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Accuracy

Far-Field Bistatic RCS From Near-Field Measurements
R.A. Marr (Air Force Research Laboratory),R.V. McGahan (Air Force Research Laboratory), T.B. Hansen (MATCOM Corp.), T.J. Tanigawa (Air Force Research Laboratory), U.W.H. Lammers (MATCOM Corp.), November 2003

Bistatic radar cross sections of targets are computed from field measurements on a cylindrical scan surface placed in the near field of the target. The measurements are carried out in a radio anechoic chamber with an incident plane-wave field generated by a compact-range reflector. The accuracy of the computed target far field is significantly improved by applying asymptotic edge-correction techniques that compensate for the effect of truncation at the top and bottom edges of the scan cylinder. The measured field on the scan cylinder is a “total” near field that includes the incident field, the field of the support structure, and the scattered field of the target. The background subtraction method determines an approximation for the scattered near field on the scan cylinder from two measurements of total near fields. The far fields of metallic sphere and rod targets are computed from experimental near-field data and the results are verified with reference solutions.

Cam RCS Dual-Cal Standard, The
W.D. Wood (Air Force Institute of Technology),P.J. Collins (National RCS Test Facility), T. Conn (EG&G), November 2003

We introduce a new calibration standard geometry for use in a static RCS measurement system that can simultaneously offer multiple “exact” RCS values based on a simple azimuth rotation of the object. Called the “cam,” the new calibration device eliminates the problem of frequency nulls exhibited by other resonantsized cal devices by shifting the nulls through azimuthal rotation. Furthermore, the “cam” facilitates the use of dual-calibration RCS measurements without the need to mount a second cal standard. The “cam” is practical to fabricate and deploy; it is conducting, composed of flat and constant-radius singly-curved surfaces, and is compatible with standard pylon rotator mounts. High-accuracy computational results from moment-method modeling are presented to show the efficacy of the new standard.

Evaluation of Radome Performance From Cylindrical Near-Field Measurements
B. Dixon (Chelton Radomes Ltd.),D.J. van Rensburg (Nearfield Systems Inc), November 2003

This paper describes the installation and implementation of a Cylindrical Near-field Test Facility at Chelton Radomes Ltd, Stevenage, (formerly British Aerospace Systems and Equipment Ltd.), in the UK for the testing of large radome/antenna combinations. Test site commissioning and validation activities to determine measurement accuracy & repeatability for the radome performance parameters of transmission loss and boresight error, are discussed. Test data from actual measurements are presented.

Interactions Between Probe Arrays and Antenna Under Test in Cylindrical and Spherical Near-Field Test Ranges: Numerical Assessment and Compensation Schemes
A. Ziyyat (Mohammed first University),D. Picard (Supélec), J-Ch Bolomey (Supélec), November 2003

While probe arrays are now recognized to allow rapid and accurate near-field measurements, the interaction with the Antenna Under Test (AUT) is still sometimes considered as a potential limitation, especially for electrically large directive antennas [1]. Based on numerical simulations, this paper reports the results of a thorough investigation of the interaction mechanism and analyses its impact on the far-field pattern accuracy. The most often, interaction effects can be maintained at an acceptable level, thanks to an appropriate design of the probe array element and structure. However, the efficiency of a posteriori compensation schemes has also been investigated. The Pattern Coherent Averaging Technique (PCAT) [2], which is well known for compensating plane wave deviations in the quiet zone of antenna far-field test ranges or interactions from single probe near-field facilities, also proved very efficient to reduce the interaction effects with a probe array.

Readily Made Comparison Among the Three Near-Field Measurement Geometries Using a Composite Near-Field Range
D.W. Hess (MI Technologies), November 2003

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.

Antenna Beam Shape - Effects on RCS Measurement
L. Oldfield (Defence Science Technology Laboratory),G. Wilson (Hutton Moor Measurement Facility), November 2003

This work is part of the UK Ministry of Defence initiative to examine causes of uncertainties in RCS measurements and to establish a network of certified facilities. Having developed a ‘best practice’ guide where causes of uncertainty were listed, the effect of polar diagram was selected as a priority topic. Correction algorithms for RCS measurements require knowledge of the beam shape and resolution in crossrange of the significant scatterers. Accordingly, the accuracy of polar diagram measurement, the effect of amplitude ripple and the applicability of the correction algorithms to near-field data were addressed. Measurements were made on two targets, a long cylinder and a small aircraft. Two antennas and two ranges were used to achieve 1dB, 3dB and 6dB illumination tapers across the cylinder. The 6dB taper situation was modelled for three different numbers of points. The work demonstrated that polar diagram effects are significant for point scatterers or simple targets, like the cylinder; however, for the small aircraft with a large number of distributed scatterers, the overall effect is less significant.

Test and Assessment of a Direction Finding Antenna Measured on the Nose of an MH-47A Helicopter
S. Abbott (US Army), November 2003

One basic Direction Finding (DF) technique for Radar is Amplitude Based Comparison DF. Multiple directional antennas are placed around an aircraft to get a 360 deg view of the area. By placing these antennas on the aircraft, the antennas are subjected to reflections from the aircraft, which distorts the antenna characteristics. This antenna distortion causes errors in the measurement of the angle of arrival. The work presented here describes the measurement of the antenna characteristics of a cavity backed spiral antenna both by itself and attached to the nose of an MH- 47A helicopter nose measured in an anechoic chamber. The spiral antenna’s pattern was changed when it was measured on the helicopter. The effect this change in pattern has on the DF accuracy is discussed.

High Accuracy Wide Band Compact Antenna Test Range
P. Bengtsson (Ericsson Microwave Systems AB),H. Eriksson (Ericsson Microwave Systems AB), M. Boumans (ORBIT/FR-Europe GmbH), November 2002

ORBIT/FR-Europe is in the process of finishing a new Compact Antenna Test Range for Ericsson Microwave Systems (EMW) in Sweden. The design was presented at AMTA 2001. Here the progress in the project is presented. Most subsystems are now installed, and system level acceptance will follow in the near future.

Algorithms and Mechanics Employed for Successful Portable Imaging Via the SCI-Xe Microwave Imaging System
J. Ashton (Sensor Concepts, Inc.),S. Gordon (Sensor Concepts, Inc.), November 2002

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.

Development of Highly Accurate Measurement Techniques for State-of-the-Art Antenna Test Facilities
J. Hartman (Astrium GmbH),H.J. Steiner (Astrium GmbH), J. Habersack (Astrium GmbH), R. Kis (Intelsat Global Service Corporation), D. Fasold (Munich University of Applied Sciences), November 2002

Contoured multi-beams achieved by multi-feed reflector antennas, realized in modern communication satellites, like Intelsat VIII and IX generations satellite, require an economic measurement of their antenna characteristic. Further, highly accurate, but also fast and therefore real-time measurements are assumed to be applied for the testing of the antenna performance. For that aim, the Compensated Compact Range CCR 75/60, applied at e.g. Space Systems Loral (SSL) in Palo Alto (USA), at ALCATEL in Cannes (France), at the MISTRAL facility in Toulouse (France) and at Astrium GmbH (Germany) was developed and installed by Astrium GmbH. In order to optimize the measurement accuracy of the CCR, detailed error analyses and investigations for improvement measures were performed. Within this paper, the accuracy analyses and improvement steps will be presented in order to establish accuracy values, which can be realized in state-of-the-art compact range test facilities.

An Evaluation of Errors Encountered Using the NUWC/NPT Overwater Arch Antenna Measurement Range
P. Mileski (Naval Undersea Warfare Center),D.A. Tonn (Naval Undersea Warfare Center), P.E. Giles (Naval Undersea Warfare Center), November 2002

The NUWC/NPT Overwater Arch Antenna Range consists of a 70 ft radius measurement arch located over an elevated 90 ft x 65 ft salt water pool. This facility, located outdoors, presents mechanical and electrical challenges. Measurement accuracy and precision are a function of environmental parameters (including unwanted signals), physical plant and instrumentation characteristics. Measured data variation will be presented along with techniques which could be employed to improve range performance.

Improved Antenna Radiation Pattern Measurements Using an Equalization Technique
P.S.H. Leather (Fizzle Technologies Limited (UK)),D. Parsons (Fizzle Technologies Limited (UK)), November 2002

This paper describes a novel system that overcomes the inaccuracies in antenna radiation pattern measurements caused by multipath propagation. The system operates by specifically compensating for the effects of unwanted signals rather than by attempting to remove, or minimize, their effects through the use of screens or baffles or an anechoic chamber. Compensation is achieved through the use of an equalization technique, the parameters of the equalizer(s) being determined from a special measurement of the antenna range under consideration. The method is generally applicable; it may be implemented ab initio in new indoor or outdoor ranges, or retrofitted to existing ranges to improve accuracy. Most importantly, however, the basic idea leads to the design of a completely new type of real-time 3-D range in which sensors are placed on the surface of an imaginary sphere surrounding the antenna under test (AUT), and an anechoic chamber is not required.

Dielectric Positioners for Measurements of Omni-Directional Antennas
V. Vinogradov (ORBIT/FR Inc.),K. Flood (ORBIT/FR Inc.), John Aubin (ORBIT/FR Inc.), November 2002

With the continued growth of mobile communications and the emergence of wireless LAN and personal area networks (PAN), there is an increased need to accurately measure the antenna properties for omnidirectional antennas and antenna systems. Furthermore, it is very desirable that antenna measurement systems be flexible to support a variety of antenna configurations and form factors. In this paper, we assess the performance of two measurement configurations utilizing dielectric positioners. These configurations comprise a traditional roll-over-azimuth antenna positioner and an arm-overturntable system such as that used in ORBIT/FR’s Advanced Spherical Cellular Near-Field (ASCENT) product. The results show that both configurations offer demonstrable improvements over conventional metallic positioners, and the arm-based system provides the highest accuracy for omnidirectional antennas.

Inaccuracy in Spherical Near Field Antenna Measurements Due to Anechoic Chamber Reflectivity
S.N. Pivnenko (Technical University of Denmark),J.M. Nielson (Technical University of Denmark), O. Breinbjerg (Technical University of Denmark), November 2002

The need for a well-defined accuracy estimate in antenna measurements requires identification of all possible sources of inaccuracy and determination of their influence on the measured parameters. For anechoic chambers, one important source of inaccuracy is the reflection from the absorbers on walls, ceiling, and floor, which gives rise to so-called stray signals that interfere with the desired signal. These stray signals are usually quantified in terms of the reflectivity level. For near-field measurements, the reflectivity level is not sufficient information for estimation of inaccuracy due to the stray signals since the near-to-far-field transformation of the measured near-field may essentially change their influence. Moreover, the inaccuracies are very different for antennas of different directivity and with different level of sidelobes, and for different parts of the radiation pattern. In this paper, the simulation results of a spherical near-field antenna measurement in an anechoic chamber are presented and discussed. The influence of the stray signals on the directivity at all levels of the radiation pattern is investigated for several levels of the chamber reflectivity and for different antennas. The antennas are modeled by two-dimensional arrays of Huygens' sources that allow calculation of both the exact near-field and the exact far-field. The near-field with added stray signals is then transformed to the far-field and compared to the exact far-field. The copolar and cross-polar directivity patterns are compared at different levels down from the peak directivity.

Low Frequency Spherical Near Field Measurement Facility at CNES
P. Dumon (CNES),D. Belot (CNES), L Duschene (SATIMO), P. Garreau (SATIMO), November 2002

In a conventional manner, a majority of compact ranges are currently used between 2 GHz and 200 GHz. Mechanical stiffness limits compact ranges at high frequency and diffraction effects are dominant at low frequency. However, CNES has installed a single reflector with dedicated serrations to perform accurate measurements between 800 MHz and 2 GHz. These serrations are 2 meters long and minimize the ripple in both amplitude and phase within the quiet zone. In order to further improve its measurement capabilities at lower frequencies, CNES has installed, in co-operation with SATIMO, a spherical near field measurement system directly inside of its compact range building. The goal is to measure antennas within the frequency range 80 MHz – 400 MHz with a relatively good accuracy. The spherical near field measurement facility has been tested and validated with four antennas that had been previously measured in the compact range of CNES and other external ranges. This paper focuses in this smart approach, which allows to extend the lower frequency domain of compact ranges. This paper describes in details the measurement facility, the test and the validation of the system.

Simultaneous Axis Motion Applications in Antenna and Radome Measurements
J.F. Aubin (ORBIT/FR, Inc.),C.J. Arnold (ORBIT/FR, Inc.), K. Flood (ORBIT/FR, Inc.), November 2002

This paper describes the use of simultaneous axis motion for various antenna, RCS, and radome applications, and the use of off the shelf hardware to support the corresponding measurement requirements. This is particularly relevant to polarization, low reflectivity target characterization, and radome measurements. Specific motion profiles required to accomplish various classes of tests are discussed, along with the implications on the mode of operation of the measurement system in order to achieve the most efficient collection of the required data. These simultaneous axis motion requirements may typically be user defined from the available set of axes composing the positioning system. Evaluation of the speed and real time tracking capability of the multiple axes are examined as they relate to the accuracy of the measurements that are required.

Gravity Deformation Measurements of 70m Reflector Surfaces
M. Brenner (Engineering Metrology Services),M.J. Britcliffe (California Institute of Technology), W.A. Imbriale (California Institute of Technology), November 2001

Two of NASA’s Deep Space Network (DSN) 70-meter reflectors are measured using a Leica TDM-5000 theodolite. The main reflector surface was measured at five elevation angles so that a gravity deformation model could be derived that described the main reflector distortions over the entire range of elevation angles. The report describes the measurement equipment and accuracy and the results derived from the data.

Evaluation of Antenna Tracking Systems
B. Dybdal (The Aerospace Corporation),D. Pidhayny (The Aerospace Corporation), November 2001

Antenna tracking systems are an important part of practical system designs. The goal of antenna tracking for communication applications is to provide sufficient accuracy to limit pointing loss, while for radar applications, to determine the target’s position as accurately as possible. Antenna tracking systems are reviewed describing both open and closed loop designs. Corresponding measurement techniques to quantify system performance are described.

A New Antenna Laboratory for 3D Antenna and RCS Measurements
A. Lysko (Norwegian University of Science and Technology),E. Eide (Norwegian University of Science and Technology), November 2001

A system has been developed for acquiring an antenna’s complete (3D) radiation pattern and radar cross-section (RCS) measurements. The system consists of a motion controller, a network analyser and tower assembly. The tower assembly is in an anechoic chamber. The tower has a novel design. It uses three motors in a special configuration, thereby allowing 2 ½ degrees of freedom. This freedom gives the ability to run complete antenna or RCS measurements automatically. Another advantage stemming from the degrees of freedom is expansion of the range of measurements. This is enabled by a variety of possible positions inside the chamber. Tests have also been carried out on system performance. The data acquisition rate becomes crucial when dealing with 3D pattern measurements. The performance of an HP 8720 or 8753 network analyser series can be dramatically increased by using the power sweep mode for data acquisition. Together with the “external trigger-on-point” mode, this gives the best positioning accuracy. The six-month experience has demonstrated the flexibility and reliability of the set up and ideas.

Helendale Measurement Facility Uncertainty Analysis
J.R. Newhouse (Lockheed Martin Aeronautics),J.M. Stinson (Lockheed Martin Aeronautics), N.E. Dougherty (Lockheed Martin Aeronautics), R.D. Nichols (Lockheed Martin Aeronautics), T.J. Fischer (Lockheed Martin Aeronautics), November 2001

This paper reviews the Helendale Measurement Facility (HMF) ground plane range uncertainty analysis and associated data collection. Range uncertainty analysis is a requirement for ISO-25/ANSI-Z-540 range certification and is a priority one section in the Helendale Range Book. Targets used for the analysis were two sets of right circular “squat” calibration cylinders. These cylinders are the dual calibration cylinders for HMF. Calibration measurement uncertainties are established statistically from a large number of repeated measurements at S, C, X, and Ku bands. Each measurement was taken at two target support locations down range. The field data collected included monostatic scattering from two calibration cylinders, backgrounds with no target and support, and drift data for quality control. I and Q imbalance, frequency stability, range accuracy, linearity, and field uniformity at target locations were considered in the analysis. The uncertainty analysis is based on RSS addition of errors and assumes all errors are additive and that targets are not LO. The statistical approach used to perform the uncertainty analysis reported in this paper was developed cooperatively at AFRL and Mission Research Corporation.







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