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The Geoscience Laser Altimeter System (GLAS) on the NASA Ice, Cloud and land Elevation Satellite (ICESat) has provided a view of the Earth in three dimensions with unprecedented accuracy. Although the primary objectives focus on polar ice sheet mass balance, the GLAS measurements, distributed in 15 science data products, have interdisciplinary application to land topography, hydrology, vegetation canopy heights, cloud heights and atmospheric aerosol distributions. Early laser life issues have been mitigated with the adoption of 33‐day operation periods, three times per year, designed to document intra‐ and inter‐annual polar ice changes in accordance with mission requirements. A variety of calibration/validation experiments have been executed which show that the elevation products, when fully calibrated, have an accuracy that meets the science requirements. The series of papers in this special ICESat issue demonstrate the utility and quality of the ICESat data.

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Comparison of satellite altimetry against a high‐quality network of tide gauges suggests that sea‐surface heights from the TOPEX altimeter may be biased by ±5 mm, in an approximate piecewise linear, or U‐shaped, drift. This has been previously reported in at least two other studies. The bias is probably caused by use of an internal calibration‐mode range correction, included in the TOPEX “net instrument” correction, which is suspect owing to changes in the altimeter's point target response. Removal of this correction appears to mitigate most of the drift problem. In addition, a new time series based on retracking the TOPEX waveforms, again without the calibration‐mode correction, also reduces the drift aside for a clear problem during the first 2 years. With revision, the TOPEX measurements, combined with successor Jason altimeter measurements, show global mean sea level rising fairly steadily throughout most of 24 year time period, with rates around 3 mm/yr, although higher over the last few years.

We present the first winter season of surface height and sea ice freeboards of the Arctic Ocean from the new Ice, Cloud, and Land Elevation Satellite (ICESat‐2; IS‐2) mission. The Advanced Topographic Laser Altimeter System onboard has six photon‐counting beams for surface profiling with a 10‐kHz pulse rate (interpulse distance ~0.7 m) and footprints of ~17 m. Geolocated heights assigned to individual photons scattered from the surface allow significant flexibility in the construction of height distributions used in surface finding. For IS‐2 sea ice products, a fixed 150‐photon aggregate is used to control height precision and obtain better along‐track resolution over high reflectance surfaces. Quasi‐specular returns in openings as narrow as ~27 m, crucial for freeboard calculations, are resolved. The fixed photon aggregate results in unique variable along‐track resolutions and nonuniform sampling (17 m × 27 m to 17 m × 200 m for the strong beams) of the surface. The six profiling beams—three pairs separated by 3.3 km with a strong and weak beam in each pair—provide correlated statistics at regional length scales for assessment of beam‐to‐beam retrieval consistency and accuracy. Analysis shows along‐track height precisions of ~2 cm and agreement in the monthly freeboard distributions across the strong beams to 1–2 cm. In this paper, we describe briefly the approaches used in surface height and freeboard retrievals from Advanced Topographic Laser Altimeter System photon clouds and detail the key features of these along‐track sea ice products, focusing on the first release of data collected over the Arctic Ocean, which spans the period between 14 October 2018—the start of data collection—and the end of March 2019.