ICESat‐2 science requirements are dependent on the accurate real‐time pointing control (i.e., geolocation control) and postprocessed geolocation knowledge of the laser altimeter surface returns. Prelaunch pointing alignment errors and postlaunch pointing alignment variation result in large geolocation errors that must be calibrated on orbit. In addition, the changing sun‐orbit geometry causes thermal‐mechanical forced laser frame alignment variations at the orbit period and trends from days, weeks, and months. Early mission analysis computed precise postlaunch laser beam alignment calibration. The alignment calibration was uploaded to the spacecraft and enabled the pointing control performance to achieve 4.4 ± 6.0 m, a significant improvement over the 45 m (1 σ) mission requirement. Laser frame alignment calibrations are used to reduce the alignment bias and time variation, as well as the orbital variation contributions to geolocation knowledge error from 6 to 1.7 m (1 σ). Relative beam alignment of the six beams is calibrated and shown to contribute between 0.5 ± 0.1 m and 2.4 ± 0.2 m of remaining geolocation knowledge error. Independent geolocation assessment based on comparison to high‐resolution digital elevation models agrees well with the calibration geolocation error estimates. The analysis demonstrates the ICESat‐2 mission is performing far better than its geolocation knowledge requirement of 6.5 m (1 σ) after the laser frame alignment bias variation and orbital variation calibrations have been applied. Remaining geolocation error is beam dependent and ranges from 2.5 m for beam 6 to 4.4 m for beam 2 (mean + 1 σ).

中文翻译：

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ICESat-2指向校准和地理位置性能

ICESat-2的科学要求取决于准确的实时指向控制（即地理位置控制）和激光高度计表面返回的后处理地理位置信息。发射前指向对准误差和发射后指向对准误差会导致较大的地理位置误差，必须在轨道上进行校准。此外，不断变化的太阳轨道几何形状会导致热机械强迫激光框架在轨道周期内发生对准变化，并有几天，几周和几个月的趋势。早期任务分析计算出了精确的发射后激光束对准校准。对准校准已上载到航天器，并使指向控制性能达到4.4±6.0 m，比45 m（1σ）的任务要求有了显着改善。激光框架对准校准用于将对准偏差和时间变化以及轨道变化对地理定位知识误差的贡献从6减少到1.7 m（1σ）。校准了六个光束的相对光束对准，并显示出它们在剩余的地理定位知识误差中的贡献范围为0.5±0.1 m至2.4±0.2 m。基于与高分辨率数字高程模型进行比较的独立地理位置评估与校准地理位置误差估计非常吻合。分析表明，在应用了激光框架对准偏差变化和轨道变化校准之后，ICESat-2任务的性能远远优于其6.5 m（1σ）的地理位置知识要求。剩余的地理位置误差取决于波束，范围从波束6的2.5 m到波束2的4.4 m（平均值+ 1σ）。