INSAT-2A spacecraft carries on-board a Very High Resolution Radiometer (VHRR) as one of the major payloads for the purpose of meteorological observations. The thermal infrared (IR) detector of the Radiometer requires cooling to cryogenic temperature for its operation at any of the set points in the range of 105 K to 115 K for optimum performance. A passive radiant cooler has been designed and realised indigenously to meet this requirement. A computer oriented mathematical model has been developed which simulates the on-orbit thermal environment and predicts the performance of the cooler for various sun illumination conditions. INSAT-2A spacecraft has completed more than one year of its successful operation on-orbit now. It has been observed that a minimum patch control power margin of 4.7 mW exists for beginning of life (BOL) condition i.e. 5th August, 1992.