A possible long-term trend of the total solar irradiance could be a natural cause for climate variations on Earth. Measurement of the total solar irradiance with space radiometers started in 1978. We present a new total solar irradiance composite, with an uncertainty of ± 0.35 W m−2. From the minimum in 1995 to the maximum in 2002 the total solar irradiance increased by 1.6 W m−2. In between the minima of 1987 and 1995 the total solar irradiance increased by 0.15 W m−2.

Radiometers for the measurement of the total solar irradiance from space have been developed by D. Crommelynck at the Royal Meteorological Institute of Belgium (RMIB) since 1970. These radiometers, also used in the set of instruments defining the “World Radiometric Reference”, have successively flown on SPACELAB 1, ATLAS 1, EURECA, ATLAS 2, ATLAS 3, SOHO and HITCHHIKER, among them the SOLCON radiometer on the last STS107 flight. Other radiometers are under construction or integration for the upcoming missions: one as part of the SOVIM experiment on the International Space Station and SOVAP on the PICARD micro satellite. These radiometers have a symmetrical side by side cavity design. They are operated using the differential active cavity radiometric principle. In order to construct a long time series and to detect any variation between two cycles, it is important to know all the instrument parameters and make a clear distinction between what will lead to uncertainty on an absolute level and what guarantees the repeatability of the measurements. The purpose of this paper is to review these parameters; after a description of its principle, the instrumental equation will be established. The electrical parameters and the determination of remaining instrumental parameters are done during the characterisation phase at the RMIB. The results from the DIARAD/VIRGO radiometer on SOHO are presented. The repeatability of the measurements is found to be of the order of 100 ppm after correction of the in-flight aging. The accuracy on an absolute level is about 1.06 W m−2 for DIARAD left cavity. This accuracy is limited by the characterisation accuracy with a dominant role of the precision aperture area determination. The understanding of the behaviour of the DIARAD type instrument allows improving the design of the new radiometers on the upcoming missions.

PICARD is an investigation dedicated to the Sun-climate connection. The mission objectives are the study of the solar forcing on the Earth's climate, and the physics of the Sun. It consists of simultaneous measurements of the absolute total and spectral solar irradiance, the diameter and solar shape, and to carry out helioseismologic observations to probe the Sun's interior. These measurements obtained all along the mission will allow to study their variations as a function of the solar activity. The observations will be carried out by three instruments placed on board a microsatellite built by Centre National d'Etudes Spatiales (France). Ground-based observations will complement the space observations.