AMTA Paper Archive
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Monostatic RCS Calibration of Radar Target Using Extrapolation Method in Millimeter-wave Frequency Band
In this paper, we propose a calibration method for monostatic radar cross section (RCS) of simple radar targets (e.g. trihedral corner reflectors and square flat-plate reflectors) using extrapolation method. By the proposed method, we can calibrate the monostatic RCS of radar targets from 1-port S-parameter measurements. In our system, the applicable size of radar targets are 75 mm to 125 mm for corner reflectors and 40 mm to 75 mm for square flat-plate reflectors, respectively. The nominal RCS of reflector targets calculated by physical optics ranges from +3 dBsm to +15 dBsm in W-band. The measured results are agree well with simulation results calculated by method of moment (MoM).
SAR-ISAR Blending Using Compressed Sensing Methods
Inverse Synthetic Aperture Radar (ISAR) measurements are used in this study to obtain images of full scale targets placed on a turntable. The images of the targets are extracted using compressed sensing methods. The extracted target images are edited and merged into measured Synthetic Aperture Radar (SAR) images. Airborne SAR field trials are complicated and expensive. This means that it is important to use the acquired data efficiently when areas with different background characteristics are imaged. One would also like to evaluate the signature of targets in these background scenes. Ideally, each target should then be measured for many orientations as well as illumination angles which would result in a large number of measurement cases. A more efficient solution is to use ground based ISAR measurements of the desired targets and then blend these images into the SAR scene. We propose a SAR blending method where a noise free image of the target is extracted from the RCS measurement by using the compressed sensing method Basis pursuit denoise (BPDN) and then solving for a model consisting of point scatterers. The target signature point scatterers are then merged into a point scatterer representation of the SAR background scene. The total point scatterer RCS is evaluated in the frequency-angle domain followed by using that RCS for back projection to form a seamless SAR image containing the target with the desired orientation and aspect angle. A geometrically correct shadow, constructed from a CAD-model of the target, is edited into the background. The process is completed by adding noise to the image consistent with the estimated SNR of the SAR-system. The method is demonstrated with turntable measurements of a full scale target, with and without camouflage, signature extraction and blending into a SAR background. We find that the method provides an efficient way of evaluating measured target signatures in measured backgrounds.
Review of Cross-Eye Jamming
This paper gives a review of cross-eye (CE) jamming using the retro-directive channel implementation. CE jamming is an electronic warfare self-protection technique in which the phase-front of an electromagnetic wave, transmitted towards a threat radar, is distorted in a way similar to radar glint. A retro-directive channel is used in the implementing of the CE jammer to avoid prohibitive tolerance requirements on the electronic warfare (EW) system. In a practical implementation of the CE jammer in an EW system, active electronically scanned array antennas (AESA) can be used to fulfil effective radiated power requirements. The achievable reciprocity i.e. similarity between the transmission and reception radiation patterns in the AESA is central to the performance of the CE jammer system. Effects of the CE jammer on mono-pulse radar are presented and described. The effects include the mixing of a CE jammer signal and a target echo. The CE jammer can induce false target angles and prevent the radar from finding a stable settling angle. The origin of CE jamming is in the field of radar multipath phenomenon such as glint and reflections from water surfaces. The CE jamming technique has previously been described and analyzed in various literature. This paper summarizes the most recently published results and gives references to the publications.
Scattering Effects of Traveling Wave Currents on Linear Features
Backscattering responses of range-extended objects include those attributed to traveling-wave effects, typically caused by the termination of the object. Diagnosing the nature of scattering mechanisms contributing to the composite response is essential to modifying the object's radar signature. ISAR images reveal essential information on the location and nature of scattering features by decomposing the frequency/angle data into basis functions corresponding to independent point scatterers. When applied to responses of objects exhibiting other basis functions, such as those for traveling-wave scattering, ISAR images reveal unexpected results that can obscure proper interpretation of the scattering mechanisms. Because traveling-wave lobes are restricted to limited ranges of grazing angles and are frequency dependent, however, localizing their effects from high-resolution images can be elusive. Specifically, the traveling-wave responses are not readily distinguished from direct or diffracted responses. The paper deals with backscattering data collected on a slender cylindrical rod of 183cm length and 0.635cm diameter for aspect angles 0-180 degrees over a frequency range of 2-10GHz with polarization parallel to the plane of incidence, intended to emphasize effects of traveling waves at the rod's grazing angles. In spite of its relatively simiple geometry, the linear rod object presents complicated responses owing to the combined effects of traveling waves and multiple diffractions. Although ISAR images properly locate point scatterers, an understanding of the imaging process provides clues on the expected location of image elements corresponding to more complicated scattering features. The angle and frequency dependence of each scattering mechanism is illustrated in the paper by frequency responses, range responses, and ISAR images. The total scattering resonse of the rod for grazing incidence is characterized by at least 5 distinct scattering mechanisms, each interacting with the others as a function of viewing angle. Because images of traveling- and diffracted-wave components overlap for some grazing angles, their relative responses preclude separation. The results provide an example of the complex nature of scattering from a simple shape subject to traveling-wave effects.
Development of a Precision Model Positioning System for a Multi-Use Electromagnetic Test Facility at NASA Langley Research Center
This paper describes the mechanical design, control instrumentation and software for a precision model positioning system developed for use in the Experimental Test Range (ETR) electromagnetic test facility at NASA Langley Research Center. ADC has a contract to design, build, and install major components for an updated indoor antenna characterization and scattering measurement range at NASA Langley Research Center. State-of-the-art electromagnetic systems are driving a demand to increase the precision and repeatability of electromagnetic test ranges. Sophisticated motion control systems can help meet these demands by providing electromagnetic test engineers with a level of positioning fidelity and testing speed not possible with previous generation technology. The positioning system designed for the Experimental Test Range at NASA Langley Reseach Center consists of a rail positioning system and four rail positioning carriages: an antenna measurement positioner, scattering and RCS measurement pylon, an azimuth rotator to support foam columns, and an electric personnel lift for test article access. A switching station allows for rail positioning carriages to be quickly moved on and off of the rail system. Within the test chamber there is also a string reel positioning system capable of positioning test articles within a 40’ x 40’ x 25’ volume. Total length of the rail system is 112’ with laser position encoding for the final section of the rail system. Linear guide rails are used to support the carriages and each carriage is position with a rack and pinion drive. Rails mount to steel weldments that are supported with 8” diameter feet. Capacity of the rail system is 7,300 lbs. A switching station allows for positioning components to be moved off of and onto the rail system independently and a place to dock positioning components when they are not in use. A curved linear guide rail supports the switching station so that the platform can be rotated manually. Hardened tapered pins are used to align the switching station with mating rail segments. The scattering and radar cross section (RCS) measurement pylon is a 4:1 ratio ogive shape and has a 3,000 lb load capacity. A pitch rotator tip or spline driven azimuth tip can be mounted to the pylon. The spline drive shaft can be removed to allow for the pitch tip to be mounted to the end of the pylon. Total height of the pylon is 18’ from the floor to the pitch positioner mounting plate. Keywords: RCS, Scattering, Pylon, Positioner, Antenna Design, Rotator, Instrumentation, electromagnetic, Radio Frequency, Radar
Advances in Instrumentation and Positioners for Millimeter-Wave Antenna Measurements
Applications using millimeter-wave antennas have taken off in recent years. Examples include wireless HDTV, automotive radar, imaging and space communications. NSI has delivered dozens of antenna measurement systems operating at mm-wave frequencies. These systems are capable of measuring a wide variety of antenna types, including antennas with waveguide inputs, coaxial inputs and wafer antennas that require a probing station. The NSI systems are all based on standard mm-wave modules from vendors such as OML, Rohde & Schwarz and Virginia Diodes. This paper will present considerations for implementation of these systems, including providing the correct RF and LO power levels, the impact of harmonics, and interoperability with coaxial solutions. It will also investigate mechanical aspects such as application of waveguide rotary joints, size and weight reduction, and scanner geometries for spherical near-field and far-field measurements. The paper will also compare the performance of the various mm-wave solutions. Radiation patterns acquired using some of these near-field test systems will be shared, along with some of the challenges encountered when performing mm-wave measurements in the near-field.
Nearfield RCS Measurements of Full ScaleTargets Using ISAR
Near field Radar Cross Section (RCS) measurements and Inverse Synthetic Aperture Radar (ISAR) are used in this study to obtain geometrically correct images and far field RCS. The methods and the developed algorithms required for the imaging and the RCS extraction are described and evaluated in terms of performance in this paper. Most of the RCS measurements on full scale objects that are performed at our measurement ranges are set up at distances shorter than those given by the far field criterion. The reasons for this are e.g., constraints in terms of budget, available equipment and ranges but also technical considerations such as maximizing the signal to noise in the measurements. The calibrated near-field data can often be used as recorded for diagnostic measurements. However, in many cases the far field RCS is also required. Data processing is then needed to transform the near field data to far field RCS in those cases. A straightforward way to image the RCS data recorded in the near field is to use the backprojection algorithm. The amplitudes and locations for the scatterers are then determined in a pixel by pixel imaging process. The most complicated part of the processing is due to the near field geometry of the measurement. This is the correction that is required to give the correct incidence angles in all parts of the imaged area. This correction has to be applied on a pixel by pixel basis taking care to weigh the samples correctly. The images obtained show the geometrically correct locations of the target scatterers with exceptions for some target features e.g., when there is multiple or resonance scattering. Separate features in the images can be gated and an inverse processing step can be performed to obtain the far field RCS of the full target or selected parts of the target, as a function of angle and frequency. Examples of images and far field RCS extracted from measurements on full scale targets using the ISAR processing techniques described in this paper will be given.
Measurement of Operational Orientations Using Coordinate Transforms and Polarization Rotations
Antenna and Radar Cross Section (RCS) measurements are often required for orientation sets (cuts) that are difficult or impossible to produce with the positioning instrumentation available in a given lab. This paper describes a general coordinate transform, combined with a general polarization rotation to correct for these orientation differences. The technique is general, and three specific examples from actual test programs are provided. The first is for an RCS measurement of a component mounted in a flat-top test fixture. The component is designed to be mounted in a platform at an orientation not feasible for the flat-top fixture, and the test matrix calls for conic angle cuts of the platform. The transforms result in a coordinated, simultaneous two-axis motion profile and corresponding polarization rotations yielding the same information as if the component had been mounted in the actual platform. The second example is for a pattern measurement of an antenna suite mounted on a cylindrical platform (such as a projectile). In this case, the test matrix calls for a roll-cut, but the range positioning system does not include a roll positioner. The transforms again result in a coordinated, simultaneous two-axis motion profile and corresponding polarization rotations to provide the same information as the required roll-cut but without the use of a roll positioner. Finally, the third example is for an antenna pattern measurement consisting of an extremely large number of cuts consisting of conic yaw cuts, roll cuts and pitch cuts. The chosen method involves the use of the Boeing string suspension system to produce great-circle cuts at various pitch angles combined with the use of the coordinate and polarization transforms to emulate, off-line, any arbitrary cut over any axis or even multiple axes. Keywords: Algorithm, Positioning, Polarization, Coordinates, RCS
Application of Huygens' Principle to a Dual Frequency Constant Beamwidth Reflector Operating in the Focused Near-Field
A technique is presented for determining the pattern of an antenna in the focused near-field from cylindrical near-field measurements. Although the same objective could be achieved by conventional near-field to far-field transformation followed by a back projection, the proposed technique has an intuitive appeal and is considerably simpler and faster. The focused near-field antenna pattern is obtained by applying Huygens’ principle, as embodied in the field equivalent principle, directly to near-field measurements and by including an “obliquity factor” to suppress backlobe radiation. The technique was experimentally verified by comparison with far-field patterns obtained by conventional cylindrical near-field to far-field transformation and by EM simulations. Excellent agreement in sidelobe levels and beamwidth was achieved. The technique was applied to the 25 in diameter reflector antenna of a harmonic radar operating at 5.8 GHz and 11.6 GHz. Since the operating range of this radar is less than 40 ft, the reflector is the near-field at both frequencies. By defocusing the reflector at the harmonic frequency the beamwidths and gains at both frequencies can be made the same. The defocusing is accomplished by exploiting the frequency dependent phase center displacement of a log-periodic feed.
Reflector Panel Gap Analysis for the U. S. Army’s Electronic Proving Ground Outdoor Range
The outdoor range at the U. S. Army’s Electronic Proving Ground Antenna Test Facility features a large reflector in order to facilitate radar cross-section and antenna performance evaluation with large targets. Constructed during the late 1980s and early 1990s, this range features a 67-foot diameter reflector to satisfy quiet zone size specifications. The reflector is composed of 138 individual panels with nominal panel separation of 0.06 inches. This research investigates the impact of these gaps between reflector panels on the field received at the quiet zone. GTRI’s physical optics computational code was used to analyze the existing range design at the frequencies of interest, from C- through Ka-band, taking into account edge diffraction from the panels. In research presented at AMTA 2013, a range modification of the ground between the range source antenna and the reflector was performed to minimize ground reflections. This range modification has been incorporated with current research to provide quiet zone field analysis which includes reflector gaps as well as ground reflections.
"RF DNA" Fingerprinting for Non-Destructive Antenna Acceptance Testing
Abstract- Quality control is critical for all industrial processes, but often times defect detection is labor intensive. A novel approach to industrial defect detection is to use a random noise radar (RNR), coupled with Radio Frequency "Distinctive Native Attributes (RF-DNA)" fingerprinting processing algorithms to non-destructively interrogate microwave devices and classify defective units from properly functioning units. Example applications include assembly line inspection of automotive collision avoidance systems, wireless or cellular antenna defect detection during manufacture, and phased array element defect detection prior to RF system assembly. The RNR is uniquely suitable since it uses an Ultra Wideband noise waveform as an active interrogation method that will not cause destructive damage to microwave components. Additionally, it has been demonstrated that multiple RNRs can operate simultaneously in close proximity, allowing for significant parallelization of defect detection systems resulting in increased process throughput. Using this method, 100% sampling for quality control may be attainable in many cases. RF-DNA has previously demonstrated “serial number” discrimination of Orthogonal Frequency Division Multiplexed (OFDM), Direct Sequence Spread Spectrum (DSSS) network signals, GSM, WiMAX signals and others with classification accuracies above 80% using Multiple Discriminant Analysis and Generalized Relevance Learning Vector Quantification classification algorithms. Those cases all involved discrimination of passive emissions. This approach proposes to couple the classification successes of the RF-DNA fingerprinting with a non-destructive active interrogation waveform.
Modelling and Simulation of a Resistance Loaded Bow Tie Antenna
The modelling and simulation of a modified bow tie antenna optimized for radar cross section measurement is described. The bow tie antenna shows improved transient response for radiating Ultra Wide Band pulses with decreased late time ringing. In applications such as radar cross section measurement, late time ringing caused by reflections at the open ends can mask objects of interest in close proximity. The antenna reduces reflections by resistive loading based on works by Lestari and Wu-King. Full wave modelling and simulation is done using CST Microwave Studio. S-Parameter and VSWR optimization by modification of the conductivity profile is demonstrated. Experimental verification of the model has been carried out and confirms both the properties of the antenna and the simulation.
Polarimetric Weather Radar Antenna Calibration Using Solar Scans
The US NEXRAD weather surveillance Doppler radar (WSR-88D) was recently upgraded to polarimetric capability. This upgrade permits identification of precipitation characteristics and type, thus providing the potential to significantly enhance the accuracy of radar estimated rainfall, or water equivalent in the case of frozen hydrometeors. However, optimal benefits are only achieved if errors induced by the radar hardware are properly accounted for through calibration. Hardware calibration is a critical element in delivering accurate meteorological information to the forecast and warning community. The calibration process must precisely measure the gain of the antenna, the Polarimetric bias of the antenna, and the overall gain and bias of the receive path. The absolute power measurement must be accurate to within 1 dB and the bias between the Polarimetric channels must be known to within 0.1 dB. These requirements drive a need for precise measurement of antenna characteristics. Engineers and scientists with the NEXRAD program employ solar scanning techniques to ascertain the absolute gain and bias of the 8.53 m parabolic center fed reflector antenna enclosed within a radome. They are also implementing use of daily serendipitous interference strobes from the sun to monitor system calibration. The sun is also used to adjust antenna gain and pedestal pointing accuracy. This paper reviews the methods in place and under development and identifies some of the challenges in achieving the necessary calibration accuracies.
Electromagnetic Scattering Analysis of Possible Targets for Orbital Debris Remediation
After decades of international launches and varying space expeditions the low Earth orbit (LEO) has become littered with man-made objects and debris. With over 22,000 objects larger than a softball, and hundreds of thousands in smaller size existing, remediation efforts must take place to ensure the continuation of both collision free space flight and orbits. While smaller objects are difficult to track, and would consume more resources, the larger bodied debris offer a means to collect greater volumes of orbital debris clutter with less operations. In an effort to assist in the architectural design of microwave remote sensors, for the detection, tracking and identification of the large tumbling bodies, apriori knowledge of their relevant electromagnetic scattering parameters is essential. This paper work focus on the scattering phenomenology from possible large bodied orbital debris, such as rocket bodies, whose geometries are publically available. The results will strengthen existing data sets, Radar architectures, required signal processing, and even guidance navigation and control (GNC) routines that would be supported by resultant sensor information. Data products developed from commercially available electromagnetic simulation software will be presented, and the induced phenomenological scattering differences from the geometric variations between the possible targets will also be discussed.
Revising the Relationships between Phase Error and Signal-to-Noise Ratio
Within RF measurement systems, engineers commonly wish to know how much phase ripple will be present in a signal based on a given signal-to-noise ratio (SNR). In a past AMTA paper (Measurement Considerations for Antenna Pattern Accuracy, AMTA 1997), John Swanstrom presented an equation which demonstrated how the bound on the phase error could be calculated from the peak SNR value. However, it can be shown that the Swanstrom bound is broken when the signal has a peak SNR value of less than approximately 15 dB. This paper introduces a new equation that bounds the maximum phase error of a signal based on the signal’s peak SNR value. The derivation of this new bound is presented, and comparisons are made between the old Swanstrom bound and the new bound. In addition, the inverse relationship (i.e., calculating the SNR value of a signal from phase-only measurements) is investigated. In the past, analytical equations for this relationship have been presented by authors such as Robert Dybdal (Coherent RF Error Statistics in IEEE Trans. on Microwave Theory and Techniques) and Jim P.Y. Lee (I/Q Demodulation of Radar Signals with Calibration and Filtering in a Defense Research Establishment Ottawa publication). The analytical equations for calculating the SNR value using phase-only measurements are reviewed and discussed, and a brand new numerical relationship based on a polynomial curve fitting technique is proposed.
Distinguishing Localized and Non-Localized Scattering for Improved Near-Field to Far-Field Transformations
Historically, the inverse synthetic aperture radar (ISAR) reflectivity assumption has been used in the implementation of Image-Based Near Field-to-Far Field Transformations (IB-NFFFT) to estimate monostatic far field radar cross-sections (RCS) from monostatic near field radar measurements. The ISAR assumption states that all target scattering occurs at the location of the incident field excitations, i.e., the target is composed entirely of non-interacting localized scatters. Certain non-localized scattering phenomenon cannot be effectively handled by the IB-NFFFT approach with the ISAR assumption. Here we have used the adaptive Gaussian representation, which is a joint time-frequency decomposition technique, to coherently decompose near field measured data into two subsets of scattering features: one subset of localized scatterers and the other of non-localized scatterers. The localized scattering features are processed through the IB-NFFFT as typical, which includes compensating for the R4 fall-off present in the near field measured data. The non-localized scattering features, more appropriately scaled, are then coherently added back in to the post-NFFFT localized scattering phase history. Although this does not properly transform the non-localized scattering features into the far field, it does avoid the over-estimation error associated with improperly compensating distributed non-localized scattering features by a R4 power fall off based strictly on downrange position.
The Study on a New Type of Low-profile and Passive Radar Retro-reflector
Abstract—Nowadays, radar retro-reflector has been widely applied as a decoy, to seduce an incoming assault away from the target, or towards a less vulnerable part of it to communication systems and remote identification as their characteristics of low-profile, low-cost and Radar Cross Section(RCS) enhancement. A passive retro-reflector is a device which can be used to be reflected most of the energy incident upon it in the direction of the in-going wave. The Luneberg lens and a sphere are widely used as their self characteristics. In this paper one of the retro-reflector, is paid more attention as time goes by, is introduced. The retroreflector is consist of patch antenna arrays and feeding system and can be defined as Retro-directive arrays (RDA). It has a very simple structure and can focus outgoing waves back at the direction of incident waves. The character of the re-radiation pattern affected by the size and type of patch and width and length of feeding network related are optimized by the HFSS. The final results are validated experimentally.
Implementation of a Burst-Mode Technique and Variable Coherent Integration to Minimize Radar Data Collection Time
Abstract— Compact ranges are ideal settings for collecting low-RCS measurement data at high pulse rates. However, until recently, two operating constraints have limited the efficiency of instrumentation radar systems in this setting: (1) system delays limiting Pulse Repetition Frequency (PRF) and (2) fixed integration across frequency resulting in more time spent on certain frequencies than required. In this paper, we demonstrate the capability to significantly increase data throughput by using a Burst-Mode to increase the usable PRF and a frequency table editing mode to vary integration levels across the frequency bandwidth. A major factor in the choice of PRF for a specific application is system hardware delays. We describe the use of a Burst-Mode of operation in the MkVe Radar to reduce delays caused by physical layout of the instrumentation hardware. Burst-Mode essentially removes setup time in the system, reducing the time between pulses to the roundtrip time of flight from the antenna to the target. Most pulsed-IF instrumentation radar users fix the coherent integration level for the entire measurement waveform, even though the set level of integration may not be required at all frequencies to achieve the desired sensitivity. We describe the use of a frequency table Parameter Editor Mode in the MkVe that allows the integration level to vary for each step in the waveform. We demonstrate the use of both methods to reduce data collection time by a factor of seven using a MkVe Radar installed in a compact range.
Radar Cross Sections in the Shorter Millimeter-wave Region: Characterization and Calculation for Targets that include Rough Surfaces
Abstract— Measurements and strategies for the calculation of radar cross-sections in the shorter millimeter wave region, especially of objects that include rough surfaces, are discussed. Because of decreasing wavelength, roughness becomes more significant in this spectral region, but also more difficult to characterize. A tabletop radar cross-section measurement system was set up to measure scattering from canonical objects and rough objects with regular or random patterns using a swept frequency continuous wave system. Random, rough objects of different surface roughnesses were measured and fit to statistical distributions governed by optical speckle theory. In this paper we consider the inclusion of optical speckle theory in the electromagnetic codes to address both issues associated with the characterization of target surfaces and the time required for numerical calculations.
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