Report on an H-F Type Cavity Radiometer after Six Years Exposure in Space Aboard the LDEF Satellite

A cavity radiometer of the H-F type was included as the total irradiance monitor of the Advanced Photovoltaic Experiment (APEX) aboard the Long Duration Exposure Facility (LDEF) satellite. This is the same type of cavity radiometer which has returned data from Nimbus-7 for almost twelve years. The LDEF was retrieved by the Space Shuttle Columbia on 12 January 1990 after almost six years in space. Initial results show that the electrical properties of the thermopile and heater circuits were unaffected. There was a slight visible change to the painted surface of the cavity, possibly caused by atomic oxygen impingement late in the mission. This apparently has not affected the agreement with ground-based reference standards. Intercomparisons to date indicate agreement of approximately 0,1%, but with an uncertainty at this same 0,1% level. The radiometer was included in the International Pyrheliometric Comparison (IPC VII) in September 1990. In-flight data was taken by an on-board data system (no telemetry) for the period from 7 April 1984 to 16 March 1985. Digital flight data from which solar pointing and irradiance information can be calculated has recently become available.