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Four aspects of the radiometry of the Landsat-5 Thematic Mapper were characterized over the 20+ year mission lifetime for the six reflective bands: relative gain (the radiometric gain of each detector within a band relative to other detectors in that band), bias, performance of the Internal Calibrator (IC) system, and noise. Relative gain was found to be stable or slowly varying and could be described as a linear function of time for most detectors; the maximum change was approximately 0.5%/year. These relative gain characterizations provide an alternate source of destriping information that, in general, compares favorably with that obtained from currently used scene-specific methods. Much of the variability in instrument bias levels was found to be related to temperature effects; long-term changes in bias levels were less than 0.5 DN overall. The lamp-based IC system, though stable over the short term, showed both individual lamp phenomena and changes in overall behavior that complicated the ability to monitor the system's stability. Using the best behaved lamp and some assumptions about expected lamp behavior, characterization of response with a simple model was achieved through the year 2000. The model shows an initial 5% to 10% decay in response over the first three years of operation, depending on the band. Noise levels and signal-to-noise ratio in the instrument appear to be stable throughout the lifetime.