# Pre-Processing of Inner CCD Image Stitching of the SDGSAT-1 Satellite
## 1 Introduction  
The Sustainable Development Goals Satellite 1 (SDGSAT-1), launched in November 2021, is specially designed for the \"Big Earth Data Science Engineering\" project supported by the Chinese Academy of Sciences. There are three sensors assembled on the platform: an infrared thermal sensor, a night-time light sensor, and a multispectral sensor. The infrared thermal sensor is sensitive to temperature variations of 0.2 \({}^{\circ}\)C, which is helpful for mapping energy distributions and the interaction between human activities and nature. The night-time light dataset, collected on a 1000:1 scale, is useful for detailed analysis of global human settlement patterns. The multispectral sensor covers seven channels, and canwarn of changes in water quality in rivers, lakes, and oceans [1]. The acquisition breadth of the imaging swath is about 300 km, and it can achieve global coverage within 11 days. With the combination of multiple sources and all-day observation from the SDGSAT-1 satellite, it is of great significance to characterize the traces of human activities in order to provide exclusive data support for Sustainable Development Goal (SDG) indicators that can represent the interaction between humankind and nature. Table 1 provides the main characteristics of the SDGSAT-1 satellite. A significant change in earth observation research has been brought about by the use of Time-Delayed and Integration Charge-Coupled Device (TDI-CCD) sensors [2]. Remote sensing images produced by TDI-CCDs have the benefits of adequate exposure, small volume, and good mobility [3]. Standard products offered by image vendors include stitched images, which are necessary due to the restricted field of view (FOV) of single CCD sensors. Figure 1 illustrates typical types of staggered CCD elements based on the IKONOS, Pleiades, WorldView-2, and ZiYuan-3 (ZY-3) satellite platforms. As shown in Figure 1a,b, multiple inner CCDs are staggered in two or more lines mechanically, as this is the simplest and most practical method. To reduce the influence of imaging time delay between different inner CCDs, the imaging sensors on the Pleiades satellite are arranged in an arc, as shown in Figure 1c. A semi-transparent mirror is applied on the focal plane of the ZY-3 satellite by projecting all inner CCD images into the same plane, as shown in Figure 1d. As for the SDGSAT-1 satellite, multiple inner CCDs are arranged in a similar layout as the design of the ZY-3 satellite. Influenced by various factors, stitched products derived from inner CCD images often suffer from low accuracy and obvious distortions [4; 5]. As such, the stitching accuracy between adjacent inner CCD images is a critical factor for the geometric performance of stitched images. A large number of investigations based on images from SPOT-5, QuickBird, ALOS/PRISM, ZY-3, and other satellite platforms have been carried out for multiple CCD image processing. The orientation of time-dependent single CCD lines requires that the calibration, attitude accuracy and time interval requirements for recording attitude information be satisfied. Three phases for processing the SPOT-5 images were carried out in 2002, including CCD distortion calibration and relative camera orientation [6]. In [7], the authors used IKONOS images to generate digital surface models (DSM). The accuracy of produced DSMs reached about 2 m after calibration of the interior parameters. Similarly, different combinations of stereo and tri-stereo Pleiades images were applied for the production of DSM in [8]. After a detailed analysis, ref. [9] found that the stitching accuracy of IKONOS imagery greatly influenced the reconstruction of the Digital Elevation