The aim is to present a first approach for the on-ground radiometric characterization of the new infrared sensor technology (NIRST) instrument. NIRST is an infrared radiometer on board the SAC-D/Aquarius mission, launched on June 10, 2011. It is composed of a middle-wave infrared and a long-wave infrared camera, with three arrays of 512 microbolometers each, and has also a pointing Be mirror. In order to perform the on-ground radiometric characterization, several measurements are taken using blackbody sources. Aiming to obtain a set of absolute radiometric coefficients for each pixel of each microbolometer array, relating digital numbers and brightness temperature or its equivalent in radiated power, polynomial fits are performed. Interpixel characterization to obtain relative calibration coefficients is also performed, relating the counts of an arbitrary pixel to those of a reference pixel. The choice of polynomial order for both absolute and relative calibration functions, as well as the election of reference pixels, are analyzed. Finally, a pointing angle characterization is performed. This approach leads to high polynomial orders for both absolute and relative calibrations, indicating that a new approach for NIRST radiometric characterization is required to catch-up the nonlinearity.