Total Solar Irradiance Monitor for Chinese FY-3A and FY-3B Satellites – Instrument Design

The Total Solar Irradiance Monitor (TSIM) instrument is designed to perform daily observations of total solar irradiance (TSI) in space on the Chinese FY-3A and FY-3B satellites. Three absolute radiometers are constructed for the TSIM to achieve measurements with traceability to SI with an absolute accuracy better than 550 ppm. The absolute radiometers are implemented based on the principle of electrical substitution. The design of the absolute radiometers and their electrical system, operation modes in space, and uncertainty evaluation are described. A method for calculating the electrical power in the observation and reference phases is proposed to maintain the primary cavity at a nearly constant temperature.