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Corner cube retro‐reflectors (CCRs), passive optical components, are used to independently evaluate the geolocation accuracy and effective laser footprint diameter of NASA's laser altimetry mission, ICESat‐2, at two specific study sites: White Sands Missile Range in New Mexico and along a segment of the 88°S line of latitude in Antarctica. The CCR methodology provides ICESat‐2 the ability to monitor these altimeter performance metrics throughout the mission lifetime as an indicator of the health of the instrument and the quality of the observations for science applications. The results using this technique reveal a mean geolocation accuracy of the ICESat‐2 measurements of 3.5 m ± 2.1 m, meeting the mission requirement of 6.5 m. For those instances where multiple CCRs are illuminated, the mean effective laser footprint diameter is 10.9 m ± 1.2 m, with the variability assumed to be due to the influence of atmospheric conditions, but warrants further investigation.

The Advanced Topographic Laser Altimetry System (ATLAS) is the sole instrument on the Ice, Cloud, and land Elevation Satellite 2 (ICESat‐2). Without some method of reducing the transmitted data, the volume of ATLAS telemetry would far exceed the normal X‐band downlink capability or require many more ground station contacts. The ATLAS Onboard Flight Science Receiver Algorithms (hereinafter Receiver Algorithms or Algorithms) control the amount of science data that is telemetered from the instrument, limiting the data volume by distinguishing surface echoes from background noise, and allowing the instrument to telemeter data from only a small vertical region about the signal. This is accomplished through the transfer of the spacecraft's location and attitude to the instrument every second, use of an onboard Digital Elevation Model, implementation of signal processing techniques, and use of onboard relief and surface type reference maps. Extensive ground testing verified the performance of the Algorithms. On‐orbit analysis shows that the Algorithms are working as expected from the ground testing; they are performing well and meeting the mission requirements., Key Points Algorithms onboard ICESat‐2 limit telemetry data volume using onboard signal detection, along with elevation and relief databasesExtensive prelaunch instrument testing showed consistent results meeting mission requirementsAfter launch, the Algorithms have performed very well, with results consistent with those from the prelaunch instrument testing

The ICESat‐2 Precision Orbit Determination (POD) system computes the precise position of the laser altimeter instrument in inertial space, a measurement necessary for accurate geolocation of individual photon surface returns. ICESat‐2 POD solutions are generated by the reduction of Global Positioning System (GPS) double‐difference carrier phase observable residuals with NASA Goddard Space Flight Center's GEODYN software. Independent satellite laser ranging (SLR) measurements are withheld from the POD solutions and used as an independent arbiter of POD performance. Measurement model updates in the form of estimated GPS antenna phase center and satellite laser ranging retro‐reflector tracking point offset vectors as well as a GPS antenna phase center variation map have led to significant improvement in POD performance. GPS residual analysis and orbit precision analysis show no significant anomalous temporal or geographic POD performance issues. The independent SLR residual analysis indicates that POD solutions have achieved a radial orbit accuracy just below 1.5 cm, thus exceeding the mission radial orbit accuracy requirement of 3.0 cm by more than a factor of 2. Future POD processing updates have been identified and will lead to even further improvements in POD performance.