The Belgian DIARAD Total Solar Irradiance Observations, Historical Objectives, Achievements and Particularities.

The origin of the Total Solar Irradiance (TSI) observations at the Royal Meteorological Institute of Belgium is the will to monitor the Earth Radiation Budget (ERB) whereoff the TSI is one of the terms, the two others are the reflected solar radiation and the Earth emitted radiation actually measured operationnaly by the Geostationnary Earth Radiation Budget (GERB) experiment on board of EUMETSAT MSG. This concerns atmospheric physics as well as climate. The new design of a differential absolute radiometer (DIARAD) characterised in air as well as in vacuum, led to its incorporation in the set of instruments defining the World Radiometric Reference (WRR); adapted for space, it flew repetitively on the space shuttle as well as on the european retrievable carrier (EURECA). Since 1996 DIARAD operates contineously without any failure from VIRGO on SOHO. Our strategic principles required to insure a long range high quality TSI data base are developed . It incorporates the usage of Space Absolute Radiometric Reference (SARR) coefficients to normalise the observations in time. The compared characteristics of DIARAD and PMO instruments are presented as well as the chronological events and behaviours of the radiometers flying on VIRGO/SOHO. It is concluded that a minimum of three simultaneous flying radiometers are required to guarantee the value of the TSI data base.