## 2 BDS/GNSS for Earth Observations
### GNSS Ionosphere Monitoring  
GNSS ionospheric monitoring is a technology that uses GNSS signals to observe and analyze the states and variations of the ionosphere. For a long time, scientists have been monitoring the ionosphere mainly through ionospheric altimetry, radar, sounding rockets, and other detection instruments, but these traditional methods have some limits. GNSS has the characteristics of all-weather functionality, high efficiency, and low cost. It is capable of tracking whole ionospheric activity over an extended time series, offering a crucial foundation for uncovering the temporal and spatial distribution of ionospheric Total Electron Content (TEC) variations and understanding the patterns of ionospheric activity.  
Given the different operating positions of GNSS signal-receiving equipment, ionospheric monitoring technology may be divided into ground-based and space-based GNSS detection. Ground-based GNSS detection is based upon a signal receiver on the ground. Nowadays, there are more GNSS tracking stations on the ground. The largest and most extensively distributed network of ground-based stations is represented by the International GNSS Service (IGS). Currently, there are more than 500 IGS stations worldwide. Additionally, due to the swift advancement of GNSS technology, the IGS has started the establishment of a multi-GNSS observation network that operates across multiple frequencies and systems (MGEX). In addition to the global GNSS network, different countries and regions have also established their own regional GNSS networks. The EUREF Permanent GNSS Network, the Crustal Movement Observation Network of China, and the Australian Regional GNSS Network are notable examples of existing ground-based GNSS systems. These networks utilize global ground-based GNSS technology to monitor the ionosphere, offering robust all-weather observation data. However, a significant limitation of these ground-based GNSS stations is their predominant distribution over land areas, which results in a notable scarcity of observation data collected over oceanic regions. To overcome this limit, space-based GNSS ionospheric monitoring technology has been developed, which is able to provide comprehensive measurements over oceans, where ground-based data is lacking. The space-based GNSS observation is a new technology that utilizes a signal receiver on a LEO satellite to monitor the ionosphere and thereby realizes the remote sensing of the ionosphere. According to the mutual motion of the satellites and the Earth, when the radio occultation occurs, the GNSS receiver can obtain information such as TEC and bending angle at different heights, and then invert the ionospheric electron density parameters. A series of geo-orbiting satellites with GNSS occultation observation capabilities have been launched internationally, such as CHAMP, COSMIC, GRACE, Fengyun-3C and Metop-A/B/C. The connected facilities of space-based GNSS and ground-based GNSS have been widely used for ionospheric monitoring and research [21; 22]. In the future, the establishment and development of more Earth-orbit satellites will also lay a foundation for ionospheric detection with high spatial and temporal resolution.