Uncertainty of total solar irradiance: an assessment of the last twenty years of space radiometry

A complete set of total solar irradiance measurements from space is availab le dating from November 1978, yielding a time series of more than twenty ye ars. From measurements made by different space radiometers (HF on NIMBUS-7, ACRIM I on SMM, ACRIM II on UARS and VIRGO on SOHO) a composite record of solar total irradiance can be compiled. The corrections needed for each rad iometer, mainly to compensate for degradation, are based on a detailed stud y of the behaviour of the VIRGO (Variability of Solar Irradiance and Gravit y Oscillations) radiometers. Moreover, several errors arising from operatio nal changes or interruptions, which influence the overall uncertainty more than corrections for degradation, have to be corrected. A composite of the corrected time series is constructed in order to estimate the long-term sta bility, which is much better than the estimated 1 part in 10(5) per annum d educed as an upper limit from VIRGO radiometry. The absolute accuracy of th e data, however, is more difficult to estimate.