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Accuracy

Measurements for the verification of antenna temperature calculations for reflector antennas
K.M. Lambert (Analex Corporation),R.C. Rudduck (Ohio State University), November 1992

One antenna characteristic that is difficult to predict accurately is the antenna temperature. There are two basic reasons this is true. First, the effect of the full volumetric radiation pattern of the antenna must be taken into account. Secondly, the antenna temperature calculation requires knowledge of the noise power incident on the antenna, from the environment in which it is operating. This paper describes a measurement program which was undertaken to establish the accuracy of a model which is being used to predict antenna temperature for earth based reflector antennas. The measurements were conducted at 11 GHz, using an 8-foot diameter Cassegrain reflector antenna in an outdoor environment. The measurements are compared to predictions generated by The Ohio State University Reflector Antenna Code. Use of the reflector code allows the full volumetric pattern of the antenna, including all sidelobes, backlobes and cross-polarized response, to be included in the calculation. Additionally, the contribution to the antenna temperature from the various regions of the pattern can be calculated separately and analyzed.

A Hologram type of compact antenna test range
J. Tuovinen (Helsinki University of Technology),A. Raisanen (Helsinki University of Technology), A. Vasara (Helsinki University of Technology), November 1992

The applications of conventional reflector type compact antenna test ranges (CATR), becomes increasingly difficult above 100 GHz. The main problems are the tight surface accuracy requirements for the reflector, and therefore the high manufacturing costs. These problems can be overcome by the use of a new hologram type of compact range, in which a planar hologram structure is used as a collimating element. This new idea is described, and its performance is studied with theoretical analyses and measurements at 110 GHz.

Characterizing compact range performance for space communication antenna applications
S. Brumley (Boeing Defense and Space Group), November 1992

This paper addresses measurement requirements for space communication antennas and identifies antenna parameters most influenced by indoor compact range quiet zone quality. These parameters include sidelobe level, beam pointing, and gain. The compact range mechanisms limiting measurement accuracy are identified and discussed. Proven methods for characterizing quiet zone performance are described and demonstrated through illustration and example. Analysis is presented which related quiet zone quality characteristics to antenna measurement accuracy. The paper summarizes typical measurement results and error levels achievable for modern compact range systems. Methods for improving compact range performance for satellite antenna testing are also presented.

Dynamic air-to-air imaging measurement system
R. Harris (METRATEK, Inc.),B. Freburger (METRATEK, Inc.), J. Hollis (The Northrop Corporation), R. Redman (METRATEK, Inc.), November 1992

METRATEK has completed a highly successful program to prove the feasibility of high-resolution, air-to-air diagnostic radar cross section imaging of large aircraft in flight. Experience with the system has proven that large aircraft can indeed be imaged in flight with the same quality and calibration accuracy that can be achieved with indoor and outdoor ranges. This paper addresses the results of those measurements and the Model 100 AIRSAR radar and processing system that were used on this program.

Stereo optical tracker for compact range models
W.D. Sherman (Boeing Defense & Space Group),J.M. Saint Clair (Boeing Defense & Space Group), M.D. Voth (Boeing Defense & Space Group), P.F. Sjoholm (Boeing Defense & Space Group), T.L. Houk (Boeing Defense & Space Group), November 1992

A Precision Optical Measurement System (POMS) has been designed, constructed and tested for tracking the position (x,y,z) and orientation (roll, pitch, yaw) of models in Boeing's 9-77 Compact Radar Range. A stereo triangulation technique is implemented using two remote sensor units separated by a known baseline. Each unit measures pointing angles (azimuth and elevation) to optical targets on a model. Four different reference systems are used for calibration and alignment of the system's components and two platforms. Pointing angle data and calibration corrections are processed at high rates to give near real-time feedback to the mechanical positioning system of the model. The positional accuracy of the system is (plus minus) .010 inches at a distance of 85 feet while using low RCS reflective tape targets. The precision measurement capabilities and applications of the system are discussed.

Design considerations for a planar near-field scanner
J.H. Pape (Scientific-Atlanta, Inc.),A.L. Wilcox (Scientific-Atlanta, Inc.), J.D. Huff (Scientific-Atlanta, Inc.), November 1992

Planar Near-Field scanning is becoming the method of choice for testing many types of antennas. These antennas include planar phased arrays, space deployable satellite antennas and other antennas either too large to move during the test or otherwise sensitive to the gravity vector. The planar scanner is a major component of the measurement system and must provide an accurate and stable platform for moving the RF probe across the test antenna's aperture. This paper describes basic design requirements for a planar near-field scanner. Based on recent development activity at Scientific-Atlanta several design considerations are presented. Scanner parameters discussed include basic scanner concepts and geometry, scanner accuracy and stability, RF system including cabling and accuracy, load carrying requirements of the RF probe carriage, position and readout systems and drive and control systems. A scanner will be presented which incorporates many of the design features discussed.

Speed and accuracy for near-field scanning measurements
D.W. Hess (Scientific-Atlanta, Inc.),D.R. Morehead (Scientific-Atlanta, Inc.), S.J. Manning (Scientific-Atlanta, Inc.), November 1992

Rapid data acquisition is crucial in making comprehensive near-field scanning tests of electronically-steered phased array antennas. Multiplexed data sets can now be acquired very rapidly with high speed automatic data acquisition. To obtain high speed without giving up accuracy in probe position a feature termed subinterval triggering has been devised. To obtain simultaneously reliable thermal drift or tie scan data a feature termed block tie scans has been devised. This paper describes these two features that yield speed and accuracy in planar near-field scanning measurements.

The Commissioning of a fast planar near-field facility
K.S. Farhat (ERA Technology Ltd.),N Williams (DRA (Maritime Division)) E H England (DRA (Maritime Division)), November 1992

Some of the novel mechanical and electronic subsystems features on a recently installed high specification planar near-field scanner are described together with a discussion of the problems encountered during the commissioning period. The test facility incorporates a number of novel design concepts both in terms of its instrumentation, control and processing subsystems. Features of the facility are the speed of data acquisition and the accuracy of the acquired near-field data. Scan speeds of up to 0.8 m/s and positional accuracies of 30 microns in the Z-axis have been achieved, and the near-field data is acquired, displayed and measured on the fly, hence allowing a typical 3m x 3m scan to be executed and the measured near-field results to be displayed and processed within a period of thirty minutes.

Practical considerations for millimeter wave antenna measurement instrumentation
S.R. Gibson (Hewlett Packard Company), November 1991

As millimeter wave antenna systems become increasingly popular, engineers are challenged to develop effective methods for testing them. A practical method of designing a millimeter wave antenna measurement instrumentation system is presented in which frequency range, accuracy, dynamic range, and speed are considered.

Ramp sweep accuracy of RCS measurements using the HP 8530A
R. Shoulders (Hewlett-Packard), November 1991

The frequency accuracy of the HP 8530A receiver and HP 8360 Series synthesizers in ramp sweep is measured using a delay line discriminator. The effect of the frequency error on measurement accuracy is derived for radar cross section (RCS) measurements of one and two point constant-amplitude, scatterers and for background subtraction. The results of swept and synthesized frequency measurements are compared, showing that the errors due to ramp sweep are negligibly small for practical RCS measurements.

Swept frequency gain measurements from 33 to 50 GHz at the National Institute of Standards and Technology
M.H. Francis (National Institute of Standards and Technology),R.C. Wittmann (National Institute of Standards and Technology), November 1991

As part of an effort to provide improved measurement services at frequencies above 30 GHz, scientists at the National Institute of Standards and Technology (NIST) have completed development of a swept frequency gain measurement service for the 33-50 GHz band. This service gives gain values with an accuracy of ± 0.3 dB. In this paper we discuss an example measurement and the associated errors.

Method of determining the phase center location and stability of wide beam GPS antennas
P.I. Kolesnikoff (Ball Communication Systems Division), November 1991

Some proposed attitude control systems will require sub-millimeter position accuracy and GPS signals. Toward this end, two antenna parameters must be determined and optimized. These two parameters are phase center location and phase center stability. The phase center location is defined as the point whose ideal spherical phase front has the minimum RMS difference between itself and the measured phase data. Phase center stability is the effective movement of a GPS antenna’s phase center caused by the deviation of the radiated phase front from an ideal spherical phase front. The RMS difference between the ideal phase and the measured phase is a good measure of phase center stability. This paper describes a method of determining the phase center location of a wide-beam GPS antenna. Once the phase center is determined, the phase center stability throughout the coverage region (usually a hemisphere) is characterized. Finally, some sources of error are identified. Methods of minimizing the effects of these error sources are addressed.

Compact range bistatic scattering measurements
E. Walton (The Ohio State University ElectroScience Laboratory),S. Tuhela-Reuning (The Ohio State University ElectroScience Laboratory), November 1991

This paper will show that it is possible to make bistatic measurements in a compact range environments using near field scanning. A test scanner is designed and operated. Criteria for the accuracy of positioning and repositioning are presented. Algorithms for the transformation of the raw data into bistatic far field calibrated RCS are presented. Examples will be presented where comparisons with theoretical bistatic sphere data are shown. Bistatc pedestal interaction terms will be demonstrated.

Some differences between gated CW and pulse radars in RCS and imaging measurements
R.H. Bryan (Scientific-Atlanta, Inc.), November 1991

This paper compare some of the features and capabilities of gated CW and pulse radars for RCS and imaging measurements. At the conceptual level, these two types of radars are very similar. The primary conceptual difference is that a pulse radar has a relatively high bandwidth receiver while a gated CW system has a relatively narrow bandwidth receiver. The measures of performance of an RCS and imaging system include sensitivity, measurement time, clutter rejection, dynamic range and accuracy. Other considerations such as inter-pulse modulation may be important in some cases. For some applications, typically where long ranges are involved, a pulse system has significant performance advantages. For many applications, the performance advantage of a pulse system is not significant, particularly when viewed in light of the large difference in cost. This is particularly true of Quality Assurance applications which are normally characterized by both short range and lower budgets. Typically, the price of a gated CW system is in the range of ¼ to ½ the price of a comparable pulse system. This paper discusses general similarities and differences in the fundamental operating characteristics of the two systems. Specific performance measures are discussed including system sensitivity, gate performance, clutter rejection, and measurement times. Other considerations such as pulse modulation are discussed. A summary of the various considerations is presented in order to give the reader an understanding of the applications for which a gated CW system is more appropriate.

Radar cross section measurements for computer code validation
S. Mishra (Canadian Space Agency),C. Larose (Canadian Space Agency) C.W. Trueman (Concordia University), November 1991

Computer codes for the computation of scattering are based on physical, mathematical, and numerical assumptions and approximations that impact the accuracy of the results in ways that are not obvious or quantifiable analytically. This paper stresses the usefulness of a concurrent measurement program to provide reliable RCS data for targets of special interest in establishing the range of validity of the various assumptions upon which a specific computer code is based. This in turn assists in developing “modelling guidelines” restricting the design of computer models for input to the code such that reasonable accurate results are likely to be obtained.

Range instrumentation performance verification and traceability
D. Lynch (Hewlett-Packard Company), November 1991

This paper will discuss the need for performance verification, or calibration, of the transmitter and receiver systems used in an antenna or RCS range. Errors introduced by the range and positioning system means the instrumentation’s performance must be measured independently of the range and positioner. The performance verification should insure that the measurement system exceeds the manufactures’ specifications by a reasonable margin. The verification must be performed with the equipment installed on the range to insure adequate performance on the range. The system must als be verified as a system, rather than individual instruments. This guarantees that measurement errors in each instrument will not add together to exceed the system’s specifications. Testing of the system should be easy and repeatable to insure accuracy of the verification by the test technician. The tests should also be documented for later reference. The measurements should be traceable to a local standard such as NIST to certify the accuracy and stability of the measurement. The verification should be repeated on a regular basis to insure continued accuracy of the measurement system.

Range field compensation
D.N. Black (Georgia Institute of Technology),E.B. Joy (Georgia Institute of Technology), M.G. Guler (Georgia Institute of Technology), R.E. Wilson (Georgia Institute of Technology), November 1991

The accuracy of antenna measurements can be improved by compensating for the effects of extraneous fields present in an antenna range using analytical compensation techniques. Range field compensation is a new technique to provide increased measurement accuracy by compensating for extraneous fields created by refection and scattering of the range antenna field from fixed objects in the range and by leakage of the range antenna RF system from a fixed location in the range. The range antenna field must be the dominant field in the range, and the range field cannot change for different AUTs. Existing compensation techniques are limited in the amount of compensation they can provide. The range field is measured over a spherical surface encompassing the test zone using a low gain probe. The measured range field is used in subsequent antenna measurements to compensate for the effects of extraneous fields. This technique is demonstrated using measurements simulated for an anechoic chamber far-field range.

Measurement receiver error analysis for rapidly varying input signals
O.M. Caldwell (Scientific-Atlanta Inc.), November 1991

An assessment of instrumentation error sources and their respective contributions to overall accuracy is essential for optimizing an electromagnetic field measurement system. This study quantifies the effects of measurement receiver signal processing and the relationship to its transient response when performing measurements on rapidly varying input signals. These signals can be encountered from electronically steered phased arrays, from switched front end receive RF multiplexers, from rapid mechanical scanning, or from dual polarization switched source antennas. Numerical error models are presented with examples of accuracy degradation versus input signal dynamics and the type of receiver IF processing system that is used. Simulations of far field data show the effects on amplitude patterns for differing rate of change input conditions. Criteria are suggested which can establish a figure of merit for receivers measuring input signals with large time rates of change.

Applications of portable near-field antenna measurement systems
G. Hindman (Nearfield Systems Incorporated), November 1991

Portable near-field measurement systems can provide significant flexibility to both large companies seeking to increase their antenna test capabilities, and small companies looking for their first investment in a test range. There are many unique applications for portable near-field antenna measurement systems in addition to their use for standard antenna performance measurements. Some additional applications include flight-line testing, anechoic chamber quiet zone imaging, and EMI testing. Many of NSI’s near-field systems have been portable designs, capable of being set up in a small lab or office and easily relocated. Key features required for use of a portable system are rapid setup, simplicity of use, low cost, and accuracy. This paper will be focused on practical experience with installing, calibrating, and operating portable near-field measurement systems. It will also cover tradeoffs in their design, and usage in a variety of applications.

General analytic correction of probe-position errors in spherical near-field measurments
L.A. Muth (National Institute of Standards and Technology), November 1991

A recently developed analytic technique that can correct for probe position errors in planar near-field measurements to arbitrary accuracy [1] is shown to be also applicable to spherical near-field data after appropriate modifications. The method has been used to successfully remove errors in the near-field, hence leading to more accurate far-field patterns, even if the maximum error in the probe’s position is as large as 0.2?. Only the error-contaminated near-field measurements and an accurate probe position error function are needed to be able to implement the correction technique. It is assumed that the probe position error function is a characteristic of the near-field range, and that it has been obtained using state-of-the-art laser positioning and precision optical systems. The method also requires the ability to obtain derivatives of the error contaminated near-field defined on an error-free regular grid with respect to the coordinates. In planar geometry the derivatives are obtained using FFTs [1], and, in spherical geometry, one needs to compute derivatives of Hankel functions for radical errors, and derivatives of the spherical electric and magnetic vector basis functions for errors in the ? and Ø coordinates. The error-correction technique has been shown to work well for errors in and of the spherical coordinates r, ? or Ø. Efficient computer codes have been developed to demonstrate the technique using computer simulations.







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