FY-3E

FY-3E Mission

Various space instruments have measured TSI since 1978. One of the latest is the Joint Total Solar Irradiance Monitor (JTSIM) on the Fengyun-3E (FY-3E) satellite. FY-3E is the 5th flight unit of the FY-3 series. Its main mission is operational meteorology. It will provide a substantial contribution to ocean and ice monitoring, climate monitoring, atmospheric chemistry and space weather.  JTSIM has two absolute radiometers on board: the Digital Absolute Radiometer (DARA) developed by PMOD/WRC, and the Solar Irradiance Absolute Radiometer (SIAR), developed by the Changchun Institute of Optics, Fine Mechanics and Physics / Chinese Academy of Sciences (CIOMP/CAS), China. FY-3E was launched on 4 July 2021 at 23:28 UTC according to CASC (China Aerospace Science and Technology Corp.) on a Long March 4C vehicle from the Jiuquan Space Launch Center in China. The orbit is sun-synchronous and near-circular (inclination of 98.7°) with an altitude slightly higher than 800 km. The nominal lifetime of the satellite is eight years. The JTSIM experiment is part of the solar activities monitoring package.

The measurement of solar irradiance with accuracy sufficiently high to detect and reliably follow small changes (~0.1%) exhibited by the Sun is a remarkable achievement. Measuring the variability of the Sun remains a particular challenge because of the large brightness contrast of the Sun compared to other astronomical objects. Hence, maintaining a photometric calibration using comparisons with stars is difficult.  A high precision and accurate measurement record of the spectrally-integrated total solar irradiance (TSI) arriving at the top of the atmosphere is essential to understand both the energy balance of the Earth’s climate system and the impact of TSI variations on decadal and centennial timescales. TSI is defined as the value of the integrated solar energy flux over the entire spectrum arriving at the top of the terrestrial atmosphere at the mean Sun-Earth distance of 1 AU (the astronomical unit, AU). These measurements are critical inputs to help understand the effects of solar variability on climate.

Source and credits: CIOMP, CMA, PMOD/WRC

PMOD/WRC Instrument: JTSIM-DARA

Scientific Objectives

The JTSIM-DARA/FY-3E instrument is a cooperation with CIOMP (Changchun Institute of Optics, Fine Mechanics and Physics of the Chinese Academy of Sciences), and the China Meteorological Administration (CMA). Key aspects of the FY-3 satellite series include collecting atmospheric data for intermediate and long-term weather forecasting and global climate research. The idea is to operate a standard group of (originally) three electrical substitution radiometers of a different make and model on one satellite, similar to the ground-based World Standard Group (WSG) located at PMOD/WRC, to provide improved long-term stability of TSI measurements. The first realisation of a space standard group will consist of a JTSIM-DARA radiometer, and a SIAR radiometer designed by CIOMP.

Instrument Design and Characteristics

DARA has three electrical substitution cavity radiometers (Channels A, B and C). These are aligned in a triangle. Compared to the VIRGO/PMO6 radiometer, DARA inverts the aperture geometry to eliminate stray-light. The design of the new cavity (and the heat sink system) reduces the so-called non-equivalence effect. The non-equivalence effect is caused by a slightly different thermal configuration of the active cavity in the measurement/calibration phases (i.e. difference in the shutter-closed phases). A different thermal configuration can cause spurious heat flows (e.g. through the surrounding air, or radiation) that are not exactly equal.

The design of the radiometer is based on the early DARA prototype [Suter, 2014] and the Compact Lightweight Absolute Radiometer (CLARA on NorSat-1) [Walter, 2017]. DARA-JTSIM is calibrated and traceable to the World Radiometric Reference of the WSG at PMOD/WRC.The pre-flight calibration and analysis is described in several publications [Song, 2021; Song, 2022a; 2022b; Montillet, 2022].

References

Source and credits: CIOMP, CMA, PMOD/WRC