Jack Paden

Postlaunch radiometric validation of the Clouds and the Earth's Radiant Energy System (CERES) Proto-Flight Model on the Tropical Rainfall Measuring Mission (TRMM) Spacecraft through 1999

Abstract Each Clouds and the Earth’s Radiant Energy System (CERES) instrument contains three scanning thermistor bolometer radiometric channels. These channels measure broadband radiances in the shortwave (0.3–5.0 μm), total (0.3–>100 μm), and water vapor window regions (8–12 μm). Ground-based radiometric calibrations of the CERES flight models were conducted by TRW Inc.’s Space and Electronics Group of Redondo Beach, California. On-orbit calibration and vicarious validation studies have demonstrated radiometric stability, defined as long-term repeatability when measuring a constant source, at better than 0.2% for the first 18 months of science data collection. This level exceeds by 2.5 to 5 times the prelaunch radiometric performance goals that were set at the 0.5% level for terrestrial energy flows and 1.0% for solar energy flows by the CERES Science Team. The current effort describes the radiometric performance of the CERES Proto-Flight Model on the Tropical Rainfall Measuring Mission spacecraft over t...

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.

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.

Effects of atmospheric emissivity on clear sky temperatures

Abstract The accurate determination of atmospheric temperatures from outgoing longwave radiation depends upon the effective atmospheric emissivity used in Stefan-Boltzmanns law. We reviewed the literature dealing with the atmospheric emissivity equations for clear sky and studied their effects on the clear sky atmospheric temperatures. The clear sky outgoing longwave radiation data were taken by the National Aeronautics and Space Administrations (NASA) Earth Radiation Budget Satellite (ERBS) scanning radiometers. The five years (1985–1989) of global annual-mean of clear sky atmospheric temperatures with different emissivity values are presented and discussed in this paper. The effect of emissivity on the retrieval of clear sky atmospheric temperatures are found to vary by 7–8 K (Kelvin).

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.

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.

On-orbit calibrations of the ERBE active-cavity radiometers on the Earth Radiation Budget Satellite (ERBS): 1984-2002

From October 1984 until September 30, 1999, on-orbit, the Earth Radiation Budget Satellite (ERBS)/Earth Radiation Budget Experiment (ERBE) nonscanning, active cavity radiometers (ACR) were calibrated using observations of the incoming total solar irradiance, and of reference irradiances from an on-board tungsten lamp and blackbodies in order to determine drifts and shifts in the ACR responses. On October 7, 1999, the ERBE elevation drive system failed near the earth nadir viewing configuration. Thereafter, the elevation failure prevented observations of the on-board, built-in calibration systems. On July 23, August 8, and December 10, 2002, the ERBS was pitched 180 degrees to observe cold space, representative of a 3 Kelvin blackbody, in order to determine the ACRs zero-irradiance offsets. On December 4, 2002, the ERBS was pitched 180 degrees away from the earth in order to observe the sun, and to determine the ACRs gains. In this paper, the 2002, 180-degree pitch calibrations are compared with the earlier 1984-1999, calibrations which were obtained using the on-orbit, built-in calibration systems. In addition, the 2002 calibrations are compared with earlier scheduled November 21, 1984, and October 20, 1985, 180-degree pitch calibrations, as well as with deep space calibrations from unscheduled July 2, 1987, January 16, 1999, and November 16, 2000, ERBS spacecraft tumbles. The 2002 ACR offsets were found to be consistent with 1984-2000 offsets at the 1.0 Wm-2. 1984-1999, ERBE top-of-the-atmosphere (TOA), and satellite altitude (SA) earth irradiances are presented. Analyses of the TOA ERBE earth irradiances indicate that the TOA irradiance time series exhibited a 1.7 Wm-2 increase as a result of 1988-1992, and 1998-2002 satellite altitudinal decreases during periods of maximum solar magnetic activity.

Determination and validation of slow mode coefficients of the Clouds and the Earth's Radiant Energy System (CERES) scanning thermistor bolometers

The Clouds and the Earth’s Radiant Energy System (CERES) scanning thermistor bolometers have a response time of approximately 9 ms for 98 to 99% of the signal, after which there is a slow change for the remaining 1 to 2% of the response due to a slow mode. This paper describes the theoretical and experimental procedures used in producing the slow mode coefficients for the CERES Flight Models 1 and 2 instruments aboard the Terra spacecraft, which was launched on December 18,1999. The response behavior for the total thermistor bolometer (0.3 - > 100 µm) and window channel (8-12 µm) were determined by analyzing the internal blackbody calibration ground data while the shortwave thermistor bolometer (0.3 - 5 µm) was determined using shortwave internal calibration source ground data obtained at the TRW calibration facility at Redondo Beach, California. These slow mode coefficients agree with the coefficients obtained by analyzing the in-flight calibration data. A numerical filter removes the effects of the slow mode from the measurements. The method may be applicable to other instruments which have spurious transients.