization System MROCS: 1&2), and a Materials Measurement System (MMS) which provides complex free space, bistatic attenuation and reflectivity data on Radar Absorbing Materials (RAM), paints, nets and specialized coatings/materials. This paper will describe the instrumentation systems, calibration procedures and measurement techniques used for data collection as well as several applications which support modelina and simulation activities in the Smart Weapon community.

RATSCAT Technical Enhancements and Upgrades
J.H. Eggleston,G.V. Jones, S.J. Gray, November 1999

RATSCAT has pursued a wide gamut of technical enhancements and upgrades to its Mainsite and RATSCAT Advanced Measurement System (RAMS) locations. Acquisition of three radar systems has provided RATSCAT with the most capable radar systems available. RAMS is capable of acquiring full scattering matrix (FSM) data from 120 MHz to 36 GHz. Mainsite is capable of acquiring bistatic FSM data from 2 GHz to 18 GHz and monostatic FSM data from 1 GHz to 36 GHz. RATSCAT is pursuing unparalleled background levels through the acquisition of new pylon technology at RAMS and is expanding its target handling capability via construction of additional target storage as well as the addition of a mobile target handling shelter and new 50' and 14' pylons at Mainsite. RATSCAT has acquired a full feature data processing capability at both sites that uses a reflective memory interface between data acquisition and data processing resulting in faster validation of data cuts. Through acquisition programs and partnership with industry RATSCAT has improved their RCS test capability to become the technical leader in outdoor static RCS testing.

Boeing Near-Field Test Facility (NFTF) Upgrades & Design Tradeoffs
P.J. DeGroot,M. Westerhold, November 1999

The Boeing Near Field Test Facility (NFTF) in St. Louis, MO was constructed in 1991 to conduct near field RCS measurements of production parts, models, and full-scale operational aircraft. Facility upgrades were identified in 1997 to support operational aircraft testing, such as the F/A-18 E/F. Target rotation mechanization, measurement antennas, and the test radar were identified as requiring upgrades. The target rotation hardware was upgraded to a 40-foot diameter turntable capable of handling production fighter aircraft. Antennas were mounted in an elevation box, which also contains the radar and an absorber aperture. The elevation box translates vertically, and pitches in elevation for different view angles. A new Lintek Elan radar, with a frequency range of 2ml8 GHz, 200 Watt Traveling Wave Tube (TWT) amplifiers, and Programmable Multi-Axis Controller cards (PMAC), controls all motion in the facility. In addition, modifications to the facility were completed to improve efficiency and ergonomics.

Precision Boresight Measurements for Doppler Radar Systems Measured on a Near-Field Range
V. Moore,B. Schluper, November 1998

Airborne Doppler Velocity Sensors require precise boresight information in determining a Doppler solution. Far-field ranges have been extensively used to provide this boresighting capability. This paper discusses an empirical investigation to determine the feasibility of using near-field techniques to fulfill the boresighting requirement.

Development and Applications of a 16 Channel UHF/L-band Noise Radar
E.K. Walton,S. Gunawan, November 1998

In this paper, we will demonstrate a 16 channel multi-antenna UHF/L-band noise radar system. We will show applications to building penetration and ground penetration. We will show noise radar responses for humans walking on the other side of building walls, and buried objects, including land mines. We will discuss classification techniques, and show some techniques that yield initial success.

Three-Dimensional Imaging From Compact Range Data
D.L. Mensa,K. Vaccaro, W.T. Yates, November 1998

The collection of radar scattering data necessary for imaging targets with three-dimensional resolution requires frequency diversity, combined with angular diversity over two orthogonal axes fixed to the target. Although the necessary data can easily be collected using modern instrumentation systems when the target is outfitted with an embedded two-axis rotator, some applications preclude the intrusion of the rotator. This paper describes an alternative method for obtaining the required data which uses conventional target rotation in the azimuth plane, combined with a linear displacement of the compact range feed along the vertical axis of the collimator's focal plane. Frequency diversity is provided by a stepped-frequency radar and angular diversities in the horizontal and vertical directions are provided by the target rotation and vertical feed displacement, respectively. The data-collection scheme samples a wedge-shaped volume of the target spatial spectrum (k-space) with radial and angular extents de­ terminated by the bandwidth and target rotation relative to the radar axis. A three-dimensional image is formed by processing a three-dimensional array of data, typically consisting of 128xll8x128 data samples. The paper describes the experimental set-up used to collect Ku-band data and presents two- and three­ dimensional images obtained from the data. Considerations of the following issues are addressed in the paper. 1. Aberrations resulting from displacing the feed from the collimator focal point. 2. Control of the linear feed displacement, target rotation, and radar operation to automate the data collection. 3. Methods for calibrating and aligning the data. 4. Signal processing methods which combine wide­band, ISAR and spotlight SAR processing for three-dimensional applications. 5. Clutter suppression using zero-Doppler filtering.

RCS Data Improvement Using Two-Dimensional Parametric Signal History Editing (PSHE)
B.E. Fischer,J.W. Burns, November 1998

Target support and clutter contamination can be a limiting factor in radar cross section (RCS) measurements of signature­ controlled targets. Conventional ISAR image editing methods can be used to remove contamination, but their performance degrades rapidly when the available resolution is insufficient to identify and separate the support returns from those of the target. ERIM International, Inc. (EI) has developed and successfully demonstrated data post-processing techniques based on 1-D parametric spectral estimators for removing additive contamination from low resolution swept frequency measurements [1, 2]. To further enhance performance and take advantage of the cross-range resolution afforded by target aspect information, EI has investigated the use of coherent 2-D spectral estimation techniques for improved identification and mitigation of measurement contamination in frequency and angle diverse data. In particular, parametric signal history editing (PSHE) algorithms based on 2-D TLS-Prony [3] and 2-D MEMP [4] have been developed and exercised on numerical simulations and measured data. The paper demonstrates 2-D spectral estimation in representative measurement situations, identifies strengths and limitations, and quantifies mitigation algorithm performance. In addition, automated filtering of spectral representations using energy level ordering, Cramer Rao Bounds (CRBs), and spatial filtering are discussed.

3-D Radar Imaging Using Range Migration Techniques
J.M. Lopez-Sanchez,A.J. Sieber, J. Fortuny-Guasch, November 1998

This paper presents an efficient three­ dimensional (3-D) SAR imaging algorithm us­ ing range migration techniques. The algorithm is used to form 3-D radar reflectivity images of targets measured in anechoic chambers. As an input, the algorithm requires frequency domain backscatter data which have been acquired us­ ing a stepped frequency system equipped with an antenna that synthesizes a 2-D planar aperture. Resolution in the vertical and horizontal cross-range directions is given by the dimensions of the synthesized aperture, whereas resolution in ground-range is provided by the synthesized frequency bandwidth. The presented formulation has been justified by using the stationary phase method. Results both with syn­ thetic and measured backscatter data show the high efficiency of the technique. The extension of the algorithm when the antenna synthesizes a 2-D spherical aperture has been addressed. Re­ sults with this aperture geometry show that the technique is still highly efficient.

Mitigation of Target Illumination and Multipath Errors in Ground Plane RCS Measurements
J.F. Stach,J.W. Burns, November 1998

The close proximity of the ground to the radar antenna and the target under test is often hard to avoid at an outdoor RCS measurement range. Ground reflection of energy from the antenna leads to target illumination errors, and target-ground interactions lead to multipath errors. By proper positioning of the antenna and target, ground reflections of the antenna illumination can be exploited to increase overall system sensitivity by concentrating more energy on the target; however, this is only effectivefor narrowband measurements over a limited target region [1]. Reducing target-ground interactions by increasing the target height above the ground generally has limits due to mechanical restrictions on both the radar antennas and the target. This paper will present a model-based data post-processing technique to mitigate illumination errors and target-ground interactions in ground plane range RCS measurements. The algorithm is an extension of the network model multipath mitigation technique previously developed for indoor RCS measurement ranges [2,3,4]. The technique will be described and demonstrated using a numerical simulation of the RCS measurement of a canonical target over a ground plane.

Radar Cross Section (RCS) Range Enhancements at Eglin's Seeker Test & Evaluation Facility (STEF)
W.C. Parnell, November 1998

The Seeker Test and Evaluation Facility (STEF) located on Range C-52A at Eglin AFB FL. is used to perform high-resolution multispectral (EO-IR-RF-MMW) signature measurements of US and foreign ground vehicles primarily to support the Research, Development, Test and Evaluation (RDT&E) of smart weapons (seekers, sensors and Countermeasure techniques). In order to support two major DOD signature measurement programs in 1997 this facility required significant range upgrades and enhancements to realize reduced background levels, increase measurement accuracy and improve radar system reliability. These modifications include the addition of a 350'X 120' asphalt ground plane, a new secure target support facility, a redesigned low RCS shroud for the target turntable and a new core radar system (Lintek elan) and data acquisition/analysis capability for the existing radars Millimeter-Wave Instrumentation, High Resolution, Imaging Radar System - MIHRIRS). This paper describes the performance increase gained as a result of this effort and provides information on site characterization and radar instrumentation improvements as well as examples of measured RCS of typical ground vehicle signatures and ISAR imagery

Physics Based Modeling of Target Signatures
J. Berrie,B. Welsh, G. Wilson, H. Chizever, November 1998

The scattered field from an arbitrary target may include a variety of scattering mechanisms such as specular and diffraction terms, creeping waves and resonant phenomena. In addition, buried within such data are target-mount interactions and clutter terms associated with the test environment. This research presents a method for decomposing a broadband complex signal into its constituent mechanisms. The method makes use of basis functions (words) which best describe the physics of the scattered fields. The MUSIC algorithm is used to estimate the time delay of each word. A constrained optimization refines the estimate and determines the energy for each. The method is tested using two far-field radar cross section (RCS) measurements. The first example identifies target­mount interactions for a common calibration sphere. The second example applies the method to a low observable (LO) ogive target.

Full Polarimetric Calibration for RCS Measurement Ranges: Performance Analysis and Measurement Results
B.M. Welsh,A.L. Buterbaugh, B.M. Kent, L.A. Muth, November 1998

Full polarimetric scattering measurements are increasingly being required for radar cross-section (RCS) tests. Conventional co-and cross-polarization calibrations fail to take into account the small amount of antenna cross-polarization that will be present for any practical antenna. In contrast, full polarimetric calibrations take into account and compensate for the cross-polarization the calibration process. We present a full polarimetric calibration procedure and a simulation-based performance study quantifying how well the procedure improves measurement accuracy over conventional independent channel calibration.

Fresnel-Zone Measurement and Analysis of a Dual-Polarized Meteorological Radar Antenna
D.B. Hayman,G.C. James, T.S. Bird, November 1998

The use of dual polarization in meteorological radars offers significant advantages over single polarization. Recently a standard single-polarization Cuband radar was upgraded to operate in dual-polarization mode. The antenna has a 4.2m diameter parabolic reflector with a prime-focus feed. A spherical Fresnel-zone holographic technique was used to obtain the radiation pattern for the upgraded antenna. The sidelobes were higher than predicted and so the data was analyzed to identify the relative contributions of shadowing from the feed crook and surface errors in the dish. This paper describes practical considerations in the measurement of this antenna and the analysis of the results.

DATE - Depot Aperture Tuning Equipment for the ERIEYE Airborne Early Warning System
H. Eriksson, November 1998

DATE is a portable, rapid assembled, planar near field measurement system for ERIEYE Airborne Early Warning System. DATE shall be used both as a production range at Ericsson Microwave Systems (EMW) and as a maintenance equipment delivered with the ERIEYE AEW System. Up to now ERIEYE has been measured and phase aligned at EMW's large nearfield range. The active antenna is interfaced through a Beam Steering Computer (BSC) and hardware interface. The disadvantages with this approach is a slow communication speed and reduced Built In Test. Since the large nearfield range is designed to meet the requirements from many different antenna types the transport, mounting, alignment and range error analysis are very time and personnel consuming. The DATE-scope is to provide a portable planar near field test system that's custom-made for ERIEYE. The time from stored system to completed measurement shall be very short and performed by a "non antenna test engineer". This is done by: • Incorporate the BSC as a radar-mode. • Use the radar receiver and transmitter for RF­ measurement. • Reduce alignment time and complexity by a common alignment system for antenna and scanner. Scanner alignment for very high position accuracy. • Automatic Advanced Data Processing: Transformation from near field to far field to excitation to new T/R-module setting-up-table in one step.

Low-Cost Portable Near-Field Antenna Measurement System
D.P. Woollen,A.R. Tillerson, G. Lear, J.M. Snow, W. Slowey, November 1998

The Marine Corps desired a portable test system for the AN/TPS-59 radar antenna (a large, 15.2 feet by 29.1 feet, L-band phased array antenna) to verify on site performance. The test system was also required to be capable of antenna acceptance testing at the overhaul depot. An innovative mechanical design using commercial off-themshelf (COTS) products paved the way for the development of this low-cost system. The low-frequency, moderate-sidelobe antenna characteristics allowed for flexibility in mechanical scanner design. The near-field scanner attaches directly to the antenna and is aligned in place. The Hewlett-Packard 8530 Antenna Measurement System is employed for data collection. An interface from the computer to the antenna was designed for beam­ steering control (BSC). LabVIEW software controls the HP8530, the near-field scanner, BSC, and other miscellaneous RF hardware. Digital Visual Fortran 5.0 and Matlab are used to run the National Institute of Standards and Technology (NIST) near­ field programs.

Results from Inter-Laboratory Comparison Measurements at the Boeing 9-77 Range
M.D. Bushbeck,A.W. Reed, C.N. Eriksen, P.S.P. Wei, November 1998

Recently, RCS measurements were made of several common calibration objects of various sizes in the Boeing 9-77 Range. A study was conducted to examine the accuracy and errors induced by using each as a calibration target with a string support system. This paper presents the results of the study. Two of the objects, i.e., the 14"-ultrasphere and the 4.5"-dia. cylinder, are found to perform the best in that they exhibit the least departures (error) from theory. The measured departures of 0.2 to 0.3 dB are consistent with the temporal drift of the radar in several hours.

Full Scattering Matrix Calibration with Error Analysis
R.J. Jost,R.F. Fahlsing, November 1998

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. It is almost a certainty, that if someone proposes a way to calibrate RCS data, someone else will come up with reasons as to why the "new" approach will not yield results that are "good enough." In the case of full scattering matrix RCS measurements, the lack of information concerning calibration techniques is even greater. The Air Force's Radar Target Scattering Facility (RATSCAT) at Holloman AFB, NM,has begun an effort to refine monostatic and bistatic cross polarization measurements at various radar bands. For the purposes of this paper, we have concentrated on our monostatic cross polarization developments. Such issues as calibration targets and techniques, system stability requirements, etc. will be discussed. During several programs we have attempted to collect sufficient data to do full scattering matrix corrections. In a previous paper, "Bistatic Cross-Polarization Calibration," our collected data had a high background which obscured much of the cross polarized return. The data presented here is from a program conducted at RATSCAT recently which utilized the Ka band. Because of the sensitivity of measurements at Ka to many effects, an error estimate was required. This paper presents this error estimation and some results of full scattering matrix correction of RCS data. This analysis is based upon "The Proposed Uncertainty Analysis for RCS Measurements", NISTIR 5019, by R. C. Wittmann, M. H. Francis, L. A. Muth and R. L. Lewis. This paper was aimed at principle pole measurements, e.g. HH and VV. The tabular data presented in the paper are from this paper with additions for errors associated with cross polarization and cross polarization correction.

Polarimetric Calibration Standards for Reciprocal Monostatic Radar Systems
L.A. Muth,A.L. Buterbaugh, B.M. Kent, B.M. Welsh, P.D. Domich, November 1998

Calibration standards for radar systems are being developed cooperatively by NIST and DoD scientists. Our goals are to develop standard procedures for polarimetric radar calibrations and to improve the uncertainty in the estimation of system parameters. Dihedrals are excellent polarimetric calibration artifacts, because (1) the consistency between dihedral scattering data and the mathematical model of scattering can be easily verified, and (2) symmetry properties of the dihedral data provide powerful diagnostics to reveal system problems. We apply Fourier analysis to polarimetric data from dihedrals over a full rotation about the line of sight to reduce the effects of noise and clutter, misalignment, and other unwanted signals. An extension of the analysis to satisfy nonlinear model constraints allows us to monitor data quality and to further improve the calibration. We obtain the system parameters from the Fourier coefficients of the data in a simple manner. We illustrate these concepts using polarimetric radar cross section calibration data obtained as part of a national interlaboratory comparison program.
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