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Calibration
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.
Radar target scatter (RATSCAT) division low frequency range characterization
M. Husar (Air Force Development Test Center),F. Sokolowski (Johnson Controls World Services, Inc.), November 1996
The RATSCAT Radar Cross Section (RCS) measurement facility at Holloman AFB, NM is working to satisfy DoD and program office desires for certifies RCS data. The first step is to characterize the Low Frequency portion of the RATSCAT Mainsite Integrated Radar Measurement System (IRMS). This step is critical to identifying error budgets, background levels, and calibration procedures to support various test programs with certified data. This paper addresses characterization results in the 150 – 250 MHz frequency range. System noise, clutter, background and generic target measurements are presented and discussed. The use of background subtraction on an outdoor range is reviewed and results are presented. Computer predictions of generic targets are used to help determine measurement accuracy.
Evaluation of a CPTR using an RCS flat plate method
M.A.J. van de Griendt (Eindhoven University of Technology),V.J. Vokurka (Eindhoven University of Technology) J. Reddy (European Space Agency) J. Lemanczyk (European Space Agency), November 1996
Compact Payload Test Ranges (CPTR) for test zones of 5 meters or larger can be used for both payload and advanced antenna testing. In both cases accurate calibration, including amplitude and phase characteristics across the test zone, is required. Accurate data analysis is needed in order to establish corresponding error budgets. In addition, boresight determination will be required in both measurement types for most applications. Since it may be difficult or even impossible to scan the test zone field using a (planar) scanner, application of a large reference target (a rectangular or circular flat plate) can be seen as in interesting alternative. RCS measurements are then performed and test-zone field characteristics are determined in both amplitude and phase. Time- and spectral domain techniques can provide valuable information as to the location of possible disturbances. The evaluations is complemented with the measurement of a VAlidation STandard (VAST) antenna in combinations with an advanced APC technique. These techniques have been demonstrated at the CPTR at ESTEC, Noordwijk, the Netherlands. Results and practical considerations are presented in this paper.
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.
Polarimetric calibration of nonreciprocal radar systems
L.A. Muth (National Institute of Standards and Technology),R.C. Wittmann (National Institute of Standards and Technology), W. Parnell (Air Force Development Test Center), November 1996
The calibration of nonreciprocal radars has been studied extensively. A brief review of known calibration techniques points to the desirability of a simplified calibration procedure. Fourier analysis of scattering data from a rotating dihedral allows rejection of noise and background contributions. Here we derive a simple set of nonlinear equations in terms of the Fourier coefficients of the data that can be solved analytically without approximations or simplifying assumptions. We find that independent scattering data from an additional target such as a sphere is needed to accomplish this. We also derive mathematical conditions that allow us to check calibration data integrity and the correctness of the mathematical model of the scattering matrix of the target.
Time and direction of arrival estimation of stray signals in a RCS/antenna range
I.J. Gupta (The Ohio State University ElectroScience Laboratory),E. Walton (The Ohio State University ElectroScience Laboratory), W.D. Burnside (The Ohio State University ElectroScience Laboratory), November 1996
A method to generate time and direction of arrival (TADOA) spectra of the quiet zone fields of a RCS/ antenna range is presented. The TADOA spectra is useful for locating the stray signal sources in the RCS/ antenna range. To generate the TADOA spectra, quiet zone fields along a linear scan over the desired frequency band are probed. The probed data are calibrated to remove the magnitude and non-linear phase variation versus frequency. A calibration technique is also proposed in the paper. The TADOA spectra for simulated probed data as well as experimental probed data are shown.
RFI measurement system for field sites, An
R.B. Dybdal,G.M. Shaw, T.T. Mori, November 1995
A portable system for measuring the RF environment at remote sites is described. A frequency range between 500 MHz and 18 GHz is covered by this system. The design, calibration and use of this system are discussed.
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.
Architecture and algorithms for real-time ISAR imaging of dynamic targets
J. Trischman, November 1995
The Naval Command, Control and Ocean Surveillance Center RDT&E Division (NRaD) has been using a 500 MHz Linear Frequency Modulated (LFM) radar to collect measurements of flying aircraft. These data have been used to generate high resolution Inverse Synthetic Aperture Radar (ISAR) images of the targets [l]. Digital Signal Processing (DSP) hardware had been added to the radar and algorithms have been implemented to perform ISAR processing on the data in real time. A VME bus architecture has been developed to provide a scaleable, flexible platform to test and develop real-time processing software. Algorithms have been developed from a system model, and processing software has been implemented to perform pulse compression, motion compensation, polar reformatting, image formation, and target motion estimation.
Polarimetric calibration of reciprocal-antenna radars
L.A. Muth,R. Lewis, R.C. Wittmann, November 1995
We discuss how RCS target depolariza­ tion enhances cross-polarization contamination, and we present a graphical study of measurement error due to depolarization by an inclined dihedral reflector. Error correction requires complete polarimetric RCS measure­ ments. We present a simple polarimetric calibration scheme that is applicable to reciprocal antenna radars. This method uses a dihedral calibration target mounted on a rotator. Because the calibration standard can be ro­ tated, there is no need to mount and align multiple sepa­ rate standards, and clutter and noise may be rejected by averaging over rotation angle.
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.
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.
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.
INTA's free space NRL arch system and calibration for absorber material characterization
I. Montiel, November 1995
In order to measure the performance of microwave absorbing materials a broadband free- space measurement system constructed in INTA. The is a kind of N RL Arch that gives us the possibility of measurements in d ifferent configurations. It comprises a set of dielect ric loaded rectangular waveguide antennas, coaxial vector analyzer, sample support and a computer. A TRL calibration technique in the plate near field is developed taking advantage of the calibration functions implemented in the network analyzer and the time domain gating. We introduce the use of typical RCS calibration standards as the calibration reflect standards. It gives us the possibility of performing the near field free space calibration in the same way that it is usually done in waveguide, but for directions di fferent to the normal. This calibration allows us to check the edge diffraction behaviour of the samples in the measurement and is thought to be adecuated for thin materials.
Radar cross section calibration measurements using helicopter suspended spheres
M.J. Prickett, November 1994
The Naval Command, Control and Ocean Surveillance Center, Research, Development, Test and Evaluation Division (NRaD) is tasked by the Navy to collect and evaluate full-scale radar cross section (RCS) measurements on ships and aircraft. The Radar Branch at NRaD, operates a radar range west of Pt Loma, San Diego, CA. This radar range has been used to collect X-band and Ku-band calibrated data on Naval ships for the past seven years. The NRaD radar calibration helicopter procedures are the focus of this paper. Using helicopters to suspend and measure "isolated" spheres in space as the primary reference is a major calibration element. A 1700-ft Kevlar line is used to suspend the sphere from the helicopter. This length of line is sufficient to isolate the helicopter from the sphere; thus, the helicopter is not in the significant antenna sidelobes.
High resolution one dimensional radome characterization
A. van der Merwe,C.W.I. Pistorius, D.J. Janse van Rensburg, November 1994
In this paper radome evaluation based on high resolution imaging techniques is described. It allows anomalies on a large radome to be detected very accurately. It required scanning of the radome through only a small angular section using an inverse synthetic aperture radar approach. The one dimensional image formed from field data provides a linear distribution of scattering source locations. The calibration necessary to compensate for the translation and rotation of the antenna is discussed. The technique is demonstrated through measurements performed on a large fibre glass radome.
Three antenna gain methods on a near field range
W.G. Scott,G. Masters, November 1994
The Three-Antenna gain method is commonly used on far-field ranges to determine an antenna's absolute gain. This is especially true when no other calibrated antenna is available. This method has been used for years by calibration laboratories such as NIST to calibrate probes and gain standards for far and near-field ranges. In some cases, the calibration is too costly or does not meet the schedule requirements of the near-field test range. An alternative is to calibrate the probe or gain standards directly on the near-field range. In this paper we present the results of a study done to show the accuracy of the Three-Antenna gain method when used on a near-field range. An extensive error analysis is presented validating the utility of this method.
Laboratory accreditation issues in antenna measurement and application - The Australian experience
J.C. Mitchell,J.D. Hunter, November 1994
This paper describes the of the National Association of Authorities (NATA) which is for the accreditation of calibration and aboratories in Australia. The development of accreditation criteria in the area of antenna measurements is with the roles of NATA's Electrical Registration Advisory Committee and the National Measurement Laboratory being defined. The evolving EMC regulatory environment which has driven the demand for calibration and testing facilities is outlined. Specific issues addressed include the acceptability of calibrations in terms of traceability from either within Australia or overseas, the validation of test sites, the determination of uncertainties of measurement and the relationship of uncertainties to test limits and specifications. Some of the specific problems encountered at laboratory assessments are highlighted. Finally, NATA's international linkages, which have been established via mutual recognition agreements, are discussed together with their significance for accreditation and the acceptability of measurement results.
Implementation of a national voluntary laboratory accreditation program (NVLAP) for calibration laboratories
J.L. Cigler, November 1994
This paper describes the implementation of the National Voluntary Laboratory Accreditation Program (NVLAP) for calibration laboratories at the National Institute of Standards and Technology (NIST) NVLAP Office in Gaithersburg Maryland. It chronicles the efforts of the National Conference of Standards Laboratories (NCSL) Total Quality Management (TQM) Committee for Calibration Systems Requirements in demonstrating the need for the program, and NIST's efforts in reaching the decision to implement the program in response to a petition from the NCSL. The development of Calibration Laboratories Program Handbook (NIST Handbook 150), and a companion Calibration Laboratories Technical Guide (NIST Handbook 150-2) for the eight fields of calibration covered by the program are discussed. The recruitment and training of Technical Experts (TEs) who are used in the assessment of laboratory competence are outlined. The issue of proficiency testing as it relates to determination of laboratory competence, and the importance of the program as it relates to international markets via recognition by international accreditation community, are discussed.
Experimental RCS analysis of a communications antenna mounted on a large cylinder
E. Walton,H.W. Tseng, November 1994
The radar scattering from a small communications antenna mounted on a large cylinder was measured at the Ohio State University ElectroScience Laboratory compact range. This paper will describe the experimental measurement techniques and the details of the analysis of the experimental. The small (5 cm) blade/slot/cavity antenna was mounted on a 1.82 meter long cylinder of 0.61 meter diameter. The cylinder was treated with RAM on the ends to reduce the direct and interactive end scattering effects, and was mounted in the OSU compact RCS measurement range. Measurements over the 2 to 18 GHz band both with and without the antenna were made and the results subtracted during the calibration effects to further remove the end effects. We will demonstrate these techniques and evaluate their effectiveness. ISAR imaging of both the antenna and the scattering term associated with the load on the end of the antenna transmission line will be shown. This will demonstrate that the transmission line and loan can be separately evaluated using such techniques. A time frequency distribution (TFD) analysis technique will also be demonstrated as a means of extracting various antenna resonance terms from the data. A description of the theoretical computation of the scattering will also be given and the special aspects of this problem outlined. The theoretical RCS data will be compared to the experimental measurements of the RCS.


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