Up to now, dozens of series of satellites, such as China’s Fengyun Meteorological Satellites, Europe’s Geostationary Operational Environmental Satellites (GOES), and the USA’s European Meteorological Satellite (METEOSAT), have been launched to provide real-time remote sensing data globally. Often, it is necessary to utilize remote sensing data from various sensors on multiple satellite platforms to study long-term change trends due to the limited functionality and lifespan of a single satellite. Therefore, enhancing the on-orbit radiometric calibration accuracy of remote sensing devices is crucial for facilitating mutual comparison of measurement data from different sensors. It is essential to trace the radiometric scale of remote sensors back to the international system of units (SI) and maintain long-term stability. The radiometric calibration of multispectral or hyperspectral remote sensing payloads still relies on lamp-board or solar diffuser, which cannot be traced to SI on-orbit due to the influence of the launch process and long-term attenuation. The Moderate Resolution Imaging Spectrometer (MODIS), multi-angle imaging spectrometer (MISR) and ocean wide field scanner (SeaWiFS) have made significant efforts in on-orbit radiometric calibration. However, MODIS achieves a reflectance uncertainty of 2%, which is the minimum among them. Moreover, there is no comparability of observation data from different countries, different series within the same country, and even different satellites within the same series, since the satellite payload radiometric calibration system cannot trace to the radiometric benchmark on orbit. For example, there is a 10% deviation between the radiometric remote sensing data of MODIS and MISR. Currently, there is a 0.3% deviation among the total solar irradiance observed by multiple payloads, making it difficult to describe the periodic solar change of 0.1% over a decade, which highlights the technical challenge of high-precision space absolute radiation measurement.

The establishment of a space radiation measurement benchmark traceable to the SI is one of the hot issues in international research. Currently, scientists from China, Europe, and the United States are making efforts to establish the benchmark of space radiation measurement. However, there exists a huge technical challenge to apply ground metrology means to space since cutting-edge technologies such as the Cryogenic Absolute Radiometer and phase-change blackbody will make the calibration system cost more than the payload itself. Therefore, it is not economically feasible to equip an expensive calibration system for each payload. In 2006, a Chinese expert group on earth observation and navigation with the National High-Tech R&D Program proposed the concept of Chinese Space-based Radiometric Benchmark (CSRB). The CSRB project has been under development since 2014. The goal of the CSRB project is to launch a radiometric benchmark satellite to completely solve the radiometric traceability problem of remote sensing satellites by adopting a new in-orbit calibration system instead of using solar diffusers, standard lamps, vicarious calibration methods, and ground-based calibration techniques. The National Physical Laboratory (NPL) proposed the Traceable Radiometry Underpinning Terrestrial-and Helio-Studies (TRUTHS) project in 2003, serving as an “international standard laboratory” in space. This project conducts absolute radiation measurement of the 0.4-2.35 μm solar reflected waveband and takes the measurement value as the reference standard to establish a radiometric calibration system traced back to SI, providing a reference benchmark for the space optical remote sensing instruments of other satellite platforms. The National Aeronautics and Space Administration (NASA) proposed the Climate Absolute Radiation and Refraction Observation Platform (CLARREO) project in 2007 to develop solar and Earth radiation measurement instruments in three phases. The CSRB coincides with the TRUTHS proposed by Europe and the CLARREO proposed by the United States to solve the radiometric observation traceability problem of remote sensing satellites thoroughly. The space cryogenic absolute radiometer is an electric alternative radiometer working in the 20 K temperature zone, mainly used for high-precision measurement of in-orbit optical power. Currently, the development of the core detector of the space cryogenic absolute radiometer has been completed, measuring an absorption ratio of 0.999981. At the three angles of 0°, 90°, and 180°, the standard deviation of the absorption ratio of the black cavity in ±2 mm is less than 0.0003%, and the maximum deviation is less than 0.001% (Fig. 3). The space cryogenic solar radiation monitor is developed based on space cryogenic radiation measurement technology, planned to be carried on the Fengyun-3 meteorological satellite-10 star, aimed at verifying the feasibility of the in-orbit application of the space cryogenic absolute radiation meter. At present, the principle prototype has been developed, and the performance test and optimization have been preliminarily completed. The results show that the measurement repeatability of 5 mW laser power is better than 0.01%. The measurement repeatability of 0.5 mW radiation power is better than 0.03%, and the relative deviation from the standard trap detector measurement results provided by the China Institute of Metrology is less than 0.01%. The Earth-Moon imaging spectrometer is primarily used for measuring Earth reflection radiance and lunar irradiance. The imaging spectrometer consists of an off-axis triple reverse front view system, a visible-near infrared spectrometer, and a short-wave infrared spectrometer. The front system adopts a double Barbinet principle to reduce polarization sensitivity; the spectrometer uses an Offner structure with a convex grating as the spectral element. According to the design parameters, the signal-to-noise ratio of the imaging spectrometer in the working band is better than 300 (Fig. 8). The main function of the satellite reference transfer link is to realize the in-orbit traceability of the Earth-Moon imaging spectrometer to SI, mainly composed of the transfer radiometer, solar monochromator, and uniform light integral sphere. At present, the optical design of the solar monochromator and transfer radiometer has been completed. The working band range of the solar monochromator is 350-2400 nm, with a spectral resolution better than 8 nm (Table 2). The uncertainty component of the reference load in the solar reflection spectrum mainly includes the measurement uncertainty of the space cryogenic absolute radiometer, which is 0.03%, the uncertainty of the reference transfer link, which is 0.47%, and the inaccurate measurement introduced by the imaging spectrometer, which is 0.48%. Hence, the on-orbit traceability uncertainty of the imaging spectrometer is estimated to be 0.68%, enabling the measurement uncertainty of Earth reflected radiance to be better than 0.8% (Table 5).

The research results of our paper provide a theoretical and experimental basis for the development of the reference load of the solar reflection spectrum segment. The reference payload will significantly improve the accuracy and long-term stability of spectral remote sensing data and offer high-precision remote sensing data for climate change studies. In addition, the radiation scale of the reference load can be transmitted to other space optical remote sensing devices through cross-marking, unifying the on-star radiation scale of different remote sensing payloads.