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Radar
RCS characterization on a portable pit with a foam column at VHF/UHF
M. Husar,J.H. Eggleston, November 1997
The RATSCAT radar cross section (RCS) measurement facility at Holloman AFB, NM is working to satisfy DoD and customer desires for certified RCS data. This paper discusses the low frequency characterization of the RATSCAT VHF/UHF Measurement System (RVUMS). The characterization was conducted on a portable pit with a 30' foam column at the RAMS site. System noise, clutter, backgrounds and generic target measurements are presented and discussed. Potential error sources are examined. The use of background subtraction and full polarimetric calibration are presented. Potential errors, which can occur from using certain cross-pol calibration techniques, are discussed. The phase relationship between each polarization components of the scattering matrix and cross-pol validation techniques are considered.
UWB noise radar using a variable delay line
E. Walton,I. Theron, S. Gunawan, November 1997
The Ohio State University ElectroScience Laboratory (OSU/ESL) has built a series of radars that transmit UWB random noise. On receive, the signal is cross correlated with a delayed version of the transmitted signal. When the response of the system is taken as a function of the delay time, the result is proportional to the impulse response of the system. After background subtraction and calibration, the impulse response of the target results. We will present a description of the variable delay line system and show an example ISAR image made from measurements taken in the OSU compact range.
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.
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.


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