**3. Basic technical framework of spaceborne LiDAR 3D mapping**

The preferred approach to the acquisition of global laser point cloud data is to use spaceborne instruments. Since the beginning of the twentieth century, two generations of satellite-based LiDAR measurement technology have been developed [30, 31]: the first generation is represented by ICESAT-1, which uses a laser linear detection regime characterized by high-energy pulses, full waveform detection, and high accuracy of elevation measurement to the decimeter level [32]. Limited by the volume of power consumption, it is generally a single beam or several beams with a large laser footprint, which to some extent affects the planimetric positioning accuracy to about 10 m [33]; at the same time, the sampling frequency is low, generally 2–3 Hz, and the density of acquisition points is low, which requires long-period continuous

#### **Figure 5.**

*NASA's 20-year development plan for laser imaging radar satellites.*

observation to achieve dense global coverage. The second generation, represented by ICESAT-2, adopts a photon-counting laser detection system [34, 35], characterized by micro-pulse and multi-beam detection, which improves the detection effectiveness by three orders of magnitude while maintaining high accuracy of elevation measurement, and significantly increases the sampling frequency and data sampling density, providing an effective technical way to achieve global high-precision and high-density 3D point cloud data acquisition [28]. With the support of the National Major Science and Technology Project "High Resolution Earth Observation System," China has broken through the key technology of second-generation satellite-based LiDAR measurement after more than 10 years, which has laid a solid foundation for the development of independent laser mapping satellites.

**Figure 6** shows the basic technology framework for rapid construction of 3D digital geospatial information based on global laser point cloud. The framework

#### **Figure 6.**

*The setup diagram of 3D digital geospatial information framework to rapid reconstruct based on global laser point cloud.*

system mainly includes three parts: global photonic laser point cloud data acquisition, data processing and data application, and the ultimate goal is to form a global point cloud database to support the construction and update of global mapping database.
