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.