Studying vegetation canopy with ICESAT-2

Although the primary mission goal of ICESat-2 is to monitor changes in the cryosphere, ICESat-2 also collects elevation data over the Earth’s land surfaces providing geodetic measurements to support a wide range of terrestrial applications. In the temperate and tropical regions, ranging measurements from ICESat-2 are used to produce estimates of terrain and canopy heights of the worlds’ forests. The density and vertical distribution of the returned photons from within the canopy can be utilized to infer information regarding forest biomass, canopy volume, habitat mapping, biodiversity, and parameterization of land-climate models. The ICESat-2 satellite is in a polar orbit (92 degrees) and is the only space based laser altimeter capable of collecting ranging measurements over all land surfaces.

The instrument onboard ICESat-2 is the Advanced Topographic Laser Altimeter System (ATLAS) and ATLAS is sensitive to detect single photons reflected from the surface. ATLAS uses a 532 nm laser that fires at a rate of 10 kHz (or every 70 cm on the Earth’s surface) which facilitates high spatial resolution in the along-track direction. A diffractive optical element splits the ATLAS laser into 6 beams; 3 beam pairs approximately 3 km apart. Each beam pair is comprised of a strong beam and weak beam. Because ATLAS is sensitive at the photon level, solar background noise can present a challenge in the analysis or photon counting data. ICESat-2 is a profiling lidar and a result of the beam configuration is high resolution in the along-track direction; however gaps exist between beams. In the mid-latitudes, ICESat-2 operates in vegetation mode which consists of off-nadir pointing the satellite to a different ground track each 91-day repeat cycle. Thus, rather than repeating an orbit every 91 days, ICESat-2 will point to a different location on the Earth to improve the spatial sampling. Over a period of two years, this series of off-pointing maneuvers has reduced the distance between tracks at the equator from 29 km to approximately 2 km. As the mission continues through time with more off-pointing maneuvers, the distance between ground tracks will continue to decrease.