A five-channel ac radiometer is mounted at 45° to the satellite spin axis. In order to obtain radiation measurements independent of satellite temperature, an optical chopping system is used which utilizes the near-zero radiation from space as a reference. These five radiometers have coincident 5°×5° instantaneous fields of view which scan arcs over the earth’s surface as the satellite spins at 10 rpm. The five channels are filtered to respond to different spectral regions in the visual and infrared which are of meteorological significance. A two-channel, wide-angle, unchopped radiometer is mounted parallel to the spin axis and sees a 50° circular field of view which progresses slowly over the earth’s surface with the orbital motion of the satellite. This radiometer uses two thin thermistor disks mounted at the apex of two reflecting cones and coated "black" and "white," respectively. These radiometers measure solar reflection and total emission of the earth. A third infrared device having a small angular field of view normal to the satellite spin axis produces reference pulses as it crosses the earth’s horizon. This sensor provides information to determine the inclination of the spin axis and the spin rotation rate.
R. W. ASTHEIMER, R. DEWAARD, and E. A. JACKSON, "Infrared Radiometric Instruments on TIROS II," J. Opt. Soc. Am. 51, 1386-1393 (1961)