Single-photon, Dual-color, Polarimetric Laser Altimeter Measurements of Lake Ice Freeboard, Roughness and Scattering Properties

The Slope Imaging Multi-polarization Photon-counting Lidar (SIMPL) is an advanced technology airborne laser altimeter developed with a focus on remote sensing of ice sheets and sea ice including their melt state. Its development was sponsored by the NASA Earth Science Technology Office Instrument Incubator Program. SIMPL utilizes micropulse single photon laser ranging at 532 nm (green) and 1064 nm (near-infrared) wavelengths in a four-beam push-broom configuration. Currently, the instrument is capable of flight altitudes of up to 5000 m; this spreads the 4 profiles over a cross-track distance of 30 m providing an estimate of both along-track and cross-track slope magnitudes and directions. For both wavelengths on each beam, depolarization is measured as the ratio of received energy perpendicular and parallel to the plane-polarized transmit beams. The precision of the single photon ranges is 8 cm and a range observation is acquired every 5 to 10 cm at airborne flight speeds. This performance enables measurement of ice freeboard and surface roughness at 5 m length scales based on the height dispersion of single photon ranges aggregated along the profiles. The depolarization ratio is a function of the scattering properties of the target, specifically the proportions of specular reflection and surface and volume scattering. The relationship between surface roughness and depolarization at green and near-IR wavelengths will be illustrated using data acquired during flights over Lake Erie ice cover in February 2009, an analog for sea ice. Observed in simultaneously acquired digital video frames, the ice cover appears to be a heterogeneous amalgamation of ice types, thicknesses and ages. The lake ice is covered by snow in places and contains numerous open water leads to enable ice freeboard detection relative to the water surface. The depolarization ratio differentiates open water, young clear ice, older granular ice and snow cover. The variability of the ratio along a profile is an indicator of the ice cover homogeneity at 10 m length scales. The four parallel profiles spaced apart by 10 m exhibit highly correlated depolarization ratios demonstrating that the measurement is reproducible within distinct lake ice types. This airborne test bed provides a means to assess the value of this implementation of advanced measurement techniques for remote sensing of the cryosphere. The results are directly applicable to the ICESat-2 spaceflight mission now in development. A multi-beam, micropulse, single photon ranging instrument is the implementation currently planned for ICESat-2.