The Landsat Data Continuity Mission (LDCM) continues Landsat data into a fourth decade with a two-sensor platform that includes the Thermal Infrared Sensor (TIRS). TIRS will operate in pushbroom mode with two spectral bands (10.8 and 12 micrometer) and a required spatial resolution of 120-m and 185-km swath. The radiometric assessment of TIRS relies on NIST-traceability of the radiometric calibration, knowledge of out-of-band spectral response, and characterizing and minimizing stray light. Calibration testing will take place at NASA GSFC using a vacuum-compatible system that is capable of filling the entrance aperture of TIRS while permitting evaluation of the entire field of view. Spectral response of the sensor will be determined using a monochromator source external to the test chamber. The error budget for the calibration system shows that the required 2% radiometric accuracy for scene temperatures between 260 and 330 K is well within the capabilities of the system.