Calibration Constants Research Articles

Planck2013 results. V. LFI calibration

We discuss the methods employed to photometrically calibrate the data acquired by the Low Frequency Instrument on Planck. Our calibration is based on a combination of the Orbital Dipole plus the Solar Dipole, caused respectively by the motion of the Planck spacecraft with respect to the Sun and by motion of the Solar System with respect to the CMB rest frame. The latter provides a signal of a few mK with the same spectrum as the CMB anisotropies and is visible throughout the mission. In this data release we rely on the characterization of the Solar Dipole as measured by WMAP. We also present preliminary results (at 44GHz only) on the study of the Orbital Dipole, which agree with the WMAP value of the Solar System speed within our uncertainties. We compute the calibration constant for each radiometer roughly once per hour, in order to keep track of changes in the detectors' gain. Since non-idealities in the optical response of the beams proved to be important, we implemented a fast convolution algorithm which considers the full beam response in estimating the signal generated by the dipole. Moreover, in order to further reduce the impact of residual systematics due to sidelobes, we estimated time variations in the calibration constant of the 30GHz radiometers (the ones with the largest sidelobes) using the signal of a reference load. We have estimated the calibration accuracy in two ways: we have run a set of simulations to assess the impact of statistical errors and systematic effects in the instrument and in the calibration procedure, and we have performed a number of consistency checks on the data and on the brightness temperature of Jupiter. Calibration errors for this data release are expected to be about 0.6% at 44 and 70 GHz, and 0.8% at 30 GHz. (Abriged.)

Application of the Incremental Hole-Drilling Method on Thick Film Systems—An Approximate Evaluation Approach

A new evaluation procedure is presented regarding the application of the incremental hole-drilling technique to determine in-depth residual stress distribution in thick film systems. Using a direct correction of the measured strain relaxation, obtained after each incremental depth, to take into account the effect of the layered structure of the material, the standard evaluation procedures can still be used without any further modification. For example, the standardized integral method uses specific geometrical calibration constants, which are only valid for isotropic, homogeneous and linear elastic materials. Using artificial neural networks, for the prediction of the correction function to be applied to the measured strain relaxation in thick layered systems, the standard integral method can still be applied. The validation of the proposed approach is shown using finite element (FE-) simulations. In addition, the new evaluation strategy is applied for residual stress determination in a thermally sprayed Aluminum oxide coating on a steel (S690QL) substrate.

Calibration of a novel instrument for the investigation of small permeation fluxes of gases through membranes

In this study the calibration and the determination of the sensitivity and reproducibility of a permeation measurement equipment are presented. It is showed that the calibration constant describing the relationship between the material current entering a vacuum chamber through a membrane and the pressure increase over the base pressure in this chamber is dependent on the type of gas used but independent of the membrane temperature. The reproducibility of the calibration constant is approx. ±13%, which can be accepted as the reproducibility of the equipment. The results indicate that the minimum material current required for the determination of the transport parameters in a mounted membrane is in the range of 10-11⌊mols-1⌋ and the maximum measurable permeating current is 5 orders of magnitude higher than this. Using various mountable membrane thicknesses this means that the transport properties of a wide range of materials can be investigated with the equipment.

Validation of the hypercapnic calibrated fMRI method using DOT–fMRI fusion imaging

Calibrated functional magnetic resonance imaging (fMRI) is a widely used method to investigate brain function in terms of physiological quantities such as the cerebral metabolic rate of oxygen (CMRO2). The first and one of the most common methods of fMRI calibration is hypercapnic calibration. This is achieved via simultaneous measures of the blood-oxygenation-level dependent (BOLD) and the arterial spin labeling (ASL) signals during a functional task that evokes regional changes in CMRO2. A subsequent acquisition is then required during which the subject inhales carbon dioxide for short periods of time. A calibration constant, typically labeled M, is then estimated from the hypercapnic data and is subsequently used together with the BOLD–ASL recordings to compute evoked changes in CMRO2 during the functional task. The computation of M assumes a constant CMRO2 during the CO2 inhalation, an assumption that has been questioned since the origin of calibrated fMRI. In this study we used diffuse optical tomography (DOT) together with BOLD and ASL – an alternative calibration method that does not require any gas manipulation and therefore no constant CMRO2 assumption – to cross-validate the estimation of M obtained from a traditional hypercapnic calibration. We found a high correlation between the M values (R=0.87, p<0.01) estimated using these two approaches. The findings serve to validate the hypercapnic fMRI calibration technique and suggest that the inter-subject variability routinely obtained for M is reproducible with an alternative method and might therefore reflect inter-subject physiological variability.

Assessment of Langley and NASA-GISS Calibration Techniques for MFRSR Aerosol Retrieval

Sun photometric measurements are an important method of measuring the column amount and optical properties of atmospheric aerosols and are frequently used to better understand the impact of aerosols on the Earth's radiative budget. This paper assesses two calibration techniques used for multifilter rotating shadowband radiometers (MFRSRs) and presents the results obtained at Long Island, NY, during the July 2011 Aerosol Life Cycle Intensive Operational Period (IOP) campaign. The instrument calibration constants are validated internally against each other and against an independent quantitative technique based on the solar irradiance at the top of the atmosphere, convolved with the MFRSR filter response. In addition, the aerosol optical parameters obtained from the MFRSR are assessed against the same parameters retrieved with a recently calibrated CIMEL sun/sky radiometer collocated with the MFRSR instrument. These comparisons indicate generally good agreement between the two instruments for both calibration techniques. The advantages and disadvantages of the two techniques are presented. Additionally, an analysis of the optical and physical properties of aerosols from the MFRSR and their chemical compositions obtained by an in situ high-resolution time-of-flight aerosol mass spectrometer, together with back trajectories, indicate that the principal source of high concentrations of fine aerosols observed during July 18-24 was forest fires in western Canada.

Residual Stress Measurements in Finite-Thickness Materials by Hole-Drilling

Hole-drilling measurements of residual stresses are traditionally made on materials that are either very thick or very thin compared with the hole diameter. The calibration constants needed to evaluate the local residual stresses from the measured strain data are well established for these two extreme cases. However, the calibration constants for a material with finite thickness between the extremes cannot be determined by simple interpolations because of the occurrence of local bending effects not present at either extreme. An analytical model is presented of the bending around a drilled hole in a finite thickness material and a practical procedure is proposed to evaluate the corresponding hole-drilling calibration constants.

FT-IR spectra of 17O-enriched CO2 in the ν3 region: High accuracy frequency calibration and spectroscopic constants for 16O12C17O, 17O12C17O, and 17O12C18O

We report new Fourier transform infrared (FT-IR) spectroscopy measurements of the 17O-enriched isotopologues of CO2 in the ν3 region (2200–2450cm−1, 65–75THz) with absolute calibration accuracies better than 3×10−6cm−1 (∼100kHz) for strong, isolated transitions. Spectra were obtained using the JPL Bruker IFS-125HR Fourier Transform Spectrometer (JPL-FTS). The JPL-FTS performance was achieved by vigorously maintaining the optical alignment, regularly monitoring its modulation efficiency while minimizing phase errors to deliver absolute frequency calibration and line position accuracies comparable to typical sub-millimeter/THz spectroscopy. Fits of the 00011←00001 fundamental bands for 16O12C17O (627), 17O12C17O (727), and 17O12C18O (728) were obtained with RMS residual values of 2.9×10−6cm−1 for all three isotopologues. These results provide a complete and accurate set of transition frequencies required for remote sensing of planetary atmospheres, especially Venus and Mars, and represent a new level of Doppler measurement accuracies.

Photoacoustic measurement of the Grüneisen parameter using an integrating sphere.

A method that uses an integrating sphere as a platform for photoacoustic measurement of the Grüneisen parameter Γ of absorbing liquids is developed. Derivation of a simple equation for determining Γ is presented. This equation only requires the voltage peak-to-peak value of the photoacoustic signal detected by a flat transducer and the relative energy of the incident light measured by a photodetector. Absolute detector sensitivities are not required. However, a calibration procedure is necessary. An experimental setup is constructed in order to implement and verify the method. Aqueous ink solutions are used as absorbing liquids to determine the calibration (instrument) constants. Validation of the equation is done by determining Γ of ethanol at room temperature. The obtained value of Γ(ethanol) = 0.72 ± 0.06 has a 7% relative difference to the calculated value from known thermal properties reported in literature.

Active Pulse Vision Systems and Algorithms for Determining Distances to Objects

It is found analytically and experimentally that four characteristic distances intervals can generally be distinguished in the formation of images of objects in active pulse vision systems. A physical interpretation of their origin is given. Four algorithms are proposed for determining the distance to objects using the characteristic distances or for measuring calibration constants. The depth of the visibility zone is found to depend on the sum of the durations of the laser illumination pulse and the strobe pulse of the receiver system.

Retrieval of characteristic parameters for water vapour transmittance in the development of ground-based sun–sky radiometric measurements of columnar water vapour

Abstract. Sun–sky radiometers are instruments created for aerosol study, but they can measure in the water vapour absorption band allowing the estimation of columnar water vapour in clear sky simultaneously with aerosol characteristics, with high temporal resolution. A new methodology is presented for estimating calibration parameters (i.e. characteristic parameters of the atmospheric transmittance and solar calibration constant) directly from the sun–sky radiometer measurements. The methodology is based on the hypothesis that characteristic parameters of the atmospheric transmittance are dependent on vertical profiles of pressure, temperature and moisture occurring at each site of measurement. To obtain the parameters from the proposed methodology some seasonal independent measurements of columnar water vapour taken over a large range of solar zenith angle simultaneously with the sun–sky radiometer measurements, are needed. In this work high time resolution columnar water vapour measurements by GPS were used as independent data set, but also the case when such measurements are not available was considered by developing the surface humidity method (SHM). This methodology makes it possible to retrieve the needed independent data set of columnar water vapour using the standard surface meteorological observations (temperature, pressure and relative humidity) more readily available. The time pattern of columnar water vapour from sun–sky radiometer retrieved using both the methodologies was compared with simultaneous measurements from microwave radiometer, radiosondings and GPS. Water vapour from sun–sky radiometer, obtained using GPS independent measurements, was characterized by an error varying from 1% up to 5%, whereas water vapour from SHM showed an error from 1% up to 11%, depending on the local columnar water occurring at the site during the year. These errors were estimated by comparing water vapour series from sun–sky radiometer against measurements taken by GPS at a nearby station. The accordance between retrievals from sun–sky radiometer and simultaneous measurements from the other instruments was found always within the error both in the case of SHM and of the GPS independent data set. Water vapour obtained using characteristic parameters of the atmospheric transmittance dependent on water vapour was also compared against GPS retrievals, showing a clear improvement with respect to the case when these parameters are kept fixed.

Human vision noise model validation for the U.S. Army sensor performance metric

Image noise originating from a sensor system is often the limiting factor in target acquisition perfor- mance, especially when limited by atmospheric transmission or low-light conditions. To accurately predict target acquisition range performance for a wide variety of imaging systems, image degradation introduced by the sen- sor must be properly combined with the limitations of the human visual system (HVS). This crucial step of incor- porating the HVS has been improved and updated within NVESD's latest imaging system performance model. The new noise model discussed here shows how an imaging system's noise and blur are combined with the contrast threshold function (CTF) to form the system CTF. Model calibration constants were found by presenting low-contrast sine gratings with additive noise in a two alternative forced choice experiment. One of the principal improvements comes from adding an eye photon noise term allowing the noise CTF to be accurate over a wide range of luminance. The latest HVS noise model is then applied to the targeting task performance metric respon- sible for predicting system performance from the system CTF. To validate this model, human target acquisition performance was measured from a series of infrared and visible-band noise-limited imaging systems. © 2014

Analytical techniques for retrieval of atmospheric composition with the quadrupole mass spectrometer of the Sample Analysis at Mars instrument suite on Mars Science Laboratory

The Sample Analysis at Mars (SAM) instrument suite is the largest scientific payload on the Mars Science Laboratory (MSL) Curiosity rover, which landed in Mars׳ Gale Crater in August 2012. As a miniature geochemical laboratory, SAM is well-equipped to address multiple aspects of MSL׳s primary science goal, characterizing the potential past or present habitability of Gale Crater. Atmospheric measurements support this goal through compositional investigations relevant to martian climate evolution. SAM instruments include a quadrupole mass spectrometer, a tunable laser spectrometer, and a gas chromatograph that are used to analyze martian atmospheric gases as well as volatiles released by pyrolysis of solid surface materials (Mahaffy et al., 2012). This report presents analytical methods for retrieving the chemical and isotopic composition of Mars׳ atmosphere from measurements obtained with SAM׳s quadrupole mass spectrometer. It provides empirical calibration constants for computing volume mixing ratios of the most abundant atmospheric species and analytical functions to correct for instrument artifacts and to characterize measurement uncertainties. Finally, we discuss differences in volume mixing ratios of the martian atmosphere as determined by SAM (Mahaffy et al., 2013) and Viking (Owen et al., 1977; Oyama and Berdahl, 1977) from an analytical perspective. Although the focus of this paper is atmospheric observations, much of the material concerning corrections for instrumental effects also applies to reduction of data acquired with SAM from analysis of solid samples.

Airborne SAR radiometric calibration using point targets

The trihedral corner reflector, with high stability, large RCS and little change within a wide angle range, is widely used in SAR radiometric calibration. Results from airborne SAR overflights of corner reflectors are utilized to compute calibration constant, transfer function of the system and backscattering coefficient of different targets. The key step of SAR calibration is the derivation of calibration constant. In this paper, we used two methods, the peak and integral method, to compute the calibration constant. Through real flight data, we found that, using point target for SAR calibration is simple and practicable.

Calibration of a Sky radiometer (Prede) using observations obtained from Hanle and Merak high-altitude stations in Ladakh

The present work was carried out to check the performance of a Prede Sky radiometer and the stability of the solar calibration constant (F0s) at two high altitude pristine stations, Hanle and Merak in Ladakh region during 2008 to 2012. Two procedures for F0s calculation were applied: one based on the normal Langley plot technique, the other based on a modified method that allows determining F0s from measurements of sky irradiance. The instrument measures sun/sky irradiance at five wavelengths in the visible and near infrared regions, and the data were processed using 4.2 version of the Skyrad.Pack software. The inputs of the software are solid view angles (SVA) at the five wavelengths and sky irradiances at different scattering angles normalized by the direct irradiances. Auxiliary sets of data such as surface albedo, atmospheric pressure, and total column ozone of the station are required to run the software. The software has two processing levels. In the first scheme, Aerosol optical depth (AOD) is retrieved along with single scattering albedo (SSA), phase function, and volume size distribution as the products of radiative transfer calculation. In the second scheme, AOD and the other parameters are calculated from Lambert–Beer law after estimating the F0s from the first scheme. On an average, the deviation of SVA during the last five year period was found to be stable with less than 1.0% from the SVA values set during installation. The estimated F0s from normal and improved Langley methods were quite agreed. The estimated F0s show temporal variation and the uncertainties of estimated F0s in the present study were within 2%. Further, uncertainties of AOD and SSA due to ±2% variation in the F0s were within 0.02 and 0.2, respectively. The estimated coefficients of variation (CV) of the determined F0s show a dependence on the observed AOD.

Online Software Time Calibration for a Continuous Air Shower Array

Time calibration is a crucial item for a shower array performance as it uses the time of flight method to reconstruct the arrival direction of the primary particle. This paper presents a software time calibration algorithm exploiting the continuous detector feature and based on the assumption of locally flat shower front; accordingly, taken a small portion of the detector (tens of m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), a simple time-position fit of the arriving particles provides the time calibration constants of that part of detector. In a second step, the time offsets among the different portions are measured and the complete detector calibration is obtained. The implementation of this algorithm in an online environment like the ARGO-YBJ data acquisition system has been studied. The results of this investigation are reported and discussed.

Photoluminescence imaging for determining the spatially resolved implied open circuit voltage of silicon solar cells

Photoluminescence imaging has widely been used as a characterisation tool for the development of silicon solar cells. However, photoluminescence images typically only give qualitative information due to the presence of an unknown calibration constant. In this work, quasi-steady-state photoconductance measurements on partially processed solar cells and I-V measurements on finished solar cells are used to determine the calibration constants to yield spatially resolved implied open circuit voltage images. This technique is then applied to determine the implied open circuit voltage of laser doped selective emitter solar cells at various stages of cell fabrication after the formation of the full area aluminium back surface field when other characterisation techniques such as photoconductance cannot be used.

Voltage calibration of luminescence images of silicon solar cells

A variety of methods extracting spatially resolved information of solar cell parameters using luminescence imaging on silicon solar cells have been introduced in the past years. Nearly, all methods base on calibrating the local luminescence intensity to local junction voltage. The different methods, however, use different approaches to calibrate the luminescence images to local junction voltage. The assumptions made by the voltage calibration approaches are fundamental for a correct voltage calibration and hence for a correct extraction of other relevant local solar cell parameters. In this work, we review the different voltage calibration approaches and analyse the validity and the effect of the underlying assumptions carefully using circuit simulation results. We present experimental results of voltage calibrated luminescence images of a multicrystalline silicon solar cell using different voltage calibration approaches. We show the relevance of accurate voltage calibration by performing a sensitivity analysis investigating the sensitivity of local cell parameter results obtained by a method known from literature with respect to changes in the calibration constant. We conclude proposing an approach for voltage calibration which is the most reasonable trade-off between accuracy and sufficiently low data acquisition times.

BALATON FELETT MÉRT GLOBÁL- ÉS REFLEXSUGÁRZÁS ÉRTÉKEK KORREKCIÓJA

Albedo measurements have been carried out since 2007 above the Lake Balaton near Keszthely and Siofok. It turned out that a systematic offset error was superposed to both the global and the reflex radiation. The value of this systematic error was approximately constant per pyranometer within a year but on the other hand it varied from year to year and from pyranometer to pyranometer. In this paper the values of this systematic errors were determined with two different methods. The difference between the values measured at night-time and the intrinsic thermal offset error of pyranometers were examined with both methods. The base of the first method is the empirical observation, that the values measured at night-time by a global radiometer are typically negative whereas by a reflex radiometer are typically positive. The substance of the second method is utilizing the air temperature measured within 1 as well as 5 hours before the radiation measuring to +select the fully overcast nights, when the thermal offset error of the global radiometer is zero. In addition, the cases where the thermal offset error of the reflex radiometer is zero were selected on the basis of the difference between water and air temperature. When the thermal offset error is zero the measured value is equal to the systematic error. Comparing the results of the two methods showed that the systematic error of the global radiometer were determined with uncertainty of 1 Wm–2, whereas that of the reflex radiometer with uncertainty of 2 Wm–2. The calibration constants were recalculated from the values being in the calibration reports taking the systematic errors into account.

FAO-56 Penman-Monteith DailyETofrom Linear Regression Calibrated Hargreaves Equation with Wind Terms in Tropics with Limited Data

Hargreaves equation (HG), which lacks a wind speed (u2) term, was modified, through a linear regression calibration method, into LHGu which hasu2terms. LHGu is effectively a simplified method for approximating FAO-56 Penman-Monteith equation (FPM) daily reference evapotranspiration (ETo) in tropics with only temperature data. In LHGu, the “0.0023” constant term in HG was calibrated as a shifted power function ofu2, and the calibration constant was parametrized as a quadratic function ofu2. LHGu was developed using simulated constantu2data and historical temperature data for four sites in West Africa: Abidjan, Accra, Daloa, and Lome. LHGu matched FPMETobetter than HG over a wide range ofu2: for Accra, foru2range 0.5–6.0 m/s, the modified coefficient of efficiency,E1, varied narrowly (0.83–0.98) for LHGu but widely (0.14–0.95) for HG optimized foru2=2.0 m/s; the corresponding MBE ranges were −0.05–0.01 mm/d for LHGu and 0.02–0.63 mm/d for HG which cannot respond to varying dailyu2. LHGu is useful for quickly computing practically accurate estimates of FPMETofor varying dailyu2where only temperature data are available.

A parametric model for predicting cut point of hydraulic classifiers

A new parametric model was developed to predict the cut point of hydraulic classifiers. The model directly uses operating parameters including pulp flowrate, feed particle size characteristics, pulp solids content, solid density and particles retention time in the classification chamber and also covers uncontrollable errors using calibration constants. The model applicability was first verified using a bench scale classifier and then validated at industrial scale for a coal classifier. The results showed that the new model can predict the cut point more precisely compared to the conventional Masliyah model, i.e. the accuracy values of 80% and 37% for the new and Masliyah models, respectively. Sensitivity study showed that the model was extremely sensitive to the particle size distribution of feed while being least sensitive to the particles retention time.