Potential performance characteristics of ICESat-2/ATLAS for canopy height retrievals in different ecosystems

Abstract:

With an expected launch in late 2017 (or early 2018), the ICESat-2 satellite will provide a global distribution of geodetic measurements of both the terrain surface and relative canopy heights which will provide a significant benefit to society through a variety of applications including forest structural mapping and improved global digital terrain models. The Advanced Topographic Laser Altimeter System (ATLAS) instrument on ICESat-2 will utilize a photon counting lidar which utilizes low power laser pulse with detectors sensitive at the single photon level. Due to this type of detector, any returned photon whether from the reflected signal or solar background can trigger a detection event. The ATLAS instrument will record the arrival time associated with a single photon detection that can occur anywhere within the vertical distribution of the reflected signal, that is, anywhere within the vertical distribution of the canopy. One of the uncertainties facing the ecosystem community is a comprehensive understanding of the performance of ICESat-2 will be for various ecosystems. Specifically, which ecosystems –and to a greater extent- what amount of canopy cover will create constraints on the ability to derive relative canopy height from ICESat-2 measurements. This paper aims to provide the science and user community of the ICESat-2 land/vegetation data products with a realistic understanding of the performance characteristics and subsequent data quality and the associated errors. To simulate ICESat-2/ATLAS data, data from small-footprint waveform lidar are the best surrogate for empirically deriving simulated ICESat-2 data as the entire temporal profile of the laser energy is recorded on each waveform. As such, a normalized waveform is the probability of where a photon is returned from the reflecting surface along the laser line-of-sight. Using this empirical approach, simulated ICESat-2 data can be developed for a variety of ecosystems ranging from sparse shrublands to dense forests. Relative canopy heights from simulated ICESat-2 data are evaluated against the canopy heights derived from source small-footprint lidar data to provide a baseline uncertainty for specific ecosystems.