Potential radiometric impact of atmospheric attenuation on ICESat-2 land retrievals

ICEsat-2 measures the height of the Earth's surface using a photon counting laser altimeter, ATLAS. The laser on ATLAS is split into 6 beams and reflected photons are geolocated and subsequently used to estimate surface heights. The number of reflected photons per shot, referred here as the radiometry, is a function primarily of the transmitted laser energy, solar elevation, surface reflectance, and atmospheric scattering and attenuation. For highly reflective surfaces (e.g ice sheets) and clear skies, approximately 10 signal photons from a single strong beam are detected and recorded by the ATLAS instrument for a given transmit pulse. Over vegetated land where the surface reflectance is considerably less than snow or ice surfaces, the signal level is anticipated to be considerably less. In this paper we explore the relationship between detected scattering and attenuation in the atmosphere against the radiometry for terrestrial surfaces and the subsequent impact, if any, on the accuracy of surface heights retrievals. We also explore the radiometry and height retrieval errors as a function of day versus night. In this study, we show that ground height retrievals are more accurate in day acquisitions compared to night acquisitions in the boreal forests of Finland.