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Accuracy

A Low Cost and High Accuracy Optical Boresighting and Alignment System using Video Cameras
J. Demas,Phan Q., November 2004

ABSTRACT This paper describes a novel optical boresighting and alignment system used to mechanically align antennas on a compact antenna range at the North Island Naval Air Depot in San Diego, CA. The antenna range has a 5-axis (roll/upper slide/azimuth/elevation/lower slide) positioner used to measure various airborne antennas for production testing. The video alignment system implemented on this range uses two video cameras outfitted with telephoto lenses, one on the roll stage and the other on an antenna-mounting fixture. The system has been demonstrated to yield an accuracy of ±0.005 degrees. Prior to the start of testing the positioner is commanded to a “0” position and the cameras focus on a fixed optical target to provide the operator with a quick visual confirmation that the positioner is accurately aligned prior to testing. The video alignment system described has numerous advantages over other mechanical alignment techniques, is low cost, easy to use, and can be adapted to a variety of testing configurations.

Compact Range Rolled Edge Reflector Design, Fabrication, Installation and Mechanical Qualification
J. Proctor,A. Fenn, D. Smith, G. Somers, M. Shields, P. Martin, November 2004

This paper describes the methodologies and processes used for the development, installation, alignment and qualification of a Compact Range Rolled Edge Reflector purchased by the MIT Lincoln Laboratory and installed at their test facility located at Hanscom Air Force Base. The Ohio State University, under contract to MIT Lincoln Laboratory, performed the electromagnetic design and analysis to determine the desired surface shape and required mechanical accuracy of various zones of that surface. The requirement for operation over a very broad frequency range (400 MHz to 100 GHz) resulted in a surface specification that was both physically large (24 ft × 24 ft) and included extremely tight tolerance requirements in the center section. The mechanical design process will be described, including the generation of a solid “Master Surface” created from the “cloud” of data points supplied by The Ohio State University, verification of the “Master Surface” with The Ohio State University, segmentation of the reflector body into multiple panels, design, fabrication and factory qualification of the structural stands, panel adjustment mechanisms, and panels. Results of thermal cycling of the reflector panels during the fabrication process will be presented. The processes used for installation of the reflector and the alignment of each panel to the “Master Surface” will be presented and discussed. Final verification of the surface accuracy using a tracking laser interferometer will be described. Color contour plots of the reflector surface will be provided, illustrating the final surface shape and verifying compliance to the surface accuracy requirement

Rapid Spherical Near-Field Antenna Test System for Vehicle Mounted Antennas
J. Graham,P. Iversen, November 2004

More and more wireless services such as satellite radio (SDAR), navigation systems, OnStar, and mobile telephones are installed in GM vehicles. This has created a need to make quick and accurate vehicle antenna measurements. For the frequency range of 500 MHz to 6 GHz, one solution is to use a spherical near-field system. The Satimo rapid probe array technology was selected to develop a vehicle antenna test system (ATS) to reduce test time and maintain data accuracy. The ATS was designed to operate inside of an existing GM electromagnetic compatibility (EMC) anechoic chamber equipped with a nine-meter turntable. The ATS was completed and received XM certification in the first quarter of 2004. The ATS performs multi-frequency dual-polarized complex measurements for every one-degree in azimuth and elevation, over a full hemisphere, in approximately five minutes. The autonomous transport and deployment system, allows the ATS hardware to be removed and the chamber returned to its EMC configuration. This paper presents the ATS design and a summary of the verification test results. A detailed uncertainty budget, as defined by NIST, is also presented.

Implementation of a Geometric-Error Correction System for Extremely High Probe Position Accuracy in Spherical Near-Field Scanning
S. Pierce,J. Langston, November 2004

In this paper, we describe a new method for improving the true-position accuracy of a very large, spherical near- field measurement system. The mechanical positioning subsystem consists of 10-meter diameter, 180 circular- arc scanner and an MI Technologies MI-51230 azimuth rotator and position controller. The principle components of the error correction method are the error measurement system, the position correction algorithm, and a pair of very high precision, mechanical error correction stages. Using a tracking laser interferometer, error maps are constructed for radial, planar and elevation errors. A position correction algorithm utilizes these discrete-point error maps to generate error correction terms over the continuous range of the elevation axis. The small position correction motions required in the radial and planar directions are performed using the mechanical correction stages. Corrections to the position of the elevation axis are made using the primary elevation axis drive. Results are presented that show the geometry of the spherical scanning system before and after error correction. It is observed that the accuracy of the radial, planar and elevation axes can be significantly improved using the error correction system.

High Accuracy Horizontal Scanner Operating in X-band for the Measurement of a Spaceborne Synthetic Aperture Radar (SAR) Antenna
S. Dreizin,Y. Sharay, November 2004

ELTA is now in the process of designing and building a new spaceborn SAR “TECSAR” – Israel Synthetic Aperture Radar (SAR) X-Band lightweight satellite. TECSAR contains an ultra-light weight high accuracy Paraboloid deployable reflector antenna. TECSAR’s electronic beam steering capability is achieved by using a feed array in the focal plane. For future testing at ELTA, Israel, an horizontal Planar near-field antenna test range (7m x 8m scan) has recently been completed by ORBIT/FR to allow testing of large fully integrated space antennas as stand alone as well as integrated with a satellite The paper will describe: o Short TECSAR SAR antenna description o The special requirements of the measurement system o System design and measured performance

Probe Array Concepts for Fast Testing of Large Radiating Structures
P. Barreau,A. Gandois, L. Foged, L. Duchesne, P. Iversen, November 2004

Satimo’s STARGATE probe array systems are now well established as an efficient tool for testing radiated performances of wireless devices and antennas. Since 1998, about forty STARGATE measurement systems have been successfully installed worldwide. Recently, a range of new applications have also demonstrated the suitability of probe arrays for large radiating structures and directive antennas. These new generation of measurement set-ups present innovative aspects regarding their rapidity, dynamic range, and accuracy. This paper will describe several novel antenna testing concepts based on probe arrays that cover automotive, aerospace, and military applications and a wide range of frequencies. The basic difference between traditional approaches using single probe and the STARGATE approach using an array of probes will be explained along with probe array calibration procedures. An error analysis budget using the conventional NIST error terms will be presented including the specific terms related to the use of probe arrays. Also a discussion will be made on some of the key technical challenges to making large probe arrays including such issues as dynamic range, mechanical tolerances, and data truncation effects.

High Accuracy Heavy Load Positioning System for Compact Range
M. Pinkasy,Roni. Braun, Y. Bitton, November 2004

Large satellites antennas are best measured in specially designed compact range systems designed for aerospace applications, located in a clean room environment. This testing requires very large, high accuracy positioners to accommodate full size satellites. Typical requirements include positioning accuracy of 0.003 degrees for a payload of 5 tons. ORBIT/FR has recently delivered to Astrium a unique payload positioner system specifically built for such high accuracy applications. This positioner provides the ability to accurately locate satellite payloads in the Astrium compact range system chamber to within the tolerances necessary to perform all radiated payload tests for specification compliance. In order to realize the required accuracy performance, an extremely stable positioner construction is required, such that near-perfect orthogonality between the rotary axes is maintained, and minimum structural bending is exhibited. This level of construction quality is realized by a unique elevation axis bearing configuration, in conjunction with an adjustable counter-weight system. In addition, very high accuracy absolute optical encoders are used; these exhibit higher accuracies than the traditional Inductosyn type of encoder. All axes are equipped with brakes on the primary axis to eliminate backlash. Alignment requirements further accentuate the need to be able to position to within a few thousandths of a degree. This in turn places difficult requirements on low speed operation and on the control system. This paper details the design and performance of such a positioning system as measured for two compact range installations utilized for satellite antenna testing applications.

Estimating the Uncertainties Due to Position Errors in Spherical Near-Field Measurements
A.C. Newell (Nearfield Systems Inc.), November 2003

Probe position errors, specifically the uncertainty in the theta and phi position of the probe on the measurement sphere, are one of the sources of error in the calculated far-field and hologram patterns derived from spherical near-field measurements. Until recently, we have relied on analytical results for planar position errors to provide a guideline for specifying the required accuracy of a spherical measurement system. This guideline is that the angular error should not result in translation along the arc of the minimum sphere of more than ?/100. As a result of recent simulation and analysis, expressions have been derived that relate more specifically to spherical near-field measurements. Using the dimensions of the Antenna Under Test (AUT), its directivity, the radius of the sphere (the minimum sphere) enclosing all radiating surfaces and the frequency we can estimate the errors that will result from a given position error. These results can be used to specify and design a measurement system for a desired level of accuracy and to estimate the measurement uncertainty in a measurement system.

Axial Ratio Errors When Using Linearly Polarized Probes in Planar Near-Field Measurements
P.R. Rousseau (The Aerospace Corporation),C.M. Turano (The Aerospace Corporation), M.S. Yonezaki (The Aerospace Corporation), W.C. Wysock (The Aerospace Corporation), November 2003

For a planar near-field range, it is sometimes convenient to use a linearly polarized probe to measure a circularly polarized antenna. The quality of the circular polarization of the test-antenna is determined by the measured axial ratio. This requires the amplitude and phase from two near-field scans, one scan with the probe polarization oriented horizontally and another vertically. A lateral probe position error between the horizontal and vertical orientations can occur if the probe is not aligned properly with the probe polarization rotator. This particular probe position error affects the accuracy of the axial ratio in the main beam if the beam of the test antenna is not perpendicular to the scan plane. This paper presents analysis and measurement examples that demonstrate the relationship between the errors in the axial ratio and the lateral probe position. It is shown that the axial ratio, within the main beam, is not sensitive to the lateral probe position error when the beam is normal to the scan plane. However, the error in the axial ratio in the main beam can be quite significant with a small lateral probe position error if the antenna beam is tilted at an angle with respect to the scan plane. A simple phase correction algorithm is presented that is useful for measured data from an electrically large aperture.

An Augmented Three-Antenna Probe Calibration Technique for Measuring Probe Insertion Phase
A. Frandsen (TICRA),D.W. Hess (MI Technologies), O. Breinbjerg (Ørsted-DTU), S. Pivnenko (Ørsted-DTU), November 2003

Probe calibration is a prerequisite for performing high accuracy near-field antenna measurements. One convenient technique that has been used with confidence for years consists of using two auxiliary antennas in conjunction with the probe-to-be-calibrated. Inherent to this technique is a calibration of all three antennas. So far the technique has mostly been applied to measure polarization and gain characteristics. It is demonstrated how the technique can be extended to also measure an antenna’s phase-versus-frequency characteristic.

Accurate Determination of a Compact Antenna Test Range Reference Axis and Plane Wave Quality
H. Garcia (Alcatel Space),B. Buralli (Alcatel Space), C. Bouvin (Alcatel Space), H. Jaillet (Alcatel Space), H. Kress (EADS Astrium GmbH), J. Habersack (EADS Astrium GmbH), J. Hartmann (EADS Astrium GmbH), J. Steiner (Alcatel Space), O. Silvestre (Alcatel Space), November 2003

Highly accurate antenna and payload measurements in antenna test facilities require highly accurate alignment and boresight determination. The Angle of Arrival (AoA) of the plane wave field in the quiet zone of the CCR Compensated Compact Range CCR 75/60 of EADS Astrium GmbH, installed at Alcatel Space in Cannes . France, has been measured using three different methods (optical geometrical determination using theodolites, Radar Cross Section (RCS) maximization, planar scanner phase plane alignment). The proposed paper describes the three methods and the performed measurement campaign and provides the correlation between the resulting angles via a comparison of the results. The achieved absolute worst case values of lower than 0.005° demonstrates the high level of accuracy reached during the campaigns.

Compact Range Performance Effects in Interferometer Testing and Related Statistical Analysis of Field Probe Measurements
J.F. Aubin (ORBIT/FR, Inc.),M.A. Bates (ORBIT/FR, Inc.), November 2003

This paper describes and discusses relevant performance issues concerning the quiet zone illumination of a baseline interferometer antenna using a compact range system. Typical baseline interferometer antennas are utilized for precision direction finding applications, and are designed on the principle of detecting the incoming phase wave front as a means to determine the direction of arrival of the detected signal. Quiet zone illumination of the antenna using a compact range deviates from the ideal illumination by introducing some levels of amplitude and phase taper and ripple. Unwanted relative differences in the illumination of the individual elements of the interferometer antenna will introduce errors in the subsequent analysis of the direction finding accuracy and precision of the array. Sources of these errors are examined in this paper, and relevant compact range performance trade-offs are discussed to optimize the range. Considerations are given to both utility of the range, as many interferometer antennas are broadband EW type arrays, and thus require single feed, single test broadband measurements, as well as to the accuracy in characterizing the performance of the interferometer over its full operating bandwidth. In addition, this paper discusses the analysis of high precision compact range field probe data, and the subsequent application of relevant statistical parameters to characterize the data. The analysis techniques utilized highlight the important performance features required of the compact range to effectively test baseline interferometers. The implementation of an automated utility is described that applies the relevant corrections, and applies the statistical algorithms, to the data to effectively reduce the data and summarize it in a fashion that provides immediate utility to the field probe test operator.

Compact Antenna Test Facility for Link Antennas
Z. Frank (MTI Wireless Edge Ltd.),G. Pinchuk (ORBIT/FR Eng.), M. Boumans (ORBIT/FR-Europe GmbH), M. Pinkasy (ORBIT/FR Eng.), November 2003

MTI Technology and Engineering Ltd. in Israel has installed an antenna test facility for the development and production testing of communication link antennas. Link antennas are typically high gain, medium size (< 2 ft) and medium to high frequency (10 to 50 GHz), with strict requirements on sidelobes, back-radiation and cross-polarization. Production testing is typically done on the main cuts. The facility is also used for PTMP and WLL antennas down to 2 GHz. This is an ideal requirement for a small size compact range. The ORBIT/FR single reflector compact range with a cylindrical quiet zone of a size 4 x 4 ft (diameter x length) was selected. The performance is compliant to international regulations (e.g. FCC, ETSI, DTI-MPT), and has a cross polarization as low as –40 dB for 0.4-m antennas. The total chamber size is 31 x 18 x 15 ft (L x W x H). The positioner system is roll over model tower over azimuth over lower slide. The instrumentation is Agilent 8530 based. The system was installed and qualified in late 2002. Qualification was performed from 2 to 50 GHz for quiet zone field probing and antenna sidelobe level accuracy testing. A system description, as well as an excerpt of the qualification data are presented in the paper.

Preliminary Investigations of Cohering Distributed Aperture Measurement Data
J. Kemp (Georgia Tech Research Institute),J. Holder (Georgia Tech Research Institute), November 2003

Preliminary investigations for cohering multiple apertures into a single distributed aperture were performed at the Georgia Tech Research Institute. Data were collected on complex targets in near realtime with two individual HP8510 Network Analyzer systems controlled by a single data acquisition computer as an interferometeric measurement. The data were analyzed and presented for high-accuracy angular resolution by examining the amplitude and phase difference between the two network analyzers. In addition, further upcoming tests on the Georgia Tech Research Institute far-field range will be outlined, showing how both measured angular resolution improvement and power-aperture gain product will be collected over a wideband frequency range.

The AFRL RF Materials Measurement Laboratory
G.R. Simpson (Air Force Research Laboratory), November 2003

The Air Force Research Laboratory (AFRL) Materials Measurement Laboratory (MML) is a state of the art facility for the characterization of the electromagnetic properties of materials at radio frequencies. The two-fold mission of the MML is to provide material characterization services to AFRL and to conduct R&D to develop or improve RF material characterization technology. The goal of the MML is to perform—or develop the ability to perform—material property measurements to the highest degree of accuracy possible with state of the art test equipment. Characterization measurements range from determination of RF reflection or transmission loss to the extraction of the dielectric permittivity and magnetic permeability of material samples. The MML has the ability to characterize material samples from below 100 MHz to above 18 GHz over material test sample temperatures ranging from – 150oC to greater than 1000oC. While maintaining capabilities using ‘standard’ material measurement techniques (circular coax and rectangular waveguide), the MML’s most highly utilized system is based on the GTRI focused arch apparatus. The MML also employs resonant cavity fixtures, open-ended coax probes and impedance meters to provide a capability to evaluate material samples of a wide variety of shapes and sizes.

A Broadband Materials Measurements Technique Building Upon the Implementation of Coaxial Probes
T. Holzheimer (Intelligence and Information systems), November 2003

A Technique is presented that allows for broadband nondestructive material electrical parameter measurements. Electrical parameters of a large number of materials are not readily available over extremely broad bandwidths (multiple octaves as an example). This information is required for accurate modeling of microwave circuits and antenna(s). These parameters consist of complex permittivity and complex permeability that result in loss due to the types and thickness of materials to be used. A Method is required that allows for fast, accurate and low cost measurements of the materials under test. The technique of using dual coaxial probes provides a solution that can be applied to numerous materials including thin films. It takes advantage of the full frequency extent of the network analyzer. This measurement uses dual coaxial probes, as compared to the implementation of cavity resonators, coaxial lines, waveguides and free space measurements, and performs the measurement in a 2-port calibration procedure. The resultant analytical solution is a transcendental equation with complex arguments. The Coaxial probes are described and can be easily made with available components where the only limitation is the valid component frequency bandwidth. Several material examples show the expected accuracy versus frequency range of this measurement technique.

Extreme Accuracy Tracking Gimbal for Radome Measurements
J.M. Hudgens (MI Technologies),G.W. Cawthon (MI Technologies), November 2003

Modern radome measurements often involve scanning the radome in front of its antenna while the antenna is actively tracking an RF signal. Beam deflections caused by the radome are automatically tracked by the antenna and its associated positioning system, which is typically a two-axis (pitch & yaw) gimbal. The motion required to accurately track the beam can be very demanding of the gimbal. High structural stiffness, zero drivetrain backlash, and extremely accurate angle measurement are all necessary qualities for radome beam deflection measurement. This paper describes a new, advanced, two-axis gimbal that embodies those qualities. The new gimbal incorporates direct-drive motors to achieve zero backlash. The motors are mounted directly to the rotating gimbal elements, thereby eliminating the usual causes of drivetrain compliance. Rated torque of the motors is not high, and the antenna is therefore fully counterweighted. Each of two optical encoders is mounted on the same rotating gimbal element as its associated motor. The encoders are directly mounted; no flexible coupling is used. The antenna is mounted to those same rotating elements. Antenna positioning error due to windup of the structure and drivetrain is virtually eliminated. Eccentricity of the encoder disk, which is the primary source of direct-drive encoder errors, is adjusted by virtue of a remarkable in situ process.

Array Element Phase Determination From Time-Domain Measurements
H.M. Aumann (Massachusetts Institute of Technology),F.G. Willwerth (Massachusetts Institute of Technology), K.A. Tuttle (Massachusetts Institute of Technology), November 2003

A technique is presented for determining the insertion phase of array elements directly from time domain measurements. It is shown that the Inverse Discrete Fourier Transform (IDFT) commonly used in swept frequency time delay measurements may yield unreliable phase results. A compensation to the IDFT is proposed which allows the phase of an array element to be accurately estimated from time domain data without gating and without taking a second DFT. The technique is applied to determine the insertion attenuation and phase of the elements in a linear L-band phased array. Compared to conventional array calibrations, the removal of extraneous range reflections implicit to the time domain technique resulted in a significant improvement in measurement accuracy.

How Far is Far Enough for System-Level Testing of DF Interferometer Arrays
N. Isman (ORBIT/FR Engineering ltd.), November 2003

The restriction of ? 2D2 R = is a commonly employed criterion for the minimum required separation between the range antenna and the Antenna Under Test (AUT) in a Far-Field (FF) antenna test range. However, this criterion, which is suitable for most common and simple cases, may not be adequate for more specialized test applications. Direction-finding (DF) interferometer antenna array testing is one such example. In a DF interferometer antenna array the phase difference between any two antennas serves as an Angle-Of-Arrival (AOA) discriminator for the radiation impinging on the array. At the system level, the array must be tested in order to calibrate its AOA discrimination function and to evaluate its accuracy, which, in many cases is done using a FF test range. In this paper, interferometer array FF testing is analyzed and an expression is developed for estimating the required separation between the range antenna and the array under test, in order to satisfy certain angle discrimination accuracy requirements. The results are compared with the common FF criterion and with restrictions imposed by other considerations.

Aspects of Antenna Pattern Characterization of an L-Band Space Radiometer
S. Pivnenko (Technical University of Denmark),J.M. Nielsen (Technical University of Denmark), O. Breinbjerg (Technical University of Denmark), November 2003

This paper deals with different aspects of the on-ground antenna pattern characterization of the MIRAS radiometer for ESA’s SMOS mission. Various technical challenges of the project are briefly described. Special attention is given to the effect of multiple reflections between the antenna under test and the measurement probe. A series of antenna measurements of the MIRAS radiometer antennas is now on-going at the DTU-ESA Facility. The main objectives of these are to investigate the accuracy of the forthcoming antenna characterization, to find solutions to the already known problems, to identify new possible difficulties, and to establish an optimal measurement strategy, which should allow for the tight error requirements and minimize the overall time of the measurement campaign.







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