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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.
Modern day remote sensing spacecraft often feature multiple payloads sharing a common bus (spacecraft platform). RE02 emission testing (1, 2) characterizes the emission signature of a given payload in order to assess electromagnetic compatibility with respect to other payloads (i.e. “victims”) on the bus. Typically, a simple path loss model based on 1/r2 power variance (ref: Friis path loss equation) is used to account for the distance between the emitting and victim payloads using measured amplitudes taken during RE02 measurements. RE02 measurement technique (2) dictates that emissions testing take place at a fixed radial distance of one meter from the radiating instrument. At certain frequencies, however, this measurement takes place in the near field of the emitter. In general, power density amplitudes are greater in the near field than its far field counterpart. This paper investigates any potential error incurred by not accounting for this effect. A simple math model for a common mode radiator is developed to estimate this error and attempt to better understand the field relationships at lower frequencies where the near field predominates.
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.
This paper discusses design aspects related to a tiltable lightweight near-field scanning system for use at sub-millimeter frequencies. It addresses design issues as they relate to accuracy and scanner distortions from multiple causes. Calibration methods to measure and correct for anticipated and unanticipated errors are briefly addressed. Actual test results are presented. The tiltable scanner being discussed was designed for the Atacama Large Millimeter/submillimeter Array (ALMA) [1] and is being used by the National Radio Astronomy Observatory (NRAO) [2]. It has many other applications by virtue of its light weight (approx. 120 lbs) and ability to be oriented at different angles. These include flight-line testing and other in-situ antenna test applications.
Focused-beam measurement systems are commonly employed in material characterization measurements due to their inherent broadband capability. Calibration of this system is typically performed using a simple response calibration in conjunction with gating techniques to eliminate unwanted reflections. An undesirable artifact of this calibration technique is the extracted permittivity and permeability measurements can be highly dependent on the width and shape of the gate. This paper explores a three-short full two-port calibration technique which eliminates the need for gating. The two-port calibration consists of three independent short measurements in both the forward and reverse directions (i.e., S11 and S22), two isolation measurements (S21 and S12) and four empty measurements (S11, S21, S12, S22). Material parameter extraction measurements based upon this calibration technique were performed using both a low-frequency (0.5 - 2 GHz) and high-frequency (4 - 18 GHz) focused-beam system. Initial results show the technique’s viability and the dependency on accurate positioning of the shorts used in the calibration process and possible interaction between the sample and the sample holder.
While using squat cylinders for calibrations, we study the MoM-simulated data in terms of surface waves. We have found that the fine structures in both the amplitude and the phase are related to the target geometry. Key Words: RCS calibration, simulation, polarization
We use a rotating dihedral to determine the cross-polarization ratios of radar cross section measurement systems. Even a small amplitude drift can severely degrade the calibration accuracy, since the calibration relies on accurate determination of polarimetric data over a large dynamic range. We show analytically how drift introduces errors into the system parameters, and outline an analytic procedure to minimize the in.uence of drift to estimate system parameters with greater accuracy. We show that only very limited information about the drift is needed to provide measured system parameters accurate to second order in the error-free parameters. Higher-order accuracies can be achieved by using more detailed information about the drift. We use simulations to explain and illustrate the analytic development of this theory. We also show that, using cross-polarimetric measurements on a cylinder, we can recover the exact system parameters. These .ndings show that we can now calibrate polarimetric radar cross section systems without the large uncertainties that can be introduced by drift.
A probe station based antenna measurement setup is presented. The setup allows for measurement of complex impedance and radiation patterns of an on-wafer planar antenna, henceforth referred to as the device under test (DUT), radiating at broadside and fed by a coplanar waveguide (CPW). The setup eliminates the need for wafer dicing and custom-built test fixtures with coaxial connectors or waveguide flanges by contacting the DUT with a coplanar RF probe. In addition, the DUT is probed exactly where it will be connected to a transceiver IC later on, such that no de-embedding of the measured data is required. The primary sources of measurement errors are related to calibration, insufficient dynamic range (DR), misalignment, scattering from nearby objects and vibrations. The performance of the setup will be demonstrated through measurement of an on-wafer electrically short slot antenna (.0/35 × .0/35, 5 mm2) radiating at 2.45 GHz.
This paper presents the design, development and testing of an inverted Stewart platform for suspending and positioning targets during RF antenna and signature testing. Previous string target support systems use multiple string attachment point configurations that do not allow the target roll or pitch to be modified during the azimuthal data collection. This presentation will discuss an in-house development of a scale model target support system that allows for high accuracy simultaneous target roll and pitch positioning. The inverted Stewart platform also offers unique stability of the target by damping out the torsional pendulum motion typically encountered in conventional string support systems. In this paper we will also discuss the advantages and disadvantages of the string support concepts and provide design guidance for a building an inverted Stewart platform support system. If possible, a simple squat calibration standard will be measured to assess the quality and precision of this novel support system.
The AN/SPS-48E antenna is a three dimensional air search antenna that is currently installed on 27 US ships. Currently the 48E antenna is removed from the ship after five to seven years to be overhauled at NSWC Crane Division. The new San Antonio Class ships (LPD 17 – 25) have a new enclosed mast design, the Advanced Electromagnetic Mast/Sensor (AEM/S), in which the 48E antenna and others are installed inside the enclosed mast. The cost of removing the enclosed mast led to the decision that the 48E antenna systems (antennas and pedestals) will not be removed for overhaul and maintenance on these ships as is currently done for all other installations. As a result, new fixtures and procedures need to be developed to allow maintenance inside of the mast. The most challenging of the new fixtures is a near-field scanner, which will be used to re-tune the antenna and characterize the RF performance parameters. This paper discusses the design and development effort currently underway for this Enclosed Mast Antenna Calibration System (EMACS), most notably the mechanical design constraints placed on the scanner by the enclosed mast regarding equipment movement, installation, alignment and testing.
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.
Three methods are described to characterize ground antennas in frequency bands where satellite beacons exist. This measurement method is useful when the antenna to be tested cannot be easily measured using conventional general purpose facilities or radio source measurement techniques. The measurement methods are described, and the factors that result in measurement uncertainties are discussed. Key Words: Antenna Measurements, Gain Calibration, Ground Terminals
The calibration of a prototype system to monitor the on-orbit performance of heritage and modernized GPS satellites is described. While the monitor can measure other GPS parameters of interest, the calibration to accurately determine the received signal levels is described here. The calibration determines the monitor’s receive antenna gain and relates the received power at the antenna terminals to the indicated output of the monitor’s receiver. Key Words: System Calibration, Error Budgets, Satellite Measurement
Abstract— Microwave, total-power radiometers are calibrated by observing sources of known brightness temperature. Free-space calibrations are usually performed assuming far-.eld interactions, although the actual calibration source may be well into the near-.eld region of the radiometer antenna. Here, we simulate the free-space coupling between radiometer and calibration target to explore the potential contribution of proximity effects to measurement uncertainty.
A large compact range facility required a foam column for RCS testing where the center of the quiet zone was six meters above the floor level. The RCS measurement after vector background subtraction, had to be accurate down to a –50 dBsm level from 1.5 GHz to 40 GHz. A foam column was constructed from a single billet of material. The foam column was evaluated as to its RCS level in both whole body and ISAR imaging modes. This paper describes the specification, construction and RCS evaluation of this column in the compact range facility. The column was evaluated at single frequencies and with RCS images from 2 GHz to 36 GHz using a gated CW radar. Data is presented that shows the effects of the column on the response of a calibration sphere and the response of the column itself. A study of the foam column imaging response used as the background for vector background subtraction is also described. Targets in the –60 dBsm range were successfully imaged with vector background subtraction of the foam column.
ABSTRACT We have developed a new compact spherical near-field measurement system using a photonic sensor as a probe and successfully measured the 3D antenna patterns of a double-ridged horn antenna from 1 GHz to 10 GHz. This system consists of a compact spherical scanner and a photonic sensor that is used for the probe of the spherical near-field measurements. In our system, only one probe can be used for the wide frequency range measurements and the probe compensation is not needed in the measurements. For the system, we propose a simple calibration method using a double-ridged horn antenna for our system. We calibrate the system by measuring the double-ridged horn antenna on the reasonable assumption that the antenna efficiency is 100 %. Comparing the absolute gain obtained by the proposed calibration method with the one decided by using three-antenna method at far-field range, we show that the agreement is good within 1 dB over the whole frequency range.
We examine how accurately the transmit and receive parameters of a radar cross section measurement system can be determined by use of a rotating dihedral as the polarimetric calibration device. We derive expressions for the errors due to misalignment in the angle of rotation. We obtain expressions for the angles a0,hv and a0,vh for which the measured cross-polarization ratios of a target vanish. Since the theoretical cross-polarization of a cylinder is 0, we can .nd the calibration bias-correction angles. We use simulated and real data to demonstrate the robustness of this bias-angle correction technique. We derive expressions for the uncertainty in the polarimetric system parameters.
In this paper is presented an experimental investigation of conventional array calibration in the presence of various kinds of joint discontinuities between array panels. Two rigid array panels were positioned such that the element lattice was continuous across a narrow joint. Three kinds of discontinuities were applied to the joint: (1) an angle, (2) a gap (including an edge), and (3) a step between panels. Each type was investigated for joints oriented in the E-plane and the H-plane. Each discontinuity was also varied in magnitude so as to observe parametric effects. Planar near-field-range (NFR) measurements were made in a conventional array calibration mode and a near-field pattern mode. Processing included separating the pattern component due to element transmission (impedance) change from that due to pattern shape change. Results show that conventional calibration methods quickly become inadequate to calibrate these discontinuities because they change element pattern shapes.
Abstract— A method is proposed that will optimally select the placement of sources to aid in the calibration of a phased array of scalable panels that is mounted on a stationary, ground-based, non-rigid frame. A cost function based on the Cramer-Rao Lower Bound is optimized through constrained minimization. The array is constructed from idealized (non-deforming) subarray panels that have unknown perturbations in orientation and location. To demonstrate the proposed method, several case studies are investigated involving combinations of known calibration sources.
In this paper, reduction of the near-field scanplane in calibration of phased array antennas is discussed. In general, truncation of near-field data can give a considerable reduction of acquisition time. This particularly applies in a larger extent to phased array measurements, where a high number of channels is measured in the calibration process. Also, relative small equipment can be used to measure relative large antennas, which can be cost-effective. In this paper, it is shown that under certain conditions the scanplane, and therefore acquisition time, can be reduced substantially. Based on an example, different scanplane sizes and reduction techniques are considered to investigate and estimate the influence of truncation size on the error in the calibration parameters.