Radiometric calibration plays a key role in quantifying the responsivity of remote sensors, correcting on-orbit response decay, and verifying the accuracy of data products for Fengyun meteorological satellites. An independent and complete technical system of calibration consists of laboratory calibration before the launch, on-board calibration, and site calibration during on-orbit operation. Pre-launch laboratory calibration coefficients are generally not suitable for operational data processing due to sensor on-orbit degradation. On-board calibration and site calibration are the main data quantification methods during on-orbit operation. On-board calibration requires the co-design of software and hardware of calibrators and remote sensors, which requires the support of valuable on-board resources, and the technical iteration is relatively cautious. Before solving the problem of space radiation benchmarks traceable to the international unit (SI), the performance decay of the on-board calibrator mainly limits the accuracy and stability. Site vicarious calibration realizes system-level calibration under the on-orbit operation of remote sensors. Meanwhile, its technical upgrade does not affect the operation of the remote sensors, and the calibration facilities can obtain the calibration support of the metrology laboratory. Site calibration has maintained technical evolution for more than 20 years and provides the most reliable calibration results at present. The site calibration implementation significantly depends on the measurement ability of spectral radiation characteristics of the atmosphere, surfaces, and surrounding environment. Additionally, the performance of site calibration instruments directly affects the application effect of site calibration. Before 2015, site calibration was implemented by manually performing instruments in field measurements. To obtain at least three rounds of qualified data in suitable weather and satellite overview conditions, site calibration generally needs to last for 10-30 days. Generally, manual calibration can only be implemented once a year in summer and autumn, and the update frequency of the calibration coefficient is difficult to reflect the actual state of the on-orbit sensors in time. Site calibration via large-scale manual observation can no longer meet the development requirement of multi-satellite constellation, high-efficiency observation, and long-term stable data accuracy of meteorological satellites. The development and applications of the ground calibration platform with automatic operation and site calibration networks, as well as real-time sharing of calibration data, are urgent requirements for improving calibration frequency to correct the sensor decay in time and ensure the data quality.

In the past ten years, many institutions in China have independently developed visible thermal infrared field calibration instruments, established laboratory testing and calibration facilities for the instruments, and obtained CNAS and CMA certifications. These field instruments significantly improve the high-precision traceability, reliability, and long-term stability in the solar reflection band. Compared with the R&D and application capabilities of the instrument in the early stage, the progress in site calibration instruments in the past 10 years is mainly reflected in the following aspects: 1) manual operation is upgraded to unattended automatic operation to improve data repeatability; 2) multi-channel and hyperspectral observation capabilities are equipped to improve the completeness of on-orbit fine spectral calibration parameters; 3) key weather-resistance technology is developed to adapt to complex working environments in the field to improve the long-term stability and reliability of data; 4) site self-calibration integration is realized to achieve timely SI traceability and decay correction to maintain long-time observation accuracy; 5) software and hardware are collaboratively developed to achieve more convenient data processing, analysis and sharing. On-orbit degradation of the remote sensors is continuous. Practical site calibration experiences in the past 20 years show that a few calibration sites are insufficient to monitor and correct on-orbit decay or update calibration coefficients in time. Technically, calibration sites and instruments are not the only calibration data sources. Upgrading and maintaining a large number of sites equipped with high-performance observation instruments is hardly sustainable for long-term operation. A moderate number of equipped sites can be employed as benchmark sites, with focusing on continuous upgrading of high-precision instruments and maintaining reliable traceability to national metrology standards. With the continuous performance improvement in satellite sensors and calibration capability of the benchmark sites, satellite sensors calibrated by the benchmark sites can continuously obtain quantitative and high-quality observation data of the global surface and atmosphere. Mining the data will screen and massively increase the number of global calibration sites. Exploitation and applications of calibration data sources mark important progress in site calibration technology in the past decade. According to the calibration site requirements, a screening algorithm is developed to select hundreds of calibration sites around the world suitable for on-orbit radiometric calibration. Meanwhile, a calibration site network with different geographical locations, altitudes, spectral characteristics, and radiation dynamic ranges is constructed. The global calibration site network is integrated into the radiometric calibration software, which mainly includes the site database and automatic planning module of calibration tasks. Site databases manage site basic information, surfaces and atmospheric characteristics, and satellite sensor information. Additionally, data is evaluated, graded, and updated according to the application effect to continuously improve quantity, quality, timeliness, and reliability. Based on remote sensor orbits, imaging mechanisms, bands, and spatial and spectral resolutions, the calibration task module automatically selects and quickly matches the site type, geographical location, spatial uniformity, satellite observation and sunlight illumination angle, dynamic range, site scale, and atmospheric conditions, and determines the best calibration time according to the real-time meteorological data. The global calibration site network realizes the long-time series absolute radiometric calibration of sensors such as FY-3B, and quantifies the continuous on-orbit response changes. High-frequency calibration significantly reduces data diversity by only one site. The annual average change rate of the calibration coefficient obtained by linear fitting reveals the degradation trend of the remote sensor more accurately, which provides credible basic data for diagnosing the working state and correcting the decay of satellite sensors. Global calibration sites provide a wide dynamic range of surface reflectivity and facilitate nonlinearity characterization of remote sensors. For example, FY-3C MERSI nonlinear response correction significantly reduces the influence of nonlinearity on data accuracy. Historical data of FY-3A/B/C MERSI is recalibrated by employing the global calibration sites. Inconsistency correction of each remote sensor generates long-term data series obtained by the three remote sensors at a common radiometric scale.

The most significant progress in site calibration technology in the past decade is reflected in the following two aspects. The R&D and applications of automated site instruments have solved key techniques such as site calibration, field weather resistance, and remote wireless measurement and control, and have the calibration ability with high timeliness and long-term sequences. Site calibration frequency has been increased from about once a year to more than once a week, only limited by weather conditions. Automatic operation of instruments greatly reduces the manual measurement workload and systematic error introduced by the operation level difference of different personnel and significantly reduces the operation cost. The exploitation and in-depth applications of calibration data sources have increased hundreds of calibration sites with favorable natural conditions and annual stability around the world, with a wide range of geographical distribution, surface radiation characteristics, and atmospheric transmission characteristics. The global calibration site network has increased the calibration data amount by magnitude, which has improved the data traceability of site calibration and made it possible to recalibrate historical data. The merging of equipped benchmark sites and the global digital site network can meet the needs of high-precision absolute calibration and high-frequency decay correction, which embodies a new technological approach of “integration of calibration means and calibration objects, and unification of observation and calibration processes”. The benchmark sites, digital global site network, space radiation benchmark, and new technologies such as high-altitude calibration sites, nighttime calibration, big data and machine learning calibration technology, and space radiometric benchmarks, are expected to build a new generation of calibration technology system for meteorological satellites to meet the requirements of continuously improving the accuracy and stability of data products.