A method is presented for the calibration of infrared radiometers on geostationary satellites using calibrated infrared radiometers on an orbiting satellite. This method relies on similarities between the weighting functions corresponding to the radiometers on geostationary satellites like METEOSAT or the GOES series and the weighting functions of some of the channels on the TIROS-N Operational Vertical Sounder (TOVS). It makes use of iso-secant observations of the same scene from both satellites. Many such observations are available every day resulting in daily calibration curves defined by several hundred points. This calibration method is shown to be very sensitive, accurate and tractable. This method does not require the collection of radiosonde data or any kind of in situ experiments and may be completely automated. Abstract A method is presented for the calibration of infrared radiometers on geostationary satellites using calibrated infrared radiometers on an orbiting satellite. This method relies on similarities between the weighting functions corresponding to the radiometers on geostationary satellites like METEOSAT or the GOES series and the weighting functions of some of the channels on the TIROS-N Operational Vertical Sounder (TOVS). It makes use of iso-secant observations of the same scene from both satellites. Many such observations are available every day resulting in daily calibration curves defined by several hundred points. This calibration method is shown to be very sensitive, accurate and tractable. This method does not require the collection of radiosonde data or any kind of in situ experiments and may be completely automated.