Sun is the key driver of the Earth’s climate system, it is essential to understand the interaction between the Sun and the Earth. In this paper, the past space based Total Solar Irradiance (TSI) measurement is presented. According to the instrument operation mode and whether there is a sun tracking platform or not, the space based TSI measurements are divided into three groups: passive sun tracking, dedicated TSI missions and the experiment with an accurate sun tracking system. The configuration and characteristics of solar radiometers of each kind is introduced in detail. The TSI value, which used to be named as solar constant normalized at one astronomical unit is changed from 1365 W/m-2 to 1361 W/m-2 (during the 2008 solar minimum period). The justification of the new lower TSI value is briefly recalled and discussed. A series of solar irradiance references such as International Pyrheliometric Scale (IPS) 1956, World Radiometric Reference (WRR), and the TSI Radiometer Facility (TRF) are also separately recalled. The WRR has an estimated accuracy of 0.3% and guarantees the worldwide homogeneity of radiation measurements within 0.1% precision, which is 0.34%$ higher than the international system of units (SI), while the uncertainty of the TRF facilities to the SI is evaluated and both systems agrees in $0.01\%$. The Solar Irradiance Monitoring (SIM) experiment on FY-3C meteorological satellite measured a TSI value consistent with the new lower values after tracing to the SI scale.
% Please include a maximum of seven keywords \keywords{TSI, WRR, SI, Absolute Radiometer}