G. Louis Smith

Clouds and Earth radiant energy system: an overview

The Clouds and Earth radiant energy system (CERES) instrument was first flown aboard the TRMM spacecraft whose 35 inclination orbit allowed for the collection of radiation budget data over all local times, i.e. all solar zenith angles for the latitude range. Moreover, this instrument has gathered the only bidirectional radiance data covering all local times. An additional quartet of CERES instruments are now operating in pairs on both the TERRA and AQUA spacecrafts. Thus far, these instruments have collected several years of Earth radiation budget observations and continue to operate. For each of the TERRA and AQUA spacecrafts, one CERES instrument operates in a cross-track scan mode for the purpose of mapping the Earths outgoing longwave radiation and reflected solar radiation. The other operates in an azimuthal rotation while scanning also in zenith angle for the purpose of gathering measurements for the angular distribution of radiance from various scene types, to improve the computation of fluxes from radiance measurements. The CERES instruments carry in-flight calibration systems to maintain the measurement accuracy of 1% for measured radiances. In addition to retrieving fluxes at the top of the atmosphere, the CERES program uses data from other instruments aboard the spacecraft to compute the radiation balance at the surface and at levels through the atmosphere. 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

Terra spacecraft CERES flight model 1 and 2 sensor measurement precisions: ground-to-flight determinations

On December 18, 1999, the Clouds and the Earth’s Radiant Energy System (CERES) flight models 1 (FM1) and 2 (FM2) sets of scanning thermistor bolometer sensors were launched into orbit aboard the NASA Terra Spacecraft. The sensors measure earth radiances in the broadband shortwave solar (0.3 µm - 5.0 µm) and total (0.3 µm - >100 µm) spectral bands, as well as in the 8 -12 micrometer water vapor window, narrow-band spectral band. In order to measure sensor response drifts or shifts, inflight blackbody and evacuated tungsten lamp calibration systems were built into the CERES instrumentation. These systems were used to determine the sensor responses during the ground/pre-launch, ground to orbit, and on-orbit phases of the sensor calibrations. Analyses of the pre-launch, vacuum ground calibrations indicated that the CERES sensor responses can change as much as 0.6% between vacuum and ground ambient atmospheric pressure environments. The sensor responses were found to vary directly with the temperature as much as 2% between the 311 K and 270 K thermal environment of the vacuum calibration facility. From the vacuum ground calibration through the on-orbit calibration phases, the Terra Spacecraft CERES broadband total and shortwave sensor responses and in-flight calibration sources maintained their radiance measurement ties to an International Temperature Scale of 1990 (ITS-90) radiometric scale at precision levels approaching ± 0.3% (0.3 Wm-2sr-1). Analyses of the ground and on-orbit calibrations are presented and discussed using built-in, reference blackbody and lamp observations.

Validation of CERES/TERRA Data

12 There are 2 CERES scanning radiometer instruments aboard the TERRA spacecraft, one for mapping the solar radiation reflected from the Earth and the outgoing longwave radiation and the other for measuring the anisotropy of the radiation. Each CERES instrument has on-board calibration devices, which have demonstrated that from ground to orbit the broadband total and shortwave sensor responses maintained their ties to the International Temperature Scale of 1990 at precisions approaching radiances have been validated in orbit to +/- 0.3 percent (0.3 W m-2sr-1). Top of atmosphere fluxes are produced by use of the CERES data alone. By including data from other instruments, surface radiation fluxes and radiant fluxes within the atmosphere and at its top, shortwave and longwave, for both up and down components, are derived. Validation of these data products requires ground and aircraft measurements of fluxes and of cloud properties.

On-orbit solar calibrations using the Terra Clouds and Earth's Radiant Energy System (CERES) in-flight calibration system

The Clouds and the Earths Radiant Energy System (CERES) spacecraft scanning thermistor bolometers measure earth- reflected solar and earth-emitted longwave radiances, at the top- of-the-atmosphere. The bolometers measure the earth radiances in the broadband shortwave solar (0.3 -5.0 µm) and total (0.3 - >100 pm) spectral bands as well as in the 8 -12 µm water vapor window spectral band over geographical footprints as small as 10 kilometers at nadir. In December 1999, the second and third sets of CERES bolometers were launched on the Earth Observing Mission Terra Spacecraft. Ground vacuum calibrations define the initial count conversion coefficients that are used to convert the bolometer output voltages into filtered earth radiances. The mirror attenuator mosaic (MAM), a solar diffuser plate, was built into the CERES instrument package calibration system in order to define in-orbit shifts or drifts in the sensor responses. The shortwave and total sensors are calibrated using the solar radiances reflected from the MAM. Each MAM consists of baffle-solar diffuser plate systems, which guide incoming solar radiances into the instrument fields of view of the shortwave and total wave sensor units. The MAM diffuser reflecting type surface consists of an array of spherical aluminum mirror segments, which are separated by a Merck Black A absorbing surface, overcoated with silicon dioxide. Thermistors are located in each MAM plate and baffle. The CERES MAM is designed to yield calibration precisions approaching 0.5 percent for the total and shortwave detectors. In this paper, the MAM solar calibration techniques are presented along with on-orbit measurements.

Overview of CERES sensors and in-flight performance

The Clouds and Earth Radiant Energy System (CERES) instrument is designed to measure the Earths radiation budget and also to make measurements from which the anisotropy of reflected solar radiation can be computed. The instrument design, which is based on the Earth Radiation Budget Experiment (ERBE), and its operations are described. The instrument can scan in elevation and azimuth simultaneously. The azimuthal rotation is important for gathering data to describe the anisotropy of the reflected solar radiance field. The ground vacuum calibration facility ties the calibration of the instrument to the International Temperature Scale of 1990. In-flight calibration sources are included to maintain and demonstrate the required 1 percent accuracy of each mission. Flight operations to achieve the accuracy are also discussed. The CERES Proto-Flight Model is flying on the Tropical Rainfall Measurement Mission spacecraft and successive models are scheduled to fly aboard the EOS/AM-1 and EOS/PM-1 platforms. The objectives of each flight of the instrument are discussed.

Reality check: a point response function (PRF) comparison of theory to measurements for the clouds and the Earth's radiant energy system (CERES) tropical rainfall measuring mission (TRMM) instrument

The first Clouds and Earth Radiant Energy System (CERES) scanning radiometer is scheduled to be launched on the joint US/Japanese Tropical Rainfall Measuring Mission in late 1997. The use of data from the CERES with those from higher resolution imagers requires a detailed knowledge of the CERES PRF, which describes the response of the radiometer to a point at a given location in the field-of-view. The PSF is determined by the field-of-view of the instrument, its optical design, and the time response of the thermistor- bolometer and the associated signal-conditioning electronics. The field-of-view is limited by an elongated hexagonal aperture in the field stop. The PSF has been measured in the laboratory; however, the finite solid angle of the beam used for measurement of the PSF complicates the interpretation of these measurements. This paper discusses the estimation of the PSF of the CERES instruments based on the effects of the time response, the finite solid angle of the beam used in the laboratory calibration, and the experimental output of the instrument. The paper compares the actual instrument output with the predicted results based on a finite solid angle uniform source.

Optical design of the CERES telescope

The Clouds and Earth Radiant Energy System (CERES) instrument was designed to make measurements of solar radiance reflected from the Earth (0.2 to 0.5 microns) and radiance emitted from the Earch (5.0 to 50+ microns) with 1% accuracies. The CERES design evolved from the Earth Radiation Budget Experiment instrument which had similar objectives. The CERES also had a channel to measure radiance in the 8 to 12 micron window emitted by the Earth for studying the effects of water vapor on the Earths radiation budget. A CERES instrument flew on the Tropical Rainfall Measuring Mission and 2 are operating on the TERRA spacecraft. One instrument will map the geographical distribution of radiation and the other will measure the anisotrophy of the radiance field. Two CERES instruments will also fly on the AQUA spacecraft. The design features of the telescope and the rationales are described. These aspects of the instrument should be understood by users of the data for a number of purposes. Each channel has its separate telescope to gather radiation onto its detector, which is a thermistor-bolometer. There is a total channel which measures radiances over the range 0.2 to 50+ microns. The shortwave (0.2-5.0 micron) and window (8-12 micron) channel each have filters to provide the desired band. The emitted radiation is computed as the total minus the shortwave radiance.

Critical overview of radiation budget estimates from satellites

Abstract The progress of Earth radiation budget research has been due to the improvement of instrument accuracy and reliability, and also due to the vast progress in computer capability. The increase of instrument reliability and the resulting longer measurement records has brought the organizational support which is necessary for the improvement of analysis methods for generation of data products and to bring together the science teams which are needed to make good use of the data products. Computers today can process enough data so that we can bring in data streams from more than a single instrument and perform extensive computations. As a consequence, the field has progressed from producing monthly global maps at the top of the atmosphere to producing daily maps and producing radiation profiles through the atmosphere. Radiation data are now used together with other data types to study the weather and climate as a system. Scientific questions addressed by radiation data have evolved from monthly-mean zonal descriptions toward smaller time and space scales.

On-orbit radiometric calibrations of the Earth Radiation Budget Experiment (ERBE) active-cavity radiometers on the Earth Radiation Budget Satellite (ERBS)

Between November 1984 and July 2002, the Earth Radiation Budget Satellite (ERBS)/Earth Radiation Budget Experiment (ERBE) nonscanning, active cavity radiometers (ACR) were used to measure incoming total solar irradiance, earth-reflected solar irradiance, and earth-emitted outgoing longwave radiation (OLR) irradiance. The ERBE shortwave wide field-of view (SWFOV) and toal wide field-of-view (TWFOV) ACRs measured irradiances from the entire earth disc in the shortwave (0.2-5.0 μm) and total (0.2-100 μm) broadband spectral regions. On-orbit, the ACRs observations of the incoming total solar irradiance, and of reference irradiance from on-board tungsten lamp and blackbodies were used to determine drifts and shifts in the ACR responses/gains. In the cases of the SWFOV ACR, its response/gain changed as much as 8.8% while the TWFOV response was stable at levels better than 0.1%. The precise measurements of gain and offset variations have permitted the generations of ERBE level 1 data products [earth-reflected solar (≈240 Wm-2)and earth-emitted (≈100 Wm-2) irradiances] at the precision levels better than 0.3 Wm-2. In this paper, the ACR radiometric on-orbit calibration approaches and systems are outlined.

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.