Removing Unwanted Fluctuations in the AVHRR Thermal Calibration Data Using Robust Techniques

Abstract

The study deals with analysis of thermal calibration of the Advanced Very High Resolution Radiometer (AVHRR) aboard National Oceanic and Atmospheric Administration (NOAA) spacecrafts. In particular, the effects caused by various types of contamination or corruption of the thermal calibration data are investigated. These phenomena lead to perturbations of the true signal, referred to here as unwanted fluctuations. They must be removed or corrected to maximum possible extent to reduce the error in the calibrated data. It is shown that methods currently employed in operational practice at NOAA and the Canada Centre for Remote Sensing (CCRS) frequently fail to remove some of the unwanted fluctuations in calibration data that may lead to biases in brightness temperature exceeding 1 K. A complex method for removing unwanted fluctuations in the thermal calibration data specifically designed for the AVHRR radiometers is proposed. The procedure is based on combining robust statistical procedures and Fourier transform filtering techniques. Application of the method is considered for various components of calibration data: temperature sensors, blackbody, and space count, as well as gain in all thermal channels. High Resolution Picture Transmission (HRPT) data and Global Area Coverage (GAC) data are analyzed. Power spectra analysis of the calibration data has been conducted to estimate impact of various frequency harmonics. The method proposed may be useful for the development of calibration techniques for similar radiometers and the future National Polar-Orbiting Operational Environmental Satellite System.