Calibration and Validation for NOAA18 HIRS/4 Measurements

A new polar-orbiting environmental satellite - NOAA18, was successfully launched in May 2005. Among the instruments carried aboard NOAA18, the new model of the High Resolution Infrared Sounder (HIRS/4) provides multi-spectral data for direct radiance assimilation, and retrievals of the atmosphere's vertical temperature profile, water vapor, ozone, and clouds. Major postlaunch calibration and validation efforts are made to verify the instrument performance by analyzing instrument noise, calibration bias, and intersatellite consistency. First, a trending system is developed to monitor the instrument performance, including instrument component temperature, space view and blackbody view counts, calibration coefficients, and noise equivalent differential radiance (NEdN). Secondly, intersatellite comparisons between NOAA17 and NOAA18 for each HIRS channel is performed by using Simultaneous Nadir Overpass (SNO) observations at two satellite's orbital intersections. The bias between HIRS/3 and HIRS/4 are quantified and the root causes investigated. Finally, forward calculations using the line-by-line radiative transfer model (LBLRTM) are used to quantify the effects due to small differences in the spectral response functions between HIRS/3 and HIRS/4. The effect of field of view difference are also investigated. The findings may have significant impacts on the understanding of on-orbit instrument performance as well as on applications using HIRS radiance data.