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Errors

Radar cross section range characterization
L.A. Muth (National Institute of Standards and Technology),B. Kent (Wright-Patterson Air Force Base), J. Tuttle (Naval Air Warfare Center) R.C. Wittmann (National Institute of Standards and Technology), November 1996

Radar cross section (RCS) range characterization and certification are essential to improve the quality and accuracy of RCS measurements by establishing consistent standards and practices throughout the RCS industry. Comprehensive characterization and certification programs (to be recommended as standards) are being developed at the National Institute of Standards and Technology (NIST) together with the Government Radar Cross Section Measurement Working Group (RCSMWG). We discuss in detail the long term technical program and the well-defined technical criteria intended to ensure RCS measurement integrity. The determination of significant sources of errors, and a quantitative assessment of their impact on measurement uncertainty is emphasized. We briefly describe ongoing technical work and present some results in the areas of system integrity checks, dynamic and static sphere calibrations, noise and clutter reduction in polarimetric calibrations, quiet-zone evaluation and overall uncertainty analysis of RCS measurement systems.

Accuracy of RCS measurements
S. Mishra (Canadian Space Agency),C.W. Trueman (Concordia University), November 1996

Some precautions necessary for accurate RCS measurements using a short model range are discussed. Sources of error in these measurements such as non ideal range geometry, misalignment of the target and inappropriate time domain gating are discussed. A simple technique to estimate possible errors in RCS measurements due to factors such as bistatic angle due to finite separation of source and receive horns and finite length of the measurement range, is presented. The range of RCS values that can be measured within defined error bonds is identified.

A Top-down versus bottom-up RCS range certification approach
W.D. Burnside (The Ohio State University ElectroScience Laboratory),E. Walton (The Ohio State University ElectroScience Laboratory), I.J. Gupta (The Ohio State University ElectroScience Laboratory), J.D. Young (The Ohio State University ElectroScience Laboratory), November 1996

A new approach for certification of RCS ranges is discussed. This new approach is based on evaluating the major expected sources of errors in a RCS range rather than evaluating each and every error source and then defining the error bar for a given RCS measurement. The new approach is, therefore, called a top-down approach. Based on our experience with many indoor RCS ranges, we can say that the main sources of errors in RCS measurements are range related. (stray signals, chamber drift, target/mount interactions etc.) One should, therefore, critically evaluate these errors such that the performance level of the range can be verified. A test approach is defined to characterize the range related errors. Various tests are based on the RCS measurement of specific targets, and thus, can be easily performed using standard RCS measurement procedure. This approach will provide range operators with the needed information to justify the use of their range to measure RCS of a given target. Also, one can spend more effort fixing the error sources which lead to large RCS measurement errors.

Enhanced image editing by peak region segmentation
J. Stach (ERIM),E. LeBaron (ERIM), November 1996

For the past seven years, ERIM has been studying RCS measurement error sources and processing methods by which these errors can be reduced. Image editing is an extension of range-gating where scattering measurements are improved by removing undesired scattering phenomena in the range-crossrange image domain. Conventional image editing methods rely on a user-supplied polygon to segment an image into desired and undesired scattering regions. However, the polygon method suffers from variability due to user and display characteristics, provides little hope for automation, and cannot be easily extended beyond two dimensions. An alterative approach based on peak region segmentation minimizes or eliminates these limitations and adds an element of optimally that can also improve the performance of image editing techniques. In this paper, we will discuss the application of peak region segmentation to the image editing problem and show examples that demonstrate some of the advantages of this approach.

Mismatch errors in insertion-loss measurements using harmonic mixers
J. Guerrieri (National Institute of Standards and Technology),D. Tamura (National Institute of Standards and Technology), K. MacReynolds (National Institute of Standards and Technology), N. Canales (National Institute of Standards and Technology), November 1996

In this paper we discuss proper RF system design for performing insertion-loss measurements using a microwave receiver and harmonic mixers. Specifically we will deal with problems caused by changing reflection coefficients of the devices which feed the mixer. When broadband mixers and coaxial isolators are used problems may be caused by the changing load seen by the local oscillator. This is due to local oscillator leakage through the mixer and isolator. We will elaborate on this problem, noting its impact on the measurement and suggest a procedure to properly minimize its effect.

On reducing primary calibration errors in radar cross section measurements
H. Chizever (Mission Research Corporation),Russell J. Soerens (Mission Research Corporation) Brian M. Kent (Wright Laboratory), November 1996

To accurately measure static or dynamic Radar Cross Section (RCS), one must use precise measurement equipment and test procedures. Recently, several DoD RCS ranges, including the Advanced Compact RCS Measurement Range at Wright-Patterson AFB, established procedures to estimate measurement error. Working cooperatively with the National Institute of Standards and Technology (NIST), Wright Laboratory established a baseline error budget methodology in 1994. As insight was gained from the error budget process, we noted that many common RCS measurement calibration techniques are subject to a wide variety of potential error sources. This paper examines two common so-polarized calibration devices (sphere and squat cylinder), and discussed techniques for evaluating calibration induced errors. A rigorous “double calibration” methodology is offered to track calibration measurement error. These techniques should offer range owners fairly simple methods to monitor the quality of their primary calibration standards at all times.

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.

3-D low frequency radar target imaging
M.J. Gerry,E. Walton, November 1995

The imaging of radar targets is typically accom­ plished by measuring the radar cross section (RCS) of the target as a function of frequency and az­ imuth angle. We measure a third dimension of the RCS by tilting the target and collecting data for conical cuts of the RCS pattern. This third dimension of data provides the ability to estimate the three-dimensional location of scattering centers on the target. Three algorithms are developed in order to process the three-dimensional RCS data.

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.

Interpretation of area target amplitude and dimensions in ISAR images
D. Flynn, November 1995

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 to far-field transformation of RCS measurements
D. Mensa,K. Vaccaro, November 1995

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.

Method to quantify target-support interaction terms, A
J. Matis, November 1995

Target support interaction terms often drive Radar Cross Section Measurement limitations. These limitations are when mask needed information, or render interpretation difficult. Although support improvement is desirable and studied, there is a fundamental problem. Perhaps we can create a support that is 10 dB better than existing supports. The technology producing that improvement will usually be applicable to targets. Result: The same ratios recur. Modern instrumentation Radar possesses many acquisition agility's. Processing power currently available permits handling huge volumes of data. This paper studies evaluation and/or elimination of interaction terms using these agility's. Interactions within the test article are often significant. Controlled of this method would select and retain, or remove the terms.

Triband radome measurement system: installation and testing results, A
V. Jory,G.W. Pearson, J.R. Jones, L.L. Oh, S.J. Manning, T.L. Norin, V. Farr, November 1995

In an earlier paper ("System Engineering for a Radome Test System," John R. Jones, et al, AMTA, October 1994) the system level design of a compact range enhancement for the testing of the Triband Radome was presented. This paper will discuss the installation and testing of the radome measurement system in the compact range. The purpose of the radome measurement system is to determine (within close tolerances) boresight shift, transmission loss, antenna pattern changes and polarization effects caused by the radome. Unique features include novel coordinate transformation and correction by means of a laser autocollimator and data reduction algorithms. Also featured is the tracking subsystem which consists of a specially designed two-axis track pedestal, an autotrack controller, and three five-horn compact range feed arrays operating at X, K, and Q-bands. The performance of the triband radome measurement system in the compact range setting will be presented.

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.

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.

Near-field to far-field transformation of RCS measurements
D. Mensa,K. Vaccaro, November 1995

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.

Numerical methods for measurement error mitigation
J. Stach, November 1995

For the past six years, ERIM has been studying RCS measurement error sources and processing methods by which these errors can be reduced. Typical errors that can be mitigated by processing techniques include near-field effects, multipath sources, and target support interactions. In this paper we briefly discuss image editing and spec­ tral decomposition methods which can be applied to error mitigation when the target sjze and bandwidth are suffi­ cient to resolve scattering centers. More details on these methods will be presented in other papers at this confer­ ence. We then describe in detail the netwcrk model approach which is best suited to applications where the target size is electrically small and the bandwidth is nar­ row. We show that the network model is a logical extension of the other techniques and discuss its application to error mitigation.

Influence of noise and calibration errors on HRR and ISAR
M.R. van der Goot,V.J. Vokurka, November 1995

Several approaches are known for the identification of non­cooperative air-borne targets with radar. Assuming that the tar­ get can be tracked during a certain flight path, observations from different aspect angles will be obtained. High-resolution radar (HRR) systems use these observations to create one-dimensional range profiles. With Inverse Synthetic Aperture Radar (ISAR) the data from all observed aspect angles are combined to obtain two-dimensional images. In recent years, techniques for resolution enhancement have been developed for both techniques. The choice for one of the two approaches should depend on the applicability of the target representation for identification. ISAR is the most suitable for reproduction on a display and identification by human observers. In case of identification by a machine, for example an algorithm on a computer, the choice is not straight­ forward. In this paper an overview of the influence of several errors on the performance of HRR and ISAR will be given. The error sources that will be evaluated are: • uncertainty of the absolute distance of the target; • errors in the mutual alignment of observations; • additive noise. The errors are generated numerically and applied to data from simulations and low-noise measurements. The influence of the bandwidth and angular span on the quality of the target reconstruction will be regarded as well as the performance of some high-resolution techniques. Finally, conclusions are drawn concerning the applicability of ISAR and HRR.

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.

Simulation of errors in near-field facilities
D.J. Janse van Rensburg,G. Seguin, S. Mishra, November 1995

A technique for estimating measurement errors in near­ field facilities is presented. Known mechanical and electrical errors can be accounted for in simulation and such results are presented here. Unknown factors like chamber reflection and instrumentation drift can be estimated via selective measurement and the error induced by such anomalies may be combined with the simulated findings to provide error patterns for a particular test antenna and facility. Results are shown where these patterns are used to calculate measurement error limits. The software presented here also allows the generation of parametric curves which show the impact of a parameter of interest.







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