Presented at the 2013 Aquarius/SAC-D Science Team MeetingA methodology to perform absolute calibration for NIRST on board the SAC-D/Aquarius satellite is presented here. At-sensor radiances of the thermal infrared 10.8 um NIRST sensor band were simulated for a set of cloud free images acquired over Lake Tahoe for different seasons, within a selected period of time that went from late August 2012 to early April 2013. The lake temperatures were obtained from Lake Tahoe Validation website run by the Jet Propulsion Laboratory (JPL/NASA). These temperatures were propagated to the equivalent at-sensor radiances by means of the radiative transfer code MODTRAN 4, using the sensor response function and a standard atmosphere model. A linear relationship is assumed between the at-sensor radiances and the digital numbers (DN's) taken from NIRST acquisitions of the lake area. The correlation coefficient found was >0.95.