Ice, Cloud, and Land Elevation Satellite-2 Photon-Counting Lidar for 3-Dimensional Ocean Observation: A Comprehensive Review of Algorithms, Applications, and Synergistic Opportunities

This review systematically elucidates the transformative impact of the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) on profiling remote sensing science, with a particular focus on its unexpected yet revolutionary applications in oceanography. Launched in 2018, ICESat-2 carries the Advanced Topographic Laser Altimeter System, the first spaceborne photon-counting light detection and ranging (lidar), which provides vertical profile information of the Earth’s surface and subsurface at unprecedented resolution. While its primary mission is cryospheric science, this report details how ICESat-2 has become a cornerstone for a new generation of ocean science, enabling direct bathymetric mapping with reported accuracies approaching 0.5-m root mean square error and the retrieval of diffuse attenuation coefficient ( K d ) profiles validated against Biogeochemical-Argo floats with a mean absolute percentage difference of 15.7%. We analyze the core technologies, data products, and retrieval algorithms that underpin these capabilities, alongside a critical assessment of the system’s advantages and inherent limitations. A central theme is the power of synergy: This review explores how ICESat-2 data, when fused with passive optical sensors (e.g., Moderate Resolution Imaging Spectroradiometer, Sentinel-2, and Plankton, Aerosol, Cloud, Ocean Ecosystem), other active sensors (e.g., Cloud-Aerosol Lidar with Orthogonal Polarization and Surface Water and Ocean Topography), and in situ platforms (e.g., Biogeochemical-Argo) through advanced methods such as machine learning, overcome the constraints of any single system. Finally, ICESat-2 is placed within the broader evolutionary trajectory of spaceborne lidar technology, looking ahead to the future of next-generation high-spectral-resolution lidar missions (e.g., Earth Clouds, Aerosols and Radiation Explorer and Daqi-1) and dedicated ocean-optimized systems (e.g., Cloud Aerosol Lidar for Global Scale Observations of the Ocean–Land Atmosphere System and Guanlan), which promise to build upon the foundation laid by ICESat-2 to construct a fully integrated, 3-dimensional Earth observation system.