ICESat-2/ATLAS Onboard Flight Science Receiver Algorithms: Purpose, Process, and Performance

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. Plain Language Summary The Advanced Topographic Laser Altimetry System (ATLAS) is the sole instrument on the Ice, Cloud, and land Elevation Satellite 2. ATLAS measures distances to the Earth's surface by firing a green laser pulse 10,000 times per second and measuring the round-trip time of flight from when each laser pulse leaves the instrument to when it returns to the instrument after bouncing off the surface. Green solar photons reflected from the ground and atmosphere, and reflections of the laser off of the atmosphere, also enter the instrument, and their arrival times are measured. To reduce the quantity of data transmitted to the ground, the Receiver Algorithms have been implemented in the flight hardware and software to distinguish the ATLAS laser surface bounces from the light coming from the Sun or laser atmosphere reflections. The Algorithms use a statistical method for finding groups of photons with a high probability of containing surface bounces. Real-time spacecraft location and onboard databases describing the Earth's surface support this process. The Algorithms were extensively tested and verified during ground testing of the instrument. On-orbit results show good performance of the Receiver Algorithms and are consistent with the results seen during ground testing.