# Spaceborne LiDAR Systems: Evolution, Capabilities, and Challenges
## 1 Introduction  
LiDAR(Light Detection and Ranging) is an active remote sensing technique that has advanced our capability to observe and measure the Earth's surface, atmosphere, and planetary science [1, 2]. Since its inception in the 1960s, shortly after the discovery of lasers, LiDAR technology has evolved considerably from ground-based systems to sophisticated spaceborne instruments capable of taking detailed measurements of Earth's topography, vegetation, and atmospheric constituents [3, 4]. LiDAR systems for terrestrial applications can be broadly classified based on their deployment platforms into three distinct categories: (1) ground-based LiDAR, (2) airborne LiDAR, and (3) spaceborne LiDAR [5]. Initially developed in the 1960s [6], ground-based LiDAR systems pioneered the application of laser technology for precise distance measurements and atmospheric studies. These early systems laid the foundation for atmospheric studies, including measurements of cloud height and monitoring of air pollution [7]. Modern ground-based LiDAR systems have evolved to include scanning capabilities, enabling the creation of high-resolution three-dimensional models of structures, urban landscapes, and vegetation. These systems are widely used in geotechnical monitoring, civil engineering, and environmental research due to their ability to provide highly precise spatially localized measurements [7, 8]. Later, in the 1970s and 1980s, LiDAR technology extended its range and applications through integration with airborne platforms [9]. These airborne systems provided greater flexibility and coverage, enabling topographic mapping of large andinaccessible areas by generating accurate Digital Elevation Models (DEMs) by providing high-density ground point measurements, enabling the creation of accurate and detailed elevation grids in accordance with established geospatial definitions [10]. DEMs themselves are essential tools in various fields including hydrological studies, urban planning, disaster risk assessment, and archaeology [10; 11; 12; 13]. In addition, airborne LiDAR became an asset in forest applications such as quantifying canopy height, biomass, and forest structure and helped to understand carbon sequestration and biodiversity [14; 15]. Lastly, airborne LiDAR transformed archaeological explorations, applications of airborne LiDAR have revealed discovering previously hidden structures beneath dense vegetation, such as Mayan city layouts in Central America [12]. The need for global-scale observations with consistent accuracy and resolution drove the transition towards spaceborne LiDAR, where LiDAR serves as a payload on the satellite/spacecraft to provide continuum measurements [2]. A critical milestone was the deployment of the first spaceborne LiDAR system called LiDAR In-Space Technology Experiment (LITE). In the 1990s [16], LITE and subsequent spaceborne LiDAR missions contributed to our understanding of Earth's topography, ice sheet dynamics, and forest canopy structure and provided data for a wide range of scientific, environmental and commercial applications, such as climate change research and environmental monitoring [2; 7; 17; 18]. Figure 1 provides an overview of important milestones of the development of LiDAR technology. Beyond its terrestrial applications, LiDAR has also played an increasingly important role in planetary science, commencing with the Apollo 15 mission in 1971, where a laser altimeter--a precursor to modern LiDAR systems--mapped the Moon's surface and provided valuable topographic data [22; 23]. This paper comprehensively reviews spaceborne LiDAR technology, focusing on its evolution, underlying principles, applications, and future prospects, particularly in terrestrial applications and contributions to Earth observation. Section 2 describes the fundamental working principles of LiDAR systems, detailing their key components, the LiDAR equation, and the system characterization.