Understanding the state of the Earth's Polar Regions is a key component of understanding Earth's dynamic climate (Meredith et al., 2019) . Observations of the cryosphere are required to quantify changes, observe trends, and contribute to improvements of predictive models for both the present climate and future change (e.g., National Research Council, 2012). Elevation measurements facilitate these studies and from the vantage point of space satellites provide broad coverage of both the Arctic

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... Antarctic regions but also create an opportunity to collect global heights for a multitude of additional studies over a broad range of scientific disciplines. Laser altimetry is a proven technology for elevation measurements from airborne and space-based platforms. The Ice, Cloud, and land Elevation Satellite (ICESat) was operational from 2003 to 2009 and collected climate-critical elevation measurements with the dedicated onboard altimeter, the Geoscience Laser Altimeter System (GLAS; Schutz et al., 2005) . When ICESat was decommissioned, the successor was already in development. This next mission would continue the time series of elevation change begun by IC-ESat and extended through large-scale airborne measurements from NASA's Operation IceBridge (Koenig et al., 2010). ICESat-2, similarly to ICESat, was designated to carry a single laser altimeter instrument but Abstract The Advanced Topographic Laser Altimetry System (ATLAS) onboard the NASA Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) is the newest Earth observing satellite for global elevation studies. The primary objectives for ICESat-2 follow the objectives of its predecessor, ICESat and also focus on providing cryospheric measurements to determine ice sheet mass balance, and monitor both sea ice thickness and extent. However, the global observations support secondary science objectives as well such as biomass estimation, inland water elevation, sea state height and aerosol concentrations. Since launch of ICESat-2, ATLAS has collected more than a trillion measurements. This study provides a mission overview, a description of the operational components that enable the altimeter products for science, on-orbit observatory performance, and assessment of the spacecraft attitude control systems that enable repeat measurements to within 10 m and pointing control within ±45 m. These metrics should be considered for ground-based validation campaigns or science investigations. Plain Language Summary Space-based remote sensing provides an unequaled point of view for observing changes on Earth's surface. Collecting precise elevation data from this perspective with modern measurement technology holds promise for a wide range of science disciplines given the coverage over all surface types (e.g., land ice, sea ice, inland water, ocean, and vegetation). Over time this high quality data can not only reveal global elevations but elevation change in those regions with repeat measurements. ICESat-2 carries a state-of-the-art laser ranging system for accurate elevation measurements and utilizes onboard instrumentation and databases to help control the laser pointing enabling repeat measurements and individual geodetic (latitude and longitude) points of interest. The repeat measurements are critical for looking at elevation change over time, while the collection of elevations over specific locations on the Earth enable studies for science and validation efforts. ICESat-2 has been on-orbit for over two years and has collected nearly a trillion measurements. Evaluation of the repeat measurements over time indicate the satellite can point to within 10 m while the ability to collect a measurement of a single position on the surface is within the mission specification of ±45 m. MAGRUDER ET AL. Key Points: • ICESat-2 has proven the on-orbit ability of photon-counting lidar for precise and accurate space-based altimetry • ICESat-2 onboard attitude control systems exceed the ±45 m mission requirement for pointing control • The satellite is able to follow reference ground tracks to provide repeat measurements within 10 m on average