The total solar irradiance during the recent solar minimum period measured by SOHO/VIRGO.

Various space missions have measured the total solar irradiance (TSI) since 1978. Among them the experiments Precision Monitoring of Solar Variability (PREMOS) on the PICARD satellite (2010-2014) and the Variability of Irradiance and Gravity Oscillations (VIRGO) on the mission Solar and Heliospheric Observatory, which started in 1996 and is still operational. Like most TSI experiments, they employ a dual-channel approach with different exposure rates to track and correct the inevitable degradation of their radiometers.

Solar autocalibrating XUV-IR spectrometer system (SOLACER) for the measurement of solar spectral irradiance.

The accurate measurement of solar spectral irradiance (SSI) from space is challenging because it requires a system that can reliably exclude the impact of degradation on the instruments for the full duration of a long-term mission. The new Autocalibrating XUV-IR Spectrometer System (SOLACER) presented here meets this requirement. It is a compact and moderate-cost instrument that allows repeated onboard calibration of the spectrometers (SPs) providing SSI data of significantly increased accuracy.