Calibration of AVHRR Reflectance Channels NOAA Office of Global Programs has funded STAR to lead an effort to derive a consensus historical AVHRR reflectance calibration. The availability of MODIS data now makes this feasible. Simultaneous Nadir Observations (SNO) occur when the ground track of two satellites cross at the same time. (see cartoon on right) SNOs offer a new tool for calibrating the reflectance channels on the AVHRR which have no onboard calibration. SNO’s occur between all polar orbiting satellites. For example, SNOs between AVHRR and MODIS allow for transfer of the MODIS calibration to the AVHRR SNO’s also allow for monitoring of the relative calibration for historic AVHRR data and greatly improves its potential for climate studies. Example Results from MODIS/AVHRR SNO Verification of Results – Stability of Greenland’s Reflectance over last 25 years AVHRR Ch1 Count MODIS Ch1 Reflectance