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The UCLA hi-polar planar near-field scanner has a novel implemen tation which results in a polar sampling grid. The scanner was used to perform measuremen t comparisons using three sampl in g arrangements: polar, thinned-polar, and linear-spiral sampling. The data acquired using each was processed to the far-field for both simulated and measured near-field data. Excellent agreement was observed.
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.
The development* of a real time electronic system to accurately measure the pattern of high gain, ultralow sidelobe level antennas in the presence of multipath scatterers is described. Antenna test ranges contain objects that scatter the signal from the transmitting antenna into the main beam of a receiving antenna under test (AUT), thereby creating a multipath channel. Large measurement errors of low sidelobes can result. The design and computer simulation of an Antimultipath System (AMPS) is complete. Fabrication of a feasibility demonstration model AMPS to operate with rotated AUTs to suppress indirect (scattered) components and permit accurate pattern measurements is almost done. Results to date show the likelihood of measuring sidelobe levels 60 dB below the main beam. * This project is sponsored in part by the Air Force Material Command under Rome Laboratory Contract Nos. F30602-92-C-0009, Fl9628-92-C-0130 and F 19628-93-C-02 l4.
Optical surveying techniques with theodolites have been utilized for many years for static measurements of reflector antennas. This paper reports on updated optical surveying systems used to measure the accuracies and structural deformations of reflector antennas. Deformations of large Cassegrain tracking antennas during elevation rotation and a fixed, billboard-style compact range reflector over time are discussed. A simple surveying method is shown for the integrated measurement of Cassegrain antennas (both primary and secondary reflectors) from near the primary vertex. Other topics covered include accurate prediction of interpolated gravity deformations of rotating reflectors based on a small measurement sample, and a method for taking differences between measurements. The use of EDM (Electronic Distance Measurement) theodolites as well as angle-only devices is described, along with software which manages both the measurement and data-reduction systems.
In this paper, we present a new technique for measuring the input impedance of balanced antenna systems. The process uses standard two-port scattering parameters for balanced antennas, feeding each of the balanced input ports as the port of a two-port. The scattering-parameters will be related to the designed input impedance which may be obtained by post-processing the data. In addition, the scattering-parameters may be used to check for the assumed balance of the system. Both experimental and simulated results will be presented to validate the technique.
The identification of targets with radar is frequently based on a priori knowledge of the RCS characteristics of the target as a function of frequency and viewing angle. Due to the complex ity of most targets, it is difficult to predict their RCS signature accurately. Furthermore, complex and large reference libraries will be required for identification purposes. In most cases, a complete knowledge of the RCS is not required for successful identification. Instead, a target representation composed of the contributions of the main scattering centers of the target can be sufficient. This means that a corresponding target representation based on an estimation with Geometrical Optics (GO) or Physi cal Optics (PO) techniques will contain enough information for target identification purposes. In this paper, a new technique is described which is based on a reconstruction of the scattering centers. These are found at locations where the normal to the surface points in the direction of the angle of incidence. The RCS at these positions depends mainly on the local radii of curvature of the surface. Further more, PO and GO approximations are known as high-frequency techniques, assuming structures that are large compared to the wavelength. At low frequencies, which may be of interest for certain class of identification procedures, and for small physical radii of curvature, the RCS prediction is often difficult to determine numerically. Results from measurements show that this approach is also valid at lower frequencies for the classes of targets as mentioned, even for structures that are significantly smaller than the wavelength. As a consequence, it is expected that even complex targets can be represented adequately by the simplified model.
The identification of targets with radar is frequently based on a priori knowledge of the RCS characteristics of the target as a function of frequency and viewing angle. Due to the complex ity of most targets, it is difficult to predict their RCS signature accurately. Furthermore, complex and large reference libraries will be required for identification purposes. In most cases, a complete knowledge of the RCS is not required for successful identification. Instead, a target representation composed of the contributions of the main scattering centers of the target can be sufficient. This means that a corresponding target representation based on an estimation with Geometrical Optics (GO) or Physi cal Optics (PO) techniques will contain enough information for target identification purposes. In this paper, a new technique is described which is based on a reconstruction of the scattering centers. These are found at locations where the normal to the surface points in the direction of the angle of incidence. The RCS at these positions depends mainly on the local radii of curvature of the surface. Further more, PO and GO approximations are known as high-frequency techniques, assuming structures that are large compared to the wavelength. At low frequencies, which may be of interest for certain class of identification procedures, and for small physical radii of curvature, the RCS prediction is often difficult to determine numerically. Results from measurements show that this approach is also valid at lower frequencies for the classes of targets as mentioned, even for structures that are significantly smaller than the wavelength. As a consequence, it is expected that even complex targets can be represented adequately by the simplified model.
The amplitude of a point target observed in an ISAR image is equal to their free space RCS when effective sidelobe windowing is used. Likewise, its location in the image is identical to its actual location. The interpretation of observed amplitude and dimension of area targets is not as easy. The ISAR image of a rectangular flat plate formed by rotating it around its longer axis is significantly different from an ISAR image of the same plate rotated about its shorter axis. Both the amplitude and the size of the plate's image are different. In this paper, the theory of physical optics is reviewed in conjunction with the principles of ISAR processing to explain these differences.
Near field inverse synthetic aperture radar 3D is performed utilizing data for arbitrary, but known, positioning of the target. The imaging method was implemented and is described. This straightforward approach has many advantages. It geometrically correct in near field. Field corrections can be independently for each frequency, antenna position and point of interest in the target volume. The main disadvantage is that the processing using the algorithm is very time consuming. However, in many cases it is only necessary to perform the analysis on a few cuts through the object volume.
Compact ranges have found wide application for antenna measurements, RCS measurements, and, most recently, for spacecraft payload measurements. Each of these ap plications requires certain special features of the range optics, positioning systems, electronics, and software. The system design of a compact range measurement sys tem for making all these types of measurements presents a number of challenges. This paper will discuss the system aspects of the design of a multi-purpose compact range facility. Items of inter est include the RF electronics design, the positioning sys tem design, the optimization of the reflector and feeds and the specialized software design.
The ESAT-TELEMIC division at Katholieke Universiteit Leuven (KUL) has three antenna ranges: an indoor Far-Field range, an indoor planar Near-Field range and an outdoor Far-Field range. The positioning equipment is of a variety of manufacturers. The division launched an effort to modernize the range complex and add automatic measurement capabilities, while still retaining control of all three ranges from one control console and using one positioner controller, one angle readout and a single receiver to save costs. The system upgrade included some electrical refurbish ment of the positioning equipment and the replacement of all the old control and data recording equipment with Orbit Positioner Controller/Programmer, Power Control Unit and combined Near-Field and Far-Field software. Control of all three sites is achieved using a special Orbit Junction Box. With the new configuration all three ranges can be operated in fully automatic mode, one range at a time. The software package controls both Near-Field and Far Field measurements using compatible data formats and human interfaces.
ERIM is currently investigating several near-field to far-field transfonnations (NFFFfs) for predicting the far-field RCS of targets from monostatic near-field measurements. Each of the techniques uses approximate tions and/or supporting information to overcome the need for the bistatic near-field data which is required to rigorously transfonn a target's scattered field from the near zone to the far zone. Our focus has been on spheri cal near-field scanning, since this type of collection geometry is most compatible with existing RCS ranges. One particular NFFFT is based on the reflectivity approximation commonly used in ISAR imaging to model the target scattering. This image-based NFFFT is the most computationally efficient technique under con sideration, because, despite its theoretical underpinnings, it does not explicitly require image fonnation as part of its implementation. This paper presents an efficient discrete implementation of the image-based NFFFT, along with numerically-simulated examples of its perfonnance. The advantages and limitations of the technique will be discussed. A simplified version which applies to high aspect ratio (length-to-height) targets and requires only a single great circle (waterline) data in the near field is also summarized.
The RCS of extended objects measured in the near field is subject to errors induced by the spherical nature of the incident and scattered wavefields. A number of techniques have been applied to estimate far-field responses from results of monostatic near-field measurements. While the results indicate successful transformations for linear scatterers, the lack of a sound theoretical basis brings into question the appli cability to general objects. The paper explores the theoretical basis of the far-field transformation of RCS data and the consequence of the limited data obtained from monostatic measurements. The limitations of approaches reported to date [1-4] are explored from conceptual and physical con siderations with the goal of establishing reasonable expectations for practical methods. Examples using simulated and measured near-field data are presented to illustrate successes and failures of the algorithms in transforming results to far-field RCS.
Near field inverse synthetic aperture radar 3D is performed utilizing data for arbitrary, but known, positioning of the target. The imaging method was implemented and is described. This straightforward approach has many advantages. It geometrically correct in near field. Field corrections can be independently for each frequency, antenna position and point of interest in the target volume. The main disadvantage is that the processing using the algorithm is very time consuming. However, in many cases it is only necessary to perform the analysis on a few cuts through the object volume.
Planar near-field measurements are the usual choice when testing phased array antennas. NSI recently delivered a large state-of-the-art near field measurement system for testing a multi beam, solid state phased-array antenna. The critical sidelobe and beam pointing accuracy specifications for the antenna required that special attention be paid to near-field system design. The RF path to the moving probe was implemented using a multiple rotary joint system to minimize phase errors. Additional techniques used to minimize system errors were an optical probe position correction system and a Motion Tracking Interferometer (MTI) for thermal drift correction.
The ESAT-TELEMIC division at Katholieke Universiteit Leuven (KUL) has three antenna ranges: an indoor Far-Field range, an indoor planar Near-Field range and an outdoor Far-Field range. The positioning equipment is of a variety of manufacturers. The division launched an effort to modernize the range complex and add automatic measurement capabilities, while still retaining control of all three ranges from one control console and using one positioner controller, one angle readout and a single receiver to save costs. The system upgrade included some electrical refurbish ment of the positioning equipment and the replacement of all the old control and data recording equipment with Orbit Positioner Controller/Programmer, Power Control Unit and combined Near-Field and Far-Field software. Control of all three sites is achieved using a special Orbit Junction Box. With the new configuration all three ranges can be operated in fully automatic mode, one range at a time. The software package controls both Near-Field and Far Field measurements using compatible data formats and human interfaces.
From prototyping to consulting, the McClellan Near-Field Team has worked on the installation of four near-field( NF) facilities. Currently this group operates and maintains two planar near-field test systems. This paper will discuss considerations McClellan used in building or specifying near-field facilities of increasing complexity. NF software will be examined for functional growth and graphical improvement. RF system improvements, over four systems, will be examined for frequency range, dynamic range and cable loss. The evolution of the antenna under-test (AUT) stand will be examined for weight, and precision factors. Changes in alignment procedures will also be discussed. Additionally, the NF construction issues of anechoic material and safety features will be examined for improvement. Finally, changes will be discussed in the NF operation and maintenance procedures over the four McClellan ranges.
ERIM is currently investigating several near-field to far-field transfonnations (NFFFfs) for predicting the far-field RCS of targets from monostatic near-field measurements. Each of the techniques uses approximate tions and/or supporting information to overcome the need for the bistatic near-field data which is required to rigorously transfonn a target's scattered field from the near zone to the far zone. Our focus has been on spheri cal near-field scanning, since this type of collection geometry is most compatible with existing RCS ranges. One particular NFFFT is based on the reflectivity approximation commonly used in ISAR imaging to model the target scattering. This image-based NFFFT is the most computationally efficient technique under con sideration, because, despite its theoretical underpinnings, it does not explicitly require image fonnation as part of its implementation. This paper presents an efficient discrete implementation of the image-based NFFFT, along with numerically-simulated examples of its perfonnance. The advantages and limitations of the technique will be discussed. A simplified version which applies to high aspect ratio (length-to-height) targets and requires only a single great circle (waterline) data in the near field is also summarized.
ERIM is currently investigating several near-field to far-field transfonnations (NFFFfs) for predicting the far-field RCS of targets from monostatic near-field measurements. Each of the techniques uses approximate tions and/or supporting information to overcome the need for the bistatic near-field data which is required to rigorously transfonn a target's scattered field from the near zone to the far zone. Our focus has been on spheri cal near-field scanning, since this type of collection geometry is most compatible with existing RCS ranges. One particular NFFFT is based on the reflectivity approximation commonly used in ISAR imaging to model the target scattering. This image-based NFFFT is the most computationally efficient technique under con sideration, because, despite its theoretical underpinnings, it does not explicitly require image fonnation as part of its implementation. This paper presents an efficient discrete implementation of the image-based NFFFT, along with numerically-simulated examples of its perfonnance. The advantages and limitations of the technique will be discussed. A simplified version which applies to high aspect ratio (length-to-height) targets and requires only a single great circle (waterline) data in the near field is also summarized.
The RCS of extended objects measured in the near field is subject to errors induced by the spherical nature of the incident and scattered wavefields. A number of techniques have been applied to estimate far-field responses from results of monostatic near-field measurements. While the results indicate successful transformations for linear scatterers, the lack of a sound theoretical basis brings into question the appli cability to general objects. The paper explores the theoretical basis of the far-field transformation of RCS data and the consequence of the limited data obtained from monostatic measurements. The limitations of approaches reported to date [1-4] are explored from conceptual and physical con siderations with the goal of establishing reasonable expectations for practical methods. Examples using simulated and measured near-field data are presented to illustrate successes and failures of the algorithms in transforming results to far-field RCS.