A primary objective of the effort described here is to optimize the performance of a modeling environment for radiometric instruments capable of predicting their complete end-to-end behavior, integrating the optical, electrothermal, and electronic systems. The numerical environment consists of a Monte Carlo ray-trace (MCRT) model of the optical system coupled to a transient three-dimensional finite-difference electrothermal model of the detector assembly with an analytical model of the signal-conditioning circuitry. The resulting model provides a complete simulation of the dynamic optical and electrothermal behavior of the instrument. The modeling environment has been used to create an end-to-end model of the CERES scanning radiometer, and its performance compared to the calibration performance of an operational CERES total channel as a benchmark. To optimize the accuracy of the electrothermal model, the nominal properties of certain key parameters in that model are modified using an evolutionary search algorithm such that the model's simulated output exactly matches the actual instrument ground calibration data. Results indicate that varying the layer thickness, effective thermal conductivity, and effective thermal capacitance of the thermistor, kapton, and epoxy layers in the thermistor bolometer within reasonable uncertainty bounds provides an excellent match with the recorded instrument data.
The objective of the current research is to study the theoretical uncertainty of the CERES ERBE-like level 1 and level 2 instantaneous filtered and unfiltered radiance data products. The instrument views incident radiation from an Earth scene, which is then focused on a blackened thermistor bolometer, where it is converted to an electrical output. The measured digital counts are converted to a filtered radiance by means of instrument calibration coefficients. The filtered radiance is then converted to an unfiltered radiance with an algorithm that utilizes the instrument's spectral response function. Uncertainties in the calibration sources and the spectral response function of the instrument can negatively affect the quality of the final data products. A statistical study of the data products' sensitivity to various instrument and calibration parameters is performed using high-fidelity first-principle numerical models of the CERES instrument. Once the key parameters are identified, fidelity intervals of the data products are calculated using nominal parameter values and uncertainty distributions.
The objective of the current research is to minimize the theoretical uncertainty of the CERES ERBE-like level 1 instantaneous filtered and unfiltered radiance data products. The instrument's measured digital counts are converted to a filtered radiance by means of instrument calibration coefficients. The filtered radiance is then converted to an unfiltered radiance with an algorithm that utilizes the instrument's spectral response function. Uncertainties in the calibration sources and the spectral response function of the instrument can negatively affect the quality of the final data products. To reduce this effect, we are seeking to increase our understanding of the relative impact that various instrument and calibration parameters have on the level-1 filtered and unfiltered data products. Results of a statistical study of data products sensitivity to various instrument and calibration parameters are presented. The sensitivity of the level-1 data products to the spectral response of the instrument when viewing non- Planck Earth scenes is also discussed.
In a previous effort the authors developed a methodology for describing uncertainty in thermal radiation Monte Carlo ray- trace (MCRT) analyses. An application to radiometric channels used in space-based observations such as those provided by NASA's Clouds and the Earth's Radiant Energy System (CERES) was reported.. In the previous study preliminary attempts were presented to confirm the theory. In the current effort extension and modifications of the previous theory are formulated and new examples are presented to confirm the extended theory. A generic MCRT- based computational environment that simulates radiative exchange among surfaces in enclosures is used to obtain performance estimates of a simple cavity-type thermal radiation detector. Standard statistical methods are used to interpret the results as uncertainties and their related confidence intervals. The example problem is used as a vehicle to verify the modified theory. The authors then demonstrate the theory in a more complex situation that of a high-level numerical model used to predict the dynamic opto- electro thermal behavior of the CERES radiometric channels.
Scientists at NASA's Langley Research Center, in collaboration with researchers at Virginia Tech, are developing the next generation of thermal radiation detectors composed of new space-age materials, including carbon-doped Larc-Si and aerogels. In order to accurately model and design these detectors, it is necessary to determine the in situ thermoelectric properties of these new materials, including thin-film effects and contact resistance. The authors present an approach to determine these properties through the use of simultaneous parameter estimation methods in which experimental results obtained from detector prototypes are compared with results predicted from analytical models. Parametric values are varied using an optimization method to minimize the least-squares error between the experimental and model results. A numerical study is presented to validate the use of this approach. Simulated experimental results were produced using a model based on nominal parameter values. These results were then introduced into a parameter estimation algorithm that was able to recover the parameter values without the benefit of a priori knowledge about the material properties. Genetic algorithms, stochastic hill climbers, and a hybrid of the two methods were investigated for use in parameter estimation.
Atmospheric aerosols play an important role in determining the Earth's radiant energy budget and hence its climate. Aerosol measurement are needed both for intensive aerosol/climate field studies and to validate satellite measurements of aerosols and radiation. Solar aureolemeters measure the angular distribution of sunlight scattered by atmospheric aerosols in the circumsolar sky. The data obtained are interpreted in terms of aerosol single- scattering phase function, aerosol size distribution, and aerosol index of refraction. Recently interest has turned to tracking airborne instruments capable of sounding vertical profiles of these quantities. A significant engineering problem inherent to practical aureolemeters is the need to occult the solar disk while viewing the circumsolar sky. This is accomplished with an occulting disk mounted a meter or more in front of the instrument aperture. It is clear that such a long protrusion into the air stream is unacceptable for a tracking airborne instrument. The novel concept reported here uses two-mirror reflective optics in which radiation from the solar disk is captured by a cooled cavity behind an aperture at the center of the secondary mirror. This innovation makes it possible for the first time to implement a compact aureolemeter whose physical length is on the order of centimeters rather than meters.
Described is thermal radiation detector conceived for possible deployment on GERB (Geostationary Earth Radiation Budget). It consists of a linear array of 256 elements, each 60 micrometer square and separated by a 3-micrometer gap. Each element is the active junction of a single-junction-pair zinc- antimonide/platinum thermopile. The reference junction is mounted on an isothermal substrate, and the active junction is thermally isolated from the substrate by a thin layer of parylene. The detector is mounted on one wall of a wedge- shaped, mirrored cavity intended to increase the effective absorptivity and improve the spectral flatness of the detector through multiple reflections. A dynamic opto-electrothermal model of the detector/cavity combination has been formulated in order to facilitate its optimal design. The optical part of the model is based on a Monte-Carlo ray trace that takes into account diffraction at the entrance slit as well as the diffuse and specular components of reflectivity of the cavity surfaces. Heat absorption and diffusion through the thermopile structure has been modeled using the finite element method. The model has been used to validate a method for eliminating optical cross-talk among elements of the array through post- processing of data.
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