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Phaseless measurements are going to represent a viable and less expensive alternative to standard near field techniques since they allow to reduce to a very large extent the complexity of an indoor set-up. In fact, they require "scalar" receivers, probe positioning systems with less strict mechanical requirements, and present no cabling problem. Furthermore the anechoic environment extension can be reduced and low dynamic range receivers used as "truncated" data can be managed. In this paper we outline the main advantages of an approach to the solution of the problem of the far field reconstruction from phaseless near field measurements. Conditions to reliably process the collected data can be put forward so circumventing the main difficulties of most solution algorithms for non linear inverse problems. Experimental results are also included for the planar geometry.
In polarimetric RCS measurements, the cross-polarization levels which are required in the test zone, correspond closely to those which are realizable with most Compact Antenna Test Ranges (CATR). On the other hand, such a performance may not satisfy the accuracy requirements in cross-polarization measurements of high performance microwave antennas. These applications include spacecraft antennas, ground stations for satellite communications or microwave antennas for terrestrial applications, where two polarizations are used simultaneously.
Composites of the electrically conducting polymer polypyrrole with paper, cotton cloth and polyester fabrics have been evaluated for use in radar absorbing structures. Reflectively measurements on the composites in the range 8-18 GHz and transmission line modelling have revealed impedance characteristics with a common transition region. Relationships between substrate material, polymer loading and electrical performance have been explored. Polarization characteristics have also been measured. The electrical model has been successful in predicting the performance of Salisbury screen and Jaumann multi-layer designs of RAM.
The plane wave quality of a compact range (CR) is usually specified in terms of the crosspolar level and the magnitude and phase ripple in the test zone. The way these deviations from the ideal plane wave affect the measurement of different antenna types can be treated by the application of the reciprocity principle between the transmitting and receiving antenna in a measurement set-up. By the application of the sampling theorem, it is found that the measured antenna pattern can be expressed as a summation of the plane wave spectrum components of the field at the test zone weighted by the true radiation pattern of the antenna under test (AUT) evaluated at the CR plane wave directions in the rotated coordinate system of the AUT. The inverse procedure can be used to extract the CR plane wave information (and therefore the CR field at the test zone by means of the Fourier series) from the measurement of a standard antenna with a known radiation pattern.
Time-frequency distributions (TFD) describe a signal in terms of its joint time and frequency content. In this paper, it will be shown that TFDs are particularly useful for the analysis of frequency-dispersive electromagnetic scattering. A TFD based on the wavelet transform (WT) of the scattering data is presented. As an example, measured scattering from a waveguide cavity is considered. It is shown that the wavelet TFD can provide good time resolution for specular/point scattering features, and good frequency resolution for resonant features. Application to the scattering data from the KC-135 aircraft in flight shows that the WT is capable of detecting the resonant modes of the engine outlets of the aircraft.
To gain greater insight into the design of surface ships with reduced radar cross-section characteristics, a structure resembling a ship deckhouse was physically modeled and measured. The structure was represented as a truncated pyramid. Four scaled pyramids were fabricated, all identical except for the radii of the four vertical (slanted) edges. The pyramids were measured at the University of Massachusetts, Lowell Research Foundation, submillimeter laser compact range. Measurements were made a scaled X-band using a laser-based system that operates at 585 GHz with the pyramids scaled at a ratio of 1:58.5. These shaper were measured at 0.75 degrees depression angles on a smooth metal ground plane at both HH and VV polarizations. The goal of this study was to determine if small changes in the radius of the curvature of the slanted edges could significantly affect the radar cross-section of the pyramid. In this paper the results of measurements of the pyramids will be presented. The data are compared with computer code predictions and the differences are discussed.
There has been a great deal of interest in microstrip antennas and arrays in the past decade or so due to their low cost, light weight, and conformability. Most research to date on microstrip antennas has been focused on developing techniques for characterizing their radiation properties. However, interest in evaluating the scattering properties of such antennas is increasing. The RCS of three configurations of circular patch antennas have been measured versus frequency and are compared to Moment Method predictions; a single open-circuited element, a single element terminated in a 50 ohm load, and a 3 x 3 array of open-circuited elements. In most cases, the measurements and predictions are in good agreement.
The design of many modern RCS instrumentation systems is driven by the time required to complete a measurement which establishes the throughput rate of the RCS facility and therefore impacts the operating cost and efficiency. Time considerations are of particular importance when wideband systems are used to measure large targets with low RCS because multiple observations are required to span the frequency band or to increase sensitivity by coherent integration. Although significant improvements have been made to minimize inefficiencies in instrumentation systems, the fundamental limit of measurement time is governed by physical considerations of power, energy, noise, target dimension, and RCS. Evaluating the performance of a particular radar design can be facilitated by comparing the predicted measurement time with a theoretical optimum. The purpose of this paper is to develop estimates of the minimum measurement time under optimum conditions. Although likely precluded by practical considerations, the theoretical limits provide estimates of the maximum degree of radar performance and measures of optimality in practical systems.
ORBIT's String Reel Target Manipulation System is used to support and rotate a target during RCS measurements. One of the challenges in this kind of RCS measurement is to accurately determine the position of the target in space, since the weight and moment of inertia of the target and the string flexibility do not allow measuring its position with conventional methods (linear encoder, etc.). In order to overcome this problem, the Non-Contact Optical Measurement System (NCOMS) has been developed and tested at ORBIT. The system provides the capability for precision tracking of the target position (X, Y, Z) and orientation (ROLL, PITCH, YAW). NCOMS is a computer-controlled system and operates by using two standard CCD cameras (stereo technique), as well as by use of a single camera with insignificant accuracy degradation. Another advantage of NCOMS is that the system operation does not require accurate camera positioning. The only requirements for CCD camera installation are target visibility and use convenience.
The presence of the sea surface has a powerful influence on the scattering characteristics of marine targets during radar cross section (RCS) measurements. To obtain accurate RCS measurements of a large, distributed marine target, the radar site must satisfy various requirements. The major requirement is to provide quality RCS data without strong multipath distortion of the target return signal. In this paper multipath effects on a large scatterer measured at both low-and high-elevation radar sites are summarized. It is observed that multipath effects contribute strongly to the RCS of the target measured at a low elevation radar site. The data show large RCS fluctuations of more than 15 dB when a scatterer is measured at difference altitudes or ranges. The quality of the data measured at a low-elevation radar site then becomes questionable, which creates difficulties in assessing the true RCS of the target. For diagnostic purposes, it may be necessary to change the target range or altitude several times to make a credible assessment of RCS. The same target measured at a high-elevation site has less multipath influence on the RCS data, making assessment of the true RCS feasible.
New results from numerical simulations and preliminary experiments pertaining to the dynamics of conical rotation of a string-suspended target are reported. For the simplest case of a model target suspended by three strings at three points, we find that its equilibrium position and the pitch, roll and yaw are uniquely dependent on its center of gravity (CG). By assuming that the yaw angle is mainly provided by the rotation of the upper turntable (UTT), we study the change in pitch and roll by controlling the strings while using a precision optical measurement system (POMS) to monitor the actual position and angular orientation of the target. In addition, two natural modes of small oscillations for the transverse and the longitudinal motions of the model target are observed to be of periods 1.45 and 2.46 seconds, respectively. The electrical-mechanical response time of the string-reel systems is found to be about 1.8 + 0.2 seconds. The results will be useful for the conics algorithm development.
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. Ground-to-air imaging of full-scale aircraft was demonstrated by Hughes in 1990. In early 1992, a Hughes A-3 aircraft made air-to-air radar images of a test aircraft in flight. To date, Hughes has collected imagery on nine aircraft from VHF through X-Band, including nose, side and tail aspects at several elevation angles. Reference (2) describes the VHF/UHF capability of the imaging system and this paper will describe the image processing steps developed and will display S- and X-Band radar images with resolution as fine as 6 x 4 inches. The images presented in this paper are dominated by a few very large cavity-type scatterers and do not show the ultimate sensitivity and fidelity of the system. The air-to-air images do demonstrate the spectacular diagnostic utility of this technology.
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.
Antenna pattern measurements are dominantly influenced by the presence of extraneous fields in the test zone. A fast and simple way to recognize problems in pattern measurements provides the Antenna Pattern Comparison-technique (APC). This method usually consists of recording azimuthal patterns on different positions across the test zone. Differences in the amplitude data give a rough indication for the magnitude of the interfering signal. The "Novel APC-method" (NAPC) employs both amplitude- and phase-data so that it becomes possible to separate the direct and the extraneous signals from each other. It will be shown that this method is eminently suited to correct radiation patterns of high-gain and low-sidelobe antennas. For verification purposes corrected patterns are compared with time-dated ones and the resemblance is excellent. It is concluded that the NAPC-method is promising and powerful technique for accurate antenna pattern determination, mainly because it can be easily implemented for most applications.
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.
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.
MUSIC and ESPIRIT are two popular eigen analysis based super-resolution estimation techniques. The use of these techniques in scattering analysis is of interest in this paper. A comparative performance evaluation of these techniques is reported based on the time domain study of the RSC of various targets.
The proposed synthesis method allows the calculation of the diffraction figure in the focal plane of the compact range, starting from a field distribution in linear polarization over a plane in the Fresnel zone. Applying this method (in only one dimension) to the ideal near field of a FFOC compact range, a linear array is synthesized which can be extrapolated to a planar array feeder design; providing excellent features in the quite zone.