Observations of irradiance variations

Since November 1978 a complete set of total solar irradiance (TSI) measurem ents from space is available, yielding a time series of 21 years. From meas urements made by different space radiometers (HF on NIMBUS-7, ACRIM I on SM M, ACRIM II on UARS and VIRGO on SOHO) a composite record of TSI can be com piled. The corrections which are needed for each radiometer, mainly to comp ensate for degradation, but also for operational influences are described. ACRIM I (1980-1989) and ACRIM II (1992-1995) are taken as reference because their degradation was monitored by inflight comparisons with spare radiome ters; since 1996 VIRGO is used as reference. The scale of ACRIM II is adjus ted to ACRIM I by comparison with NIMBUS-7 and ERBE. The reliability of thi s adjustment depends strongly on the stability of the HF and ERBE radiomete r during the period of the gap between the ACRIMs which is discussed in det ail together with an assessment of the overall precision of the composite.