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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.
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.
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.
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 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.
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.
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.
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.
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.
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.
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.
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.
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.
Lincoln Laboratory has developed a high resolution imaging radar in conjunction with Flam & Russell, Inc., of Horsham, PA. This highly mobile, ground-based system is capable of 2-D and 3-D imaging of targets at very close ranges to a synthetic aperture. The radar is fully-polarimetric, and operates over two frequency bands (0.05-2 GHz and 2-18 GHz). The radar is currently being used for target imaging and for foliage and ground penetration experiments. In this paper, the radar system is described. In addition, data calibration and image formation are explained. Sample imagery, both 2-D and 3-D, are shown.
The radiation characteristics for a dual-frequency, dual-polarized millimeter wave antenna for a radar operating at 33 and 95-GHz were measured at the Ipswich Research Facility. On-pole and cross-pole radiation patterns were measured using the 2600 foot far field range. In this paper we'll discuss the general design of the antenna feed system and the instrumentation ensemble used to perform the far field characterization of this high performance large aperture dielectric lens antenna.
Test sequencing flexibility and high throughput are essential ingredients to a state-of-the-art near-field test range. This paper will discuss methods used by NSI to aid the operator through the near-field measurement process. The paper will describe NSI's expert system and customer applications of a unique test and processing sequencer developed by NSI for optimizing range measurement activities. The sequencer provides powerful control of software functions including multiplexed measurements, data processing and unattended test operations.
The interdependence of accuracy and speed should be considered when analyzing measurement requirements. Tradeoffs can be made to optimize the measurement when accuracy is of primary importance, or where speed is critical. Several different measurement modes of the HP 8530A Microwave Receiver are presented, each with different measurement speed and accuracy tradeoffs. Examples are given that illustrate which acquisition modes would be appropriate to optimize the acquisition speed and accuracy in a variety of applications
A simple change to the HP8510C or HP8720C vector network analyzer block diagram coupled with the TRM (Thru Reflect Match) calibration leads to accurate measurements of the material properties of flat samples. Algorithms developed for transmission line measurements can also be used in free space measurements. A description of recent improvements in the transmission/reflection algorithms is reviewed. Free space measurement results based on the transmission/reflection algorithms found in the HP85071B materials measurement software package are presented.
The far-field radar cross section (RCS) of a conducting sphere is obtained by transforming scattered near-fields measured on a spherical surface. A simple and convenient calibration procedure is described that involves measuring the incident field directly at the target location. Although a non probe-corrected transmission formula was used in this study the importance of prove correction in practice is demonstrated.
A description of antenna measurements performed on an ocean-buoy mounted antenna array is given. The array is designed to measure the E and H fields of a received wavefront at four different heights over the ocean. Four collocated electrically-small loop-dipole antenna pairs at 2 meter height spacings were integrated into a non-conducting buoy support structure. The frequency band was 50-250 MHz. Data was taken with both the Substitution (2-antenna) and 3 Antenna Measurement Methods for comparison purposes. The ground plane range that was used is described as well as the various range setups used to accumulate all of the required data.