AMTA Paper Archive

Martin Marietta designed and brought on-line an indoor far-field chamber used for radar cross section (RCS) evaluation. The range has conductive walls on all sides except for the pyramidal absorber covered back wall. The chamber was designed such that wall/floor/ceiling interactions occur with a distance (time) delay allowing for their isolation from the test region. Software gating techniques are used to remove these unwanted signals. This paper presents an analysis of the conductive chamber using Geometrical Optics (GO). The objective was to analyze and evaluate the plane wave quality in the chamber test region. The evaluation of the plane wave was performed using the angle transform technique. The measured results were compared to analytical results and measured antenna patterns.

Measuring the radar cross section of low-observable (LO) vehicles require an RCS quality control (QC) program that will last throughout the life cycle of the vehicle, from component production to operational deployment and depot maintenance. Testing must be done at regular intervals to ensure that surface or sub-surface damage has not degraded the RCS characteristics of the vehicle beyond acceptable limits. In the past, these measurements were complicated by the requirement for and expensive, well-prepared RF test environment. The test range—usually a fixed site—is often remotely controlled. System Planning Corporation (SPC) has developed an RCS QC measurement technique that requires little or no facility improvements while offering high sensitivity inverse synthetic aperture radar (ISAR) images. The instrumentation radar system can be located at the production, maintenance, or operational site of the vehicle or component. As a result, the QC program is both economical and reliable.

To achieve optimum measurement accuracy and range throughput in antenna and radar cross-section (RCS) measurement applications requires a careful and thorough design of the measurement system. Measurement accuracy requirements, test time objectives, system flexibility, and system costs must all be balanced to achieve an optimum system design. Considering these issues independently will result in unwanted and/or unexpected system performance tradeoffs. This paper examines these issues in some detail and suggests a system design approach which balances microwave performance and measurement speed with system cost.

Lockheed’s Advanced Development Company (LADC), located in Burbank, California, has been evaluating the capability of indoor anechoic chambers to measure VHF/UHF RCS. Two chambers were available for evaluation. A 155 feet long, 50 feet high by 50 feet wide tapered horn chamber and a compact range having dimensions of 97 feet long, 64 feet high by 64 feet wide, featuring a 46 feet wide collimator. For comparison purposes, a common instrumentation radar was used in each chamber. This radar was based on a network analyzer using a Lockheed designed pulse-gate unit to increase transmit/receive isolation. Various antenna feed system were tried in both chambers to ascertain their characteristics. Theoretical and experimental data on system performance will be presented emphasizing practical implementation and inherent limitations.

The paper discusses the results from a series of experiments to measure the dynamic radar cross section (RCS) for high-velocity targets at millimeter wave (60GHz). The low observable nature and detectability of the threats at millimeter wave are addressed. Date processing will provide calibrated dynamic RCS time series, from which RCS scintillation analysis and detection modeling can proceed. The data collection, reduction, analysis and target Doppler signatures are addressed.

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.

Data collection is increasingly becoming the limiting factor in overall antenna and RCS measurement time. An equation for data collection time for multiple parameter measurements is presented along with and ordering function for determining the optimum nesting order for parameters. An example is used to demonstrate measurement speed enhancement techniques, reducing data collection time by 65 percent. Changing from stepped to linear near-field scanning reduced collection time by 75 percent.

A full RCS calibration technique using a dihedral corner reflector is presented in this paper. This scheme is valid for monostatic configuration and characterized by three aspects: (1) the frequency responses of four measurement channels can be mutually independent and thus, no special care has to be taken for signal paths; (2) only scattering matrix measurements of the dihedral at two orientations about the line-of-sight direction are needed since the transmitter and receiver are related through the reciprocity theorem; and (3) simple and useful expressions are used to solve for the calibration parameters. This technique is verified by several 2-18 GHz wideband RCS measurements performed in the OSU/ESL compact range.

Mathematical techniques (calibration, background subtraction, software range gating, imaging, etc.) have become integral to the process of generating precision radar cross section measurements. The "reference target method" is a powerful RCS correction algorithm which yields plane wave illumination results from data acquired under an arbitrary but known illumination. This method is analogous to a two dimensional RCS calibration. Measurements of long bars (at X- and Ku-bands) and of a scale model aircraft (at C-band) were performed under the cylindrical wave illumination produced by March Microwave's Single-Plane Collimating Range (SPCR) at Arizona State University. The targets were also measured under the quasi-plane wave illumination produced by a March Microwave dual parabolic-cylinder CATR. The SPCR measurements were corrected using the reference target method. The corrected SPCR measurements are in good agreement with the CATR measurements.

This paper presents precision measurements of the RCS of a simplified aircraft geometry called the "generic aircraft". The RCS is measured over a frequency range of 2 to 18 GHz, and for incidence angle from "nose-on" through "broadside" to "tail on". This data is presented in the form of RCS contours as a function of frequency and incidence angle, and is compared with the computed RCS using wire-grid modeling. The contours show distinct patterns due to airframe resonance and due to the interference of the scattered field from the nose and from the tail of the aircraft.

This paper will address the issue of estimating and measuring the RCS of simple objects on a finite sized ground plane. RCS measurements of a one inch diameter hemisphere on a ground plane were collected at X-band and are shown to compare favorably with two different models of a hemisphere on a finite pc ground plane; a simple Geometric Optics (GO) model, and a EM Body of Revolution (BOR) model. The beauty of the GO model is borne out due to the insight which is gained in understanding the scattering mechanisms taking place. With the addition of a Physical Optics traveling wave component for the ground plane, the two models can be brought into good agreement with the measured data. Measurements were also conducted for a cylinder, cone and bicone whose results are also presented.

Vibrational motion imparted on targets during RCS measurements will demonstrate a distortion phenomena equivalent to Phase Modulation (PM). Vibrational PM distortion has been witnessed outdoors resulting from wind vibrating a foam column and indoors from vibration in the target rotation mechanism. The vibrational frequency and maximum downrange scatterer movement determine the location and magnitude of effective PM sidebands in the image domain. The impact of this modulation ranges from minor distortions in the image domain to a complete invalidation of the data. This will paper (sic) provide examples and describe how conventional communication theory can be used to describe this distortion phenomena.

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.

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.