A portable thermal infrared transfer radiometer (TXR) has been developed for use in comparisons and scale verifications of sources used to calibrate thermal- infrared (TIR) channels of the National Aeronautics and Space Administration's (NASA's) Earth Observing System (EOS) flight instruments. The TXR is designed to measure the radiance temperature of large-area black-body sources in cryogenic vacuum environments, either at the National Institute of Standards and Technology (NIST) or at the EOS instrument-calibration facilities. It can be operated in ambient conditions of room temperature and pressure, or in EOS instrument thermal/vacuum chambers at temperatures as low as 77K. The TXR is a liquid-nitrogen-cooled filter radiometer with two channels: one centered at 5 [mu]m based on a photovoltaic InSb detector, and the other centered at 10 [mu]m based on a photovoltaic HgCdTe(MCT) detector. The spectral, spatial and temporal characterization of the TXR using state of the art NIST ambient IR instrumentation will be reported.