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RCS measurements with frequency and angle diversity offer the benefit of spatial resolution obtained by synthesizing the equivalent of short pulses and large apertures. Recent research in several specialized fields has been directed to spectral estimation techniques which seek to maximize the achievable resolution beyond limits imposed by traditional Fourier methods. These techniques, known as: autoregressive modeling, linear predictive modeling, super-resolution, or maximum entropy, offer the possibility of enhanced resolution and band-limited signal extrapolation. The methods apply to situations in which an estimate of a required feature is derived from an incomplete set of measured data linked to the required feature by a known relation. In RCS applications, spatial distributions of reflectivity are linked to measured band-limited frequency responses by a Fourier transform. Maximum entropy methods, therefore, apply directly to the objectives of increasing spatial resolution or extrapolating band-limited frequency responses.
A method has been developed by which the fair-field RCS of a target can be evaluated from its RCS measured in the near field. The method can compensate for the nonuniformity of the antenna pattern which can be a function of the angle, the frequency, and the target distance. A correction transform is evaluated which depends on the antenna pattern, the frequency, the target distance and the target size. The correction transform is independent of the target geometry. The RCS of a target is measured in the near field, in a band of frequencies around the frequency at which the far field RCS of the target is desired. The method can practically handle directional scattering elements, shading of the scattering elements by each other, and interactions among the scattering elements. The reconstructed RCS evaluated by this method shows excellent agreement with the actual far-field RCS.
The simultaneous illumination of the Quiet Zone by number of beams is helpful and cost-effective for broadband antenna and RCS measurements. For an application such as, for instance, Electronic Warfare development, the use of scanning beam or multiple beams gives more extensive opportunities for designers. When the antenna-under-test is small in size, the lightweight and small single reflector Compact Range is very well suited for the above applications. Such a Compact Range being moved within the test facility (anechoic chamber or outdoor range) provides additional flexibility for the tests. This paper describes the development of a small Compact Range with a rolled edge reflector and a two-foot diameter Quiet Zone. Analysis of the Compact Range is performed for different feed positions, providing the beam scan in elevation and azimuth with respect to on-axis beam.
A portable measurement system has been designed and implemented to produce focused three dimensional RCS images. The Synthetic Aperture Radar (SAR) system was especially designed to operate in harsh physical and cluttered electromagnetic environments. The acquisition system, signal processing and 3D visualization capabilities are discussed and representative data ranging from simple canonical objects to production hardware are presented. The technique meets its design goal in effectively discriminating undesired clutter.
ISAR images and RCS signatures of aircract-in-flight using a ground based and an airborne radar system are presented. The ground-based measurements were at X-band and were of a Mooney 231 aircraft, which flew in a controlled path in both clockwise and counterclockwise orbits, and successiely with gear down, flaps in the take-off position and with the speed brakes up. The air-to-air measurements were made by a radar installed in the nose of the TA-3B aircraft which followed a KC 135 airplane at a range of approximately 450 ft. and traversed a cross-range angle component of (plus or minus) 30(degrees). The data indicates that these systems are useful tools for RCS signature diagnostics of aircraft in flight.
This paper describes an effort to evaluate the effect on RCS of base closures on a metallic frustrum at various depths with conducting and electrically isolated plugs. The tests were conducted at Sandia National Laboratories using System Planning Corporation's (SPC) Mark IV radar from 8 to 18 GHz, in the step chirped Inverse Synthetic Aperture Radar (ISAR) mode. Data reduction was performed on Information Systems and Research's workstation using the KNOWBELL software package. The workstation allowed the study of the imagery data in many different modes, which assisted in determining ways to evaluate RCS matching.
Coherent subtraction algorithms, such as specular subtraction, require precision target alignment with the imaging radar. A few degrees of phase change could significantly degrade the performance of coherent subtraction algorithms. This paper provides an analysis of target position measurement errors have on ISAR data. The paper addresses how traditional position errors impact phase and image focusing. Target rotational positioning errors are also evaluated for their impact on magnitude errors from specular misalignment and polarization sensitive scattering and image phase errors from height-of-focus limitations. Several tables of data provide a useful reference to ISAR data experimenters and users.
A Precision Optical Measurement System (POMS) has been designed, constructed and tested for tracking the position (x,y,z) and orientation (roll, pitch, yaw) of models in Boeing's 9-77 Compact Radar Range. A stereo triangulation technique is implemented using two remote sensor units separated by a known baseline. Each unit measures pointing angles (azimuth and elevation) to optical targets on a model. Four different reference systems are used for calibration and alignment of the system's components and two platforms. Pointing angle data and calibration corrections are processed at high rates to give near real-time feedback to the mechanical positioning system of the model. The positional accuracy of the system is (plus minus) .010 inches at a distance of 85 feet while using low RCS reflective tape targets. The precision measurement capabilities and applications of the system are discussed.
This paper describes the electrical and mechanical design of an outdoor bistatic RCS test range at the National Defence (sic) Research Establishment (FOA) in Linkoping, Sweden. Some experimental bistatic ISAR imaging results will also be discussed. The 100 m RCS test-range uses a curved rail system. The transmitter rail cart can be moved on a constant distance from the target. This can be illuminated in bistatic angles from 0 to 105 degrees. The measurement system uses fiber optic links for transferring reference signals for coherency. The system has an excellent phase stability that enables ISAR imaging and background subtraction techniques to be used.
The target support system at Boeing Defense & Space Group's 9-77 Compact Range includes a new string support system. The string support system consists of twelve string reels, six each of the High Capacity String Reels (HCSR). The string reel system is used to suspend and manipulate a target for radar cross section (RCS) measurements, primarily at frequencies below 1.5 GHz. The string reel system is capable of supporting targets up to 10,000 pounds and 40' in length and width. The manipulation and handling of targets, is a major consideration in a RCS measurement test plan. The following paper discusses the newly installed string reel system, enhancements to the 9-77 Range equipment which directly affect the use of the string support system, and future developments planned for the system.
A reflector antenna has been designed for the RCS measurements. The antenna is dual linearly polarized and exhibits constant beamwidth over an octave bandwidth. The design principle has been to keep the effective antenna aperture constant in terms of the wavelength over an octave bandwidth. The theoretical design lead into the choice of the antenna and the feed. The reflector was an offset parabolic reflector. The feed was a ring-loaded conical corrugated horn. The measurement results of the designed reflector antenna showed very good agreement with the computer results. The V- and H- polarization characteristics of the antenna are almost identical.
The HP-85330A multi-channel, multi-even system controller was designed to take advantage of the speed, performance and flexibility of automated HP 8530A microwave receiver antenna and RCS measurement systems. In its simplest form, the HP85330A is a VXI mainframe and card that controls high-speed, high-isolation solid-state microwave switch modules. Using the FAST data modes and internal data buffers of the HP 8530A, the measurement system is allowed to run at the maximum speed and performance specification. It accomplishes this through hardware control of the triggering and timing of the HP 8530A microwave receiver, HP8360 series sources, positioner controllers, and external switch modules. For outdoor ranges, the HP 85330A is capable of hardware handshaking with other HP 85330As through balanced twisted-pair wires. Accompanying the HP85330A controller are the HP 85331A and HP 85332A SP2T and SP4T external switch modules. To facilitate remoting the switches, communication between the HP 85330A system interface and 85331/2A is by parallel, twisted pair balanced lines. The lines are capable of distances of 40 meters. The switch modules are individually addressable or can be cascaded to form complex switch trees. Actual measurement throughout data is presented.
A plasma-absorber system consists of a membrane that confines a collusional gas at atmospheric pressure and an ionization source. The ionization source generates a dense plasma that tenuous near the confinement membrane. An electromagnetic wave propagating through this plasma is attenuated. The mechanism for absorption is momentum transfer among electrons, driven by an incident wave, and a gas. Because the momentum-transfer collision rate, v, at atmospheric pressure can be as high as 870 x 10^9 s-1, the 3-dB bandwidth for absorption (~v/20) is approximately 40 GHz. The plasma thickness between the source and confinement membrane is approximately one wavelength at the lowest frequency. The magnitude of absorption depends on this thickness, the maximum electron number density, and the electron density gradient. A smooth gradient reduces reflections. This paper discusses a theoretical model that predicts general absorption and reflectivity phenomena. Experiments have quantified 40-dB performance at VHF using a 4-mil polyethylene vessel, and at X-band using a 2-mil Mylar inflatable support system. Applications to precision RCS measurements include reduction of backwall reflections and target interaction with the ground plane, and a shutter for reference targets.
For the last few years, the Periodic Moment Method (PMM) has been used to analyze the scattered fields from an infinite absorber wall. Using this approach the absorbe4r can have different periodicities in the x and y directions, as well as arbitrary shapes and any dielectic (sic) distribution. This makes this analysis method very general such that it can treat any conventional wedge or pyramidal designs. Plus, it has been used to develop new ones, which is the subject of this paper. Traditionally there have been chamber uses for both wedge and pyramidal absorber (sic). In a normal RCS range, one uses pyramidal material in forward sector around the feed, wedge absorber through the target zone and pyramids on the backwall. Using this approach, one takes advantage of the basic features of the two types of absorber. To improve wedge material, one is interested in reducing its normal incidence reflection coefficient because the long straight edge is a rather large scatterer. Through the use of the PMM analysis, curved and serrated wedge absorber designs have been developed and tested. Both show significant improvement relative to conventional material. As for the pyramidal model, one would hope to improve its size requirements especially for lower frequencies. Recall that two wavelengths at 100 MHz is twenty feet. By placing twenty foot material throughout a chamber, one greatly restricts the size of the room. Again, the PMM analysis has been used to develop a new curved pyramid design which can perform as well as a conventional pyramid twice its size. Thus, one could use curved pyramids that are ten feet at 100 MHz and achieve the same performance as the commercially available twenty-foot material.
General guidelines for using software gating are presented. Examples which demonstrate both proper and improper use of gating are presented. The effects of RAM materials on the time domain signature and the selection of the gate parameters are discussed. A brief review of the general theory of high resolution RCS measurements is presented.
The frequency accuracy of the HP 8530A receiver and HP 8360 Series synthesizers in ramp sweep is measured using a delay line discriminator. The effect of the frequency error on measurement accuracy is derived for radar cross section (RCS) measurements of one and two point constant-amplitude, scatterers and for background subtraction. The results of swept and synthesized frequency measurements are compared, showing that the errors due to ramp sweep are negligibly small for practical RCS measurements.
This paper describes the target handling system that was developed for use in the compact range facility at the University of Pretoria. The system was locally designed and manufactured and comprises of a lift platform, RCS pylon and utility trolley. The pylon utilises a unique design approach resulting in a structure with very high stiffness and surface finish.
This paper describes the technique and advancement of diagnostic radar imaging technology by comparing past SAR and ISAR techniques to the more recent advancement of Autofocus SAR techniques. This recent advancement has meant the relaxation of the stringent mechanical stability requirements needed to produce high quality, high dynamic range, calibrated RCS images.
This paper will show that it is possible to make bistatic measurements in a compact range environments using near field scanning. A test scanner is designed and operated. Criteria for the accuracy of positioning and repositioning are presented. Algorithms for the transformation of the raw data into bistatic far field calibrated RCS are presented. Examples will be presented where comparisons with theoretical bistatic sphere data are shown. Bistatc pedestal interaction terms will be demonstrated.
Radar Cross Section (RCS) measurements are typically made at linear polarizations (usually horizontal and vertical) and the transmit and receive polarizations are the same (co-polarized). In addition, however, it is sometimes desirable to measure the cross-polarized RCS of a target (i.e., transmit horizontal, receive vertical or vice-versa). A complete set of both co-and cross-polarized RCS of a target is called a scattering matrix. This paper describes the algorithm used for calibrating a scattering matrix measurement in the McDonnell Douglas Technologies Inc. (MDTI), Radar Measurement Center (RMC). Verification data collected at Ka band on various targets is included to validate the algorithm and implementing computer code.