AMTA Paper Archive
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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.
Boeing Near-Field Test Facility (NFTF) Upgrades & Design Tradeoffs
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.
3-D Radar Cross Section Imaging Using Interferometric ISAR Technique
In this paper, we present an interferometric in verse synthetic aperture radar (IF-ISAR) image processing technique for three-dimensional (3-D) radar cross section (RCS) imaging of complex radar targets. A general bistatic 3-D imaging geomet ry and the corresponding 3-D image pro cessing algorithm which relates the interferomet ric phase to the target altitude are developed. The impact of multiple scattering centers on al tit ude image formation is discussed. 3-D RCS image formation examples from both indoor and outdoor test range data are demonstrated for complex radar targets.
Assessment of the NIST DoD RCS Demonstration Project, An
During the last 6 years scientists at NIST have been focusing on radar cross section (RCS) measurements to improve RCS uncertainty analysis, and to develop new measurement and calibration artifacts and procedures. In addition, NIST has been asked to provide technical support to the DoD RCS self-certification effort. In this talk I review the technical accomplishments of the program, and will make suggestions for future research to improve RCS calibration and measurement technology. I will also present the structure of the certi fication process, and discuss NIST's role in the ongoing certification activities.
Measurement of a Large Active Planar Array for Spaceborne Radar Using Near-Field Scanning Techniques
The requirement to calibrate and test large active pulsed planar array RADAR antennas, such as the one developed for the advanced synthetic aperture radar (ASAR), places certain requirements on the measurement facility and analysis software that are perhaps not encountered in other areas of application. This paper gives a brief overview of ASAR and an introduction to some of the difficulties encountered during the test and measurement campaign. Results are presented that compare measurement with theoretical prediction. Good agreement has been obtained for both far and near field data.
Projection of Near-Field Data to Far-Field
Near-field ground-to-ground imaging systems are widely used to discover damage that could degrade the radar signature of low observable vehicles. However, these systems cannot presently assess the impact of this damage on the far-field signature of these vehicles. We describe progress made on a method to accurately project the near-field data from these to the far field. Near-field data for the algorithm development is provided by the hybrid finite element/integral equation RCS computer code SWITCH. The near-field data is processed to extract the near-field scattering centers using imaging. The imaging algorithm used differs from the usual far-field imaging formulation in that it incorporates some near-field physics. The processing algorithm, which incorporates a modified version of the CLEAN technique, verifies that the scattering centers that were extracted reproduce the original data when illuminated in the near-field. These near-field scattering centers are then illuminated by a plane wave to produce far-field data. This procedure was tested using VHF band scattering data for a full size treated planform. The near field data was projected to the far-field and then compared to data from a far-field SWITCH computation.
Motion Compensation in ISAR Imaging Using a Phase-Monitoring Subsystem
Undesired antenna motion can significantly degrade SAR and ISAR image quality on an instrumentation radar operating in an outdoor or uncontrolled environment. Antenna vibration on the order of only a few hundredths of an inch at X-band frequencies can degrade performance to the point that one cannot reliably differentiate between the true and false peaks in the radar image. This paper describes a motion compensation technique that utilizes the measurements from an auxiliary antenna pointing at a stationary target. This "Phase Monitoring Subsystem" accurately records the linear antenna motion profile, which can then be used for compensation. Data collected at the US Naval Undersea Warfare Center (NUWC) Fisher Island Test Facility on a calibration target demonstrate that this compensation technique can reduce image artifacts by more than 20 dB.
Algorithms for High-Precision Two-Dimensional ISAR Imaging on an Outdoor Turntable Range
Inverse synthetic aperture radar (ISAR) imaging on a turntable-tower test range permits convenient generation of high resolution two- and three dimensional of radar targets under controlled conditions, typically for characterization of the radar cross section of targets or to provide data for testing SAR image processing and automatic target recognition algorithms. However, turntable ISAR images suffer zero-Doppler clutter (ZDC) artifacts and near-field errors not found in the airborne SAR images they seek to emulate. In this paper, we begin by reviewing a technique to suppress ZDC while minimizing effects on the target signature. Next, turntable ISAR images of a vehicle formed at Georgia Tech's Electromagnetic Test Facility are used to demonstrate a computationally-efficient implementation of a backprojection (BP) image former. BP-formed ISAR images are free of all first order near-field errors. Finally, images generated using these techniques are compared to images obtained using electromagnetic prediction codes.
Performance of Low-Cost 3.5 GHz Ground Vehicle Detection Radar
This paper presents the development and testing of a small low-cost (COTS) 3.5 GHz Ground Vehicle Detection Radar. In this frequency band, the signal can penetrate light brush and foliage. However, the detection and tracking of radar targets where both the radar and the target are close to the ground is particularly difficult because of ground wave attenuation and foliage dynamics. The radar uses all surface mount components for small size and low cost. A VCO is used to cover the frequency band from 3.1 to 3.6 GHz. A power splitter and a quadrature mixer follow the VCO. Thus the radar operates as a base-band system for direct down-conversion. We will show the design procedure for this radar as well as test results confirming the design. We will also show detection and tracking results for vehicle targets in this foliage/brush penetration close-to-the-ground environment.
Systems Analysis of the Response From A Linear FM Radar, A
The measurement of the frequency response of complex targets of interest for the purpose of radar cross section (RCS) analysis has become a common task for modern radar ranges. When carefully done to avoid transients, the stepped frequency continuous wave (CW) method directly measures the frequency response of the target. On the other hand, dechirp-on-receive processing utilized by linear frequency modulated (LFM) radars introduces certain distortions to the measurement that are rarely fully considered. In this paper, we derive the relationship between the true frequency response of a target and what is measured with an LFM radar utilizing dechirp-on-receive. One can use this relationship to analyze the effects of the LFM processing as a function of the target geometry or scattering mechanisms and radar parameters. Radar parameters may then be selected so as to minimize the differences between the LFM measured response and the true frequency response of the target.
Radar Cross Section Measurements Amid Interfering Backgrounds
In order to better understand the target-background interaction, we present new observations on the azimuthal and frequency dependences of the backgrounds, with the upper turntable (UTT) either kept stationary or in a constant rotation. In the stationary case, vector subtraction of backgrounds measured within seconds yields the lowest achievable residual levels between -50 and -60 dBsm. For the rotating UTT, the hot spots (regions of high background) exhibit a 4-fold symmetry in the azimuth, in frequency from 0.5to 4.0 GHz, and are positively identified as due to Bragg diffraction from the periodic 2-D structure pf absorbers with a 12"-square unit cell. Subtraction of backgrounds by azimuth yields a characteristic residual which mimic the structure of the hot spots. Aluminum rods (of small ka, supported by strings from the UTT in a horizontal position) provide an opportunity for studying the background interference with the echoes in HH, VH and VV, in order of decreasing signal. The results suggest that knowledge about the hot spots is essential for choosing the low background regions for measurements on low RCS objects.
Automatic Scatterer Identification From Measured Ship RCS Data Using Underlying Physical Models
Radar cross section (RCS) is a primary determinant of ship susceptibility to attack by antiship cruise missiles. RCS management benefits from the clear association of individual scatterers on a ship with measured ship RCS data, which is the scatterer identification problem. It is an. inverse scattering problem in which the scattering object is extremely complex, and environmental effects such as multipath and ducting corrupt the measurement channel. This paper describes a new method of solution to this important problem. The approach uses high fidelity models of ship RCS, of the radar signal processing, and of the environment in a constrained optimization framework. In so doing, advances are made in the areas of scatterer identification and predictive RCS model validation. Promising experimental results are presented that directly relate scatterers in a predictive RCS model of a ship to measurements of the ship taken in a maritime environment.
Bistatic Radar Cross Section Study of Complex Objects Utilizing the Bistatic Coherent Measurement Systems (BICOMS)
The NRTF and MRC have recently completed the first bistatic RCS test utilizing the Bistatic Coherent Measurement System (BICOMS). BICOMS is the first true far-field, phase coherent, bistatic RCS measurement system in the world and is installed at the NRTF Mainsite facility. The test objects include a 10 foot long ogive and a 1/3 scale C-29 aircraft model. Full pol rimetric, 2-18 GHz monostatic and bistatic RCS measurements were performed on both targets at 17 degree and 90 degree bistatic angles. BICOMS data demonstrates excellent agreement to method-of moments RCS predictions (ogive) and indoor RCS chamber measurements (monostatic, ogive). This paper describes the BICOMS system and the test process, highlights some process improvements discovered during testing, assesses the quality of the collected data set, and analyzes the accuracy of the bistatic equivalence theorem.
Characterization of an Outdoor RCS Measurement Range
The Radar Signature Management Group of Racal Defence Electronics Limited specializes in the measurement, prediction and analysis of radar signatures. Types of measurement ranges used by the Group fall into three categories: • Indoor instrumented ranges • Outdoor measurement ranges • Full-scale trials, in which dynamic measurements are made of the target in its normal operational environment This paper describes a methodology used for characterizing the uncertainties within data from one of the outdoor RCS measurement ranges, at frequencies from 8 to 12 GHz. The results are summarized and uncertainties arising from the following sources are quantified: • Linearity • Absolute Accuracy • Stability and Repeatability • Polar Diagram The effects of background and target-to-pylon support interface are also discussed. The individual uncertainties are combined in a simple manner in order to obtain an overall uncertainty bound for the range, and recom mendations are made for reducing uncertainties against the difficulty and cost of implementation.
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