AMTA Paper Archive

A recently completed Hughes program successfully demonstrated an airborne multi-spectral (VHF through X-Band) Synthetic Aperture Radar (SAR) measurement of the radar cross section (RCS) of an aircraft in flight, producing two-dimensional (2-D) diagnostic RCS images of the test aircraft. The Air-to-Air Radar Imaging Program was a multi-phase program to develop, demonstrate and exploit this new technology for the design and evaluation of advanced technology aircraft. Radar images with resolution as fine as 6 x 4 inches were produced. To date, Hughes has collected imagery on nine aircraft from VHE through X-Band, including nose, side and tail aspects at several elevation angles. The ability to generate a radar image while in flight is a significant technical achievement. The VHF images presented demonstrate the utility of the system but the images do not show the ultimate sensitivity and fidelity of the system because the aircraft presented in this paper are dominated by a few very large cavity-type scatterers. The ability to measure the VHF/UHS RCS of an aircraft in flight and to make high resolution images is one of the major accomplishments of this program. VHF/UHF in-flight images, never achieved before this program, are a powerful diagnostic tool for use in aircraft development.

The Hughes Aircraft Company Compact Range facility for antenna and RCS measurements, scheduled for completion in 1993, is described. The facility features two compact ranges. Chamber 1 was designed for a 4 to 6 foot quiet zone, and Chamber 2 was designed for a 10 to 14 foot quiet zone. Each chamber is TEMPEST shielded with 1/4 inch welded steel panels to meet NSA standard 65-6 for RF isolation greater than 100 dB up to 100 GHz, with personnel access through double inter locked Huntley RFI/EMI sliding pneumatic doors certified to maintain 100 dB isolation. While Chamber 1 is designed to operate in the frequency range from 2 to 100 GHz, Chamber 2 is designed for the 1 to 100 GHz region. Both RCS measurements and antenna field patterns/gain measurements can be made in each chamber. The reflectors used are the March Microwave Dual Parabolic Cylindrical Reflector System with the sub-reflector mounted on the ceiling to permit horizontal target cuts to be measured in the symmetrical plane of the reflector system.

Unique instrumentation is required for dynamic (in-flight) measurements of aircraft radar cross section (RCS), jammer-to-signal (J/S), or chaff signature. The resulting scintillation of the radar echo of a dynamic target requires special data collection and processing techniques to ensure the integrity of RCS measurements. Sufficient data in each resolution aspect cell is required for an accurate representation of the target's signature. Dynamic RCS instrumentation location, flight profiles, data sampling rates, and number of simultaneous measurements at different frequencies are important factors in determining flight time. The Chesapeake Test Range (CTR), NAVAIRWARCENACDIV, Patuxent River, Maryland, is a leader in quality dynamic in-flight RCS, J/S ratio, and chaff measurements of air vehicles. The facility is comprised of several integrated range facilities including range control, radar tracking, telemetry, data acquisition, and real-time data processing and display.

RCS measurement accuracy is degraded by reflections occurring between the feed antenna, the range, and the radar subsystem. These reflections produce errors which appear in the image domain (both 1-D and 2-D). The errors are proportional to the RCS magnitude of the target under test and they are present in each of the typical range calibration measurements. Current 2-term error models do not predict or account for the above errors. An improved 8-term error model is developed to do so. The model is based on measurable reflections and losses within the range, the feed antenna, and the radar. By combining the improved error model with the commonly used 2-term RCS range calibration equation, we are able to quantify the residual RCS errors. The improved error model is validated with measured results on a direct illumination range and is used to develop specific techniques which can improve RCS measurement accuracy.

Modifications to commercial millimeter-wave receiver and target positioner systems produced significant improvements in recent W-band experiments at NAWC, Point Mugu. This paper discusses difficulties encountered in millimeter-wave measurements and presents novel methods for their resolution. Results from wideband W-Band experiments, including ISAR images, are presented.

From a study of several radar cross section (RCS) measurement facilities, we identify significant sources of uncertainty and develop methods for estimating their effect. Out goal is to provide a "reasonable" and uniform formalism for evaluating RCS measurements which can be used on a variety of test ranges to produce comparable estimates of uncertainty.

Three-dimensional imaging capability has recently been added to METRATEK's Model 200 RCS Diagnostic Radar. This paper describes the rationale and methodology for producing three­ dimensional images and gives sample images taken with the system.

Details of a public domain Microsoft Windows-based database program to organize and distribute measured antenna pattern and RCS data are described. Tools for editing, viewing and plotting data are also included.

The Naval Command, Control and Ocean Surveillance Center, Research, Development, Test and Evaluation Division (NRaD) is tasked by the Navy to collect and evaluate full-scale radar cross section (RCS) measurements on ships and aircraft. The Radar Branch at NRaD, operates a radar range west of Pt Loma, San Diego, CA. This radar range has been used to collect X-band and Ku-band calibrated data on Naval ships for the past seven years. The NRaD radar calibration helicopter procedures are the focus of this paper. Using helicopters to suspend and measure "isolated" spheres in space as the primary reference is a major calibration element. A 1700-ft Kevlar line is used to suspend the sphere from the helicopter. This length of line is sufficient to isolate the helicopter from the sphere; thus, the helicopter is not in the significant antenna sidelobes.

A versatile millimeter wave imaging radar is presented to conduct polarimetric doppler as well as wide band RCS measurements. The aim of the system is not only to acquire doppler measurements of determine the distance of an object but also to generate image-like information for classification purposes. A hardware gate controller is incorporated in the system to perform pulsed measurements. This controller can drive three different frequency extension modules covering frequency ranges from 8 to 18 GHz, 70 to 80 GHz and 75 to 77 GHz respectively. In all bands, dual polarized horns are used to allow fully polarimetric measurements. A network analyzer and a FFT analyzer are used as receivers. For both concepts the advantages and disadvantages are discussed. The transmit and the receive antenna are mounted on a positioner. Thus, a radar image using the real aperture of the antennas can be generated by mechanical scanning in azimuth and elevation.

RCS measurements of representative strategic targets in the resonant scattering regime are presented in this paper. The frequency and aspect dependent RCS signatures of various targets are shown to have close agreement to method-of-moment calculations which are based upon the known target shape and composition. Using the resonant scattering signatures, non-cooperative target recognition can be performed with high confidence using a discrete frequency sampling approach. The target set included cones, spheres, and canonical shapes which have been characterized in the VHF and UHF bands. Measurements made at the Lockheed Space Missile Company Rye Canyon facility have recorded calibrated RCS of representative hardware as a function of both frequency and aspect in the resonant region. These data compare well with prediction, and their use for non-cooperative target recognition will be explained. This effort is being conducted to develop signature models, laboratory measurements and useful discrimination algorithms which exploit the frequency variation of the resonant scattering RCS.

Accurate RCS measurements of joints and cracks on a large vehicle have been difficult to to the limited quiet-zone size of indoor RCS measurement ranges and the high cost of the test model. An approach proposed here uses both simple RCS measurements of a planar test model and efficient analysis to evaluate the three-dimensions (3-D) RCS contribution of joints and cracks on an entire vehicle. Several test models with steps and grooves on planar and curved surfaces were constructed and the results of this approach were compared with actual 3-D RCS measurements. All comparisons showed very good agreement. The main advances of the measurement technique are simplicity, cost effectiveness, and its vast application to many complex discontinuity scattering problems.

The need for practical solutions to radar scattering in high-frequency regime have led to the development of a number of approximation methods. The high-frequency asymptotic methods use approximations based on physical optics (PO), geometrical theory of diffraction (GTD) or physical theory of diffraction (PTD) and their variations. Radar scattering from electrically large conducting surface includes traveling surface wave contributions which are not accounted by the high-frequency asymptotic methods. A hybrid method integrating GTD and traveling wave theory (TW) is used for verification and to illustrate important scattering mechanisms that influence radar cross section (RCS) of a wedge. Analysis of the wedge RCS signature identifies significant contributions of the traveling surface waves to the total RCS. Both measured and predicted RCS of the wedge are considered. Using hybrid GTD-TW method very good agreement between the predicted and measured RCS patterns is observed for all angles.

In response to evolving USAF RCS measurement requirements, Lintex has developed a combined Pulsed/CW and Pulsed-IF instrumentation system for use at the Advanced Radar Cross Section Measurement Range. This instrumentation system, one of Lintek's Model 500 Series, couples the simplicity and high signal-to-noise ration of Pulsed/CW measurements with the flexibility and precise clutter rejection of Pulsed-IF systems. In this paper, a direct comparison of the Pulsed/CR and Pulsed-IF performance is presented. The theoretical sensitivity and throughput of the system as a function of duty cycle in each mode is calculated and compared to the measured results. The Pulsed-IF system is found to have better sensitivity and stability for short-range measurements due to the high PRF capability of this receiver. The Pulsed-IF mode of operation also offers much better sensitivity for measurements made at longer ranges, for which the duty-cycle losses of the Pulsed/CW mode become excessive. The wideband Pulsed-IF mode is also preferred in high-background environments, since precise time-gating may be used to reduce the clutter return. In areas of high RFI, the Pulsed/CW radar system has provided better results due to the narrow receiver bandwidth.

A compact modular instrumentation radar system has been developed for antenna, RCS, and general RF measurements. The MMS-420 system consists of a single, rack mounted, programmable mainframe controller and display into which a wide range of RF, IF and signal processing modules can be installed. A family of external RF modules has also been developed to support measurements from VHF through millimeter-wave bands. It is designed to function as a stand-alone measurement system, or interface with network analyzers and other external processing equipment. The hardware and software are easy to customize for specialized measurement applications.

Traditionally the Compact Range is not considered a viable method for conducting low frequency (VHF/UHF) antenna or RCS measurements because of the limited electrical size of the collimating reflector system. Normally, compact range measurements are conducted in the extreme near-field or the collimating system where to reflector size is on the order of 25 to 30 wavelengths minimum with at least four wavelength edge treatments. This mode of operation limits measurements to the high UHF band (800 MHz) and above for typical sized reflector systems in use today. Recent research with compact range3s indicates that acceptable VHF.UHF measurements can be conducted in the quasi far-field region of the collimating system with reflectors as small as five wavelengths and with electrically short edge treatments. A good user knowledge of this mode of operation is required to maximize its utility. A qualitative measure of acceptable quiet zone performance must also be established. This paper addresses the theory of operation, practical implementation and inherent limitations of the non-conventional use of the indoor compact range for conducting low frequency measurements.

It is shown that a phase-tapered aperture may be used to produce a uniform plane wave at Fresnel zone distances. This allows one to perform antenna/RCS measurements at reduced distances relative to a far-field range, but without the illuminator complexity and cost associated with a compact range. An asymptotic expression is obtained for the Fresnel field of a circular aperture field source distribution characterized by a large quadratic phase taper. The field is shown to be equivalent to that produced by a uniform ring source and central radiator, so that the design equations for the ring source and central radiator can be applied to plane wave synthesis using a circular phase-tapered aperture. The asymptotic expression for the field as compared with a numerical evaluation obtained using aperture integration. A simple implementation of a phase-tapered aperture using a radiating source which illuminates an aperture at a distance is presented. A quiet zone field extent which is approximately 70-80% of the source aperture extent is demonstrated.

Radar Cross Section (RCS) measurements are often performed in discrete frequency bands for a variety of reasons. Although some indoor ranges are capable of performing very wide-band measurements (with bandwidths up to or exceeding 9: 1), some are designed with very rigid illumination requirements on the coIIimating reflector(s) that can only be met over a narrow band. In addition, the bandwidth available on most outdoor ranges is limited by "ground plane" effects which make it impossible to maintain an adequate broadband field over the target. Often, RCS measurements are limited to half an octave at most. Since resolution in RCS imaging is directly proportional to bandwidth, there exists a need for concate nati ng several discrete bands of measurements into a single continuous band. This resulting band must be free of both amplitude and phase discontinuities that would affect the quality of the resultant image. This paper discusses the sources of discontinuities between measured bands on both indoor and outdoor ranges, and provides algorithms for removing them using linear filtering methods. Data is presented from an outdoor range illustrating the results on targets up to 70-feet in length.

ISAR images are formed by Fourier processing coherent wideband responses collected with angle diversity. Unfortunately, physical and practical considerations limit the frequency and angle diversities achievable. The finite diversities induce sidelobes, which are usually mitigated by application of tapered windows in the spectral domain. This procedure reduces image sidelobes at the cost of increased mainlobe width, thus degrading resolution. Spatially-Variant Apodiz.ation (SVA), a new non­ linear method developed at ERIM to improve the quality of SAR imagery, reduces sidelobe levels while preserving the mainlobe width corresponding to unwindowed data. In contrast to conventional window techniques which simply apply the same window function to every image element, SVA operates on the image by adaptively applying a window optimized for each spatial element. The algorithm uses phase information available from the coherent RCS data to distinguish processing sidelobes from correct responses. Mainlobes are passed using rectangular weighting, while sidelobes are reduced or eliminated entirely. This paper discusses the concept, theory, and implementation of SVA for ISAR imaging, and summarizes capabilities and limitations of the method. Results using SVA are presented and compared to conventionally windowed one- and two-dimensional images. The sensitivity of the procedure to additive noise and phase errors is investigated

A new approach to microwave tomography only requiring monotonic RCS data is presented. The amplitude and phase variation of signals backscattered from the target are measured in uniform angular increment and then analyzed by using wavelet transform. The wavelet transform with multiresolution property is suited for transforming the measured data into aspect vs. Doppler frequency (due to the phase variation of the rotating scatterers) domain. The scatterers location can then be derived by extracting the Doppler frequency variation and peaks occurence delay from the resultant 2-D representation, hence it makes the microwave tomography possible. Two discrete­ points targets are considered and the resultant microwave tomograms are shown. In our works, the entire processing can be completed in less then ten minutes for a 41 x 41 pixels tomogram nmning on 80486 DX-33 PC and only with single frequency illuminating signal. Furtherrnore, the scattering mechanisms are clearly identifiable in the resultant 2-D representation which can not be achieved by any other microwave tomography methods.