The Earth radiation entered the narrow-angle radiometer through a cylindrical fairing (KRS-5 crystal) and fell onto a conical scanning mirror. The radiation was reflected from the mirror through a three-lobed rotating mirror chopper that modulated the radiation flux at a frequency of 80 Hz. The chopper alternately reflected earth radiation and space radiation, which entered through a separate KRS-5 crystal window, onto one of three openings in a color filter wheel – one filter for each spectral band. The particular spectral band that was passed through then fell on an off-axis parabolic mirror that focused the radiation flux onto a bolometric receiver. Periodic calibration was made when the scanning mirror moved to a 90° angle from nadir with simultaneous turning on and viewing of a silicon standard lamp.
