Geolocating Individual Photons: Laying the Foundation for ICESat-2 Science Data Products

Abstract:

The Ice, Cloud and land Elevation Satellite-2 (ICESat-2) is NASA’s next generation spaceborne laser altimeter, currently scheduled for launch in October 2017. The sole instrument onboard is the Advanced Topographic Laser Altimeter System (ATLAS), which will use a photon-counting detection method to measure range from the satellite to the Earth’s surface with green (532-nm wavelength) laser light. Combining these measurements with precise knowledge of the position and orientation of the ATLAS instrument in orbit will yield elevation measurements to ±88° latitude around the globe. In this presentation, we provide an overview of the ICESat-2 data products with a particular focus on the Global Geolocated Photon Cloud. With two redundant lasers, operated one at a time, ATLAS sends laser pulses toward the Earth 10,000 times per second, and the outgoing beam is split into three pairs of two beams, to increase spatial coverage and enable determination of local surface slopes. Geolocation algorithms in the ground-based science data processing system will produce a latitude, longitude and ellipsoidal elevation for each photon that is returned to ATLAS, yielding a photon point cloud. These data will incorporate a suite of geophysical corrections, and the product will also provide a coarse surface classification (land ice, sea ice, ocean, land, inland water), along with initial signal-vs-noise discrimination. While higher-level, surface-specific science data products will provide the most precise estimates of surface elevation by aggregating the geolocated photons to reduce noise, we expect that this lower-level data product – which provides most of the inputs to the higher-level products – will be of interest to the general community.