# Spaceborne LiDAR Systems: Evolution, Capabilities, and Challenges
## 2 Basic Principle and Functionality
### Instrument Configuration and Signal Detection
#### 2.1.1 Transmitter Subsystem  
The transmitter in spaceborne LiDAR system typically comprises a laser system source integrated with optical elements components [24]. Commonly employed lasers include solid-state neodymium-doped yttrium aluminum garnet (Nd:YAG) or neodymium-doped yttrium orthovanadate (Nd:YVO4) lasers accompanied by the Q-Switching technique to produce near-infrared (NIR) pulses at 1064 nm. These laser outputs can be frequency-doubled or tripled to produce emissions at 532 nm (visible) and 355 nm (ultraviolet), thereby extending their application to a broader range of Earth and atmospheric observations [29; 30]. The choice of laser wavelength notably influences their performance and applicability. The 1064 nm wavelength, common in aerial and spaceborne mapping, benefits from widely available laser sources, affordable silicon-based detectors, and strong reflectivity from typical terrestrial surfaces such as vegetation, snow, and soil [29]. However, it is less suitable and effective in aquatic environments due to increased absorption by water molecules, which limits its suitability for bathymetric applications [31]. Furthermore, eye safety constraints can be more stringent at 1064 nm compared to some shorter wavelengths, requiring careful system design for orbital or aircraft-based instruments [32]. In contrast, the 532 nm wavelength penetrates water molecules [31; 33] rendering it useful for bathymetric LiDAR measurements. However, in the visible spectrum, 532 nm systems can be affected by ambient light, requiring filtering techniques to improve signal-to-noise ratios [34]. The shorter wavelength increases Rayleigh scattering; therefore, the 355 nm wavelength allows highly sensitive detection of aerosols and trace gases [35]. Q-switching is a laser technique that produces short, intense pulses by temporarily storing energy in the laser medium and then rapidly releasing it. This is achieved by controlling the laser cavity's quality factor (Q), which measures energy efficiency. Q-switching enables the generation of nanosecond-range pulses with peak powers much higher than in continuous operation [36]. The laser excitation in LiDAR systems is typically achieved through optical pumping, using either flash lamps or laser diodes [37]. This process supplies energy to the laser medium to achieve population inversion, allowing electrons to transition to higher energy states and subsequently release photons during relaxation--thereby sustaining coherent laser emission [38]. Recent advances in spaceborne LiDAR technology have focused on improving the efficiency, reliability, and compactness of laser transmitters. For instance, NASA's Goddard Space Flight Center has been advancing laser sources for space-based LiDAR and communication applications, including diode-pumped solid-state lasers (DPSSLs) and fiber-based laser technologies [39]. Additionally, advances in passively Q-switched lasers using silicon photonics have shown promise for compact and efficient laser systems suitable for spaceborne LiDAR [40]. The key high-level parameters to describe the LiDAR laser system can include the following [18; 21; 41; 42; 43; 44; 45; 46]:  
* Laser wavelength [nm]: Commonly selected based on the application's sensing requirements. * Pulse repetition frequency (PRF) [Hz]: PRF refers to the number of laser pulses emitted per second. This parameter directly influences spatial resolution and ground sampling density. Although higher PRFs enable denser sampling, they also increase power demand and thermal load. Spaceborne systems typically operate at up to 20 kHz. * Laser pulse energy [m]: Pulse energy determines the system's ability to detect weak returns from distant or low-reflectance surfaces. Higher energies support longer-range and optically complex measurements (e.g., dense clouds or thick vegetation canopies)but require greater power and thermal control. Typical values for spaceborne LiDARs range from 1 to 100 mJ. * No.