Contribution of the Solar Constant (SOLCON) program to the long-term total solar irradiance observations

On long timescales the variation of the total solar irradiance (TSI) received by the Earth is believed to be one of the climate change drivers. Therefore accurate and time-stable measurements of the total solar irradiance are necessary. The Solar Constant (SOLCON) instrument made TSI measurements in April 1992 and during the International Extreme Ultraviolet Hitchhiker (IEH) 3 STS 95 shuttle flight in autumn 1998. We assume that the SOLCON instrument remained stable within 0.01 % in between those measurements, and we verify this assumption as well as possible. From the SOLCON measurements we conclude the following: (1) The 1998 Space Absolute Radiometric Reference (SARR) adjustment coefficient applicable to the Active Cavity Radiometer Irradiance Monitor II (ACRIM II) during the IEH 3 period is 1.000438 with a one sigma uncertainty of 18 ppm, compared to the 1993 SARR adjustment coefficient of 1.000258. (2) The solar monitor on the Earth Radiation Budget Satellite (ERBS), with a 1993 SARR adjustment coefficient of 1.000453, has not aged within a one sigma uncertainty level of 130 ppm; and (3) the 1998 SARR adjustment coefficients for the Variability of Solar Irradiance and Gravity Oscillations (VIRGO) radiometers have been determined with a one sigma uncertainty of 10 ppm: They are 1.000025 for the Differential Absolute Radiometer left channel (DIARAD-L) and 1.000279 for the version 1.2 data from the PMO6-VA radiometer.