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This paper presents a comprehensive radiometric calibration approach for THEOS-1, a satellite owned by the Geo-Informatics and Space Technology Development Agency (GISTDA) in Thailand. Radiometric calibration is crucial for ensuring the accuracy and reliability of satellite imagery, particularly for applications such as environmental monitoring, disaster management, and land cover classification. In this study, we employ a combination of vicarious and cross-calibration methods to refine the radiometric calibration parameters of THEOS-1. The Taiwan Space Agency (TASA) has collaborated with GISTDA to develop and validate the radiometric calibration procedures for THEOS-1. The vicarious calibration method involves utilizing ground-based measurements of known radiance targets to establish a relationship between the satellite sensor's digital numbers and the actual radiance values. We leverage the extensive work with the support of onsite sun photometer measurements and MODIS atmospheric data to obtain high-quality atmospheric data, which is essential for accurately characterizing the radiative transfer processes in the Earth's atmosphere. Furthermore, we incorporate a cross-calibration step to enhance the accuracy of the radiometric calibration. This involves comparing the radiance measurements of THEOS-1 with those of well-calibrated reference sensors on other satellite platforms. By leveraging the radiometric consistency between multiple sensors, we can identify and correct systematic biases, thereby improving the overall calibration accuracy of THEOS-1. The proposed calibration approach is implemented and validated using a series of observational data acquired by THEOS-1 over various calibration sites with diverse surface types and atmospheric conditions. Preliminary results demonstrate significant improvements in the radiometric accuracy of THEOS-1 imagery, thereby enhancing its utility for a wide range of Earth observation applications. This research demonstrates that TASA and GISTDA have successfully developed a robust procedure for the radiometric calibration of THEOS-1', making it ready for routine calibration. The collaboration between TASA and GISTDA has resulted in a well-established calibration protocol that ensures the high quality of THEOS-1 data. In summary, this paper contributes to the ongoing efforts to enhance the radiometric calibration of THEOS-1 by leveraging both vicarious and cross-calibration methods. The calibrated imagery generated through this approach holds great promise for advancing scientific research, environmental monitoring, and societal applications reliant on satellite-based Earth observation data.
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