AMTA Paper Archive
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Diagnostic imaging radar system for the F-117A stealth fighter
The U.S. Air Force is currently building deployable Diagnostic Imaging Radar (DIR) systems to perform quality control (QC) low-observable (LO) measurements of the F-117 fighter. Each system is a stepped-pulse frequency synthetic aperture radar (SAR) built by System Planning Corporation (SPC) combined with analytical software developed by MIT Lincoln Laboratory for generating radar images that will be interpreted to ensure LO integrity. The DIR systems will be used at fixed operating sites such as the F-117A main operating base, the F-117A maintenance depot, and any sites worldwide to which the aircraft may deploy. The F-117A DIR is the first field-level deployable radar cross section (RCS) measurement system for an operational weapon platform that is designed for use by the maintenance squadron. This paper discusses the critical issues of QC measurements for LO systems. It also describes the test requirements that are driving the development of DIR, and highlights the radar and SAR positioner requirements. Also presented is an overview of the diagnostic software and the algorithms used for detecting RCS anomalies and predicting maintenance actions for problem correction by flight-line crews.
3-D imaging of a T-72M at 35 and 95 GHz
Millimeter Wave (MMW) Radar Cross Section (RCS) measurements of full scale ground vehicles are used to develop and validate scattering models for smart weapons applications (target detection, discrimination and classification algorithms) and Hardware-in-the-Loop (HITL) missile simulations. This paper describes a series of MMW RCS measurements performed at Range C-52, Eglin AFB FL on a T-72M in a field environment using an exiting instrumentation radar (with slight modifications to allow for accurate height adjustment) and in-scene phase reference. The test methodology, instrumentation systems, 3-D Imaging Algorithm and sample data sets at 35 and 95 GHz will be presented as well as a detailed sensitivity analysis and discussion of error effects.
Comparison of radar imaging using data extrapolation and adaptive FIR filters
Radar images obtained using an adaptive finite impulse response (FIR) filter are compared with the radar images obtained using extrapolated scattered field data. The scattered fields of an experimental target and an airborne target are used in radar imaging. In adaptive FIR filtering, instead of fixed weights, variable weights are used in radar imaging. In this work, adaptive sidelobe reduction (ASR) technique is used to obtain the variable weights. Also, scattered field data extrapolation is carried out using forward backward linear prediction. It is shown that is the data extrapolation is successfully carried out by a factor of two or more, than the radar images obtained using the extrapolated scattered field data have better resolution than the radar images obtained using the adaptive FIR filters.
Indoor low frequency radar cross section measurements at VHF/UHF bands
In recent years there has been much interest in developing low frequency radar cross section (RCS) measurement capability indoors. Some of the principal reasons for an indoor environment are high security, all-weather 24-hour operation, and low cost. This paper describes recent efforts to implement VHF/UHF RCS measurement capability down to 100 MHz using the large compact-range collimator in the Bistatic Anechoic Chamber (BAC) at Point Mugu. The process leading to this capability has given rise to a number of technical insights that govern successful test results. An emphasis is placed on calibration and processing methodology and on measurement validation using long cylindrical targets and comparing the results with method-of-moment computer predictions and with measurements made at other facilities.
Development of a folded compact range and its application in performing coherent change detection and interferometric ISAR measurements
A folded compact range configuration has been developed at the Sandia National Laboratories’ compact range antenna and radar-cross-section measurement facility as a means of performing indoor, environmentally-controlled, far-field simulations of synthetic aperture radar (SAR) measurements of distributed target samples (i.e. gravel, sand, etc. ). In particular, the folded compact range configuration has been used to perform both highly sensitive coherent change detection (CCD) measurements and interferometric inverse-synthetic-aperture-radar (IFISAR) measurements, which, in addition to the two-dimensional spatial resolution afforded by typical ISAR processing, provides resolution of the relative height of targets with accuracies on the order of a wavelength. This paper describes the development of the folded compact range, as well as the coherent change detection and interferometric measurements that have been made with the system. The measurements have been very successful, and have demonstrated not only the viability of the folded compact range concept in simulating SAR CCD and interferometric SAR (IFSAR) measurements, but also its usefulness as a tool in the research and development of SAR CCD and IFSAR image generation and measurement methodologies.
Radar cross section range characterization
Radar cross section (RCS) range characterization and certification are essential to improve the quality and accuracy of RCS measurements by establishing consistent standards and practices throughout the RCS industry. Comprehensive characterization and certification programs (to be recommended as standards) are being developed at the National Institute of Standards and Technology (NIST) together with the Government Radar Cross Section Measurement Working Group (RCSMWG). We discuss in detail the long term technical program and the well-defined technical criteria intended to ensure RCS measurement integrity. The determination of significant sources of errors, and a quantitative assessment of their impact on measurement uncertainty is emphasized. We briefly describe ongoing technical work and present some results in the areas of system integrity checks, dynamic and static sphere calibrations, noise and clutter reduction in polarimetric calibrations, quiet-zone evaluation and overall uncertainty analysis of RCS measurement systems.
Radar target scatter (RATSCAT) division low frequency range characterization
The RATSCAT Radar Cross Section (RCS) measurement facility at Holloman AFB, NM is working to satisfy DoD and program office desires for certifies RCS data. The first step is to characterize the Low Frequency portion of the RATSCAT Mainsite Integrated Radar Measurement System (IRMS). This step is critical to identifying error budgets, background levels, and calibration procedures to support various test programs with certified data. This paper addresses characterization results in the 150 – 250 MHz frequency range. System noise, clutter, background and generic target measurements are presented and discussed. The use of background subtraction on an outdoor range is reviewed and results are presented. Computer predictions of generic targets are used to help determine measurement accuracy.
Acceptance of the Sanders Merrimack 23 compact range for RCS measurements
In 1993, we presented the newly completed compact range and tapered chamber facility . As part of this presentation, the issue of “range certification” was presented. This paper will discuss the work that we have done with the compact range for radar cross section (RCS) measurement acceptance. For customer acceptance, we had to “prove” that the compact range made acceptable measurements for the fixtures and apertures involved. Schedule and funding did not permit the full exploitation of the uncertainty analysis of the chambers, not was it felt to be necessary . The determination of our range capabilities and accuracy was based on system parameters and target measurements. Targets that were calculable either in closed form solutions (spheres) or by numerical methods (cylinders and rods) were used. Finally, range to range comparisons with the Rye Canyon Facility  of a standard target was used. The range to range comparison proved especially difficult due to customer exceptions, feed differences, and target mounting. This paper will discuss the “success” criteria applied, the procedures used, and the results. The paper will close with a discuss of RCS standards and the range certification process.
Region-of-interest radar imagery
It is shown that for dispersive scatterers, one cannot relate the intensity of the spatial response in the radar image to a particular location on the scatterer. For such cases, an imagery capable of characterizing both the spatial (dominant scattering centers) as well as spectral (resonant modes) wave objects. In the new image reconstruction, first a focusing point is selected for the image. The section of the image close to this focusing point provides good spatial resolution. As one moves away from the focusing point, the utilized bandwidth is reduced to provide good spectral resolution. The capabilities of the proposed imagery are illustrated using several examples.
Atomic decompositions of radar signals
In this paper we present a recently-developed adaptive method for decomposing radar signals into a wide range of physically meaningful mechanisms. The signal decomposition we will pursue is based on a set of functions referred to as “atoms” in the signal processing literature. These atoms can be derived from disparate mechanisms such as scattering center responses given by the Geometric Theory of Diffraction (GTD), which are localized in range, and resonance phenomena, which are localized in frequency. The atoms populate a “dictionary” of waveforms which are used to decompose radar signals. Traditional Fourier methods have restricted the class of atoms solely to harmonic exponentials. In this paper, we consider a number of signal decomposition methods. We chose a method based on the Basis Pursuit procedure of Chen and Donoho ,which uses an L1 norm as opposed to a least squares approach to search the dictionary. We chose this technique because of its sparsity of representation and convergence properties. We will show results of this technique applied to numerically simulated data and show that disparate scattering mechanisms can be isolated and identified in the radar data.
On reducing primary calibration errors in radar cross section measurements
To accurately measure static or dynamic Radar Cross Section (RCS), one must use precise measurement equipment and test procedures. Recently, several DoD RCS ranges, including the Advanced Compact RCS Measurement Range at Wright-Patterson AFB, established procedures to estimate measurement error. Working cooperatively with the National Institute of Standards and Technology (NIST), Wright Laboratory established a baseline error budget methodology in 1994. As insight was gained from the error budget process, we noted that many common RCS measurement calibration techniques are subject to a wide variety of potential error sources. This paper examines two common so-polarized calibration devices (sphere and squat cylinder), and discussed techniques for evaluating calibration induced errors. A rigorous “double calibration” methodology is offered to track calibration measurement error. These techniques should offer range owners fairly simple methods to monitor the quality of their primary calibration standards at all times.
Polarimetric calibration of nonreciprocal radar systems
The calibration of nonreciprocal radars has been studied extensively. A brief review of known calibration techniques points to the desirability of a simplified calibration procedure. Fourier analysis of scattering data from a rotating dihedral allows rejection of noise and background contributions. Here we derive a simple set of nonlinear equations in terms of the Fourier coefficients of the data that can be solved analytically without approximations or simplifying assumptions. We find that independent scattering data from an additional target such as a sphere is needed to accomplish this. We also derive mathematical conditions that allow us to check calibration data integrity and the correctness of the mathematical model of the scattering matrix of the target.
Dynamic ground-to-air radar imagery
Dynamic ground-to-air measurement of aircraft RCS has several advantages over static measurements. The target may be measured in flight configuration and the support pylon is eliminated. Although dynamic RCS imagery has been performed since the late 1970s, the cost and complexity of such measurements have limited their utility for routine testing. In this paper, an easily deployable ground-to-air radar imaging system developed by Aeroflex Lintek is presented. This system forms images of aircraft in straight flight, requiring no on-board instrumentation or special pilot training. The radar system, flight profiles, and processing tools required for generating images of aircraft in flight are presented, along with examples of measured target data.
Intelligent cruise control radar development
As part of the 1997 Automated Highway System Demonstration, the Ohio State University ElectroScience Laboratory (OSU/ESL) developed and operated a pair of automobiles equipped with radar systems for steering and cruise control. In a national demonstration attended by six autonomous vehicle teams, the system was used to convoy three autonomous vehicles along a 7 mile stretch of closed highway lanes near San Diego. The goal of the look ahead radar system was to acquire and track the vehicle ahead.
Radar image normalization and interpretation
Calibrated radar images are often quantified as radar cross section (RCS). This interpretation, which is not strictly correct, can lead to misunderstanding of test target scattering properties. To avoid confusion, we recommend that a term such as "scattering brightness" (defined below) be adopted as a standard label for image-domain data.
Rocket motor plume measurement facility
The Plume Measurement Facility is a new outdoor facility providing the capability to characterize tactical rocket motor plumes. Radar cross section of the plume is measured by both a near field and a far field radar. Infrared/ultraviolet/visible (IR/UVNIS) charac teristics are measured by numerous instruments recording spacial, temporal, and spectral data. All instrumentation is calibrated and adjusted to realtime standard day meteorological data and all data is recorded on a common synchronized time base.
Shipboard diagnostic measurements with extended imaging
The Joint Strike Fighter (JSF) office sponsored a Navy directed limited technical demonstration of diagnostic Radar Cross Section (RCS) imaging on-board an aircraft carrier at sea. The overall objective was to obtain experience and data sufficient to assist the Navy in defining any future shipboard diagnostic imaging measurement system requirements. Measurements were conducted in the hangar bay to assess the challenges posed by the carrier environment. A technique for making diagnostic imaging measurements in spatially confined areas was developed.
Experiences with near field measurements of the active phased array radar PHARUS
Measurements of antennas with integrated electronics is an upcoming topic. In many cases the antennas can only work in pulsed mode which requires synchronisation between radar and measurement equipment. Up and down mixing by internal LO's causes additional problems, especially with Near Field measurements where amplitude and phase data is required. Based upon hands-on experience, this paper treats some of the problems and pitfalls related to the Near Field measurements of an active antenna and alignment of the elements by means of backtransformation of the data.
Compact range for radar system testing, A
This paper discusses the design, fabrication, installation, and testing of a Scientific-Atlanta Model 5702 Compact Range used for radar system testing. The unique feature of this compact range is that it provides a plane wave target source for automated closed loop radar system testing. Techniques employed for meeting and verifying stringent specifications such as phase and amplitude gradients over the quiet zone are discussed. Results from closed loop testing of radar systems in the compact range are also presented.
Wide band feed for a virtual vertex reflector, A
Sanders, A Lockheed Martin Company, measures radar cross section (RCS) and antenna performance from 2 to 18 GHz at the Com pany's Compact Range. Twelve feed horns are used to maintain a constant beam width and stationary phase centers, with proper gain. However, calibration with each movement of the feed tower is required and the feed tower is a source of range clutter. To Improve data quality and quantity, Sanders and The Ohio State University ElectroScience Laboratory designed, fabricated, and tested a new wide band feed. The design requirement for the feed was to maintain a constant beam width and phase taper across the 2 - 18 GHz band. The approach taken was to modify the design of the Ohio State University's wide band feed . This feed provides a much cleaner range which reduces the dependence on subtraction and other data manipulation techniques. The new feed allows for wide band images with increased resolution and a six fold increase in range productivity (or reduction in range costs). This paper discusses this new feed and design details with the unique fabrication techniques developed by Ohio State and its suppliers. Analysis and patterns measured from the feed characterization are presented as well. This paper closes with a discussion of options for further improvements in the feed.
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