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Accuracy

The Use of pattern comparison methods for satellite antenna testing
J. van Norel (Dornier Satellitensysteme GmbH),J. Habersack (Dornier Satellitensysteme GmbH), M. Boumans (Dornier Satellitensysteme GmbH), November 1996

Nowadays, the standard facility for accurate satellite antenna testing is the Compensated Compact Range (CCR). In order to increase measurement accuracy several techniques can be applied, which are based on antenna pattern comparison. The theory of these techniques together with experimental results have been described in several papers in the past [1][2][3]. This paper presents how pattern comparison techniques are applied for space programs and is another step to official qualification of the Advanced Antenna Pattern Comparison (AAPC) method at Dornier Satellitensysteme (DSS).

Accurate determination of main beam position and beamwidth from near field measurements
M.H. Paquay (TNO Physics and Electronics Laboratory), November 1996

For narrow beam antennas or track antennas some parameters like main beam or null position and 3 dB beamwidth need to be determined with an accuracy of less than a mill or mrad. With Near Field measurements, the Far Field is normally calculated by FFT-processing. This does, however, not provide the required accuracy. Nevertheless, the measured Near Field data contains information about any Far Field point. An iterative approach is presented to determine the Far Field antenna characteristics with high accuracy.

Cross polarization measurement accuracy improvement on a single reflector compact range
D. Cook (Scientific-Atlanta, Inc.),J.H. Cook (Scientific-Atlanta, Inc.), R. Kaffezakis (Scientific-Atlanta, Inc.), November 1996

Scientific-Atlanta has developed a new algorithm for obtaining high accuracy cross-polarization measurements from prime focus, single reflector, compact ranges. The algorithm reduced cross-polarization extraneous signals to levels that rival or exceed much more expensive dual reflector systems, but with the associated cost and simplicity of a single reflector system. This paper provides an overview of the new algorithm. It explains the limitations on conventional polarization measurements in single reflector systems and the methods for overcoming these limitations without error correction for some antennas. A method for determining if error correction is needed for a particular antenna is reviewed and the fundamentals of the error correction algorithm are explained. Preliminary test results are provided.

A Cost effective, versatile antenna and radome instrumentation test system
J.F. Aubin (Flam & Russell, Inc.), November 1996

A cost-effective, versatile instrumentation system for measuring antennas and radomes is described. The system features the use of high load capacity, high accuracy stepper motor based positioners as the primary system axes. The system is capable of being easily reconfigured to perform tests on antenna/radome systems with antennas fixed relative to the radome, or with the antenna and radome capable of movement relative to one another. Measurements may be performed at RF, IF or baseband, depending on the portions of the seeker or monopulse assembly to be included in the test. The system also contains analysis capabilities that simulated mode forming and beam forming functions to isolate antenna effects.

A Planar near-field system with high precision 22M x 8M vertical scanner
M. Pinkasy (Orbit Advanced Technologies),E. Katz (Orbit Advanced Technologies Ltd.) J. Torenberg (Orbit Advanced Technologies Ltd.) S. Dreisin (Orbit Advanced Technologies Ltd.) A. Geva (Orbit Advanced Technologies Ltd.) M. Bates (Orbit Advanced Technologies Inc.), November 1996

A new 1-50 GHz Near-Field measurement system is now in operation at Matra Marconi Space, Portsmouth, UK. The system has the largest vertical planar scanner installed so far. The planar scanner is constructed of steel and has four moving axes: 22 meter horizontal X axis, 8 meter vertical Y axis, 25 cm Z axis for probe alignment and a 540o Roll axis for polarization. Precision bearings are used to ensure straightness over the full length of the X-Y travel. The vertical Y axis is exceptionally fast, 500 mm/sec, to minimize acquisition time. The scanner has extremely high positioning accuracy and planarity - ±0.2 mm over the entire 22m x 8m range – allowing uncorrected operation (without laser) up to 26.5 GHz. To achieve higher accuracy and a higher frequency range an advanced 3-axis (X, Y, Z) laser correction system automatically creates correction tables for use by the transformation routines. The scanner’s exceptional repeatability allows the use of correction tables created off-line, without need for an on-line laser correction system, considerably reducing measurement time. To create these correction tables, the scanner is fitted with laser interferometers for X and Y axes and with a spinning-diode laser to calibrate for planarity. Additional features include a shielded constant-radius cable carrier, giving minimal phase errors due to cable flexing.

Pattern measurement of ultralow sidelobe level antennas
A.E. Zeger,B.S. Abrams, November 1995

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.

New approach for modeling of radar signatures
M.R. van der Goot,V.J. Vokurka, November 1995

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.

3-D processing and imaging of near field ISAR data in an arbitrary measurement geometry
C.U.S. Larsson,O. Luden, R. Erickson, November 1995

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.

Calibration of bistatic RCS measurements
N.T. Alexander,M.T. Tuley, N.C. Currie, November 1995

Calibration of monostatic radar cross section (RCS) measurements is a well-defined process that has been optimized through many years of theoretical investigation and experimental trial and error. On the other hand, calibration of bistatic RCS measurements is potentially a very difficult problem; the range of bistatic angles over which calibration must be achieved is essentially unlimited and devising a calibration target that will provide a calculable scattering solution over the required range of bistatic angles is difficult, particularly for cross-polarized measurements. GTRI has developed a solution for amplitude calibration of both co-polarized and cross-polarized bistatic RCS, as well as a bistatic phase-calibration procedure for coherent systems.

ISAR RCS editing via modern spectral estimation methods
S.R. DeGraaf,E. LeBaron, G. Fliss, K. Quinlan, S. Li-Fliss, November 1995

ERIM is investigating the use of modem spectral esti­ mation techniques for extracting (editing) desired or undesired contributions to RCS and ISAR measurements in two ways. The first approach involves using parametric spectral estimators to perform frequency sweep range compression and signal history editing, while the second involves using the associated stabilized linear prediction filters to extrapolate sweep data and perform "enhanced resolution" Fourier image editing. This paper summarizes our editing algorithms and illustrates RCS editing results using measurements of a conesphere target contaminated by a metal rod and foam support. The reconstructed "clean" conesphere measurements are compared quantitatively against numerically simulated ground truth. Editing was performed using three bandwidths at two center fre­ quencies to provide insight into the impacts of nominal resolution and scatterer amplitude variation with fre­ quency on editing efficacy, and to assess the degree to which superresolution algorithms can offset reduced nominal resolution.

Integrating diagnostic imaging radar into development and production programs
R. Harris, November 1995

Radar cross section measurements must be performed in a wide variety of situations throughout development of a new vehicle. In these days of smaller budgets, it is vitally important that the right things get measured, at the right time in the program, with the right accuracy, and that these measurements be integrated into the development process in the right way. After delivery, the measurement system must be confidently usable by the user organization, with a minimum of outside to ensure that the vehicle is maintained. Many of the key programs in this area were begun before modern measurement technology was known to be capable of providing detailed diagnostic measurements. Consequently, specifications did not consider what can be easily measured with today's modern diagnostic radars. This paper addresses how mcxlern diagnostic radar cross section measurements can be exploite4:l to make the specification, development, pnxluction, and testing phases much more efficient than they have been in the past.

Video photogrammetry in antenna manufacturing
D. Cohen,G. Johanning, November 1995

Photogrammetry, as its name implies, is the science of obtaining precise coordinate measurements from photographs. Until recently, photo-grammetry used film photographs taken with specially designed, high-accuracy film cameras. With the development of h igh­ resolution solid-state imaging sensors, a new era in photogrammetry has arrived. Video­ grammetry, as it is often called, provides far faster results and greater capability than film­ based photogrammetry, and therefore eliminates the major impediments to more widespread use of photogrammetry in the antenna manufacturing industry. Video-grammetry is a powerful enabling technology that not only performs many current measurement tasks faster and more efficiently th an existing technologies, but also, now makes feasible many types of measurements, that pre­ viously were not practical or possible. The capability for quick, accurate, reliable, in place measurements of static or moving objects in vibrating or unstable environments is a powerful combination of features all in one package. There are many applications for this emerging new technology in the antenna manufacturing industry. This paper will describe some of the successfu l implementation of video-grammetry into the MSA T program at Hughes Space and Communications Company located in Los Angeles, California.

Accurate boresighting and gain determination techniques
M.A.J. van de Griendt,S.C. van Someren Greve, V.J. Vokurka, November 1995

Boresight and gain determination play an important role in antenna measurements. Traditionally, on outdoor ranges, optical methods are used to determine the boresight. Accuracy requirements better than 0.001 degrees are difficult if not impossible to obtain on outdoor ranges using these method since the effect of incident electromagnetic fields are not taken into account. On indoor ranges no technique is available at present that achieves the desired accuracy demands. In this paper, an improved method for boresighting will be presented. It will be shown that using this technique, desired accuracy demands on both outdoor and indoor can be obtained. Furthermore, the method can also be combined with accurate gain calibration. Advantages and disadvantages of this technique will be discussed.

State-of-the-art near-field measurement system
K. Haner,G. Masters, November 1995

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.

Test-zone field quality in planar near-field measurements
E.B. Joy,A.H. Tonning, C. Rose, EE6254 Students., November 1995

This paper reports on the results of computer simulations of planar near-field scanning and its ability to achieve an high accuracy test-zone field over a wide range of pattern angles. An quality test-zone field was defined for this study to have less than 0.2 dB peak-to-peak amplitude variation and less than 1.5 peak-to­peak phase variation. This investigation sought the minimum scan length, for a given critical angle, ec and separation, S. The minimum scan length determined from this investigation is given by: L = D + 2S(tan(0c)) + 20/cos(0c). This scan length is approximately 60),, larger, for a critical angle of 70 degrees, than previously accepted. It is suggested that the maximum practical value of Sc is between 60 and 70 degrees. The use of raised cosine amplitude and/or quadratic phase windows to the edges of the measurement plane is shown to provide test-zone field quality improvement and/or allow scan lengths approximately 10),, smaller.

Enhancement of efficiency and accuracy of near-field measurement
G. Seguin,T. Pavlasek, November 1995

This paper examines the possibility of increasing the speed of Near-Field measurement of an Antenna, by reducing the number of measurement points and by determining the degree of truncation permissible while maintaining a prescribed degree of precision of the reconstructed far-field. The Near-Field of a planar radiating array is analysed in depth. A formulation and a procedure to correct the spectral domain of the field are established. It is shown that correction in the spectral domain can improve the accuracy of the Far-Field while using the same amount of Near-Field data. The technique has a good potential to be applied to Near­ Field data of large radiating Antennas leading to new information about the accuracy and speed of measurement achievable.

Portable 4.5m x 2.0m near-field scanner, A
D.S. Fooshe, November 1995

Portable scanners used for near-field antenna measurements are usually incapable of providing a large scan area with a high degree of probe position accuracy. This paper discusses a 4.5m x 2.0m portable scanner developed by NSI with a probe position accuracy on the order of 2 mils (0.050 mm) rms. An NSI patented optical measurement system measures the X, Y, and Z position, and provides real­ time position correction capability. This lightweight, portable scanner combined with optical correction provides enhanced accuracy while reducing overall antenna measurement system costs and improving test chamber flexibility.

General order N analytic correction of probe-position errors in planar near-field measurements
L.A. Muth, November 1995

An analytic technique recently developed at NIST [1] [2] to correct for probe position errors in planar near-field measurements has been implemented to arbitrary accuracy. The nth-order correction scheme is composed of an mth-order ordered expansion and an n - m higher-order approximation, where both n and m are arbitrary. The technique successfully removes very large probe position errors in the near-field, so the residual near-field probe position errors are substantially below levels that can be measured on a near-field range. Only the error-contaminated near-field measurements and an accurate probe position error function are needed for implementation of the correction technique. The method also requires the ability to obtain derivatives of the error-contaminated near field defined on an error-free regular grid with respect to the coordi­ nates. In planar geometry the derivatives are obtained using FFTs [1], giving an approximate operation count of (3 • 2=- 1 - 1 + (n - m)) N log N, where N is the number of data points. Efficient computer codes have been developed to demonstrate the technique. The results of simulations are more accurate than those obtained us­ ing the well-known k correction [3), which can correct for position errors in some direction in k space, but further contaminates the sidelobe levels.

Hologram accuracy determination
G. Masters, November 1995

Hologram measurements are becoming more and more popular as a reliable method for identifying bad elements and the tuning of active phased array antennas. Relying on holographic data to adjust phase shifters and attenuators in these antennas can give undesired results if the accuracy of the data is poor. Often measurements can be improved if the error sources can be isolated and quantified. This paper presents an approach to producing a hologram accuracy budget based on the NIST 18-term error budget created for near-field measurements. A set of hologram accuracy terms is identified and data is presented showing the typical hologram accuracy that can be expected from a near-field scanner.

Phase-stationary high performance antenna test body, A
H. Shamansky,A. Dominek, J. Breaks, J. Hughes, S. Schneider, November 1995

Modern low profile and conformal antennas are fre­ quently evaluated in the presence of a conducting surface. Antenna designers usually predict the an­ tenna scattering and radiation performance over an infinitely conducting ground plane. To bridge the gap between a (possibly curved) antenna host surface and the designer's infinite ground plane model, an antenna testbody is required. This testbody must possess a variety of demanding attributes, such as a very close approximation to an infinite ground plane, low testbody signature, ability to provide positional accuracy (in both azimuth and elevation), physical stability (for repeatability and background subtraction), just to name a few. The most widely regarded testbody has been the "almond" testbody [1, 2], which boasts a very low signature, and excellent fidelity when compared to an infinite ground plane. This paper addresses a new variation from the traditional "almond" testbody, in which a unique positioning design provides a phase-stationary antenna aperture center under rotation of both azimuth and elevation. This testbody will be used for a variety of antenna tests at Wright Laboratory's Radiation and Scattering Compact Antenna Laboratory (RASCAL).







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