Checking vitals of CERES scanners: parameters of a slow mode filter

Clouds and the Earth’s Radiant Energy System (CERES) instruments are scanning radiometers on board the Terra, Aqua, NPP and NOAA20 satellites that use thermistor bolometers as detectors, with a sampling rate of 0.01s. During calibration testing a slow mode of detectors was found which had a magnitude of about three percent of the signal, and a characteristic time of 0.3 sec. To reduce the effect of this mode, a numerical filter was introduced. However, an analysis of the data from the CERES instruments aboard the Terra and Aqua satellites showed a difference of radiances in the forward scan compared to radiances from the backward scan. This difference was due to the fact that parameters of the digital filtered derived based on calibration data were not characterized correctly. A focus of this work is to show results of setting slow mode filter parameters based on in-flight data. A gradient-based minimization strategy is employed to effectively find the filter parameters for each instrument to meet the performance goal, namely the divergence of 0.15% for viewing zenith angles <65 degrees. The newly designed numerical filters are used in processing the newest editions of CERES Level-0 data products, and then in the Erbe-like and SSFs.