AMTA Paper Archive
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Expanding an ISO 17025-based Assessment Program in Wireless Compliance Test Facilities to Incorporate Characterization Measurements
This paper proposes an approach for the wireless industry to use in assessing its measurement facilities to help ensure that they are providing measurement results that are accurate and repeatable, with a knowable error and uncertainty. This approach is based upon the successful development of a certification program for US RCS facilities based upon an ISO 17025-like standard. Key pieces of this program include a documentation standard for defining the facility's capabilities and operation, and a Report of Measurement and an accompanying Uncertainty Analysis. This paper will discuss the similarities and differences between an existing RCS certification program and the proposed wireless program, to include technical distinctions between the two programs. These distinctions are based upon such factors as a 1-way instead of 2-way propagation paths, the various modulation schemes in use today and the different types of measurements such as Specific Absorption Rate that are not considered in RCS measurements.
Comparison of RCS Measurement of a NASA Almond Using Classical Compact Indoor Facility and a new Phased Array Antenna
CEA-Cesta has developed a new phased array antenna for RCS dual polarization wide bandwidth measurement in V/UHF bands. This array enables us to enhance signal to noise ratio especially at low frequencies. It is composed of 3 sub arrays dedicated each to one frequency band. The innovative design allows installing it in one of CEA/CESTA RCS facilities called “CAMELIA”. In order to validate this array in the highest sub-band [700 to 2000MHz], we measured in both HH and VV polarizations the near field RCS of a 2.5m long NASA almond target. This canonical object has been made of polystyrene coated with conducting nickel varnish. It has been hung on an eight wires rotating positionner. The results are compared with the data acquired in a classical RCS compact range and with the output of the 3D finite element code called ODYSSEE developed at CEA.
A Partial Rotation Formulation of the Circular Near Field-to-Far Field Transformation (CNFFFT)
For many years now, General Dynamics has described the development, characterization, and performance of an image-based circular near-field-to-far-field transformation (CNFFFT) for predicting far-field radar cross-section (RCS) from near-field measurements collected on a circular path around the target. In this paper, we consider the CNFFFT algorithm as an azimuthal filtering process and develop a formulation capable of transforming data that is not measured over a full 360º. Such a formulation has applications in measurement scenarios where collection of a complete rotation is not practical. As part of the development, we provide guidelines for the near-field data support required to achieve a desired accuracy in the sub-360º CNFFFT result. Numerical simulations are provided to demonstrate that the results of this partial-rotation formulation are consistent with the full-circle CNFFFT results presented in past papers.
Advancements in Millimeter Wave Gated RCS Measurements
The potential transmit power, and hence dynamic range of monostatic millimeter wave RCS measurements may be limited by the feed coupling of the antenna. Time domain gating can be used to reduce the measurement errors caused by this signal, as well as other undesired signals from scattering sources in the range, but does not protect the receiver from compression. Hardware gating can allow increases in transmit power by protecting the receiver from the effects of the feed coupling return. Unfortunately, equipment capable of hardware gating at millimeter wave frequencies is difficult to obtain. In addition, the usefulness of hardware gating is limited by the duty cycle loss in the measured signal. We describe a practical system using gating of the low frequency intermediate frequency (IF) signal in the receiver and a microwave pulse modulator prior to the millimeter wave multiplier in a mono-static millimeter wave RCS measurement system. We also describe methods to minimize the loss of measurement dynamic range due to duty cycle losses in this system. We demonstrate the use of this system for RCS measurements of simple targets, and compare the results with those obtained using software gating alone.
Time domain Planar Near-Field Measurement Simulation
The UWB radar operates simultaneously over large bandwidth and the antenna parameters must refer to simultaneous performance over the whole of the bandwidth. Conventional frequency domain (FD) parameters like pattern, gain, etc. are not adequate for UWB antenna. This paper describes an UWB radar antenna planar near field (PNF) measurement system under construction to get the impulse response or transient characteristic of the UWB antenna. Unlike the conventional antenna or RCS time domain test system, the UWB radar signal instead of the carrier-free short time pulse was used to excite the antenna that can avoid the decrease of the dynamic range and satisfy the needs of SAR and the other UWB radar antennas measurement. In order to demonstrate the data analysis program, FDTD simulation software was used to calculate the E-field of M×N points in a fictitious plane at different times just like the actual oscilloscope’s sampling signals in the time domain planar near field (TDPNF) measurement. The calculated results can be considered the actual oscilloscope’s sampling output signals. Through non-direct frequency domain near field to far field transform and direct time domain near field to far field transform, we get the almost same radiation patterns comparing to the FD measurements and software simulation results. At last, varied time windows were used to remove the influences of the non-ideal measurement environment.
The RCS Calibration Uncertainty of Balloon Tethered Spheres For Outdoor RCS Measurement Systems
Hollow metallic aluminum spheres have been used for years for calibrating RCS measurement systems both indoors and outdoors. While many previous papers have identified the RCS calibration shortfalls associated with spheres [1,2], most of these papers have concentrated on indoor RCS measurement systems, where there exist a number of accurate calibration alternatives to spheres, including the so-called "squat cylinder" [3,4]. For outdoor free space RCS measurement systems, especially those designed to measure dynamically moving or changing targets, (i.e. the NASA Shuttle C-Band Debris Radar), calibration is a much tougher problem. Frequently, spheres are used to calibrate such systems, by releasing and tracking a sphere attached to a lighter-than-air balloon, or by tethering a sphere to a lighter-than -air balloon and allowing it to float through a fixed radar beam. Recently, the Air Force Research Laboratory Mobile Diagnostic Laboratory (MDL) had the opportunity to measure the clutter and uncertainty associated with balloon tethered Sphere RCS calibrations. Two spheres were measured suspended by various string types and a line under an 8 ft. diameter tethered Helium filled balloon. We will provide design guidance, signal processing techniques and measurement uncertainty to help minimize the clutter and error induced by balloon borne RCS calibration spheres.
Characterization of Passive UHF RFID Tag Performance
This paper deals with characterization of passive ultra-high frequency (UHF) radio frequency identification (RFID) tag performance. Tag’s energy harvesting properties and the significance of the backscattered signal strength and radar cross section (RCS) of the tag are discussed using two examples: dipole tag antennas of various widths and identification of industrial paper reels.
Dynamic Radar Cross Section and Radar Doppler Measurements of Commercial General Electric Windmill Power Turbines; Part 1 - Predicted and Measured Radar Signatures
Commercial windmill driven power turbines (“Wind Turbines”) are expanding in popularity and use in the commercial power industry since they can generate significant electricity without using fuel or emitting carbon dioxide “greenhouse gas”. In-country and near-off shore wind turbines are becoming more common on the European continent, and the United States has recently set long term goals to generate 10% of national electric power using renewable sources. In order to make such turbines efficient, current 1.5 MW wind turbine towers and rotors are very large, with blades exceeding 67 meters in diameter, and tower heights exceeding 55 meters. Newer 4.5 MW designs are expected to be even larger. The problem with such large, moving metallic devices is the potential interference such structures present to an array of civilian air traffic control radars. A recent study by the Undersecretary of Defense for Space and Sensor Technology acknowledged the potential performance impact wind turbines introduce when sited within line of site of air traffic control or air route radars. . In the Spring of 2006, the Air Force Research Laboratory embarked on a rigorous measurement and prediction program to provide credible data to national decision makers on the magnitude of the signatures, so the interference issues could be credibly studied. This paper, the first of two parts, will discuss the calibrated RCS measurement of the turbines and compare this data (with uncertainty) to modeled data.
THREE-DIMENSIONAL NEAR FIELD/FAR FIELD CORRECTION
The DGA/CELAR (France) (Centre d'Electronique de l'Armement: French Center for Armament Electronics) is able to measure targets in order to get their RCS (Radar Cross Section). Yet CELAR RCS measurement facilities are not compact bases and therefore the measured field is a near field. This article proposes a solution allowing the transformation of this near field to a far field and this in the three dimensions of space without limiting any dimension with Fraunhöfer criterion. Thanks to this method the RCS of a target is able to be known in any direction of space and moreover the calculation of a three-dimensional ISAR (Inverse Synthetic Aperture Radar) picture is thus possible. At first the theoretic part of our work is presented. Then a fast method in order to calculate the transformation of a near field to a far field by optimising the calculation time thanks to signal processing theory is given. Finally obtained results from simulated bright points are presented.
Indoor Spherical 3D RDC Near-field Facility
Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern. In order to perform more complete characterizations, a spherical experimental layout has been developed at CEA for indoor near field monostatic RCS assessment. The experimental layout is composed of a motorized rotating arch (horizontal axis) holding the measurement antennas. The target is located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allows full 3D near field monostatic RCS characterization. Two bipolarization monostatic RF transmitting and receiving antennas are driven by a fast network analyser : - an optimised phased array antenna for frequencies from 800 MHz to 1.8 GHz - a wide band standard gain horn from 2 GHz to 12 GHz. This paper describes the experimental layout and the numerical post processing computation of the raw RCS data. Calibrated RCS results of a canonical target are also presented and the comparison with compact range RCS measurements is detailed.
THREE-DIMENSIONNAL RADAR IMAGING USING INTERFEROMETRY
The DGA/CELAR (France) (Centre d'Electronique de l'Armement: French Center for Armament Electronics) is able to measure targets in order to get their RCS (Radar Cross Section). Once this RCS is acquired it may be very interesting to calculate RADAR pictures of these targets because RADAR picture allows emphasizing the bright points. Until now, CELAR produced images in two dimensions, but these pictures have shown their limits in order to locate problems in altitude. This article fills this gap while proposing two methods in order to get an image in three dimensions: a method using a three-dimensional Fourier transform and a method based on interferometry.
Implementation of a "Cam" as an RCS Dual-Cal Standard
The 2004 AMTA paper entitled “The “Cam” RCS Dual-Cal Standard” introduced the theoretical concept of the “cam,” a new calibration standard geometry for use in a static RCS measurement system that could simultaneously offer multiple “exact” RCS values based on simple azimuth rotation of the object. Since that publication, we have constructed a “cam” to further explore its utility. The device was fabricated to strict tolerances and its as-built physical geometry meticulously measured. Utilizing these characteristics and moment-method analysis, a high-accuracy computational electromagnetic (CEM) “exact” file required for calibration was produced. Finally, the “cam” was evaluated for its efficacy as a single device that could be utilized as a dual-cal standard. This development was conducted with a particular focus on the hypothesized improvements offered by the new standard, such as the elimination of frequency nulls exhibited by other resonant-sized calibration devices, and improved operational efficiency. In this follow-on paper, we present the advantages to and challenges involved in making the “cam” a viable RCS dual-cal standard by describing the fabrication, modeling and performance characterization.
A Compact but Highly Flexible 5-axis Positioner
ACC has developed for the ESA-ESTEC CATR a compact but highly versatile 5-axis positioner. It is composed of a roll axis, upper azimuth, elevation, translation and lower azimuth axis. The clearance between the floor and the translation stage is designed to pass over a 12” walkway absorber while the roll axis height is only 155 cm (~5 feet). The standard configuration for medium or high gain antennas is the roll-over-azimuth or elevation-overazimuth configuration with a vertical interface for the AUT. For omni-directional antennas and RCS measurements, the positioner can be configured as a low profile azimuth positioner with a horizontal interface without a blocking structure behind the AUT. The positioner can also be configured for bistatic RCS measurements and Spherical Near Field. With the addition of a linear scanner, the Quiet Zone can be scanned in a polar way but also planar scanning is possible. Other key parameters are: angular accuracy: 0.01°, accuracy of the translation axis: 0.01 mm, load capacity 100 kg.
Outdoor RCS Measurement Range for Spaceborne SAR Calibration Targets
The Microwaves and Radar Institute regularly performs calibration campaigns for spaceborne synthetic aperture radar (SAR) systems, among which have been X-SAR, SRTM, and ASAR. Tight performance specifications for future spaceborne SAR systems like TerraSAR-X and TanDEM-X demand an absolute radiometric accuracy of better than 1 dB. The relative and absolute radiometric calibration of SAR systems depends on reference point targets (i. e. passive corner reflectors and active transponders), which are deployed on ground, with precisely known radar cross section (RCS). An outdoor far-field RCS measurement facility has been designed and an experimental test range has been implemented in Oberpfaffenhofen to precisely measure the RCS of reference targets used in future X-band SAR calibration campaigns. Special attention has been given to the fact that the active calibration targets should be measured under the most realistic conditions, i. e. utilizing chirp impulses (bandwidth up to 500 MHz, pulse duration of 2 µs for a 300 m test range). Tests have been performed to characterize the test range parameters. They include transmit/receive decoupling, background estimation, and two different amplitude calibrations: both direct (calibration with accurately known reference target) and indirect (based on the radar range equation and individual characteristics). Based on an uncertainty analysis, a good agreement between both methods could be found. In this paper, the design details of the RCS measurement facility and the characterizing tests including amplitude calibration will be presented.
Measurement of System Dynamic Range in the Time Domain
The dynamic range of a measurement system is typically evaluated in the frequency domain. However, for radar-cross-section (RCS) measurements, time processing of the frequency-domain data is often utilized to determine the temporal or spatial (down-range) location of responses. Dynamic range in the time domain is thus of considerable importance in determining what range of responses can be resolved and identified. While the coherent integration inherent in the pulse-compression process can increase the time-domain dynamic range beyond that of the frequency-domain, non-linearity in the measurement system leads to signal-dependent noise which, in turn, limits the time-domain dynamic range to a much smaller value. Thus, specification and characterization of time-domain dynamic range is critical for understanding the linearity requirements and the time-domain capability of the measurement system. This paper reviews design considerations, error sources, and measurement methods relevant to optimizing dynamic range in the time domain. Examples of time-domain measurements are included.
A Technique for Materials Characterization from Backscatter Measurements
Method of moments (MoM) codes have become have become increasingly capable and accurate for predicting the radiation and scattering from structures with dimensions up to several tens of wavelengths. In particular, for simple structures like canonical shapes or antenna / RCS test fixtures, especially those with material treatments, the primary source of disagreement between measurements and predictions is often due to differences between the “as-designed” and “as-built” material parameters rather than to the underlying MoM code itself. This paper describes an algorithm that uses a MoM model combined with backscatter measurements to estimate the “as-built” materials parameters for the case where the treatments can be modeled using an equivalent boundary condition. The algorithm is a variant of the network model technique described in -. The paper presents a brief formulation of the network model materials characterization algorithm, along with numerical simulations of its performance for a simple canonical RCS shape using the CARLOS-3D™ MoM code . The convergence properties of the algorithm are also discussed.
Measurements and Calibrations on the Larger Squat Cylinders
RCS measurements of two larger squat cylinders (with dia. 18” and 15”) have been studied. Numerical extrapolation from the best available MoM-simulation is used to generate the finer oscillations (< 0.1 dB) in RCS-PO at higher frequencies. Though the uncertainties at 0.4 dB would obscure the opportunity for a comparison at this time, a smoothly silver-painted surface did yield error bars at 0.2 dB for the Ku-band.
Compact Range Evaluation by GTD Modelling
A compact antenna test range has been analysed for stray signals. The analysis is based on GTD ray trac-ing, i.e. obeying the reflection law in the chamber walls and assuming straight edges of reflectors and walls. Comparisons to an RCS as well as a time-domain measurement of the quiet-zone performance show good agreements with respect to identification of the ray paths of the stray signals. Rough estimates of the power loss at reflections and diffractions show acceptable agreements with the measured levels.
RCS Measurement Facilities Certification Program Update
The National Radar Cross Section Measurement Facilities Certification Program seeks to raise collectively the quality bar across the community. A program to accomplish this goal was initiated in 1995. It continues with facilities joining the program every year. The program has now entered the recertification phase for facilities that achieved certification five or more years ago. This paper will briefly cover the history of the program, the participants, the certification process and criteria, the recertification process, status, and the way ahead.
RCS Measurements at 320 GHz to Verify the Alignment of the PLANCK Reflector Configuration.
In the Flight Model (FM) of the PLANCK telescope, the feed horns are connected to either HEMTs or bolometers operating at cryogenic temperatures to detect the Cosmic Microwave Background radiometric signal. For the purpose of an overall alignment verification at ambient temperature, RCS measurements have been performed using an auxiliary feed horn that is terminated with a switching diode. This verification test has been conducted at 320 GHz, to benefit from the narrow beam and a high sensitivity to misalignment. To perform the RCS measurements, an additional “circulator” with low propagation loss and high isolation from transmit to return channel had to be developed. Besides that, the circulator also co-locates the phase centres of both Tx and Rx range antennas on the focal point of the CATR, which allows mono-static RCS measurements. Quasi-optical techniques have been used to design a circulator that meets these requirements. To test the feasibility of determining the feed location from the RCS measurements with an uncertainty of ±1 mm, a test campaign was conducted with the so called RF Qualification Model (RFQM). In this campaign, 9 feed locations with 1 mm separation were tested. With the Flight Model, the test was on the critical path of the planning and only one test could be conducted to verify the overall alignment.
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