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Kory J. Priestley
,
Susan Thomas
, and
G. Louis Smith

Abstract

The Clouds and the Earth’s Radiant Energy System (CERES) scanning radiometers have been operating to make raster scans of the moon on a quarterly basis to validate the point response function for the three channels of flight models 1–4 aboard the Terra and Aqua spacecraft. Instrument pointing accuracy was verified by this method to 0.2° for the total channel of FM-3. The point response functions were computed from the lunar observations and were found to be nominal with the exception of the FM-2 window channel, which was found to have a region of high sensitivity. This anomaly is attributed to a delamination of the detector flake from the heat sink in that region. The influence of this anomaly is accounted for by the in-flight calibration and has no adverse effect on the application of the data.

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G. Louis Smith
,
D. K. Pandey
,
Robert B. Lee III
,
Bruce R. Barkstrom
, and
Kory J. Priestley

Abstract

The Clouds and Earth Radiant Energy System (CERES) scanning radiometer was designed to provide high accuracy measurements of the radiances from the earth. Calibration testing of the instruments showed the presence of an undesired slow transient in the measurements of all channels at 1% to 2% of the signal. Analysis of the data showed that the transient consists of a single linear mode. The characteristic time of this mode is 0.3 to 0.4 s and is much greater than that the 8–10-ms response time of the detector, so that it is well separated from the detector response. A numerical filter was designed for the removal of this transient from the measurements. Results show no trace remaining of the transient after application of the numerical filter. The characterization of the slow mode on the basis of ground calibration data is discussed and flight results are shown for the CERES instruments aboard the Tropical Rainfall Measurement Mission and Terra spacecraft. The primary influence of the slow mode is in the calibration of the instrument and the in-flight validation of the calibration. This method may be applicable to other radiometers that are striving for high accuracy and encounter a slow spurious mode, regardless of the underlying physics.

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Kory J. Priestley
,
Bruce R. Barkstrom
,
Robert B. Lee III
,
Richard N. Green
,
Susan Thomas
,
Robert S. Wilson
,
Peter L. Spence
,
Jack Paden
,
D. K. Pandey
, and
Aiman Al-Hajjah

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 the first 19 months of scientific data collection.

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Norman G. Loeb
,
Kory J. Priestley
,
David P. Kratz
,
Erika B. Geier
,
Richard N. Green
,
Bruce A. Wielicki
,
Patricia O’Rawe Hinton
, and
Sandra K. Nolan

Abstract

A new method for determining unfiltered shortwave (SW), longwave (LW), and window radiances from filtered radiances measured by the Clouds and the Earth’s Radiant Energy System (CERES) satellite instrument is presented. The method uses theoretically derived regression coefficients between filtered and unfiltered radiances that are a function of viewing geometry, geotype, and whether cloud is present. Relative errors in instantaneous unfiltered radiances from this method are generally well below 1% for SW radiances (std dev ≈0.4% or ≈1 W m−2 equivalent flux), less than 0.2% for LW radiances (std dev ≈0.1% or ≈0.3 W m−2 equivalent flux), and less than 0.2% (std dev ≈0.1%) for window channel radiances.

When three months (June, July, and August of 1998) of CERES Earth Radiation Budget Experiment (ERBE)-like unfiltered radiances from the Tropical Rainfall Measuring Mission satellite between 20°S and 20°N are compared with archived Earth Radiation Budget Satellite (ERBS) scanner measurements for the same months over a 5-yr period (1985–89), significant scene-type dependent differences are observed in the SW channel. Full-resolution CERES SW unfiltered radiances are ≈7.5% (≈3 W m−2 equivalent diurnal average flux) lower than ERBS over clear ocean, as compared with ≈1.7% (≈4 W m−2 equivalent diurnal average flux) for deep convective clouds and ≈6% (≈4–6 W m−2 equivalent diurnal average flux) for clear land and desert. This dependence on scene type is shown to be partly caused by differences in spatial resolution between CERES and ERBS and by errors in the unfiltering method used in ERBS. When the CERES measurements are spatially averaged to match the ERBS spatial resolution and the unfiltering scheme proposed in this study is applied to both CERES and ERBS, the ERBS all-sky SW radiances increase by ≈1.7%, and the CERES radiances are now consistently ≈3.5%–5% lower than the modified ERBS values for all scene types. Further study is needed to determine the cause for this remaining difference, and even calibration errors cannot be ruled out. CERES LW radiances are closer to ERBS values for individual scene types—CERES radiances are within ≈0.1% (≈0.3 W m−2) of ERBS over clear ocean and ≈0.5% (≈1.5 W m−2) over clear land and desert.

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