Satellite measurements from the infrared (IR) channels of the Advanced Very High Resolution Radiometer (AVHRR)/NOAA have been used to derive many important atmospheric, cloud, and surface parameters for weather prediction, climate modelling, and a variety of environmental studies. Calibration accuracy of the satellite data directly affects accuracies of the derived parameters. So far, very limited attention has been given to the calibration uncertainties of the IR channels. In this study, we analyzed the calibration data of AVHRR radiometers onboard polar orbiting satellites NOAA-9 to NOAA-16. We utilized Global Area Coverage (GAC) data, approximately one orbit per month throughout the lifetime of the instruments, available from the NOAA Satellite Active Archive (SAA). AVHRR IR channels 3B, 4, and 5 are calibrated in-flight. Calibration coefficients are derived from measurements of radiance emitted from an internal calibration target (ICT) and deep-space (SP). The overall budget of uncertainties has been evaluated using an in-flight calibration system that includes four thermal platinum resistance thermometers (PRTs) to monitor the ICT temperature. The measurement noise (NEΔT) was found to vary from 0.03 K to 0.3 K at 300 K depending on the channel and radiometer, and it increases significantly as temperature decreases. Systematic degradation of the radiometric sensitivity of the IR detectors was observed during the lifetime of a radiometer, although the annual rate of degradation is rather small (typically below 1% per year). A significant correlation between the calibration gain and temperature of a radiometer is often observed. The degradation of a sensor's radiometric sensitivity reduces the radiometric resolution of the AVHRR measurements and expands the upper limit of the measured brightness temperature. PRT measurements are subject to significant orbital variation (up to 7 K) and inconsistency for some AVHRR radiometers. The inconsistency was especially large for the AVHRR onboard NOAA-12 (up to 4 K) and NOAA-14 (up to 3 K), but it is less than 0.5 K for NOAA-15 and -16. The inconsistency may signify the presence of a thermal gradient across the ICT. Some systematic differences between PRT measurements may also indicate inaccurate characterization of the PRT sensors, for example for AVHRR/NOAA-11 and -14. The impact of the varying thermal state of the AVHRR environment on the accuracy of AVHRR in-flight thermal calibration was assessed. We found this impact to be significant (up to 0.5 K and more), and proposed a physical model to explain it. We recommend this model for AVHRR operational in-flight calibration, especially during solar radiative contamination events. Estimates of the PRT thermal response time constant were derived and found to vary between 0.5 and 1.5 min among AVHRR radiometers. Overall, we found somewhat higher uncertainties in AVHRR thermal measurements than were assumed previously.