On-Orbit Radiometric Performance on ICESat-2

NASA's Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) mission measures the elevation of Earth with the Advanced Topographic Laser Altimeter System (ATLAS), a 6-beam photon-counting laser altimeter. The ICESat-2 Global Geolocated Photon data product (ATL03) is the primary source of photon information required by higher level products, along with the Calibrated Backscatter Profiles and Atmospheric Layer Characteristics data product (ATL09). ATL03 provides time-tagged, geolocated ellipsoidal photon heights for photon events downlinked from ATLAS, as well as confidence levels providing an initial classification of each photon event as signal or background. We use this photon classification to evaluate ATLAS radiometry (number of signal photons per laser pulse) over both short time scales (seconds) and over the life of the mission. Changes in ATLAS radiometry could indicate either geophysical changes (e.g. long-term changes in reflectivity of Earth) or more likely, changes in the ATLAS instrument. To the extent possible, we attempt to isolate possible causes of radiometric change within ATLAS. The radiometric performance of ATLAS will in part determine what differences in both the signal and background photon rates are significant and indicate geophysical, rather than instrumental, changes. We find that the ATLAS radiometry is very stable over short time scales (seconds) and exhibits a long-term trend over the first year and a half of the mission.