Radiance data from the High-Resolution Infrared Radiation Sounder (HIRS) have been used routinely in both direct radiance assimilation for numerical weather prediction and climate change detection studies. The operational HIRS calibration algorithm is critical for producing accurate radiance to meet the user's needs, and it has significant impacts on products at all levels. Since the HIRS does not calibrate every scan line, the calibration coefficients between calibration cycles have to be interpolated based on a number of assumptions. In the more than 25-yr history of operational HIRS calibration, several interpolation methods have been used and, unfortunately, depending on which method is used, these algorithms can produce HIRS level 1b radiance data with significant differences. By analyzing the relationship between the instrument self-emission and gain change during filter temperature fluctuations, in this paper a significant flaw in the previous operational calibration algorithm (version 3) is identified. This caused calibration errors greater than 0.5 K and periodically degraded the HIRS radiance data quality of NOAA-15, - 16, and - 17 between 1998 and 2005. A new HIRS calibration algorithm (version 4) is introduced to improve the calibration accuracy, along with better indicators for instrument noise in the level 1b data. The new algorithm has been validated in parallel tests before it became operational at NOAA/National Environmental Satellite Data and Information Service (NESDIS). Test results show that significant improvements in calibration accuracy can be achieved especially for NOAA-15/HIRS. Several areas of further calibration improvements are also identified. The new algorithm has been used for all operational satellites at NOAA/NESDIS since 28 April 2005.
