Radiometric calibration of low Earth orbit infrared imagery using a hybrid on-orbit and ground processing approach

Hydrosat’s dual band Longwave Infrared Imager (LIRI) is an uncooled thermal pushbroom imager that leverages a microbolometer, an on-board blackbody calibration source, and implements Time Delay Integration (TDI) via software. LIRI’s mission is to measure Land Surface Temperature (LST) from Low Earth Orbit (LEO). An overview of the instrument is presented, and the process to convert raw LIRI imagery into Top of Atmosphere (TOA) radiance is described. Due to data rate constraints, the application of a two-point Non-Uniformity Correction (NUC) and TDI occurs on orbit, prior to radiometric calibration. The processing to radiometrically calibrate the LIRI data then occurs on the ground. It is shown that the method to compute the NUC and TDI prior to radiometric calibration yields TOA radiance identical to applying the radiometric calibration to the raw imagery directly and then applying TDI.