The internal calibration system consists of two blackbodies and space background. The instrument can be commanded automatically to enter a calibration mode every 256 seconds. In this mode, at the beginning of a scan line, the mirror swings rapidly around (equivalent to 48 scan elements). All channels of the instrument are sampled during this period. The mirror is next moved to a position where it views a cold calibration target. Data are taken for the equivalent of 58 scan steps after which time the scan mirror is stepped to view the internal warm target. After another 56 samples, the mirror continues its motion to the start-of-scan (home) position, where it begins normal earth scan. The total calibration sequence is performed in the equivalent of three scan lines, during which time no earth data are obtained. The analog data output from the HIRS/2 sensor is digitized on board the satellite at 2880 bits per second. At this rate, there are 288 bits per step (step time = 100 ms), which includes all twenty channels. The data are digitized to 13-bits precision. Information on data formats is found in Kidwell (1988).