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Data from the solar reflectance channels of the Along Track Scanning Radiometer (ATSR) series of instruments are being used in applications for monitoring trends in many climate variables, for example, clouds and aerosols. In order to provide quantitative information, the radiometric calibrations of the sensors must be consistent, stable, and ideally traced to international standards with uncertainties quantified. In this paper, the authors describe the current methodology used to monitor the long-term drifts and determine the relative biases of the ATSR solar channel radiometric calibrations. Top-of-atmosphere bidirectional reflectance factors (BRFs) over quasi-stable desert and ice sites are extracted from level-1 images and compared against a reference BRF model derived from averages of measurements over the site from a reference sensor. This enables comparisons to be performed where there is limited or no temporal overlap between sensors. The results of the drift monitoring and intercomparisons are used to provide lookup tables to be applied by users for existing products and in subsequent reprocessing of ATSR data. The method is extended to perform comparisons against the Medium Resolution Imaging Spectrometer and the Moderate Resolution Imaging Spectrometer. Results of the comparisons are presented and show that the sensors are stable throughout the mission lifetime and biases relative to the Advanced ATSR are presented. Improvements to the methodology are discussed to account for spectral mismatches of the sensors under comparison and to increase the range of view angles that the BRF model presently covers.