Search

A conical cavity has been designed and fabricated for use as a broadband passive microwave calibration source, or blackbody, at the National Institute of Standards and Technology. The blackbody will be used as a national primary standard for brightness temperature and will allow for the prelaunch calibration of spaceborne radiometers and calibration of ground-based systems to provide traceability among radiometric data. The conical geometry provides performance independent of polarization, minimizing reflections, and standing waves, thus having a high microwave emissivity. The conical blackbody has advantages over typical pyramidal array geometries, including reduced temperature gradients and excellent broadband electromagnetic performance over more than a frequency decade. The blackbody is designed for use between 18 and 230 GHz, at temperatures between 80 and 350 K, and is vacuum compatible. To approximate theoretical blackbody behavior, the design maximizes emissivity and thus minimizes reflectivity. A newly developed microwave absorber is demonstrated that uses cryogenically compatible, thermally conductive two-part epoxy with magnetic carbonyl iron (CBI) powder loading. We measured the complex permittivity and permeability properties for different CBI-loading percentages; the conical absorber is then designed and optimized with geometric optics and finite-element modeling, and finally, the reflectivity of the resulting fabricated structure is measured. We demonstrated normal incidence reflectivity considerably below −40 dB at all relevant remote sensing frequencies.

Bunching usually occurs when faster trucks and slower trucks are mixed in a truck-shovel mining system. This paper presents the development of discrete-event simulation model to estimate the impacts of bunching on the productivity and efficiency of a truck-shovel system. The bunching effect on production, BEP, is defined to estimate the production sensitivity to the bunching effect on the truck fleet. The simulation results show that the mixed truck fleets with varying performance can cause significant bunching effect if the hauling trucks are from multiple loading sites or dumps. Depending on whether the fleet is over-trucked or under-trucked, the bunching has significant impact on both the fleet productivity and the equipment utilisation. Furthermore, the fleet with higher BEP takes a priority of overtaking the fleet with lower BEP to increase the productivity.

We directly quantify the effect of infrequent calibration on the stability of microwave radiometer temperature measurements (where a power measurement for the unknown source is acquired at a fixed time, but calibration data are acquired at variable earlier times) with robust and nonrobust implementations of a new metric. Based on our new metric, we also determine a component of uncertainty in a single measurement due to infrequent calibration effects. We apply our metric to experimental data acquired from experimental ground-based calibration data acquired from a NASA millimeter-wave imaging radiometer and a NIST radiometer (Noise Figure Radiometer-NFRad). Based on a stochastic model for the NFRad, we determine the random uncertainty of an empirical prediction model of our stability metric by a Monte Carlo method. For comparison purposes, we also present a secondary metric that quantifies stability for the case where calibration data are acquired at a fixed time, but power measurements for the unknown source are acquired at variable later times.

A modified measurement technique and nonlinear least-squares solution method is introduced for determining complex permittivity and permeability in transmission lines. In addition to a two-port S-parameter measurement, a one-port measurement of the shorted sample is employed. For low-loss samples, material property determination using the traditional method can be plagued by resonances when the sample thickness is an integer multiple of the guided half-wavelength in the sample medium. The introduction of the shorted reflectivity measurement reduces the large uncertainties inherent in the resonance effect, and increases the likelihood of determining the correct zero in the fitting of the material properties. The short-circuited sample $S_{11}$ measurement has an increased magnitude compared with the standard two-port measurement, which greatly reduces the uncertainty of this term. The resulting complex permeability and permittivity values obtained with this method are smoother improved solutions, and have lower uncertainty. Results are presented comparing the traditional Nicolson–Ross–Weir solution and National Institute of Standards and Technology iterative method with the proposed technique for a cast epoxy and a ferrite-loaded microwave absorber sample in WR42 waveguide. We also investigate finite element simulated S-parameters of a fictional material and compare the derived material parameters obtained with the proposed method to the “true” permittivity and permeability values.

We apply coherence-propagation theory to model the radiation generated by a planar passive thermal source of any state of coherence. Of our particular interest is the blackbody calibration source with partially coherent characteristic that produces the radiant intensity with its angular distribution following Lambert’s cosine law in the far field. A closed-form expression of the Poynting vector of the electromagnetic field is obtained from the theoretical framework. The formulation links the radiation field to the correlation function of the sources in a straightforward manner, though numerical computation of the Poynting vector involves evaluation of a quadruple integral and is difficult to implement directly, especially when the observation of radiation occurs at a close distance from the source. The study of the close-range radiation would, in particular, benefit the microwave remote sensing radiometric calibration that is encountered in terrestrial laboratories and space-borne satellites. To tackle the challenges in numerical calculation, we have made a few mathematical adjustments to develop a feasible scheme for improved computational efficiency, including reformulation in the angle-impact notation and various simplifications of the integration. We apply the theory and numerical techniques to simulate thermal radiation in some illustrative examples such as an isothermal blackbody source, a blackbody misaligned from the on-axis position, and a nonuniformly heated blackbody target. The coherence property of the blackbody source is shown to possess influential impacts on the radiation arising from such a source, especially in the near-field range where most measurements of the radiation take place in a practical system. The theory and technical approaches provide a systematic and reliable way to quantify the Poynting vector radiated by the blackbody source in a microwave remote-sensing radiometer.

This paper describes the development of a waveguide radiometer to measure noise from millimeter wave electronic components from 75 GHz to 110 GHz. The radiometer will estimate the noise temperature of a device under test (DUT) based on comparison with room temperature and 77K noise standards. This is a standard physical approach in other NIST microwave radiometers. The radiometer is particularly amenable to performing noise temperature as well as noise parameter measurements for amplifier and transistor characterization. As wireless communications progresses towards millimeter wave systems, noise characterization of related components and subsystems becomes essential. We report our progress in radiometer design, construction and verification for millimeter wave noise metrology at NIST.

We outline the theoretical formulation of radiometry of the free-space radiation emitted by a blackbody target. Simulation shows a much smaller drop of radiation intensity of a Lambertian source than that of an incoherent source in the near-field region, indicative of a powerful influence produced by the coherence property of the blackbody source. Further, the coupling of the radiation to a radiometer is formulated by the plane-wave scattering theory of the radiation field.

This paper discusses the design of a microwave blackbody to be used as a primary laboratory standard for passive remote sensing applications. This temperature adjustable design is required to operate and be fully characterized from 10 to 220 GHz. We discuss the challenges involved in designing this type of calibration source and address how improvements can be made to increase performance over blackbodies typically flown on airborne and space-borne instruments. A simplified electromagnetic model for absorber layer optimization is introduced as the precursor to a finite-element, full-wave solution for the calculation of emissivity. A temperature simulation predicts the physical temperature of the blackbody and surrounding chamber. The simulated data are used as inputs to a rigorous calculation of the microwave brightness temperature radiated by the blackbody source. This calculation provides an estimate of the offset between measured physical temperature and radiometrically measured brightness temperature.

A key responsibility for a depositor protection organisation is to be in a position, as soon as it begins operating, to reimburse depositors when a bank fails. A basic requirement is for the deposit insurer to know when a bank failure is going to happen. But not all deposit protection agencies know when that is going to happen given the nature of their mandates, roles and responsibilities. If, for example, the agency is a pay-box, then it might be told by the bank supervisor that a pay out is required without any advance warning. Depositor protection organisations with broader mandates might have more advance warning and even play a key role in determining whether a troubled bank that accepts deposits might be closed. In these latter circumstances, more time can be given for advance preparations. When a bank fails it is often regarded as a crisis and indeed, banking problems can lead to very expensive overhauls of a country banking system. The purpose of this paper is to set out some practical considerations regarding the advance planning that deposit protection agencies should consider doing in order to fulfil their mandates — particularly with respect to reimbursing depositor claims in a timely fashion after a bank failure.

We discuss work at NIST aimed at developing a passive microwave brightness temperature standard. By reducing uncertainty, we can provide better calibrations for future weather and climate-monitoring radiometers. We discuss the calibration procedure used, measured data, and various theoretical and simulated results that have led to an improved understanding of the various uncertainty contributions in the measurement. We overview a Monte Carlo simulation to determine the uncertainty in target brightness temperature as a function of measurement distance and target size. We also discuss other future improvements including an improved blackbody design. The achievable calibration source brightness temperature uncertainty is expected to be reduced from the current 0.7 to 1.0 K from 18 to 65 GHz to less than 0.3 K.

Microwave brightness temperature (T b ) observations by satellite sounders represent some of the most important input data for numerical weather prediction (NWP) models as well as providing vital records for climate trend detection. Each flight instrument, even if built to nominally identical specifications, has a unique calibration, typically based on the implicit assumption that the on-board black body target is well understood and characterized, but even more importantly that the transfer function between the T b received at the antenna and the thermometry embedded in the target is well determined.

We introduce a method to determine the gradient between measured physical temperature and true radiating surface temperature of a passive microwave calibration target (load or blackbody). An empirical cooling-curve fit is employed to determine heat-transfer coefficients that then allow commercial finite-element software to solve for the physical temperature at the surface of the target. Only gradients in the direction parallel to the target's pyramidal structures are determined. Two target insulation thicknesses are investigated and a mean surface radiating temperature is determined. This surface temperature differs from the internally measured physical temperature by a maximum of 0.3 K in an ambient environment. Use of a thicker insulation assembly decreases this temperature bias by 0.1 K.

The authors investigated the effects of a psychiatry clerkship that over 14 years has had a constant training philosophy and faculty and has been located in the same acute general hospital setting. In the study, 169 graduates completed a questionnaire on the effects of the clerkship on their knowledge of psychiatry, management of emotional problems in their patients, and personal development. Based on the graduates' responses, the results reveal that the clerkship indeed has had a lasting impact on its former students. Possible implications for recruitment of students into psychiatry are discussed.

We compare three methods of quantifying illumination efficiency (IE). The ratio IE describes the contribution of energy emitted from a blackbody target to the total energy measured at an antenna aperture in a free-space microwave calibration target radiometric measurement. Measurements are compared at three frequencies: 18 GHz, 22.5 GHz, and 26 GHz. An antenna pattern integration method is compared with a recently developed target-temperature fitting method. These two experimental approaches are also compared to a computational antenna pattern simulation. Results show that the simulation agrees with the experimental fitting method more closely at far-field distances, whereas the antenna pattern integration and experimental fitting method agree at closer distances.

We report a verification method for noise-temperature (NT) measurements on cryogenic low-noise amplifiers (LNAs) at liquid helium temperature. The method uses a comparison between the individual measurements of the LNA and an attenuator and the joint measurements of the tandem of the two. As a first step, we were able to determine the loss and the added NT of the cables that connected the cryogenic devices (the LNA, the attenuator, or their combination) to the test ports outside of a cryostat. The attenuator was also characterized successfully with 3% measurement uncertainty.

We describe and demonstrate a standard radiometer for making microwave brightness-temperature measurements that are traceable to fundamental noise standards. The standard radiometer is based on a National Institute of Standards and Technology (NIST) waveguide radiometer for 18–26.5 GHz, fitted with an antenna to measure radiated power. The fraction of the antenna pattern subtended by the radiating target is determined by anechoic-chamber measurements in which we vary the temperature of the target and measure the received power. Sample measurement results with uncertainties are presented. The typical standard uncertainty for a brightness temperature of around 340 K is about 1 K. The approach should be extendable to other waveguide bands where NIST has radiometers and standards.

MICROWAVE and millimeter wave remote sensing offer several advantages over infrared and visible observations. This is primarily due to the fact that microwave radiation is transparent to clouds and aerosols, thereby aiding in the accurate determination of temperature and water-vapor profiles. Radiometer calibration is the process by which a relationship is obtained between the output indicator of the radiometer usually voltage or current and the noise temperature at the radiometer input [1]. Microwave radiometer calibration targets are used in radiometer systems to provide a calibration source of known brightness temperature. Precise absolute calibration of radiometers for many Earth remote sensing applications require knowledge of the brightness temperature when viewing a calibration target to within 100 mK or better. The targets are typically periodic array of wedges or pyramids constructed of a thermally-conductive substrate coated with a thin layer of microwave absorbing material. Microwave absorbent materials are electrically dispersive in nature. The ideal radiometer calibration target should be a blackbody radiator with an emissivity of unity at the frequencies of interest. In order to efficiently design wideband calibration targets, an accurate electromagnetic and thermal analysis of these structures is essential [2]. Electromagnetic and thermal analysis of one-dimensional periodic wedge shaped structures has been presented in [3,4]. This work uses the coupled-wave approach for the electromagnetic analysis. However the work is not extended to two dimensionally periodic (doubly periodic) pyramidal structures. In this work, we have used the three dimensional Finite Difference Time Domain (FDTD) method to perform full wave electromagnetic analysis of doubly periodic pyramidal structures with a thin coating of electrically dispersive radar absorbent materials. Due to the higher accuracy required in microwave radiometry than in traditional electromagnetic or optical full wave simulations, we have developed a new precise 3D FDTD code rather than use commercial software packages. The code can be used to optimize presently used calibration targets as well as develop novel target designs.

We report on the characterization of blackbody target reflections as part of the recent progress on the development of brightness temperature standards for microwave remote sensing at the National Institute of Standards and Technology. The very low reflections from the blackbody targets used in airborne or satellite remote-sensing systems present challenges on how to extract reflection coefficients from the measurements. A full calibration technique was developed for this study by use of a flat aluminum plate used as a known standard in combination with measurements of the empty anechoic chamber. The theoretical basis and measurement procedures are presented. Calibration results validate the method by showing its independence from measurement hardware and conditions. A comparison between the theoretical prediction of reflection coefficients of a free-standing dielectric slab with well documented physical parameters and the de-embedded reflection coefficients from experiments confirms good calibration accuracy. The specific blackbody target used in this study shows well matched properties with a power reflectivity below −40 dB over the entire measurement band (18 GHz to 26 GHz).

Some important concepts in serial data communications may be demonstrated “live” in the lectureroom using a PC equipped with videoprojector, analog input board, and pickup circuitry. The apparatus displays the wave-shape of the serial signal, along with characters sent or received. It has proven useful for “seeing” the signal and for exploring hardware/software setttings, cable type, cable length and other characteristics of a serial interface./

We review the concept of reverse noise measurements in the context of on-wafer transistor noise characterization. Several different applications of reverse noise measurements are suggested and demonstrated. Reverse measurements can be used to check measurement results, to significantly reduce the uncertainty in |/spl Gamma//sub opt/|, to reduce the occurrence of nonphysical results, and possibly to directly measure or constrain parameters in models of transistors.

We present results of an interlaboratory comparison of S-parameter and noise-parameter measurements performed on 0.12 µm gate-length CMOS transistors. Copies of the same device were measured at three different laboratories (IBM, NIST, RFMD), and the results were compared. Each of the laboratories used a different measurement method, although two used similar commercial systems. Effects of different calibration reference planes are shown. The devices measured have large values of |S 11 |, |S 22 |, and |Γ opt |, and have very low minimum noise figures (below 0.2 dB) over some of the frequency range. For the most part, the measurements at the different laboratories are in reasonable agreement, though there are discrepancies. It is also evident that the noise performance of the devices is better than our ability to measure it.

Fluid dynamic forces, valve design factors and gravity interactively determine the complex motion of prosthetic heart valve occluders. Although motion has been investigated, in vitro, using high-speed image recording, the technique has significant cost and limitations on resolution.The kinematics of mechanical and biological valve occluders in the aortic and mitral positions were assessed by measuring projected dynamic valve area (PDVA). Valves were tested in a pulse duplicator system simulating normal cardiac conditions. To quantify PDVA, light passage through back-illuminated valves was measured by a calibrated photosensor with high-frequency response up to 150 kHz. Ten consecutive cycles were sampled using a PC data acquisition system. The system was calibrated under static conditions using reference areas.Several characteristics can be obtained from PDVA measurement including: maximum and minimum PDVA; rate of change of valve opening and closing PDVA; occluder rebound; and oscillatory open occluder behavior. Biological valves open more rapidly, close more gently, and exhibit no occluder rebound. They are also unaffected by gravity, and vary little in behavior from cycle to cycle compared with mechanical valves.A new method for measuring PDVA has been developed. Distinct differences in performance between valves were identified. It is hypothesized that, aside from patient factors and differences in materials, mechanical valves that mimic the PDVA behavior of biological valves, will lead to reduction of thrombogenicity, cavitation and high-intensity transient signals (HITS), and also reduce sound level and regurgitation.

The accuracy of the Gorlin equation when applied to mixed valve pathology has not been investigated. An in-vitro study was performed to determine how a range of valve regurgitations and stenoses affects the Gorlin aortic valve area.Various combinations of stenosis and regurgitation were simulated by placement of constricting orifices and minimal blockage reflux tubes within a 29-mm prosthetic pericardial valve. The orifice areas ranged from 0.7 cm2 to 1.75 cm2, and regurgitant fraction (RF) ranged from 0 to 0.35. Twenty-eight tests were performed at 70 beats/min, cardiac output of 5 l/min and systole 33-36% of the cycle. The mean pressure drops across the valve were adjusted to a value appropriate to blood density. Peripheral resistance was set to give a mean value of 1,537 dyn.s.cm(-5).The Gorlin area varied up to 0.55 cm2 from the geometric orifice area over the range of regurgitant fractions and stenoses. To improve the Gorlin equation, an amended mean volumetric forward flow rate was obtained by multiplying the cardiac output in the equation by the factor (1 - RF)(-1), to reconcile the equation for valvular regurgitation. The area predicted by the modified equation differed by0.15 cm2 from the non-regurgitant valve geometric orifice area over the range of regurgitation and stenoses simulated.The study supports the validity of the Gorlin equation predicting pure aortic valve stenosis (areas1.3 cm2); however, the equation overestimates the severity of stenoses when regurgitation is present. A modified equation is proposed, which includes regurgitant fraction. The new equation improves the calculation of valve geometric area in the presence of regurgitation and may be useful in cardiac catheterization laboratories where mixed aortic valve pathology is being evaluated.