AMTA Paper Archive

The utility of high resolution ISAR data in the devel­ opment and maintenance of low observable (LO) and conventional aircraft and the identification and charac­ terization of threat aircraft is well established. However, the task of ISAR image RCS interpretation is difficult. Often imaging effects introduced by rotating blades and jet engine modulation (JEM) can compound the already difficult interpretation task. It is easy for these effects to be obscured, ignored, or erroneously misinterpreted in ISAR down-range versus cross-range (Doppler) imag­ ery and range compressed versus time domain data. This paper presents cases of amplitude and phase modulated ISAR data collected from two airborne targets; a propel­ ler driven airplane and a helicopter, using a linear FM waveform radar. This will be supplemented with mathe­ matical models describing the modulation phenomenon and the resultant imaging effects

The radar scattering from a small communications antenna mounted on a large cylinder was measured at the Ohio State University ElectroScience Laboratory compact range. This paper will describe the experimental measurement techniques and the details of the analysis of the experimental. The small (5 cm) blade/slot/cavity antenna was mounted on a 1.82 meter long cylinder of 0.61 meter diameter. The cylinder was treated with RAM on the ends to reduce the direct and interactive end scattering effects, and was mounted in the OSU compact RCS measurement range. Measurements over the 2 to 18 GHz band both with and without the antenna were made and the results subtracted during the calibration effects to further remove the end effects. We will demonstrate these techniques and evaluate their effectiveness. ISAR imaging of both the antenna and the scattering term associated with the load on the end of the antenna transmission line will be shown. This will demonstrate that the transmission line and loan can be separately evaluated using such techniques. A time frequency distribution (TFD) analysis technique will also be demonstrated as a means of extracting various antenna resonance terms from the data. A description of the theoretical computation of the scattering will also be given and the special aspects of this problem outlined. The theoretical RCS data will be compared to the experimental measurements of the RCS.

This paper describes final results on the of non­ destructive measurement methods of missiles in terms of stealthiness. Measurements performed on full scale missiles allow to determine the reflectivity of the material and give estimation of its real RCS with to its nominal RCS. Different measurement techniques are reviewed, based on the use of coaxial transmission line, circular waveguide and spot-focusing horn lens antenna. Modeling, and characterization of spot-focusing corrugated horn lens antennas operating in the frequency range 2 - 18 GHz are presented. Finally, system configuration of full scale missile RCS measurements currently being utilized for production control is presented.

• Some Radar Cross Section (RCS) measurements contain significant contributions from the interaction of test article components. Usually the direct measurement of these terms is difficult. When these terms are not major factors, they need little attention. In other circumstances they should at least be quantified. There terms are often studied with special models, and/or Doppler measurements, and analysis. These relatively expensive methods yield the required information. For some purposes a more economical, limited method would be useful. RCS measurement and analysis facilities use software designed to present data in usable formats, with appropriate processing. This software is often run on a powerful workstation, or mainframe. McDonnell Douglas Technologies Inc. (MDTI) processing software "runs" on an .HP730 series workstation. The speed and capacity of such a system makes processing data a convenient option. MDTI demonstrated the ability to extract interaction terms from an easily acquired data set. This extraction required only the use of standard data software. Results with generic shapes demonstrate the ability to extract terms > 30 dB below the return of the test article specular return

A vertical array of antennas is used to beamform the farfield used in the measurement of Radar Cross Section (RCS) on a ground-bounce radar range. By properly weighting (attenuating) and phasing (through line length adjustments) each antenna, a desired far-field pattern can be obtained. This paper discusses some benefits of the technique and outlines a basic mathematical approach. Implementation is considered, and wide band ramifications of a practical design are discussed. At RATSCAT, this basic understanding was used to examine a simple two element array. This paper preceded that study and was originally written just for that purpose.

Typical high-resolution dynamic target imaging radars have frequency scan rates that do not properly sample the modulation from rotating structures such as aircraft propellers, engine turbines and helicopter blades. This results in the scatterer modulation energy being aliased. Moreover, if the chirp rate is too slow blurring and of the scatterer can occur in the image. Often the utility of this data for RCS signature analysis is questioned. This paper addresses the utility of images generated from undersampled data of modu­ lating scatterers. Experimental results using various combinations of chirp scan, modulation, and target-body rotation rates are presented. Fast scan rates, typical of the Linear-FM waveform, are compared to the slower scan rates commensurate with step frequency wave­ forms. Images are shown illustrating how the different chirp speeds alter the two-dimensional image of a mod­ ulating target.

ISAR imaging has proved to be a _ significant diagnostic tool for the evaluation of RCS signatures because of its ability to resolve scatterers in both the cross range and down range dimensions. There is a growing desire to extend the imaging capability to include the vertical dimension of a target, or three­ dimensional (3D) imaging. Several techniques have been suggested with varying degrees of success and complexity. These techniques include triangulation from two or more ISAR images of the same target taken at different elevation angles, tomographic algebraic reconstruction, and true 3D ISAR imaging using the FFT. Each technique requires progressively more data and more complex algorithms, but results in more resolution. This paper examines these various techniques, and evaluates their advantages and disadvantages based on actual implementations using simulated data.

A proper knowledge of clutter characteristics is critical to the design, development, and test of military seeker and radar hardware. The Clutter Mapping System under construction at Flam & Russell, Inc. is simple yet powerful tool for the evaluation of potential radar sites or the analysis of current sites. It provides a maximum 40 foot synthetic aperture that can image a 60 degree sector of terrain out to a 20 mile range and beyond. Aside from this primary mission, it has the capability to perform RCS measurement of non-cooperative ground targets or to serve as a tactical, quickly deployed imaging system. Totally self contained, and transportable, this system can fulfill a wide variety of RCS measurement needs.

One of the world's most sophisticated antenna test ranges is now fully operational. This was designed by the Deutsche Aerospace (DASA) and is operated by Siemens Plessey Systems (SPS). The presented paper will describe the pioneering design philosophy adopted to ensure the stringent performance features. Although this facility is located outside, it allows extremely high precision probing of cylindrical near field of large and very complex antenna systems, with turning diameters up to 16 meters and up to 20 GHz. Besides the RCS optimized 36 m large scanner tower the significant highlights of this facility consist of a comprehensive air-conditioning system for all accuracy dependent components, a permanent autoalignment system, which ensures high precision cylindrical measurements and an interleaved high speed data collection system, which delivers a maximum of data performance within a minimum time frame. Test results including a pattern comparison of the Ref­ erence Antenna between measurements in DASA facilities and the SPS Cylindrical Near-Field Test Facility show good range performance. The evaluation of the range performance data demonstrates the measurement integrity of the facility and proves to be qualified to characterize a wide range of antennas.

The Hughes Space and Communications Company (HSC) has recently undertaken the task to modify a RCS range once operated by Hughes Radar and Communications Systems, to accommodate the testing of Satellite Antennas. This measurement facility's configuration, design and current status will be discussed herein. This RCS range is located in El Segundo, California.

A portable system for measuring the RF environment at remote sites is described. A frequency range between 500 MHz and 18 GHz is covered by this system. The design, calibration and use of this system are discussed.

The imaging of radar targets is typically accom­ plished by measuring the radar cross section (RCS) of the target as a function of frequency and az­ imuth angle. We measure a third dimension of the RCS by tilting the target and collecting data for conical cuts of the RCS pattern. This third dimension of data provides the ability to estimate the three-dimensional location of scattering centers on the target. Three algorithms are developed in order to process the three-dimensional RCS data.

The Hughes Space and Communications Company (HSC) has recently undertaken the task to modify a RCS range once operated by Hughes Radar and Communications Systems, to accommodate the testing of Satellite Antennas. This measurement facility's configuration, design and current status will be discussed herein. This RCS range is located in El Segundo, California.

RCS data measured under near-field conditions is corrected to the far-field. The algorithm uses the HUYGEN's principle approach. The processing technique is describes and validates using anechoic chamber data and simulations taken on flat plate target at a distance from the radar R << 2D2/A, where D is the target cross range extend and A the wavelength. Good agreement with the theoretically predicted far-field RCS patterns is obtained.

The application of rapid near-field measurement systems based on the Modulated Scattering Technique (MST) and Spherical Angular Function (SAF) data processing of the measured data to extract far-zone RCS of complex targets is discussed in this paper. A first-generation Spherical near-field measurement system for efficiently determining bistatic RCS is presented.

Antenna positioners using a single pivot joint and two linear actuators are attractive for applications requiring limited two-axis motion. Such applications include antenna and RCS measurement systems, and scanning antennas. Minimum swing clearance is required. Positioners can be light, compact and stiff. Position feedback can be independent and linear for both axes. Design and selection considerations are presented. Two examples are described

We discuss how RCS target depolariza­ tion enhances cross-polarization contamination, and we present a graphical study of measurement error due to depolarization by an inclined dihedral reflector. Error correction requires complete polarimetric RCS measure­ ments. We present a simple polarimetric calibration scheme that is applicable to reciprocal antenna radars. This method uses a dihedral calibration target mounted on a rotator. Because the calibration standard can be ro­ tated, there is no need to mount and align multiple sepa­ rate standards, and clutter and noise may be rejected by averaging over rotation angle.

Historically, radar imaging sensors have been divided into two categories, SAR and ISAR systems. Even though they are solving the same imaging prob­ lems the data collection environment is dramatically dif­ ferent between the two. Consequently, the particular waveforms selected for the two have been different. The primary waveform for ISAR RCS measurement systems is stepped frequency, while the FM-chirp (linear-FM) waveform has been used much more often in SAR applications. However, recently this boundary has been blurred, in that stepped frequency radars are being applied to long range dynamic measurements, long the domain of chirped waveforms, and conversely the chirped waveform has been applied to target RCS mea­ surements of both static and dynamic targets. This paper will address the system parameter tradeoffs involved in selecting between the two waveforms for two different applications; (i) near range static target imaging, and (ii) far range dynamic target imaging. The system parameter tradeoffs involve RF bandwidth, PRF, scene size, trans­ mitter power, doppler frequency spread of target, etc. The advantages, disadvantages, and inherent limitations of each waveform will be analyzed to yield a better understanding of the tradeoffs involved, and the data collection examples will further illustrate these tradeoffs for the two specific applications.

A novel test article, the Transformable Scale Aircraft-Like Model (TSAM), which holds great promise for validating complex computational electromagnetic (CEM) codes more effectively is described. The novelty of TSAM is in the use of removable/replaceable canonical shaped structural components. The complexity in TSAM can be tailored to the modeling capabilities of the CEM code under test allowing discrepancies between measurement and simulation to be more explainable. A set of preliminary measurements on TSAM have been made and the results compared to calculations from the General Electromagnetic Model for the Analysis of Complex Systems (GEMACS) program (1), a standard CEM code.

The Naval Command, Control and Ocean Surveillance Center RDT&E Division (NRaD) has been using a 500 MHz Linear Frequency Modulated (LFM) radar to collect measurements of flying aircraft. These data have been used to generate high resolution Inverse Synthetic Aperture Radar (ISAR) images of the targets [l]. Digital Signal Processing (DSP) hardware had been added to the radar and algorithms have been implemented to perform ISAR processing on the data in real time. A VME bus architecture has been developed to provide a scaleable, flexible platform to test and develop real-time processing software. Algorithms have been developed from a system model, and processing software has been implemented to perform pulse compression, motion compensation, polar reformatting, image formation, and target motion estimation.