Accurate solar 'constant' determinations by cavity pyrheliometers

Total solar irradiance was observed outside the earth's atmosphere by three types of absolute cavity pyrheliometer in a June 1976 sounding-rocket experiment. The 1367-W/sq m average solar 'constant' result is uncertain by less than + or - 0.5%, the most accurate determination to date. Nearly simultaneous observations by the Nimbus 6 earth-radiation-budget total-irradiance detector of 1389-W/sq m exceeded the rocket result by 1.6%. These recent results are discussed in the context of a summary of solar-constant determinations made above the troposphere by cavity pyrheliometers.