On-Orbit Radiometric Performance of 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, providing time-tagged, geolocated photon heights referenced to the ellipsoid for photon events downlinked from ATLAS. ATL03 also provides 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 the life of the mission. Changes in ATLAS radiometry could indicate either geophysical changes, or more likely changes in the ATLAS instrument. To the extent possible, we attempt to isolate possible causes of radiometric change within ATLAS. Verifying that ATLAS radiometry is stable in both the signal and background photon rates, or determining the correction for ATLAS changes is important in the use of ATL03 photon data for long term climate change applications. We find that the ATLAS radiometry is very stable over the first 4 years of the mission, and show that the signal rate decreases by about 2.5% (1%) per year for strong (weak) spots, consistent with findings based on ATLAS internal measurements. We expect to regain some signal after the laser energy increase planned for autumn 2022.