A detailed analysis of the calibration procedures for the NOAA advanced very high resolution radiometer (AVHRR) based on thermal vacuum test data was performed as part of the Warm Core Rings Experiment. We find systematic differences in the derived calibration results and those reported in the various addenda to work by Lauritson et al. (1979). A weak, but important, dependence in calibration upon internal operating temperature is also observed. Calibration results for various AVHRR radiometers show instrument specific changes in the relative emittance between internal and external calibration targets. A consistent calibration methodology is developed based on this analysis of the thermal vacuum test data. Use of these newa results provides an improvement of radiometer calibration at the ¿0.2 ¿C level. such an improvement is quite important to the absolute accuracy of surface thermal fields which are derived from these data utilizing various multichannel atmospheric water vapor correction schemes. Demonstration of such accuracies with presently operational sensors is of importance to climate related studies being planned for the next decade since it shows that appropriate processing can lead to higher-quality SST fields.