Martial P.A. Haeffelin

Inter‐calibration of CERES and ScaRaB Earth Radiation Budget datasets using temporally and spatially collocated radiance measurements

Comparisons of radiance measurements from overlapping independent Earth and cloud radiation budget (ERB) missions are an important contribution to the validation process of these missions and are essential to the construction of a consistent long-term record of ERB observations. Measurements from two scanning radiometers of different design and calibration, the Clouds and the Earths Radiant Energy System (CERES) and the Scanner for Radiation Budget (ScaRaB), are compared during simultaneous operation in January and March 1999. The instruments are found to be consistent to within 0.5% and 1.5% in the longwave and shortwave spectral domains, respectively.

Early radiometric validation results of the CERES Flight Model 1 and 2 instruments onboard NASA's Terra Spacecraft

Abstract The CERES Flight Model 1 and 2 instruments were launched aboard NASAs Earth Observing System (EOS) Terra Spacecraft on December 18, 1999 into a 705 Km sun-synchronous orbit with a 10:30 a.m. equatorial crossing time. These instruments supplement measurements made by the CERES Proto Flight Model (PFM) instrument launched aboard NASAs Tropical Rainfall Measuring Mission (TRMM) spacecraft on November 27, 1997 into a 350 Km, 38-degree mid-inclined orbit. An important aspect of the EOS program is the rapid archival and dissemination of datasets measured by EOS instruments to the scientific community. On September 22, 2000 the CERES Science Team voted to archive the Edition 1 CERES/Terra Level 1b and Level 2 and 3 ERBE-Like data products. These products consist of instantaneous filtered and unfiltered radiances through temporally and spatially averaged TOA fluxes. CERES filtered radiance measurements cover three spectral bands including shortwave (0.3 to 5 μm), total (0.3 to μ m) and an atmospheric window channel (8 to 12 μm). The current work summarizes both the philosophy and results of validation efforts undertaken to quantify the quality of the Terra data products as well as the level of agreement between the Terra and TRMM datasets.

Use of first-principle numerical models to enhance the understanding of the CERES point spread function

NASAs clouds and the Earths radiant energy system (CERES) program is a key component of the Earth observing system (EOS). The CERES proto-flight model (PFM) instrument is to be launched on NASAs tropical rainfall measuring mission (TRMM) platform on 1 November 1997. Each CERES instrument contains three scanning thermistor bolometer radiometers to monitor the longwave and visible components of the Earths radiative energy budget. An integral part of analyzing these measurements will be the use of high-resolution cloud imager data in conjunction with data from the CERES instruments. The use of high-resolution cloud imager data requires that the point spread function (PSF), or the dynamic response of the radiometric channels as they scan across a far-field point source, be well characterized. The PSF is determined by the field-of-view of the radiometric channel, its optical geometry, and the time response of the thermistor bolometer and its associated signal conditioning electronics. The PSF of the CERES instruments is measured in the laboratory using a state of the art radiometric calibration facility (RCF) developed by TRW. Intrinsic difficulties in making this measurement suggest that a better understanding of the data could be obtained by the use of an independent instrument model. High-level first-principle dynamic electrothermal models of the CERES radiometric channels have been completed under NASA sponsorship. These first-principle models consist of optical, thermal and electrical modules. Accurate optical characterization of the channels is assured by Monte-Carlo- based ray-traces in which tens of millions of rays are traced. Accurate thermal and electrical characterization is assured by transient finite-difference formulations involving thousands of nodes to describe thermal and electrical diffusion within the thermistor bolometer sensing elements and the instrument mechanical structure. The signal conditioning electronics are also included in the models. Numerical simulations of the PSFs of the CERES proto-flight model (PFM) radiometric channels have been completed. This paper presents a comparison between the measured PSF and the independent numerically predicted PSF for the CERES proto-flight model total channel. Agreement between the measured and predicted PSFs is excellent. The result of this agreement is a high confidence in the model to predict other aspects of instrument performance. For example, the model may now be used to predict channel PSFs for elevation scan rates different from the nominal Earth scan rate.

Experimental and theoretical study of uncertainty in pyranometers for surface radiation

The Eppley pyranometer is widely used to measure shortwave irradiances. This instrument consists of a blackened surface in intimate thermal contact with the hot junction of a thermopile. The cold junction of the thermopile is in thermal contact with a heat sink. Shortwave radiation transmitted through two concentric hemispherical domes is absorbed by the blackened surface. The voltage developed by the thermopile is then interpreted in terms of the shortwave irradiance. Measurements obtained using these instruments are known to be influenced by thermal radiation that produces an offset from the signal that would result solely from the incident shortwave radiation. The thermal radiation emitted and reflected by the filters modifies the net radiation at the detector surface. The ongoing efforts to model these exchanges and to use experimental results to verify the model are described. The parallel experimental effort consists of determining the sensitivity of instrument response to thermal radiation effects. In this effort, thermistors are used to characterize the thermal gradients responsible for the instrument offset. The ultimate goal of the work described is to provide reliable protocols, based on an appropriate instrument model, for correcting measured SW irradiance for variable thermal radiation effects.

Numerical simulation of ground calibration of the CERES thermistor bolometer radiometers

NASAs clouds and the Earths radiative energy system (CERES) program is a key component of the Earth Observing System (EOS). Under CERES an array of radiometric instruments will be placed in Earth orbit to monitor the longwave and visible components of the Earths radiative energy budget. High-level dynamic electrothermal models of these instruments have been formulated under NASA sponsorship. Accurate optical and thermal-radiative characterization of the instruments is assured by a Monte-Carlo-based ray-trace in which tens of millions of rays are traced, and a transient finite-difference formulation involving hundreds of nodes is used to describe thermal and electrical diffusion within the thermistor bolometer sensing elements. The external electronic circuit is also correctly included in the instrument model. The actual CERES instruments will undergo pre-launch calibration in a unique thermal-vacuum radiometric calibration facility equipped with blackbodies, a cryogenically cooled active-cavity radiometer, and shortwave sources. This ground calibration can also be simulated using the high-level, dynamic electrothermal models of the CERES instruments. This offers a quick and inexpensive means of verifying the calibration procedure and anticipating any problems that may arise. The results obtained from these simulations may then be used to predict the regression coefficients in the count-conversion equation used to convert instrument readings into radiance, and to determine which parameters should be included in the count-conversion equation to maximize its sensitivity. The paper presents results of the simulated ground calibrations of the CERES total channel instrument, including predicted values for instrument accuracy during the ground calibrations.

Thermistor bolometer radiometer signal contamination due to parasitic heat diffusion

Current efforts are directed at creating a high-level end-to-end numerical model of scanning thermistor bolometer radiometers of the type used in the Earth Radiation Budget Experiment (ERBE) and planned for the clouds and the earths radiative energy system (CERES) platforms. The first-principle model accurately represents the physical processes relating the electrical signal output to the radiative flux incident to the instrument aperture as well as to the instrument thermal environment. Such models are useful for the optimal design of calibration procedures, data reduction strategies, and the instruments themselves. The modeled thermistor bolometer detectors are approximately 40 micrometers thick and consist of an absorber layer, the thermistor layer, and a thermal impedance layer bonded to a thick aluminum substrate which acts as a heat sink. Thermal and electrical diffusion in the thermistor bolometer detectors is represented by a several-hundred-node- finite-difference formulation, and the temperature field within the aluminum substrate is computed using the finite-element method. The detectors are electrically connected in adjacent arms of a two-active-arm bridge circuit so that the effects of common mode thermal noise are minimized. However, because of a combination of thermistor self heating, loading of the bridge by the bridge amplifier, and the nonlinear thermistor resistance-temperature relationship, bridge deflections can still be provoked by substrate temperature changes, even when the change is uniform across the substrate. Of course, transient temperature gradients which may occur in the substrate between the two detectors will be falsely interpreted as a radiation input. The paper represents the results of an investigation to define the degree of vulnerability of thermistor bolometer radiometers to false signals provoked by uncontrolled temperature fluctuations in the substrate.

Dynamic electrothermal model of the ERBE nonscanning channels

The Earth Radiation Budget Experiment (ERBE) consists of three satellites, each carrying a set of three scanning and four nonscanning Earth-viewing radiometric channels. These instruments are used to monitor the components of the Earths radiative energy budget, which include reflected solar radiation and Earth-emitted radiation. The current paper deals with high-order modeling of the nonscanning channels. The nonscanning instruments are active cavity radiometers which view the Earth with two fields of view, medium and wide, and in two wavelength intervals, visible (0.2 to 5.0 micrometers ) and total (0.2 to 50.0 micrometers ). The visible channels are equipped with a hemispherical fused silica filter dome which absorbs long wave radiation. The optical front-end of the nonscanning instruments, including the field-of- view limiter and the filter dome for the visible channels, is modeled using the Monte-Carlo ray-trace method. Work in progress includes a finite difference thermal diffusion model of the filter dome. When completed, this model will be integrated with an existing dynamic electrothermal model of the total channel. This will permit studies of the contamination of the cavity signal due to emission and scattering of radiation from the field-of-view limiter and filter dome. Reported are results for the optical and thermal radiative behavior of the optical front-end of the ERBE WFOV total and visible channels.

Measurement error due to radiation exchanges within pyranometers for surface radiation

The Eppley pyranometer is widely used to measure broadband shortwave irradiances on the earths surface. Measurements obtained using these instruments are known to be influenced by infrared radiation that produces an offset from the signal that would result solely from the incident shortwave radiation. Described is an effort to model the energy exchanges within the instrument to describe the measurement error as a function of external conditions. A finite element method (FEM) analysis simulates heat diffusion in the instrument, and a Monte Carlo ray-trace (MCRT) code models radiative exchange in the instrument. An FEM analysis models heat diffusion in the outer dome of the instrument and produces the dome temperature distribution resulting from specified external boundary conditions. An MCRT code is used to model the influence of radiative exchanges between the domes and the sensor surface on the sensor signal. The code confirms offsets expected from a variety of ambient conditions. The goal of the effort is to create a working MCRT model of the pyranometer that will be combined with the existing FEM model. The completed tool will allow an accurate study of the signal sensitivity to various external conditions.

High-order end-to-end model for the ERBE scanning thermistor bolometer radiometers

The Earth Radiation Budget Experiment (ERBE) consists of a suite of three scanning and four nonscanning radiometric instruments on each of three satellites which monitor the solar- reflected and Earth-emitted components of the Earths radiative energy budget. A numerical model has been formulated to study the dynamic behavior and equivalence of the ERBE scanning thermistor bolometer radiometers. The finite difference method is applied to the detector of the ERBE scanning radiometer to characterize its thermal and electrical dynamic behavior. The thermal analysis confirms the thermal time constant of the instrument claimed by the vendor. The model reveals that the instrument can be very sensitive to spatial variations of the incident thermal radiation. However, the analysis confirms that the hypothesis of equivalence is justified for viewing typical Earth scenes. The high-order numerical model described in this paper is a key component of the end-to-end dynamic electrothermal model under development for the ERBE and CERES scanning radiometric channels.