Facility for electrical calibration of the post-amplifiers is also provided. For the TIR subsystem, the signal-to-noise can be expressed in terms of an NE delta T. The requirement is that the NE delta T be less than 0.3K for all bands with a design goal of less than 0.2K. The signal reference for NE delta T is a blackbody emitter at 300K. The accuracy requirements on the TIR subsystem are given for each of several brightness temperature ranges as follows: 200 - 240K, 3K; 240 - 270K, 2K; 270 - 340K, 1K; and 340 - 370K, 2K.