SIMPL Measurements of Laser Light Penetration into Snow, Ice and Water: the 2015 Greenland Airborne Campaign in Support of ICESat-2

Abstract:

The primary objective of NASA’s upcoming ICESat-2 mission is to collect altimetry data of the Earth’s surface optimized to measure ice sheet elevation change and sea ice thickness. The mission’s multi-beam, photon-counting lidar, ATLAS, will collect that data. Understanding ranging biases in the altimetry measurements for snow, ice and water is needed to quantify errors in the elevation data for those surfaces. One potential source of bias is penetration of ATLAS’s 532 nm (green) laser pulses into those materials with the associated volume multiple-scattering introducing photon path delays. Changes in penetration depth due to seasonal and/or inter-annual changes in snow and ice optical properties could result in small errors in the trends of ice sheet elevation change. Penetration differences into sea ice versus open water could introduce errors in sea ice freeboard measurements that are the basis for determination of sea ice thickness.

To study this issue of penetration bias the airborne Slope Imaging Multiple-polarization Photon-counting Lidar (SIMPL) acquired ice sheet and sea ice data based at Thule Air Force Base in Greenland during July and August, 2015. SIMPL acquires single-photon ranging data in four cross-track beams at two co-aligned wavelengths; 532 nm and 1064 nm (near infrared). The 1064 nm photons provide a reference to the true surface elevation because the penetration depth is negligible at that wavelength. The co-incident wavelengths illuminate small footprints (30 cm diameter) so that return pulse broadening due to surface slope and roughness is minimized in order to observe the broadening due to penetration. Eight cm single photon range precision enables sub-cm level determination of penetration depth using bias calibrated range histograms. SIMPL’s measurement of laser return depolarization uniquely identifies the presence of surface water, aiding in the interpretation of the penetration results. In order to examine ice sheet penetration biases as a function of surface conditions data was acquired for higher-altitude, interior dry snow as well as western coastal ablation zone ice and melt ponds. Data for multi-year sea ice with open leads was acquired north of Greenland and Ellesmere Island. Here we present results on the instrument measurement performance for these disparate conditions.