| Radiometric calibration is the precondition and basis of quantitative remote sensing. Its calibration accuracy determines the quality of quantitative remote sensing products directly. At present, operational calibration frequency of the Chinese satellite remote sensor, once a year, has not meet the needs of quantitative remote sensing. High frequency and high precision in-flight absolute radiometric calibration has become an urgent problem to be solved for Chinese satellite remote sensor. An absolute radiometric calibration method based on Global Calibration Site Network (GCSN) was proposed. By establishing GCSN, the number of satellite overpass is increased and the calibration frequency is improved.In this paper, the general scheme and basic flow of the GCSN radiometric calibration approach was introduced. A criterion for the calibration site selection of GCSN is proposed. The GCSN with 131 calibration sites covers the whole world was established by collecting the sites that have been used and automatically searching. The spatial uniformity, temporal stability, spectral shape,bi-directional reflectance and atmospheric characteristics of calibration site were analyzed. A database based on GCSN was established to store the surface and atmospheric parameters needed for in-flight absolute radiometric calibration.The available space-based and ground-based operational observing products over the world were selected to provide basic parameters of GCSN database. The relevant GCSN database data storage table were designed according to the characteristics of each data product. In order to ensure the accuracy of the GCSN database basic data, the in-situ measured reflectance data of Dunhuang site was used to validate the reflectance data of GCSN database. Their relative deviations of MODIS band 1?6 are no more than ±3.1%. A radiometric calibration software base on GCSN was designed to manage the GCSN database and realize the long time series absolute radiometric calibration of the satellite remote sensor.The GCSN approach was conducted on Chinese satellite remote sensor GF-1 WFV4 and FY-3B VIRR to obtain long time series absolute radiometric calibration coefficient in 2013?2016. The coefficient of GF-1 WFV4 shows volatility, however,coefficient of FY-3B VIRR has a monotonic decay trend. Based on the time series calibration coefficient, a linear model is used to describe the long-term varying trend of calibration coefficients and obtain the daily coefficient. The traditional vicarious calibration of GF-1 WFV4 was carried out to validate the feasibility and accuracy of GCSN approach. The relative deviation of the calibration coefficients of the two approach is less than 2% which shows a good agreement. The calibration results of the GCSN approach are compared with the official calibration results, and the average relative deviation of the two calibration results are within ± 5.15%. It proved that the GCSN approach has good reliability and stability. Taking GF-1 WFV4 as an example,the influences of the surface and atmospheric parameters on the radiometric calibration results were analyzed.High frequency calibration in a short term can be achieved using GCSN approach.It helps eliminating the contingency of single calibration result, improving the reliability and stability of calibration. Long time series calibration based on GCSN approach can realize continuous monitoring and dynamic tracking of the radiometric characteristics of sensor, and timely correct of the performance decay of satellite remote sensor. The GCSN absolute radiometric calibration approach can improve the calibration frequency, reduce the calibration cost and improve the calibration efficiency significantly. It lays a foundation for high precision quantitative remote sensing application, and has great value. |
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