Ground-based zenith sky abundances and in situ gas cross sections for ozone and nitrogen dioxide with the Earth Observing System Aura Ozone Monitoring Instrument

High-accuracy spectral-slit-function calibration measurements, in situ ambient absorption gas cell measurements for ozone and nitrogen dioxide, and ground-based zenith sky measurements with the Earth Observing System Aura Ozone Monitoring Instrument (OMI) flight instrument are reported and the results discussed. For use of high-spectral-resolution gas absorption cross sections from the literature in trace gas retrieval algorithms, accurate determination of the instrument's spectral slit function is essential. Ground-based measurements of the zenith sky provide a geophysical determination of atmospheric trace gas abundances. When compared with other measurements, they can be used to verify the performance of the OMI flight instrument. We show that the approach of using published high-resolution absolute absorption cross sections convolved with accurately calibrated spectral slit functions for OMI compares well with in situ gas absorption cell measurements made with the flight instrument and that use of these convolved cross sections works well for reduction of zenith sky data taken with the OMI flight instrument for ozone and nitrogen dioxide that are retrieved from measured spectra of the zenith sky with the differential optical absorption spectroscopy technique, the same method to be used for the generation of in-flight data products. Finally, it is demonstrated that the spectral stability and signal-to-noise ratio performance of the OMI flight instrument, as determined from preflight component and full instrument tests, are sufficient to meet OMI mission objectives.     

Characterisation of spectrophotometers used for spectral solar ultraviolet radiation measurements

Spectrophotometers used for spectral measurements of the solar ultraviolet radiation need to be well characterised to provide accurate and reliable data. Since the characterisation and calibration are usually performed in the laboratory under conditions very different from those encountered during solar measurements, it is essential to address all issues concerned with the representativity of the laboratory characterisation with respect to the solar measurements. These include among others the instrument stability, the instrument linearity, the instrument responsivity, the wavelength accuracy, the spectral resolution, stray light rejection and the instrument dependence on ambient temperature fluctuations. These instrument parameters need to be determined often enough so that the instrument changes only marginally in the period between successive characterisations and therefore provides reliable data for the intervening period.     

Calibration transfer algorithm for automated qualitative analysis by passive Fourier transform infrared spectrometry

The automated qualitative analysis of passive Fourier transform infrared (FT-IR) remote sensing data is made difficult by the presence in the data of background and instrument-specific variation. For data collected with a single instrument, variation in the data arises from changes in the infrared background radiance, changes in the atmospheric composition within the field-of-view of the spectrometer, and changes in the instrument response function arising from temperature variation in the spectrometer. When more than one spectrometer is used, the variation in detector responses and phase signatures between instruments serves to complicate further the task of implementing an automated processing algorithm for detecting the signature of a target compound. In this work, a combination of signal processing and pattern recognition methodology is applied directly to the interferogram data collected by the FT-IR spectrometer to implement an automated compound detection procedure that is independent of background and instrument-specific variation. The key to this algorithm is the use of highly attenuating digital filters to isolate in the interferogram the frequencies associated with an analyte absorption or emission band while suppressing information at other frequencies. For the test compounds, acetone and sulfur hexafluoride, it is demonstrated that when this digital filtering procedure is coupled with either piecewise linear discriminant analysis or a back-propagation neural network, an automated detection algorithm can be developed with data from a primary instrument and then subsequently used to predict the presence of analyte signatures in data collected with a secondary spectrometer. Correct classification rates in excess of 92% are obtained for both compounds when the algorithm is applied to data collected with the secondary instrument.     

Polychromatic transmissometer for in situ measurements of suspended particles and gelbstoff in water

The beam attenuation coefficient is an optical parameter that sensitively depends on suspended and dissolved substances in water. Its measurement is not only of interest for an understanding of the radiative transfer in a water column. With appropriate algorithms for data interpretation, it also allows a fast determination of absorbing and scattering matter as time-series measurements or depth profiles that cannot easily be obtained with other methods. An instrument has been developed for measuring spectral attenuation coefficients over a wavelength range from 340 to 785 nm. The optical path length can be set between 0 and 400 mm. This allows application in a wide range of turbidity in coastal and inland (case 2 and case 3) waters and a calibration of the instrument during in-situ measurements. This makes the instrument suitable for long-term applications in which signals from conventional instruments would degrade owing to the biofouling of optical windows. From the data, the amount and the size distribution of suspended particles and the specific absorption of dissolved organic matter are derived in real time. Algorithms based on Monte Carlo methods are available for a classification of transparent particles and phytoplankton.     

Instrumentation for measuring fluorescence cross sections from airborne microsized particles

An experimental instrument for measuring a laser-induced fluorescence spectrum from a single aerosol particle is described. As a demonstration of instrument capabilities, the results of monodisperse 4.7 microm sodium chloride particles doped with fluorescent riboflavin, produced with an inkjet aerosol generator, are presented. The fluorescence of the aerosol particles is excited in the wide range from 210 to 419 nm using a pulsed, tunable optical parametric oscillator laser. The maximum of the fluorescence emission of separately measured particles is detected at 560 nm. The dependence of the fluorescence on the excitation wavelength is studied and fluorescence cross sections are estimated. Agreement between the measured fluorescence data and the literature data for riboflavin is observed.     

Instrument myopia.

The basis for instrument myopia, the tendency to accommodate inappropriately while viewing through an optical instrument, was investigated in three experiments. The first demonstrated that the distance of a peripheral surround, analogous to a field stop, influences accommodation but that the magnitude of the effect cannot account for instrument myopia. The second experiment re-examined the hypothesis that perceived distance can affect accommodation. The data indicate that perceived distance is unlikely to influence accommodation and does not provide an explanation of instrument myopia. The last experiment tested the hypothesis that instrument myopia is a manifestation of the return of accommodation to an intermediate state of rest or equilibrium in the absence of an adequate stimulus for accommodation. Implications of the intermediate-resting-state hypothesis are discussed.     

Comprehensive three-dimensional gas chromatography with parallel factor analysis

Development of a comprehensive, three-dimensional gas chromatograph (GC3) instrument is described. The instrument utilizes two six-port diaphragm valves as the interfaces between three, in-series capillary columns housed in a standard Agilent 6890 gas chromatograph fitted with a high data acquisition rate flame ionization detector. The modulation periods for sampling column one by column two and column two by column three are set so that a minimum of three slices (more commonly four or five) are acquired by the subsequent dimension resulting in both comprehensive and quantitative data. A 26-component test mixture and quantitative standards are analyzed using the GC3 instrument. A useful methodology for three-dimensional (3D) data analysis is evaluated, based on the chemometric technique parallel factor analysis (PARAFAC). Since the GC3 instrument produces trilinear data, we are able to use this powerful chemometric technique, which is better known for the analysis of two-dimensional (2D) separations with multichannel detection (e.g., GC x GC-TOFMS) or multiple samples (or replicates) of 2D data. Using PARAFAC, we mathematically separate (deconvolute) the 3D data "volume" for overlapped analytes (i.e., ellipsoids), provided there is sufficient chromatographic resolution in each of the three separation dimensions. Additionally, PARAFAC is applied to quantify analyte standards. For the quantitative analysis, it is demonstrated that PARAFAC may provide a 10-fold improvement in the signal-to-noise ratio relative to a traditional integration method applied to the raw, baseline-corrected data. The GC3 instrument obtains a 3D peak capacity of 3500 at a chromatographic resolution of one in each separation dimension. Furthermore, PARAFAC deconvolution provides a considerable enhancement in the effective 3D peak capacity.     

Solar spectral irradiometer for validation of remotely sensed hyperspectral data

A new solar spectral irradiometer that operates in the visible and near-infrared spectral ranges has been developed. This instrument takes advantage of a new concept optical head that collects the light that impinges on a hemispheric surface, thus improving the instrument angular response with respect to traditional devices. The technical characteristics of the instrument are investigated and detailed, and its radiometric calibration, performed by means of a Langley-like method, is discussed. A new simplified theoretical model that accounts for the diffuse irradiance observed in an optically thin plane-parallel atmosphere has been developed to improve the fit of the irradiance diurnal evolution. An alternative polynomial parametric representation of monochromatic diffuse irradiance evolution has been attempted, but satisfactory results were not obtained from the fitting of experimental data. The new instrument could be useful to carry out remote-sensing validation campaigns.     

Design and experimental testing of an instrument to evaluate the wear resistance of parts of transportation equipment

Calculations are performed to determine the parameters for the adjustment of a friction dilatometer with allowance for local contact strains caused by impact against structural elements of the instrument. An analysis of the results of the calculations is used as a basis for proposing a method of adjusting it for specific test conditions. Experimental data is presented that demonstrates the feasibility of using the instrument to measure the linear wear of high-strength contacts. Translated from Izmeritel'naya Tekhnika, No. 8, pp. 33–35, August, 1998.     

Comparison of two multiplicative signal correction strategies for calibration transfer without standards

Two multiplicative signal correction (MSC) algorithms are compared for the standardization of data from two near-infrared (NIR) spectrometers. Absorbance spectra were measured from 1000-2200 nm for a set of 45 jet fuel samples. Data from one instrument were standardized to match data from a second instrument using windowed MSC (W-MSC) and moving window MSC (MW-MSC). For W-MSC user-defined windows were selected and for MW-MSC the window size was optimized based on a two-step procedure: 1) assigning a cut off window to avoid over-processing and 2) selection of a specific window size based on sample leverage. For reproducibility studies performed over time on a single instrument, data extending through the last day of the study (63 days outside the calibration) required no preprocessing except a peak alignment correction on day 58. For analysis between the two instruments, successful results were obtained using a sub-region of the data from 1000–1700 nm processed by MW-MSC using a 441 point window. A method of selecting an appropriate window size is proposed based on statistical significance testing.     

Another look at the Grubbs estimators

We consider estimation of the precision of a measuring instrument without the benefit of replicate observations on heterogeneous sampling units. Grubbs (1948) proposed an estimator which involves the use of a second measuring instrument, resulting in a pair of observations on each sampling unit. Since the precisions of the two measuring instruments are generally different, these observations cannot be treated as replicates. Very large sample sizes are often required if the standard error of the estimate is to be within reasonable bounds and if negative precision estimates are to be avoided. We show that the two instrument Grubbs estimator can be improved considerably if fairly reliable preliminary information regarding the ratio of sampling unit variance to instrument variance is available.Our results are presented in the context of the evaluation of on-line analyzers. A data set from an analyzer evaluation is used to illustrate the methodology.     

Linear-array apertures for in-flight dynamic solar calibration of radiometric channels for Earth radiation-budget applications

The zero-frequency gain of nonimaging radiometers used in Earth radiation-budget applications is usually verified by a procedure that allows the instrument to view the Sun through an appropriate attenuating aperture and then equates its response to the known attenuated solar constant. However, channel intercomparison often requires that data from a low-resolution, relatively slow instrument such as an active-cavity radiometer be compared with data from a high-resolution, fast instrument such as a scanning thermistor-bolometer radiometer. In such a case, consideration of the difference in the dynamic responses of the two channels may be important. A novel technique for in-flight measurement of the radiometric transfer function of such instruments is described and then demonstrated through the use of a high-order dynamic model of the total, wide-field-of-view, nonscanning channel of NASA's Earth Radiation Budget Experiment (ERBE).     

Changes in the Radiometric sensitivity of SeaWiFS determined from lunar and solar-based measurements

We report on the lunar and solar measurements used to determine the changes in the radiometric sensitivity of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). Radiometric sensitivity is defined as the output from the instrument (or from one of the instrument bands) per unit spectral radiance at the instrument's input aperture. Knowledge of the long-term repeatability of the SeaWiFS measurements is crucial to maintaining the quality of the ocean scenes derived from measurements by the instrument. For SeaWiFS bands 1-6 (412-670 nm), the change in radiometric sensitivity is less than 0.2% for the period from November 1997 through November 1998. For band 7 (765 nm), the change is approximately 1.5% and for band 8 (865 nm) approximately 5%. The rates of change of bands 7 and 8, which were linear with time for the first eight months of lunar measurements, are now slowing. The scatter in the data points about the trend lines in this analysis is less than 0.3% for all eight SeaWiFS bands. These results are based on monthly measurements of the moon. Daily solar measurements using an onboard diffuser show that the radiometric sensitivities of the SeaWiFS bands have changed smoothly during the time intervals between lunar measurements. Because SeaWiFS measurements have continued past November 1998, the results presented here are considered as a snapshot of the instrument performance as of that date.     

Determination of combined measurement uncertainty via Monte Carlo analysis for the imaging spectrometer ROSIS

To enable traceability of imaging spectrometer data, the associated measurement uncertainties have to be provided reliably. Here a new tool for a Monte-Carlo-type measurement uncertainty propagation for the uncertainties that originate from the spectrometer itself is described. For this, an instrument model of the imaging spectrometer ROSIS is used. Combined uncertainties are then derived for radiometrically and spectrally calibrated data using a synthetic at-sensor radiance spectrum as input. By coupling this new software tool with an inverse modeling program, the measurement uncertainties are propagated for an exemplary water data product.     

Optical fiber-based laser remote sensor for airborne measurement of wind velocity and turbulence

We discuss an optical fiber-based continuous-wave coherent laser system for measuring the wind speed in undisturbed air ahead of an aircraft. The operational principles of the instrument are described, and estimates of performance are presented. The instrument is demonstrated as a single line of sight, and data from the inaugural test flight of August 2010 is presented. The system was successfully operated under various atmospheric conditions, including cloud and clear air up to 12?km (40,300?ft).     

Small-angle light scattering: instrumental design and application to particle sizing

A small-angle integrated light-scattering (SAILS) instrument was designed with the innovative use of a diffusing plate and a charge-coupled device (CCD) camera. In contrast to previous small-angle light-scattering instruments, SAILS has few optical surfaces, allowing the direct recovery of scattering data. Although this approach bypasses the need for aberration corrections that are due to lenses, geometric corrections still apply and are described in detail. The image on the diffusing plate, when photographed by the CCD camera, yields a digitized two-dimensional array, covering the observed scattering angles from 10 to 20 deg. The size distribution of the scattering particles can be obtained by a discrete inversion of the experimentally obtained intensity versus angle-scattering curve. The mean radii obtained from this inversion of SAILS data are compared with nominal sizes given by the manufacturer, and standard errors are computed. The results indicate that SAILS is an ideal instrument for the study of particulates and, because of its fast readout time, is suitable for studying aggregation phenomena. However, because of the limited Q range of SAILS it is currently not suited for the direct determination of particle diameters smaller than approximately 300 nm.     

测量仪器校准间隔的确定及其模型

讨论了测量仪器校准间隔的优化及其随机模型，该模型基于如下假设：测量仪器的校准状态可以通过一个观测参数进行监测。中还给出了不同情况下的随机漂移模型，并通过实验数据对随机模型进行了验证。     

Transfer of multivariate classification models between laboratory and process near-infrared spectrometers for the discrimination of green Arabica and Robusta coffee beans

Analogous to the situation found in calibration, a classification model constructed from spectra measured on one instrument may not be valid for prediction of class from spectra measured on a second instrument. In this paper, the transfer of multivariate classification models between laboratory and process near-infrared spectrometers is investigated for the discrimination of whole, green Coffea arabica (Arabica) and Coffea canefora (Robusta) coffee beans. A modified version of slope/bias correction, orthogonal signal correction trained on a vector of discrete class identities, and model updating were found to perform well in the preprocessing of data to permit the transfer of a classification model developed on data from one instrument to be used on another instrument. These techniques permitted development of robust models for the discrimination of green coffee beans on both spectrometers and resulted in misclassification errors for the transfer process in the range of 5-10%.