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This paper describes the development, testing and evaluation of a new, automated system for calibration and AUT alignment of a planar near-field scanner that allows the calibration system to remain in place during AUT measurement and which can be used to support AUT alignment to the scan plane. During scanner calibration, probe aperture position measurements are made using a tracking laser interferometer, a fixture that positions the interferometer retro reflector at a precise location relative to the probe aperture and a probe roll axis that maintains the proper orientation between the retro reflector and the interferometer as the probe position is moved. Aperture scan path information is used to construct a best-fit scan plane and to define a Cartesian, scanner-based coordinate system. Scan path data is then used to build a probe position error map for each of the three Cartesian coordinates as a function of the nominal position in the scan plane. These error maps can be used to implement software-based corrections (K-corrections) or they may be used for active Z-axis correction during measurements. By using a set of tooling points on the antenna mount, an AUT coordinate system is measured with the interferometer. The system then directs an operator through a set of AUT adjustments that align the AUT with the planar near-field scanner to a desired accuracy. This paper describes the implementation and testing of the system on an actual planar scanner and AUT test environment, showing the improvement in effective scanner planarity.
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.
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
Spherical near-field measurements require an increased level of sophistication and care to achieve accurate results. This paper will demonstrate an automated set of self-comparison tests, which can be used for establishing and optimizing a spherical system's performance. An over-determined set of measurements can help to qualify positioner alignment, range reflection levels, truncation effects, and additional parameters of interest. These results will help in optimizing the test configuration to achieve accurate near-field measurement results.
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.
In this paper, we describe the process used to align a large spherical near-field test system. The probe positioner consists of a cantilevered arc design with a probe path radius of five meters and a scan angle of 180°. The AUT positioner consists of an MI Technologies Model 51230 azimuth positioner with a high-precision encoder. The system is aligned using an SMX Tracker 4000 tracking laser interferometer. Alignment into a spherical system is achieved by initially defining two cylindrical systems; a primary probe positioner based system and a secondary, AUT positioner based system. Sources of mechanical error in each of these systems are identified and techniques used to control these error sources are described.
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.
In order to diagnosis array antennas we implemented the back projection technique in our bi-polar near field laboratory. Using capabilities of microwave holography we investigated actual distribution of excitation coefficient values in a variety of antenna arrays operating at 5 GHz and around 10 GHz. We investigated phase and amplitude alignment of the linear MC-8 phased array with eight elements operated in the band centered at 5000 MHz. The reconstructed phase distribution images reveal phase distributions consistent with the design values. A major technical impairment is that the resolution at the element level can not be easily assured and it is related to the element spacing.
This paper will discuss the application of alignment techniques and tools in a near-field testfacility. Standard alignment telescopes are not directly applicable in a general purpose near-field set-up because of limited dimensions of such a facility, where a direct target is not available and is often to close to the antenna to be in the focus region of the telescope itself. Self-made optical tools will be presented to overcome this problem, including some estimates about the required and obtained accuracies. Using these tools is demonstrated as a fast and accurate way to align an antenna to the measurement set-up.
In this paper I demonstrate how our current technology now very readily permits a standard of accuracy and utility to be realized, that was formerly available only in research laboratories. This is accomplished with standardly available positioning equipment and standardly available software. Accurate alignment of the range is enabled by a tracking laser interferometer. This composite nearfield scanning antenna range has afforded us the opportunity to compare readily, far-field results from the classic planar, cylindrical and spherical coodinate systems. Comparison data are presented.
Originally installed in 1992, the Advanced Compact Range (ACR) at Wright-Patterson Air Force Base was completely aligned using a Leica multi-theodolite measurement system. The Coherent Laser Radar (CLR) System provides an automated precision measurement capability which can gather significantly more data permitting a more complete characterization of the range in a relatively unobtrusive manner. This paper presents the process and results of applying Laser Radar Metrology as an optical range re-qualification tool within the Air Force Research Laboratory’s ACR.
Sensor Concepts, Inc. has developed the SCI-Xe Portable Microwave Imaging System prototype for use in the assessment of the low observable (LO) characteristics of fielded military platforms in their native environments. The SCI-Xe is a single man deployable suitcase-size system that employs a small linear rail in order to acquire Linear Synthetic Aperture Radar (LSAR) data in the 8-18 GHz frequency range. Data collections are performed via a single button push and the data is stored on a removable harddrive for comparison to an existing database for analysis. Recent deployment of the SCI-Xe prototype has provided excellent feedback on the viability of performing repeatable field measurements using alignment techniques that do not significantly affect the overall system size and weight. The SCI-Xe employs a video camera and uses video image algorithms such as edge detection, thresholding, and overlay masks to provide a simple coarse alignment to a stored baseline position. Once positioned, a single LSAR collection is performed to provide the radar data necessary for analysis, which includes a robust image registration algorithm to first, perform a quantitative assessment of the positioning accuracy and second, align the data for further image filtering and statistical processing.
The accurate measurement of static Radar Cross Section (RCS) requires precise calibration. Conventional RCS calibration objects like plates and cylinders are subject to errors associated with their angular alignment. Although cylinders work well under controlled alignment conditions, and have very low targetsupport interaction, these devices may not always suitable for routine outdoor ground-plane RCS measurements. We seek a design which captures the low interaction mechanisms of a cylinder, yet can be easily aligned in the field due to its excellent angular insensitivity. In a sense, this target has the best characteristics of both the cylinder and the sphere. This paper will describe the design of a "hypergeoid", a new calibration device based on a unique body of revolution. Calculations and measurements of some elementary hypergeoids are presented.
Z-position errors are generally the largest contributor to the uncertainty in sidelobe levels that are measured on a planar near-field range. The position errors result from imperfections in the mechanical rails that guide the motion of the measurement probe and cause it to deviate from an ideal plane. The deviations ä z (x, y) can be measured with precise optical and/or laser alignment tools and this is generally done during installation and maintenance checks to verify the scanner alignment. If the measurements are made to a very small fraction of a wavelength in Z and at intervals in X and Y approximating one half wavelength, the sidelobe uncertainty can be estimated with high confidence and is usually very small. For Z-error maps with lower resolution the resulting error estimates are generally larger or have lower confidence. This paper describes a method for estimating the Zposition error from a series of planar near-field measurements using the antenna under test. Measurements are made on one or more planes close to the antenna and on other planes a few wavelengths farther away. The Z-distance between the close and far planes should be as large as the probe transport will allow. The difference between the holograms calculated from the close and far measurements gives an estimate of the Z-position errors. This approach has the advantage of using the actual AUT and frequency of interest and does not require specialized measurement equipment.
This paper describes the experience of using a tracking laser interferometer to align and characterize the planarity of a planar near-field scanner. Last year, The Aerospace Corporation moved their planar near-field antenna range into a new larger room with improved environmental controls. After this move, the near-field scanner required careful alignment and characterization. The quality of the scanner is judged by how accurately the probe scans over a planar surface. The initial effort to align the scanner used a large granite block as a planarity reference surface and cumbersome mechanical probe measurements. However, a tracking laser interferometer was used for the final alignment and characterization. The laser interferometer was included as part of an alignment service purchased from MI Technologies. The tracking laser interferometer emits a laser beam to a mirrored target called an SMR (Spherically Mounted Retroreflector). Encoders in the tracker measure the horizontal and vertical angles while the laser interferometer measures the distance. From these measurements, the three-dimensional SMR location is determined. The laser has the ability to very accurately (within about 0.001 inch) measure the location of the scanning near-field probe. This paper includes a description of the mechanical alignment of the scanner, the tracking laser interferometer measurements, and the final planarity characterization.
Ericsson Microwave Systems (EMW) in Sweden has several outdoor and indoor test ranges in operation [1], [2], [3]. In line with future needs and requirements EMW has started building a new Compact Antenna Test Range to be used for a large range of projects and applications. The Compact Antenna Test Range will cover the frequency range of 800 MHz to 75 GHz. The test range will have the possibility for both active and passive antenna measurements at both system and subsystem / unit levels. The test zone will be 3 meters diameter. The maximum load the positioner can carry will be 700 Kg with very high position accuracy for special applications. Due to the relatively low design frequency and the desired size of the test zone, special considerations have been taken in the conceptual design of the reflector system as well as the choice of absorbers. Another important parameter in the design of the facility will be the access to the quiet zone and the time needed to change frequency bands and test objects. To accomplish this, preparations have to be made for easy alignment, very precise interfaces and a fast access to the test area.
This paper investigates the effect of alignment errors on near-field cylindrical ranges at frequencies over 18 GHz. This is of particular interest because the small probe sizes and wavelengths above 18 GHz can make the alignment of the near-field system a difficult task. Previous probe alignment investigations have been done at frequencies below 18 GHz. This paper will determine if the conclusions from the previous work are valid at higher frequencies and will expand on that previous work. Measured data will be presented to demonstrate the effect of the probe axis not intersecting the azimuth axis as well as the probe not being orthogonal to the azimuth axis of rotation.
Microwave holography is a popular method for diagnosis and alignment of phased array antennas. Holography, commonly known in the near-field measurement community as "back transformation", is a method that allows computation of the primary (aperture) fields from the secondary (far-zone) fields. This technique requires the far-zone fields to be known over a complete hemisphere and adequately sampled on a regular spaced grid in K-space. The holography technique, while known to be mathematically valid, is subject to errors just as all measurements are. Surprisingly, very little work has been done to quantify the accuracy of the procedure in the presence of known measurement errors. It is unreasonable to think that the amplitude and phase of the array elements can be trimmed to better than the uncertainty of the back-transformed amplitude and phase. This makes it difficult for an antenna engineer to determine the achievable resolution in the measurement and calibration of a phased array antenna. This study reports the results of an empirical characterization of known errors in the holography process. A numerical model of the near-field measurement and holography process has been developed and many test cases examined in an effort to isolate and characterize individual errors commonly found in planar microwave holography. From this work, an error budget can be developed for the measurement of a specific antenna.
This paper addresses the sensitivity of the cylindrical near-field technique to some of the critical alignment parameters. Measured data is presented to demonstrate the effect of errors in the radial distance parameter and probe alignment errors. Far-field measurements taken on a planar near-field range are used as reference. The results presented here form the first qualitative data demonstrating the impact of alignment errors on a cylindrical near-field measurement. A preliminary conclusion is that the radial distance accuracy requirement may not be as crucial as was stated in the past. This paper also shows how the NSI data acquisition system allows one to conduct such parametric studies in an automated way.
The majority of precision spherical positioner alignment techniques used today are based on procedures that were developed in the 1970's around the use of precision levels and auto-collimation transits. Electrical alignment techniques based on the phase and amplitude of the antenna under test are also used, but place unwanted limitations on accurately characterizing an antenna's electrical/mechanical boresight relationship. Both of these techniques can be very time consuming. The electrical technique requires operator interpretations of data obtained from amplitude and phase measurements. The autocollimation technique requires operator interpretations of optically viewed measurement data. These results are therefore typically operator dependent and the resulting error quantification can be inaccurate. MI Technologies has recently developed a mechanical alignment technique for Spherical Near-Field antenna measurements using a tracking laser interferometer system. Once the laser system has been set-up and stabilized in the operational environment; the entire spherical near-field alignment may be completed in a few hours, as compared to the much more lengthy techniques used with level/transit or electrical techniques. This technique also simplifies the quantification of the errors due to the inaccuracy of the alignment. This paper will discuss the effect of the alignment error on results obtained from spherical near-field measurements, and the procedures MI Technologies developed using a tracking laser interferometer system to obtain the precision alignment needed for a spherical near-field measurement.