摘要：NASA's ICESat‐2 mission measures Earth's elevation with the Advanced Topographic Laser Altimeter System (ATLAS), a 6‐beam photon‐counting laser altimeter. The Global Geolocated Photon data product (ATL03) is the primary source of photon information used by surface‐type‐specific higher‐level products, along with the Atmospheric Layer Characteristics product (ATL09). ATL03 provides time‐tagged, geolocated photon heights referenced to the ellipsoid and a parameter providing an initial classification of photon events as signal or background. We use this classification to evaluate ATLAS radiometry (number of signal photons per transmitted laser pulse) over short time scales and over the mission to date. The radiometric performance of ATLAS will in part determine what differences in the signal and background photon rates are significant and indicate geophysical, rather than instrumental, changes. We find the ATLAS radiometry is very stable over short time scales and exhibits a long‐term decrease of ∼1 signal photon per laser pulse (∼−12%) in the strong spots over the first 1.5 years of the mission.
