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Accuracy

The Effect of range errors on phase measurements of a spiral antenna
S. McMillan (Ball Communication Systems Division), November 1991

Phase relationships between the three dominant modes on a four armed spiral can be used to perform broad band, direction of arrival estimates, but this requires accurate estimates of the phase behavior of the antenna both in the design stage and for calibration purposes. Unfortunately, imperfections in range design make the measurement and interpretation of phase information extremely difficult. This paper describes an approach where the imperfections of the range and the behavior of the antenna are modelled, and range effects removed from antenna data through antenna motion, and frequency change. This technique obtained tremendous accuracy at the cost of large amounts of data processing.

Antenna test range validation
J. Lemanczyk (Technical University of Denmark),O. Breinbjerg (Technical University of Denmark), R. Torres (ESA-ESTEC-XEE), November 1991

Antenna specifications for space applications are very stringent in most cases requiring that antenna measurement facilities be validated before testing can proceed. One method by which this validation can be achieved is by means of antenna test range intercomparisons which entail the measurement of a suitable test antenna at several ranges wherein one range acts as a control laboratory. The problems of such an intercomparison manifest themselves in the availability of suitable validation antennas as well as a clear definition of test parameters and the standardization of comparison procedures to ensure accuracy, reliability and consistency. The several test range intercomparisons carried out by the Technical University of Denmark (TUD) under contract from the European Space Agency (ESA) provide the basis for the current effort under ESA contract to define a suitable validation antenna, design and acquire an antenna for 12 GHz operation as well as defining a Verification Test Plan.

Superresolution signal processing for RCS measurement analysis
B.W. Deats (Flam & Russell, Inc.),D. Farina (Flam & Russell, Inc.), November 1991

Superresolution (SR) processing techniques have been used for many years in direction finding applications. These techniques have proved valuable in extracting more information from a limited data set than conventional Fourier analysis would yield. SR techniques have recently proven to be an extremely powerful radar cross section (RCS) analysis tool. Typical resolution improvements of 2 to 30 times may be achieved over conventional Fourier-based range domain data in both the one-dimensional and two-dimensional image domains. Typical measurement scenarios which can most benefit from SP processing are presented. These include: VHF/UHF RCS measurements, measurement of resonant targets, and performing detailed scattering analysis on complex bodies. Measurement examples are presented illustrating the use of SR processing in a variety of test conditions. When the advantages of SR processing are combined with the accuracy of Fourier techniques, a new window is opened through which target scattering characteristics can be seen more clearly than ever.

Design your measurement system for optimum throughput
G. McCarter (Hewlett-Packard Company), November 1991

To achieve optimum measurement accuracy and range throughput in antenna and radar cross-section (RCS) measurement applications requires a careful and thorough design of the measurement system. Measurement accuracy requirements, test time objectives, system flexibility, and system costs must all be balanced to achieve an optimum system design. Considering these issues independently will result in unwanted and/or unexpected system performance tradeoffs. This paper examines these issues in some detail and suggests a system design approach which balances microwave performance and measurement speed with system cost.

VHF/UHF RCS measurements in indoor microwave facility
J. Saget (Dassault Electronique),J. Garat (CEA/CESTA), November 1990

Radar cross section (RCS) measurements were performed in the 0.1-1 GHz band in an anechoic chamber optimized for microwave frequencies. Selection of proper instrumentation, antennas, measurement techniques and processing software are discussed. Experimental results, showing the accuracy and sensibility of the system are presented.

Antenna phase measurements at 105-190 GHz
J. Tuovinen (Helsinki University of Technology),A. Lehto (Helsinki University of Technology) A. Raisanen (Helsinki University of Technology), November 1990

A novel differential phase measurement method is developed. No flexible cables or rotary joints are needed in this method. Phase center positions and phase patterns of two corrugated horns are measured at 105-115 GHz and 176-190 GHz by using this method. Good agreement between the measured values and theoretical values, calculated with the modal matching technique, is obtained. Also a new phase error correction method is introduced. This method makes possible to measure the phase error in the cable and then to remove the error numerically from the results. The accuracy of the phase error correction is limited by the phase measurement device in the system. Experimentally this method is verified at 10 GHz.

Laser tracker for radar calibration sphere position measurement
W.D. Sherman (Boeing Defense and Space Group),C.R. Pond (Boeing Defense and Space Group), M.D. Voth (Boeing Defense and Space Group), P.D. Texeira (Boeing Defense and Space Group), November 1990

A laser tracker using a computer controlled feedback loop has been designed and tested. The tracker follows a small retroreflector embedded in a radar calibration sphere. Angle encoders coupled to two orthogonal scanning mirrors give azimuth and elevation pointing angles to the target. Phase measurements of an intensity modulated laser beam give change in distance to the target, while absolute range is determined by knowing the initial 2p ambiguity interval of the target position. The crossrange accuracy of the system is limited by the scanning mirror encoders to =.063 inches rms at 105 feet (50 microradians). The downrange accuracy of the system is ˜.015 inches rms. This versatile system can be used for: a) contour measurements of models with the aid of a retroreflector moving over the surface, b) accurate determination of the coordinates of a single moving target, and c) determination of the orientation of a large extended target. Anticipated modifications of the system, with their potential precision measurement capabilities and applications, are discussed.

High performance hardware gate improves compact range performance
A.R. Lamb (Hughes Aircraft Company),H. Hgai (Hughes Aircraft Company), J. Paul (Hughes Aircraft Company), Y. Chu (Hughes Aircraft Company), November 1990

Comparative measurements have been made in a compact range to determine the performance improvements that can be achieved when adding a hardware gate to a CW-based measurement system. Starting with conventional stepped frequency CW measurements made in the time domain mode, high resolution downrange data was collected to determine the background levels of the compact range. This was followed by comparative measurements under the same conditions adding a narrow pulsed hardware gate to reject inter-horn coupling and high returns from the compact reflector. A second mode of comparison was examined by collecting aspect data with a specific range gate fixed about the target. Software gated measurements required more points to insure alias free operation, while the hardware gated measurements allowed fewer points which reduced measurement time without sacrificing any accuracy. Finally, imaging measurements were made with both software and hardware gating to compare the measurement time and accuracy

Short term stability performance of pulsed instrumentation radars using TWTAS
J. Allison (Hughes Aircraft Company),J. Paul (Hughes Aircraft Company), R. Santos (Hughes Aircraft Company), November 1990

Pulse-to-pulse amplitude and phase noise can affect the overall measurement accuracy of RCS instrumentation radars. Depending upon the measurement requirements, such noise can limit the overall performance whenever pulse-to-pulse repeatability is required in the signal processing. Radar systems using pulsed TWTAs are subject to high noise due to limitations in the performance of the TWTA modulators and power supplies. A characterization of this additive noise is important to understand the limitations in system performance. Measurements have been made on kilowatt power TWTAs at L and X band as well as 20 watt pulsed TWTAs at S, C, and X/Ku band at various duty cycles and PRFs.

Determining measurement accuracy in antenna tests
J. Boyles (Hewlett-Packard Company), November 1990

The task of making accurate antenna measurements is complicated by the numerous sources of measurement error in the antenna test range. In addition to the test system performance, the overall measurement uncertainty depends strongly upon the range configuration and user-selected operating conditions. A correct understanding of these systematic and random error sources can help optimize the test range, instrument configuration, and measurement technique to achieve the highest levels of measurement accuracy. This paper describes dominant error sources present on an antenna test range and gives methods for quantifying their effects on measurement accuracy.

Dynamic tracking method for radome characterization and measurement system
O. Porath (Orbit Advanced Technologies, Ltd.),I. Koffman (Orbit Advanced Technologies, Ltd.), N. Isman (Orbit Advanced Technologies, Ltd.), Y. Rosner (Orbit Advanced Technologies, Ltd.), November 1990

This paper describes an automated radome test and evaluation system, which quickly and accurately measures the electrical boresight shift and loss caused by the presence of a radome in front of a monopulse antenna. The system was required to measure the boresight deflection through all 60 spatial relative angles between the antenna and the radome. The conventional methods of radome characterization were useless for this range of relative angles (mechanically impossible). To overcome this problem, a unique dynamic tracking method was developed. In this methos, the antenna is mounted on a dual-axis gimbal attached to the radome. The gimbal by itself is mounted on a second dual-axis positioner. The antenna gimbal scans the radome through all the required relative angles, while the monopulse error is continuously measured and used to control the radome positioner, in order to return the antenna to the boresight position. The readings of the angles and the values of the monopulse error establish the boresight deflection results, which are highly accurate because the apparent (deflected) source is accurately tracked, and the antenna is boresighted to it. The system measures all the 60 angles in 70 minutes time, at an accuracy of 0.3mRAD.

An Economical system for RF antenna measurements
V. Autry (Hewlett-Packard Company),B. Coomes (Hewlett-Packard Company), November 1990

This paper examines antenna pattern measurements of RF frequency antennas (300 kHz-3 GHs) using an integrated source/receiver and measurement control software. Current microwave measurement systems provide sufficient measurement capability but are often too expensive to be used on ranges which require test frequencies of less than 3 GHz such as aircraft communications, cellular radio, GPS, and satellite telemetry antenna. Several system block diagrams based on the HP 8753 network analyzer will be examined with respect to system performance, measurement accuracy, and cost. System considerations for outdoor RF ranges such as RFI susceptibility will also be addressed.

The Design and structural analysis of a large outdoor compact range reflector
M.J. Brenner (ESSCO),D.O. Dusenberry (Simpson, Gumpertz & Heger Inc.), J. Antebi (Simpson, Gumpertz & Heger Inc.), November 1990

A 75 foot diameter offset paraboloidal outdoor compact range reflector was designed for operation up to 95 GHz and installed at Ft. Huachuca, Arizona. The need for high frequency operation required that a highly accurate reflector surface be maintained in the desert’s harsh thermal and wind environment. The use of thermal modeling to predict the temperature distribution in the structure, along with extensive finite element analysis to determine the structure’s distortions from thermal, wind and gravity loads were integral to the reflector design. Using the above tools, thermal isolation techniques were developed to minimize the harmful effects of the thermal environment on surface accuracy. A surface error budget based upon both calculations and measurements shows an overall rms error of 4.9 mils under optimal environmental conditions, degrading to only 6. Mils under the worst operating conditions.

An Overview of parameters determining productivity and sensitivity in RCS measurement facilities
E. Hart (Scientific-Atlanta, Inc.),W.G. Luehrs (Scientific-Atlanta, Inc.), November 1990

A major objective in the design of an RCS measurement facility is to obtain the greatest possible productivity (overall measurement efficiency) while maintaining the accuracy and sensitivity necessary for low radar cross section targets. This paper will present parameters affecting the total throughput rates of an indoor facility including instrumentation, target handling, and band changes-one of the most time consuming activities in the measurement process. Sensitivity and accuracy issues to be discussed include radar capabilities, feeds and feed clustering, compact range, background levels, and diffraction control.

The Effect of probe position errors on planar near-field measurements
J. Guerrieri (National Institute of Standards and Technology),S. Canales (National Institute of Standards and Technology), November 1990

Antenna engineers recognize that the planar near-field method for calibrating antennas provide accurate pattern and gain measurements. Bothe the pattern and gain measurements require some degree of probe position accuracy in order to achieve accurate results. This degree of accuracy increases for antennas that have structured near-field patterns. These are antennas in which the amplitude and phase change rapidly over a very small position change in the near-field scan plane. The National Institute of Standards and Technology (NIST) has recently measured an antenna with a very structured near-field pattern. This measurement was performed using a new probe positioning system developed at NIST. This measurement will be discussed and results will be presented showing how slight probe position errors alter the antenna pattern and gain.

Effects of the mechanical deformation on the accuracy of a spherical near field testing facility
L. Anchuelo (INTA),J-L. Cano (INTA), M. Manzano (INTA), R. Amaro (INTA), R. Perez (INTA), November 1990

A new spherical near field facility has been recently implemented at the Electromagnetic Propagation Area of INTA. The facility makes use of an existing big anechoic chamber (12 x 12 x 12 m.) and the near field/fair field transformation software developed by TICRA. This range has been calibrated by measuring an offset reflector antenna and comparing the results with those obtained in previous measurements of this antenna in other European testing facilities of different types. An experimental study has been carried out to check the dependence of the transformation software on the scanning parameters and different misalignments have been produced in order to determine the impact of the mechanical deformations on the accuracy of the system.

Amplitude accuracy of the PWS range probe
R.D. Coblin (Lockheed Missiles and Space Co.), November 1990

As the accuracy of antenna range instrumentation improves, multipath on the range is becoming the key limitation in antenna metrology. A fundamental requirement to improving range performance is the accurate and repeatable characterization of scattering on a range. A promising technique for range characterization is the planewave spectral (PWS) range probe. Earlier papers have demonstrated the ability of the PWS probe to locate multiple scattering centers on a range. Of equal importance to the user is the ability to correctly assess the magnitude of the scattering centers. This paper presents the problem of spectral peak broadening due to phase curvature from localized scatterers. Methods for improving the accuracy of scattering center estimation are presented along with numerical studies of the performance of these methods.

Methodology to project antenna measurement accuracy
R.B. Dybdal (The Aerospace Corporation), November 1989

Antenna measurement accuracy is often not addressed in a rigorous manner. A methodology for projecting antenna measurement accuracy is described together with some of the error components that limit measurement accuracy. Antenna measurement accuracy is approached through an error budget projection, which requires the first and second order statistics of the individual error sources. Typical error sources are described along with methods of obtaining the statistics required for the error budget.

Post processing corrections to indoor RCS VS aspect measurements
L. Pellett (Lockheed Aeronautical Systems Corporation), November 1989

This paper describes two signal processing techniques that have been used to overcome specific problems in a Lockheed Aeronautical Systems Corporation (LASC) indoor compact RCS measurement range. Both techniques are post processing techniques used to enhance the accuracy of RCS vs. Aspect measurements. These two techniques can speed up measurement time, increase measurement accuracy, and increase target sizes on a compact range.

Compact range reflector surface accuracy and quiet zone quality
L. Woodruff (Harris Corporation), November 1989

The construction of a large reflecting surface is invariably a compromise between the technical requirements and what is economically achievable. During the past three years, the compact range team at Harris has learned a great deal about this process. While aligning and testing the Harris Model 1630/1640 Compact Ranges, we have gone through a long learning experience. This paper presents some of the results of that experience.







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