**2.2 Status of development**

Satellite-based laser measurement equipment was first used in deep space exploration, such as Apollo-15, 16, 17 (1970, Moon), Clementine (1994, Moon), MGS (1996, Mars), LRO (2009, Moon), OSIRIS (2016, asteroids), Japan SELENE (2007, Moon), Chinese CE-1, 2, 3, 4 (since 2007, Moon) [9], mainly used for surface topography measurements and landing site selection. Due to the large differences between deep space exploration and earth observation in terms of detection environment and accuracy requirements [10, 11], this paper only focuses on earth observation LiDAR systems. As shown in **Table 1**, the existing and planned LiDAR systems for earth observation at worldwide [5, 12–16] mainly include SLA, ICESat, ICESat-2, ZY3–02, GF-7, terrestrial ecosystem carbon monitoring satellite, GEDI, and LIST.

NASA launched the first laser measurement satellite, the Ice, Cloud, and Land Elevation Satellite (ICESat-1), in January 2003, with the Geoscience Laser Altimeter System (GLAS) as the primary payload, with the main mission of measuring land

**Figure 2.** *Principal diagram of satellite-based LiDAR measurement.*



*Main*

**Figure 3.** *ICESat and its global laser point cloud data (395 million points, 1.36 GB).*

topography, cloud height and vertical structure, and polar ice caps [17]. During its 6.5 years of operation, the ICESat-1 laser altimeter has acquired a large amount of high-precision surface data of the earth, as shown in **Figure 3**, providing a valuable data source for scientific research and practical applications in many fields [18, 19]. Of particular note in the field of global mapping is the ability of surface elevation measurements to construct accurate medium-resolution digital elevation models of the Earth or to combine with optical stereo mapping/InSAR mapping satellite results to significantly improve the elevation accuracy of 3D terrain models [20]. Several studies have shown that the surface laser points acquired by ICESat-1 can be used as auxiliary control data [21, 22], and its elevation measurement accuracy reaches 0.1 m magnitude, which significantly improves the accuracy of aerotriangulation and mapping under uncontrolled conditions, especially in the elevation accuracy [23]. China has added similar laser altimeter payloads to the newly developed Gaofen 7 and Gaofen 14 high-precision stereo mapping satellites [24, 25], which provide support to ensure the accuracy of 1:10,000 scale elevation surveying under satellite conditions.

Given the outstanding performance of the ICESat-1 satellite, the follow-on ICESat-2 laser measurement satellite mission became one of the highest priority satellite observation missions recommended by the National Research Council for the period 2010– 2020 [26]. ICESat-2 was successfully launched in September 2018 with a primary payload of the Advanced Topographic Laser Altimeter System (ATLAS), which adopts a photon-counting, high-frequency, micro-pulse, multi-beam laser measurement scheme with a 17 m footprint on the ground and a sampling interval of 0.7 m in the along-track direction. As shown in **Figure 4**, the accuracy and reliability of the earth observation results have been greatly improved compared with the 70 m footprint and 167 m along-track sampling interval of ICESAT-1 [27, 28]. In the 2 years of ICESAT-2 operation, the global surface data collection density is 70 cm apart in the along-track direction, and the maximum spacing is less than 2 km in the vertical track direction on the equator, which provides the possibility to construct a topographic elevation model with higher plane accuracy and resolution from regional to global scales.

The planned launches of laser earth observation satellites are the Terrestrial Ecosystem Carbon Monitoring Satellite (TECMS) and the U.S. LIST (LiDAR Surface Topography) program. The carbon monitoring satellite is the first satellite mainly serving forestry in China, carrying LiDAR and high-resolution multi-angle multispectral cameras to achieve forest height and biomass inversion through synoptic measurements; LIST is proposed to use photon counting detection system to obtain global topographic information of 5 m grid size and 10 cm elevation accuracy with 1000 beams, as well as surface elevation changes of forests, lakes, and ice caps [29].

**Figure 4.**

*ICESat-2 satellite, ground beam geometry, comparison of GLAS and ATLAS laser footprint distribution (1500 times higher sampling density).*

The satellite-borne high-resolution Earth observation LiDAR is still a very cuttingedge engineering research direction internationally, and foreign research institutions, including the United States, have taken it as an important research content and development direction for future Earth observation. NASA's 20-year development plan for laser imaging radar satellites is shown in the **Figure 5**.
