# Spaceborne LiDAR Systems: Evolution, Capabilities, and Challenges
## 4 Challenges and Limitations  
LiDAR offers several distinct advantages over both passive and active remote sensing systems. Passive remote sensing techniques require an external illumination source (sun or examined target), which makes them dependent on day/night conditions [18]. Active remote sensing techniques such as SAR or LiDAR have their own source of radiation, making them suitable for day-and-night observation. A key advantage of LiDAR lies in its capability for high-precision vertical profiling, where instruments such as CALIPSO, GEDI, and ICESat-2 provide precise three-dimensional data to measure the vertical structure of clouds, aerosols, forest canopies, and ice sheets. In addition, direct range measurement through flight time can be used to determine heights and layer structures [102]. Notably, Doppler wind LiDAR systems, such as ADM-Aeolus, take advantage of the Doppler shift of scattered radiation to remotely measure wind speeds and temperature profiles, offering a unique capability to capture atmospheric dynamics even under clear air conditions [103]. However, LiDAR systems face several challenges and limitations, which vary across application domains due to differences in system design, operational environments, and performance expectations:  
* Transmitter design challenges: The transmitter laser has every component, including the laser resonator, with elements such as laser crystals, Q-switches, harmonic generator crystals, wave plates, mirrors, and other optical components. These components must meet operational lifetime without worsening performance [24]. One solution to this challenge can be component redundancy; for example, the LITE laser transmitter deployed two identical lasers, and the GLAS laser transmitter uses three identical lasers that do not operate simultaneously [21; 104].
* Spatial resolution: Compared to other remote sensing techniques, LiDARs suffer from low spatial resolution. For example, NASA's GEDI LiDAR uses 25 m diameter laser footprints spaced 60 m apart along the track (and 600 m across the track) which yields a sparse sampling of the surface rather than a continuous image. In contrast, passive optical satellites can achieve much finer horizontal resolution: commercial imagers like WorldView-3 have pixels as small as 0.31 m [105; 106].
* Spatial coverage: LiDAR is distinguished as having relatively small coverage and swath width. For comparison, Landsat-9 (passive, optical) has a swath area of 185 kilometers (km), covering the whole world every 16 days. Sentinel-1 (active radar) has a swath area of 290 km, covering the whole world every 6 days. GEDI has the widest spaceborne LiDAR swath with 4.2 km, making it possible to cover about 2-4% of the land during its 2-year mission [18; 107].
* Weather dependency: Laser light in the visible to near-infrared spectrum is strongly affected by clouds, rain, and other atmospheric conditions that can scatter or absorb the laser pulses. In contrast, SAR operates in the microwave region, which is largely unaffected by such conditions, allowing it to \"see\" through clouds and perform reliably in almost any weather [108].
* Multiple scattering signals: Cloud and aerosol measurements are complicated by multiple scattering phenomena that require complex correction algorithms [109].