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Beamspace techniques are usually employed to synthesize phased array antenna patterns of arbitrary shape. In this paper a beamspace method is used to calibrate the pattern of a 32-element linear array with a conventional array taper. By measuring the antenna pattern in specific directions the beamspace technique permits the actually applied excitation function to be determined with little mathematical effort. Iterative corrections can then be made to the excitation function to maintain low sidelobe performance, or to compensate for element failures. Since local corrections to the array pattern result in global changes to the excitation function, explicit knowledge of where an element failure has occurred is not required. The beamspace analysis was carried out using antenna patterns obtained by electronically scanning the array past a far-field source. Such pattern measurements offer the possibility of maintaining phased array performance in an operational environment.
A number of spectral analysis techniques which offer significantly higher resolution than the FFT technique have been developed in recent years. The application of these super-resolution techniques to scattering analysis is of interest. With these techniques it is possible to identify the closely spaced scattering centres even with RCS data over relatively small bandwidths. This can be of significant importance in applications where data over large bandwidths are not available. The use of Autoregressive and Eigen analysis based super-resolution techniques in the scattering analysis of two basic targets, a sphere and a cube, is investigated and the results of the study are presented in this paper.
Superresolution (SR) processing techniques have been used for many years in direction finding applications. These techniques have proved valuable in extracting more information from a limited data set than conventional Fourier analysis would yield. SR techniques have recently proven to be an extremely powerful radar cross section (RCS) analysis tool. Typical resolution improvements of 2 to 30 times may be achieved over conventional Fourier-based range domain data in both the one-dimensional and two-dimensional image domains. Typical measurement scenarios which can most benefit from SP processing are presented. These include: VHF/UHF RCS measurements, measurement of resonant targets, and performing detailed scattering analysis on complex bodies. Measurement examples are presented illustrating the use of SR processing in a variety of test conditions. When the advantages of SR processing are combined with the accuracy of Fourier techniques, a new window is opened through which target scattering characteristics can be seen more clearly than ever.
The wave constraints typically placed on high-gain microwave antennas in a space environment, such as light weight construction and unfurlable deployment, preclude the rigid construction necessary to accurately maintain a required surface configuration over extended time periods. Present designs are limited by conventional, passive fabrication techniques. The ability to measure and control the antenna figure permits operation at tens of GHz, key to presently contemplated applications. The electro-optical figure sensor monitors the phase error of an antenna surface (parabolic or planar) by viewing optical fibers attached to the antenna, thereby providing feedback for active control of the antenna to a specified shape. Least-squares fitting of measurement data permits less stressful active control to the homologically-equivalent best-fit, or even the simpler tilt-alignment. Optical analytical techniques appear applicable to large, high-frequency antennas, offering new configuration designs and simpler analysis.
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.
Transient signature representation of scattered fields and their interpretation have become common in downrange and crossrange scattering center identification. A review of the basic concepts for one dimensional transient analysis is presented. The topics included are the frequency-time domain dual representation, general characteristics of transient signatures and temporal mechanism extraction.
A plane wave spectrum method of analysis is employed to examine a hybrid approach to compact range reflector design. In order to reduce edge diffraction, an illumination taper is used in conjunction with a serrated reflector. The optimum illumination taper is determined for several serrated reflector geometries. Maximum quiet zone is the optimality criterion. The aperture illumination functions considered are -symmetric, cosinudoidal in amplitude, and uniform in phase. The reflectors considered are characterized by a circularly periodic aperture boundary. The analysis is restricted to the low frequencies at which diffraction effects are most prominent.
This paper addresses the problem of absorber scattering into the target zone of a compact range. An approximate UTD lossy dielectric corner diffraction coefficient is found, and is used to calculate the bistatic scattering from the tip of an absorber pyramid. Scattering into the target zone of a compact range from the pyramidal absorber lining the room is then investigated, for both rolled edge and serrated edge reflectors, and is compared to the levels of the direct reflector diffractions. To build confidence in these absorber scattering predictions, calculations are compared with measurements of the bistatic absorber scattering in a compact range.
This paper will describe new developments in a gated-CW radar that has been designed to improve the productivity and sensitivity of RCS measurements. Improvements in data acquisition speeds result from the design of a fast synthesizer, a data acquisition co-processor and a pulse modulator. Each of these new products have been specifically designed to take advantage of the high speed capabilities of Scientific-Atlanta’s Model 1795 Microwave Receiver. The RF sub-system has also been designed to permit continuous 2-18 GHz, full polarization data acquisitions. Critical RF components are now mounted at the feed in the chamber, improving the sensitivity and ringdown of the system. Productivity in analysis activities has been improved by the use of a multi-tasking system controller which permits simultaneous use of the system for acquisitions, analysis and plotting.
This paper deals with design and evaluation of Compact Range Antenna and RCS measurement systems. Reflector subsystem and feeders design as well as quiet zone evaluation and system performance qualification are considered. Acquisition, process and presentation software to control the whole system has been developed and successfully implemented. Two systems have been designed and are now at implementation stage. A Gregorian concept Compact Range is now been constructed at RYMSA (Spain). This facility has been fully designed by IRSA and will be operative by the end of 1990. Compact Payload Test Range (CPTR) at ESTEC (ESA) is now been tested. System Instrumentation and PAMAS (Payload and Antenna Measurement and Analysis Software) have been developed.
The reflective properties of a flat circular plate and a long thin wire are discussed in connection with the quality and calibration of the quiet zone (QZ) of a compact antenna test range. (CATR). The flat plate has several applications in the CATR. The first is simple pattern analysis, which indicated errors as function of angle in the QZ, the second uses the plate as a standard gain device. The third application makes use of the narrow reflected beam of the plate to determine the direction of the incident field. The vertical wire has been used to calibrate the direction of the polarization vector. The setup of an optical reference with a theodolite and a porro prism in relation to the propagation direction of the incident field is presented as well.
A 75 foot diameter offset paraboloidal outdoor compact range reflector was designed for operation up to 95 GHz and installed at Ft. Huachuca, Arizona. The need for high frequency operation required that a highly accurate reflector surface be maintained in the desert’s harsh thermal and wind environment. The use of thermal modeling to predict the temperature distribution in the structure, along with extensive finite element analysis to determine the structure’s distortions from thermal, wind and gravity loads were integral to the reflector design. Using the above tools, thermal isolation techniques were developed to minimize the harmful effects of the thermal environment on surface accuracy. A surface error budget based upon both calculations and measurements shows an overall rms error of 4.9 mils under optimal environmental conditions, degrading to only 6. Mils under the worst operating conditions.
An HP 8510B based RCS measurement system is presented. It can be operated in CW, hardware gating, and fast-CW modes. A VAX-3800 computer and a MAP 4000 array processor are used to speed up the data analysis and a PS 390 graphic system is used to display graphic. Three ISAR techniques, i.e., DFT approximation, focusing image processing, and diffraction limited methods, are available in the analysis program to get the target image. With an amplitude taper removing technique, this system can measure large target whose size is almost up to the size of compact range reflector.
In this paper, a general methodology for data reduction and analysis of wide-band RCS data is discussed. This methodology encompasses normal image processing, clutter removal, and noise filtering. Examples of the usefulness of the approach are presented.
Analysis and measurement activities to quantify compact range feed/subreflector time domain response are described in this paper. Reflection properties of various components are quantified and their interaction studied. Results indicate that although the feed/subreflector interaction is a factor, reverberation is dominated by instrumentation interaction particularly in the case of small compact ranges.
A microwave holographic analysis system is shown to have successfully resolved the surface deformations on an 8' symmetric Cassegrain reflector antenna known to have significant surface deformation problems. The technique is based on the Fourier transform relationship between the aperture field of an antenna and its radiated far-zone field. A signal processing technique dubbed "pattern simulation and subtraction" is discussed that increases the resolution in the transformed aperture domain by removing unwanted signals from the aperture distribution. Measurements taken on the Cassegrain reflector at 11 GHz in the OSU-ESL Compact Range provided excellent amplitude and phase stable data to be processed by the holographic analysis system. Surface deformation profiles generated by this system were then compared to an optical measurement of the main reflector surface. Excellent agreement was obtained with a worst case deviation in the adjusted profiles being 0.05 ?.
This paper describes the results of electrical boresight measurement comparisons between one far-field and two near-field ranges. Details are given about the near-field alignment procedures and the near-field error analysis. Details of the far-field measurements and its associated errors are not described here, since the near-field technique is of primary interest. The coordinate systems of the antenna under test and the measurement ranges were carefully defined, and extreme care was taken in the angular alignment of each. The electrical boresight direction of the main beam was determined at a number of frequencies for two antenna ports with orthogonal polarizations. Results demonstrated a maximum uncertainty between the different ranges of 0.018 deg. An analytical error analysis that predicted a similar level of uncertainty was also performed. This error analysis can serve as the basis for estimating uncertainty in other near-field measurements of antenna boresight.
Signal processing techniques may be used in radar signature analysis to obtain radar target impulse response. In general there is a one to one relationship between specific scattering mechanisms and the time such mechanisms appear in the impulse response. One of the difficulties of this type of analysis is that complex targets often have multiple interactions. Many of these multiple interaction mechanisms can be identified as such by the application of the bispectrum to the radar scattering data. Also, the bispectrum forms a basis for discriminating between targets. Classification of unknown radar targets based on their bispectral response is performed in this study.
Purcell's Image method is useful for measuring the absolute gain of small antennas. The method is simple to use, and utilizes only one antenna with a reflecting plane to provide an image for the receiving antenna. However, the method yields accurate results only if the antenna is matched to its waveguide. This paper describes an image method for absolute gain measurement under mismatched conditions. A gain formula was derived based on the waveguide junction analysis with the antenna terminal treated as a mismatched junction. This method does not require an accurate measurement of the radiated power, and therefore, appears most suitable for measuring the gain of small microstrip patch antennas. Experimentally, this method has been demonstrated to produce accurate results for a single rectangular patch and a two-layer parasitic patch array.
This paper considers hardware and software issues associated with accurate RCS, antenna, and near field antenna measurements. In particular we examine methods for making accurate measurements at high speed using existing network analysis equipment, such as the HP8510B. Techniques which allow for fundamental mixing are examined from the viewpoint of enhanced dynamic range and speed. Harmonic mixing techniques are also discussed and limitations related to IF bandwidth and harmonic locking are presented. The realtime requirements of software systems for these applications are presented and operating system considerations are analyzed. Interface attributes are examined with a view toward use with multi-tasking operating systems and the real-time requirements of high speed measurement systems.