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Radar Cross Section (RCS) Range Enhancements at Eglin's Seeker Test & Evaluation Facility (STEF)
The Seeker Test and Evaluation Facility (STEF) located on Range C-52A at Eglin AFB FL. is used to perform high-resolution multispectral (EO-IR-RF-MMW) signature measurements of US and foreign ground vehicles primarily to support the Research, Development, Test and Evaluation (RDT&E) of smart weapons (seekers, sensors and Countermeasure techniques). In order to support two major DOD signature measurement programs in 1997 this facility required significant range upgrades and enhancements to realize reduced background levels, increase measurement accuracy and improve radar system reliability. These modifications include the addition of a 350'X 120' asphalt ground plane, a new secure target support facility, a redesigned low RCS shroud for the target turntable and a new core radar system (Lintek elan) and data acquisition/analysis capability for the existing radars Millimeter-Wave Instrumentation, High Resolution, Imaging Radar System - MIHRIRS). This paper describes the performance increase gained as a result of this effort and provides information on site characterization and radar instrumentation improvements as well as examples of measured RCS of typical ground vehicle signatures and ISAR imagery
Full Polarimetric Calibration for RCS Measurement Ranges: Performance Analysis and Measurement Results
Full polarimetric scattering measurements are increasingly being required for radar cross-section (RCS) tests. Conventional co-and cross-polarization calibrations fail to take into account the small amount of antenna cross-polarization that will be present for any practical antenna. In contrast, full polarimetric calibrations take into account and compensate for the cross-polarization the calibration process. We present a full polarimetric calibration procedure and a simulation-based performance study quantifying how well the procedure improves measurement accuracy over conventional independent channel calibration.
Uncertainties in Measuring Circularly Polarized Antennas
Three common methods of measuring circularly antennas on a far-zone range are: using a spinning linear source antenna (SPIN-LIN), measuring the magnitude and with a linearly polarized source antenna in two orthogonal positions (MAG-PHS), and using a circularly polarized source antenna (CIRC-SRC). The MAG-PHS and CIRC-SRC methods are also used in a near-field or com pact range. The SPIN-LIN method is useful because an accur te measurement of the axial ratio and gain can be made without the need to measure phase. The MAG-PHS method is the most general method and can also completely characterize the polarization of the test antenna. The CIRC-SRC method is the simplest and least time-consuming measurement if the antenna response to only one polarization is needed. The choice of measurement method is dictated by schedule, accuracy requirements, and budget. An analysis is presented that provides errors in the measured gain, relative gain pattern, and phase of the test antenna depending on the polarization characteristics of the source and test antennas. These results are useful for deciding which measurement method is the most appropriate to use for a particular job. These results are also useful when constructing more complete error budgets.
DATE - Depot Aperture Tuning Equipment for the ERIEYE Airborne Early Warning System
DATE is a portable, rapid assembled, planar near field measurement system for ERIEYE Airborne Early Warning System. DATE shall be used both as a production range at Ericsson Microwave Systems (EMW) and as a maintenance equipment delivered with the ERIEYE AEW System. Up to now ERIEYE has been measured and phase aligned at EMW's large nearfield range. The active antenna is interfaced through a Beam Steering Computer (BSC) and hardware interface. The disadvantages with this approach is a slow communication speed and reduced Built In Test. Since the large nearfield range is designed to meet the requirements from many different antenna types the transport, mounting, alignment and range error analysis are very time and personnel consuming. The DATE-scope is to provide a portable planar near field test system that's custom-made for ERIEYE. The time from stored system to completed measurement shall be very short and performed by a "non antenna test engineer". This is done by: • Incorporate the BSC as a radar-mode. • Use the radar receiver and transmitter for RF measurement. • Reduce alignment time and complexity by a common alignment system for antenna and scanner. Scanner alignment for very high position accuracy. • Automatic Advanced Data Processing: Transformation from near field to far field to excitation to new T/R-module setting-up-table in one step.
Design and Validation of a General Purpose Near-Field Antenna Measurement Facility for the Royal Netherlands Navy
This paper describes a new multi-purpose planar & cylindrical near-field antenna test facility installed at the Royal Netherlands Navy (RNN). In this paper an overview is given of the initial list of requirements that was generated and the process of selecting the best type of measurement facility to address these. A description of the facility is given and an outline of the accuracy of the planar/cylindrical near-field scanner is presented. The paper contains details of the extensive validation program and measured data demonstrating the performance of the system.
Results from Inter-Laboratory Comparison Measurements at the Boeing 9-77 Range
Recently, RCS measurements were made of several common calibration objects of various sizes in the Boeing 9-77 Range. A study was conducted to examine the accuracy and errors induced by using each as a calibration target with a string support system. This paper presents the results of the study. Two of the objects, i.e., the 14"-ultrasphere and the 4.5"-dia. cylinder, are found to perform the best in that they exhibit the least departures (error) from theory. The measured departures of 0.2 to 0.3 dB are consistent with the temporal drift of the radar in several hours.
Evaluation Method of Radiation Patterns Caused by Residual Reflection Using the Distance Changing Technique, An
This paper presents a new method of separating and evaluating the effects of each residual reflection caused by antenna measurement environment by distance changing technique. The effects represent radiation patterns caused by residual reflections (hereafter, error patterns). The key processes of this new method are to suppress sidelobes of a Fourier spectrum applying a window as a function of the distance with the purpose of obtaining an accurate spectrum of reflections and to separate error patterns each other using a gating technique at each angle. Using this method applying the above two processes, we can evaluate the error pattern for each reflection source with accuracy. The validity of this method is confirmed by a computer simulation. This method is especially useful to detect the position of each reflection source in a case of evaluation for antenna test range.
Flexible Hardgating System as a Diagnostic Tool in Single and Double Reflector Compact Range, A
For improvement of the measurement accuracy of compact range test facilities under the constraint of maintaining the realtime measurement capability, a versatile hardgating system has been developed at the Fachhochschule Munchen. With this measurement and diagnostic tool a flexible, computer controlled variation of the pulse widths down to some ns can be performed to obtain a high spatial resolution. Besides selective measurements of the quiet zone field with suppressed interferers it is also possible to select particular inte fering field contributions in order to determine their amplitude and direction of incidence. The paper describes the hardgating system and the measurement results obtained with low and high gain antennas in the compensated compact range test facility at the Fachhochschule Munchen.
High Speed, Two-Wire Method for Controlling Antennas During Testing
Whether for speed or accuracy, it is often necessary to rapidly switch antenna beams during testing. Most current systems require a control line for each RF switch position or phase shifter bit [1,2]. Due to the need for slip rings, the number of bits that can be controlled by this method is limited. In addition, the voltage drop and interference over long lines limit the practical range lengths that can use these "wire-per-bit" techniques. A serial bit stream followed by a serial to parallel conversion is the usual approach to controlling a large number of switches with only a few lines. However, the serial bit stream approach is quite slow. This paper will present a high speed switch box that can control an arbitrary number of RF switches and phase shifters using only two control lines that can go very long distances. The electronic circuits and software interface of this box will be covered.
Alignment Concept for Spacecraft Antennas in State of the Art Test Facilities
During the design of spacecraft antennas a well defined geometrical configuration of antenna components is supposed. Also the requirements for the accuracy of the antenna integration normally will be given. The antenna alignment processes have to ensure, that the designed configuration with the required accuracy can be met. Additionally the antenna pointing has to be determined with respect to the RF measurement facility. In this paper the concepts are treated, how to determine the actual and the designed orientation and location of the components of the space antennas during subsystem and system level integration and tests. This includes also the definition of needed references for the antenna components, the creation and application of coordinates or orientation matrices at manufacturing or integration level, the used coordinate systems and the attainable accuracy for different methods. For the evaluation of the RF pattern performance, the correlation between the spacecraft coordinate system and the facility coordinate system has to be known. Basic principles of this pointing alignment and an error analysis of the measurement accuracy will be explained. The presented concepts are based on the experience at DSS' test facilities with various antenna types and agreed with different antenna manufacturers and customers.
Full Test-Zone Field Evaluation Using Large RCS Targets
Large Compact Ranges for test zone sizes of 6 meters or can be used for both payload or advanced antenna and RCS testing. In order to determine the range accuracy, test zone field evaluation is required. For physically large test zone dimensions, scanning of the test-zone fields is difficult and impractical in most situations. Furthermore, the accuracy of planar or plane-polar scanners is usually not sufficient for applications above 10 GHz. An alternative approach is the RCS reference target method where the test zone field is derived from the RCS measurement of a flat plate. Such a target can be manufactured as a single sheet aluminium honeycomb structure with rectangular or circular cross section. Reference targets with large dimensions and high surface accuracy are available. Consequently, test-zone fields can be accurately determined for test zone diameters up to about 10 meters and frequencies up to 100 GHz. In this paper the application of this method will be demonstrated at the Compact Payload Test Range (CPTR) at ESA/ESTEC. Large rectangular plate has been used for field determination within a test-zone of 5.5 meters. A 2 meter diameter circular flat plate has been used to map the residual cross-polarization level within the test zone. It will be shown that valuable information about range performance (amplitude, phase and cross-polarization) can be accurately retrieved from the RCS measurements
Large High Accuracy Plane Wave Scanner
ORBIT/FR designed and manufactured a plane wave scanner of unprecedented accuracy. It was delivered to Intespace in Toulouse, France, to verify the compact range quiet zone performance of the compact range system installed by Dornier Satellitensysteme GmbH. The design is of the plane polar type. The linear axis has an accurate travel range of 5.5 meters with additional acceleration and deceleration ranges. The polar axis has a travel range of over 180 degrees, so that a full circular plan of 5.5 meters in diameter can be evaluated. The mechanical overall planarity is better than ± 80 micrometers peak to peak. This is equivalent to ± 3.8° phase at 40 GHz. Special attention was given to the design of the RF cable track. A maximum phase variation equal to the mechanical accuracy was specified. However, no phase variation was noticed due to cable movements, even at 40 GHz. A new application for this scanner was to verify the actual boresight of the plane wave in both normal and so-called scanned boresight applications (compact range feed moved out of the focal point). For this purpose, the scanner was equipped with an optical mirror cube. Overall system alignment accuracies of 0.01° were typically achieved.
Diagnostic Tools for Multifeed Array Antennas
Diagnostic tools for the determination of the excitation coefficients of a multifeed antenna based on pattern measurements are extremely useful during a spacecraft antenna design. Due to the complexity of state of the art multifeed antennas, it is not straight forward to trace back to the location of possible error sources, if deficiencies or non-compliance's are detected during an antenna measurement campaign. Therefore a method was developed and tested at DSS which directly determines all effective excitation coefficients from pattern measurements. The method approximates the measured composite array pattern a set of computed element beam pattern, weighted by a set of unknown excitation coefficients. The resulting equation system is solved using the Method of Moments (MoM). The tool was extensively tested at DSS. The accuracy obtained for the calculations of the coefficients was in the 2% range beeing compareable to the accuracy of Beam Forming Network (BFN} measurements using a network analyser. In this paper the theoretical background of the method as well as some application cases will be described.
Error Statistics in RF Measurements
Error budget projections of measurement accuracy require statistical descriptions of the individual error sources. Thermal noise errors are well known and commonly used. Such statistics, however, have a zero mean Gaussian distribution and sadly are misapplied to the distributions of other error sources. The class of coherent RF errors, for example, has non-zero mean values and variances that differ from Gaussian values. Such statistics are described.
RCS Measurements of LO Features on a Test Body
The paper presents an example of the design process undertaken to determine the RCS response of LO features mounted on a test body. Although not unique, the example considers the various aspects which determine the accuracy of the final data in the design of the experiment and signal processing. The high quality of experimental results illustrate the potential of using an integrated approach in which the designs of the test body, the measurement process, the signal processing techniques, and validation of results are optimally applied to meet the objective not achievable by conventional means.
Characterization of Antenna Patterns by Means of Statistical Image Classification
The accuracy of near field measurements have in the past largely been judged by inspection however the authors have developed an objective measure of the accuracy and repeatability of such measurements. This paper illustrates the measurement process and the techniques associated with statistical image classification used to confirm its accuracy and repeatability. The technique will be illustrated via the correlation of data sets acquired over a variety of different frequencies and scan plane areas. The examination of these measurements will demonstrate the applicability and sensitivity of the technique when the accurate assessment of highly correlated patterns is required.
Satellite Payload Parameter Measurements in a Compensated Compact Antenna Test Range
Modern Satellite Antennas and Payloads are characterized by a lot of physical parameters like e.g. Radiation Pattern, Gain, EIRP, Flux Density, Gff and PIM, whereas the available time frame for measurements is getting shorter and shorter. The DSS Compensated Compact Range (CCR) allows a time efficient measurement of all payload parameters with high accuracy under controlled environmental conditions. The CCR consists of two doubly curved reflectors, which prevent inherent cross-polarization and create a very high constant amplitude and phase distribution in the quiet zone with a very good scanning performance. Most of the payload parameters can be measured directly or have to be calculated from a set of measurement values. For the G/T measurement of active antennas a new method for the noise power measurement was established. This paper describes the principle test set-ups with the corresponding measurement techniques to improve the measurement accuracy. Error budgets will be presented for pattern and gain measurement.
Technique for the Approximate Compensation of Antenna Illumination Taper from Near Field Measured, ISAR Data Sets, A
This paper presents an approximate, practical technique for the compensation of antenna pattern amplitude taper effects that occur in near field RCS data. The technique uses inverse synthetic aperture radar (ISAR) data sets. Complete pattern determination uses an iterative approach over target rotation angle and frequency bandwidth, with a series of near field ISAR images as input to obtain the corresponding corrected, near field, frequency/azimuth pattern data. Assumed is direct target illumination using a source with a known angular illumination pattern. The technique and its application environment in the Boeing Near Field Test Facility is described. It is then demonstrated using a near field data collection range of 100 feet from the target center of rotation. The approach is shown to be effective for target sizes with cross range extents extending to the one-way 3 dB points of the illumination taper (two-way 6 dB points). Demonstration of compensation performance and a study of accuracy achievable versus the near field image parameters used is presented.
Modelling of Time Domain Antenna Measurements in a Small Anechoic Chamber
The growing need of ultra-wide band measurements has promoted the research on real time domain (TD) antenna measurements. Theory has been already established, but practices still under development until the measurement regime becomes fully operational. In the Delft University Chamber for Antenna Measurements (DUCAT) there have been already provided outstanding results in a TD far-field configuration. A TD far field model of the facility has been developed in order to provide a key to improve the range performance and accuracy. This paper presents the model and considerations for establishing TD error correction techniques.
Low Cross-Polarized Compact Range Feeds
Compact antenna test ranges intended for low cross polarization antenna measurements require the use of feeds with polarization ratios typically greater than 40 dB across the included angle of the quiet zone as well as across the frequency band of interest. The design for a series of circular corrugated aperture feeds to meet these requirements is presented. The feeds are based on a circular waveguide OMT covering a full waveguide frequency band with interchangeable corrugated apertures to cover three sub-bands. In order to validate the design of this series of scalar feeds, high accuracy cross-polarization data was collected. The primary limiting factor in the measurement of the polarization ratio was the finite polarization ratio of the source antennas. A technique for correcting for the polarization ratio of the source is presented along with measured data on the feeds. The technique begins with the accurate characterization of the linear polarization ratio of the standard gain horns using a three antenna technique, followed by pattern measurements of the feeds, and ends with the removal of the polarization error due to the source antenna from the measured data. Measured data on these feeds is presented before and after data correction along with data predicted using the CHAMP® moment method software.
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