Abstract: In this paper,formulae are given for the determination of projection of the critical points at which a nongeosynchronous satellite or- bit is just shone by the sun on a celestial sphere,the incident angles of the sunlight at all the planes of the satellite and the instruments on board,as well as the orbit sections and the probabilities,with which the internal colder blackbody of a satellite-borne infrared spectrometer is irradiated by the sun. Finally,as an example,calculation has been made for an infrared spectro- meter on board a polar orbit meteorological satellite.The results may be of reference value to the design of some parts of the spectrometer and to the selection of launching window for polar orbit meteorological satellites.

Key words: Infrared spectrometer, Blackbody, Solar radiation, Geometry, Meteorological satellite