After over a half-century of development, bathymetric lidar is a mature and widely used technology for mapping the littoral zone in support of nautical charting, benthic habitat assessment, inundation modeling and other applications. In 2018, bathymetric lidar transitioned from a purely airborne technology to also a spaceborne capability with the launch of NASA’s ICESat-2 satellite. An important aspect of obtaining accurate seafloor elevations and horizontal coordinates in bathymetric lidar is refraction correction, which corrects for the change in the speed and direction of the laser at the air-water interface. Unfortunately, data on the refractive index of seawater needed for correction are largely lacking—especially over global extents, which are required for ICESat-2 bathymetry. This study developed and evaluated a new global refractive index of water data layer. A two-phased sensitivity analysis was conducted to investigate how systematic and random uncertainties in the refractive index layers impact bathymetric lidar uncertainty. We then developed the global refractive index of water layer using global marine datasets and evaluated it using a combination of Argo Float data and in situ refractometer measurements. The results provide a strong indication of the usefulness of the global refractive index layer, which is currently being implanted into the workflow for generating a new ICESat-2 bathymetric dataset (ATL24). To benefit other studies, the global refractive index layer is publicly available. Future improvements are possible, leveraging crowdsourced data collection to continually improve the spatial resolution and nearshore accuracy of the refractive index data set.

A major advance in global bathymetric observation occurred in 2018 with the launch of NASA’s ICESat-2 satellite, carrying a green-wavelength, photon-counting lidar, the Advanced Topographic Laser Altimeter System (ATLAS). Although bathymetric measurement was not initially a design goal for the mission, pre- and post-launch studies revealed ATLAS’s notable bathymetric mapping capability. ICESat-2 bathymetry has been used to support a wide range of coastal and nearshore science objectives. However, analysis of ICESat-2 bathymetry in numerous locations around the world revealed instances of missing or clipped bathymetry in areas where bathymetric measurement should be feasible. These missing data were due to the ATLAS receiver algorithms not being optimized for bathymetry capture. To address this, two updates have been made to ICESat-2’s receiver algorithm parameters with the goal of increasing the area for which ICESat-2 can provide bathymetry. This paper details the parameter changes and presents the results of a two-phased study designed to investigate ICESat-2’s bathymetry enhancements at both local and global scales. The results of both phases confirm that the new parameters achieved the intended goal of increasing the amount of bathymetry provided by ICESat-2. The site-specific phase demonstrates the ability to fill critical bathymetric data gaps in open ocean and coastal settings. The global analysis shows that the area of potential bathymetry approximately doubled, with 6.1 million km2 of new area in which bathymetric measurements may be feasible. These enhancements are anticipated to facilitate a range of science objectives and close the gap between ICESat-2 bathymetry and offshore sonar data.

The ICESat-2 (Ice, Cloud and Land Elevation Satellite-2) photon-counting laser altimeter technology required the design and development of very sophisticated onboard algorithms to collect, store and downlink the observations. These algorithms utilize both software and hardware solutions for meeting data volume requirements and optimizing the science achievable via ICESat-2 measurements. Careful planning and dedicated development were accomplished during the pre-launch phase of the mission in preparation for the 2018 launch. Once on-orbit all of the systems and subsystems were evaluated for performance, including the receiver algorithms, to ensure compliance with mission standards and satisfy the mission science objectives. As the mission has progressed and the instrument performance and data volumes were better understood, there have been several opportunities to enhance ICESat-2’s contributions to earth observation science initiated by NASA and the ICESat-2 science community. We highlight multiple updates to the flight receiver algorithms, the onboard software for signal processing, that have extended ICESat-2’s data capabilities and allowed for advanced science applications beyond the original mission objectives.