# On-Orbit Absolute Radiometric Calibration and Validation of ZY3-02 Satellite Multispectral Sensor
## 1 Introduction  
The Ziyuan-3 (ZY3) satellite constellation is China's civilian high-resolution stereo mapping satellite system, consisting of four satellites: ZY3-01, ZY3-02, ZY3-03, and ZY3-04. The ZY3-01 satellite was launched into orbit on 14 January 2012, is the first operational satellite of the ZY3 series, and completed its official mission and stopped operation in 2017. As its continuation, ZY3-02 was successfully launched at Taiyuan Satellite Launch Center on 30 May 2016, and is still in operation. ZY3-02 carried a multispectral imager (MUX) and three panchromatic cameras (TLC). The multispectral imagery provides repetitive acquisition of multispectral data with a high resolution of 5.8 m, while panchromatic imagery of 2.1-m resolution is mainly used for 1:50,000 scale topographic mapping. The primary purpose of the ZY3 satellite constellation is to satisfy China's need for satellite imagery for surveying and mapping and the geographic information system industry. Most studies of the ZY3-01 and ZY3-02 satellites have focused on geometric calibration, and minimal work has been performed on the radiometric calibration and validation, while quantitative applications such as surface albedo retrieval and aerosol parameter retrievals of ZY-01 and ZY-03 images require reliable radiometric information. Since the launch of ZY3-02, ZY3 satellite products have successfully played an important role in China's environmental, agricultural, and other quantitative applications, which require reliable radiometric information. The on-orbit absolute radiometric calibration of the satellite sensor is a critical activity in providing highly accurate quantitative measurements of the Earth's surface [1]. Although the technical parameters of the satellite sensor have been accurately measuredin the laboratory before launch, data from satellites are sensitive to post-launch changes, regardless of the quality of the calibration data. The absolute radiometric calibration of a satellite sensor is essential for maintaining stable satellite data quality and for identifying the relationship between the image digital number (DN) and at-sensor radiance [2; 3]. Reflectance-based vicarious calibration was recognized as one of the most reliable approaches for on-orbit absolute radiometric calibration of satellite sensors [4]. This approach has been employed for on-orbit absolute radiometric calibration of numerous satellite sensors, such as Moderate Resolution Imaging Spectroradiometer (MODIS), Landsat-7 Enhanced Thematic Mapper Plus (ETM+), Landsat-8 Operational Land Imager (OLI), and the sensors loaded on IKONOS and SPOT satellites, as well as the multispectral cameras on board China's Fengyun (FY), Ziyuan (CBERS), and Huanjing (HJ) satellites [5; 6; 7; 8; 9; 10; 11; 12]. In this paper, we describe the methods and results of a reflectance-based vicarious calibration campaign that was conducted between July and September 2016 at Baotou Site, located in Inner Mongolia, China. A series of gray-scale permanent artificial targets and infrastructure has been built in the Baotou site to provide effective support for satellite sensor on-orbit calibration. A set of several radiometric calibration tapraulins with nominal reflectance of 5%, 20%, 40%, and 60% were designed for the on-orbit absolute radiometric evaluation of the ZY3-02 satellite multispectral imager. The reflectance-based vicarious calibration approach relies on the synchronous measurement of the surface spectral reflectance and atmospheric parameters. In this campaign, the synchronous measurements of surface and atmospheric conditions (including aerosol optical depth, amount of water vapor, aerosol inversion products) at the Baotou site at the time of ZY3-02 satellite overpass. The synchronous measurement of the surface spectral reflectance and atmospheric parameters is coupled with exoatmospheric solar irradiance spectrum and relative spectral response of the sensor as inputs into a radiative transfer model to compute the at-sensor spectral radiance.