AMTA Paper Archive
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UWB Ground Wave Radar Studies
Theory and experiments for a ground wave UWB radar system for human and vehicle detection will be shown. We will consider the case where the radar uses a low gain VP antenna located 20 to 40 cm above the ground and the radar target is a moving vehicle or moving humans out to 200 meters. The nominal frequency for these tests was from 1.0 to 3.8 GHz in a step frequency scan. We will show SIN predictions using the free space radar range equation, then add ground wave attenuation effects. We will then compare these predictions with experimental measurement data for various vehicles and humans. An application using a noise radar as a UWB spread spectrum radar system in this application is our final goal.
Calibration and Error Budget in RCS Measurements
Uncertainty analysis for fundamental standards is mature, but the cost overhead has, until recently, prevented much of this work being taken up by the UK RCS measurement community. The requirement to verify the radar signature of new equipment has made it necessary to examine in detail the RCS measurement process and to create a methodology for error budgeting. The paper reviews some basic concepts in estimating uncertainties, and describes work on 'squat' cylinder calibration standards that have been manufactured following designs proposed at previous AMTA conferences. The moment method code CLASP has provided the basic theoretical solutions which have been verified on a compact range through reference to a precise 100mm spherical standard. The concept of multiple standard calibrations is discussed, and recommendations are made for overall error budgeting and the intercomparison of range types.
Interlaboratory Comparison Between the RCS Ranges at FOA Defence Research Establishment and Saab Dynamics, An
An interlaboratory comparison is made between radar cross section (RCS) measurements at the test ranges at FOA Defence Research Establishment and SAAB Dynamics, Sweden. The comparison is made in order to increase the measurement and calibration quality at the ranges. An analysis of the deviations in the measured RCS data from the ranges provides a better understanding of the sources of errors. The RCS of two generic targets are measured at the X-band. The targets are simple airplane models, length and width are approximately 1.0 m, with no cavities. A brief comparison between some theoretical results and experimental RCS data are also presented.
Wideband Radar Echoes From Cylindrical Rods
In order to assess the suitability of long thin metal rods as calibration devices for both co-polarized and cross-polarized (abbreviated as co-pol and x-pol) RCS measurements, we study RCS data from rods at broadside and compare them with 2D theoretical predictions. We find that the 45° tilt angle is optimum for calibration purposes. Near grazing incidence to a horizontal rod, the first traveling wave lobe in the HH pattern is a very prominent feature. Its angular location and amplitude have been measured as a function of frequency and compared with theory. A formerly unexplained error due to a contaminated calibration is identified.
Improvements in Static Radar Cross Section Calibration Processes and Artifacts -- Initial Measurement Results and Validation Through Inter-range Comparisons
The accurate measurement of Radar Cross Section (RCS) requires precise calibration "artifacts" as well as carefully executed measurement procedures. The Air Force Research Laboratory (AFRL) reviewed several existing common RCS calibration artifact standards and practices, and identified a number of improvements. Employing a modified "dual calibration" check procedure pioneered by AFRL, this paper demonstrates improved RCS calibration fidelity for a wide variety of static RCS calibration measurement applications. Our calibration results are verified through an industrial inter-laboratory (range) measurement program employing selected calibration artifact standards.
Performance Evaluation of the Automated Field Probe System (AFPS)
The Georgia Tech Research Institute (GTRI) under contract to the U.S. Air Force 46 Test Group, National Radar Cross Section Test Facility (NRTF) at Holloman AFB, NM, has designed and developed an Automated Field Probe System (AFPS). The AFPS operates as a one-way probe for evaluation of the electromagnetic field at the test zone and provides a mobile capability to rapidly, smoothly, and accurately probe the field at the various test-areas. The AFPS provides the ability to probe over an area as large as 40-ft x 40-ft all under computer control from the radar(s) while sweeping over 1-18 GHz and 34-36 GHz for both H and V polarization. The RF, phase reference, and control signals from the radar are transmitted to the AFPS over a microwave fiber optic link. This paper will describe the design and performance of the AFPS. Quick-look data products will be included in the presentation.
Impact of Radiation on Radar Cross Section
The purpose of this project was to determine the effects of fast neutron bombardment on the radar cross section of metal and dielectric spheres. The energetic neutrons interact with lattice atoms and, in the energy transfer that results, initiate a displacement cascade that effectiveiy damages the crystalline structure of the target material. The induced damage may change the RCS of the target via changes in the conductivity or relative permittivity. Theoretical lattice damage estimates are provided for fast neutron fluences of 1015 n/cm2 and 1016n/cm2. Limitations and potential improvement of damage estimates and measurements are also discussed.
Radar Cross Section Calibration Errors and Uncertainties
To develop standards for radar cross section measurements a complete uncertainty analysis is needed. We derive the radar cross section error equation and examine sources of measurement errors that contribute to the overall uncertainty in calibrations and measurements. We obtain expressions for upper- and lower-bound errors and uncertainties that are generally valid for monostatic measurements on any unknown target using any standard calibration artifact. The general procedure can be extended to bistatic measurements. Some experimental procedures to determine the uncertainty due to background subtraction are presented and discussed.
Real-Time Radar Cross Section Imagery
There is a growing interest in generating radar images as data collection is in progress. Such a tool is particularly useful for radar cross section verification purposes where the turnaround time is very important. With the availability of faster processing hardware, real-time radar image formation is now feasible. This paper describes the architecture, operation, and performance of a real time imaging (RTI) system that generates SAR or ISAR images while the data collection is in progress. Real-time performance of the system is benchmarked in terms of image-size and quality (imaging technique), image update rate, and image latency. Several examples of RTI are provided using a Lintek elan radar system.
SAR Imaging Through Complex Media
Classical SAR (Synthetic Aperture Radar) imaging techniques [1, 2] based on free space propagation may suffer significant distortion when a target of interest is located in a complex environment such as behind a building wall, underground or embedded in foliage. An independently derived analytical solution for electromagnetic wave propagation through a uniform dielectric wall or a uniform dielectric half-space is obtained by the authors. A new and computationally efficient model-based iterative SAR image refocusing algorithm based on the above solution is developed. The algorithm permits non-uniform spatial sampling of imaging data, and cases where a radar unit may be in the radiating near-field of a target. This algorithm is applied to both simulated and measured data. Resulting SAR images are shown to be significant improvement over those generated by the classical free-space back-projection technique.
Three-Dimensional Radar Imaging
A three-dimensional (3-D) imaging capability based on a linear FM measurement radar has been developed. This capability provides a means of resolving radar scattering centers in three dimensions, allowing the more accurate feature location and enabling the possibility of separating target returns from undesired environmental clutter. An existing portable radar cross section (RCS) measurement system was modified to incorporate a 3-D imaging capability. This modification allowed the system to remain highly portable and provide quick turnaround time with a typical measurement cycle comprising 20 minutes of data collection, followed by viewable 3D imagery within 5 minutes. The entire measurement system is comprised of a planar scanner and a single equipment rack. A 3-D RCS data set varies by frequency, azimuth, and elevation, and is obtained by scanning the radar antennas in azimuth and elevation. Innovative development of useful data visualization tools was one of the key efforts in this project. Visualization approaches include employing a mesh computer aided design (CAD) model aligned in 3-D space to the image data. The image is mapped to the surface of the model and the user can then move around the model to view it from any aspect in real time.
Iterative Information Retrieval Algorithm for Radar Applications
Phase retrieval is an important issue related to the reconstruction of SAR/ISAR images, when phase information is lost or unavailable. In this paper, an iterative algorithm is formulated which demonstrates the ability to perform phase retrieval with minimal set of constrains on the imaged object. This iterative algorithm requires only rough knowledge of the size of the imaged body and the amplitude of the received, far-field, radiation in the various frequencies and/or aspect angels (for I D or 2D image). By applying iterations between the two planes of the imaged body and the plane of the RADAR reflections (as a function of aspect angles and frequencies), a good reconstruction of the phase and the amplitude of the imaged body as well as the phase of the received radiation, are obtained. The algorithm can be used in the problem of imaging body in motion where motion compensation is difficult or in applications involving mm wave images, where phase information is lost in the turbulent atmosphere.
Technique for the Approximate Compensation of Antenna Illumination Taper from Near Field Measured, ISAR Data Sets, A
This paper presents an approximate, practical technique for the compensation of antenna pattern amplitude taper effects that occur in near field RCS data. The technique uses inverse synthetic aperture radar (ISAR) data sets. Complete pattern determination uses an iterative approach over target rotation angle and frequency bandwidth, with a series of near field ISAR images as input to obtain the corresponding corrected, near field, frequency/azimuth pattern data. Assumed is direct target illumination using a source with a known angular illumination pattern. The technique and its application environment in the Boeing Near Field Test Facility is described. It is then demonstrated using a near field data collection range of 100 feet from the target center of rotation. The approach is shown to be effective for target sizes with cross range extents extending to the one-way 3 dB points of the illumination taper (two-way 6 dB points). Demonstration of compensation performance and a study of accuracy achievable versus the near field image parameters used is presented.
Automation of Radar Image Processing of Airborne Targets
We present innovations based on pattern recognition technology that significantly reduce the level of human intervention and increase data throughput when processing radar images of airborne targets. Time consuming operator intervention is normally required to insure that images are centered and non-aliased and wireframe overlay drawings are properly registered with the target image. We have developed techniques that produce high-quality images without operator intervention. These include a template registration algorithm that can reliably orient the outline drawing with a radar image even in the presence of image artifacts such as jet engine modulation (JEM). In addition, we have developed methods that remove the average Doppler responsible for crossrange image displacement or aliasing and methods that resolve downrange ambiguities. Examples are shown which illustrate these processes applied to images of a jet aircraft in flight.
Helicopter Based RCS Measurements
A helicopter-based radar cross section (RCS) measurement system was designed and demonstrated during the past year. The system was a novel combination of modified and un-modified commercial off the shelf (COTS) equipment and software, a minor amount of new hardware, and extensive prior experience. Validation was accomplished using known calibration standards and existing test practices relevant to this type of system, and data were collected and processed for a number of targets of opportunity. The primary subsystems include the measurement radar, the helicopter, antennas and associated mount, boresighted video and recorder, and the calibration tools. The SCI1000 radar was employed because of the combination of its excellent performance at the desired test target range and its minimal physical and power demands. The Bell 500 helicopter was chosen for its size and its wide availability on the world market. Data products were RCS vs. aspect, downrange profile history, and two-dimensional imaging following pre-processing by a robust motion compensation algorithm.
Displacement of Collimator Beam for Extended Target RCS Measurements
Compact range collimating reflectors provide far-field conditions for radar signature measurements. Traditionally, the quiet zone is presented uniformly about the collimator boresight and depends upon both the size of the reflector and the beamwidth of the illuminating antenna, with a maximum determined by the reflector dimensions. Targets are placed in the center of the quiet zone and rotated about the center of gravity (cg) during measurement. Limitations on target size are defined by the quiet zone bounds. For large targets with a non-central cg location, a portion of the target may extend beyond the quiet zone boundary. A technique for synthesizing a larger quiet zone uses displacement of the collimator beam by means of feed point offset to allow far-field measurement of an asymmetrically-mounted extended target. Simultaneous measurements for each offset are then combined to produce the complete measurement. This technique was implemented for measurements of an ARIES ballistic missile target.
Automatic Detection of Radar Signature Defects
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
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
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
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.
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