Waveform characteristics comparison over Greenland Ice sheets between SARAL & Envisat Radar Altimeter

Shard Chander (SAC, ISRO, India)


Chauhan Prakash (SAC, ISRO, India)

Event: 2014 SARAL/AltiKa workshop

Session: Land ice and Sea ice

Presentation type: Type Poster

Over Ice surface, the radar waves can penetrate inside the snow layers. This penetration is mainly depend upon the radar frequency, dielectric properties of the medium and accumulation zones over the ice sheets. This study compares the waveform characteristics and their derived parameter accuracy over Greenland ice sheet using Envisat 18 Hz dataset for the period April 2009 to February 2010, and SARAL 40 Hz dataset for the period March 2013 to January 2014. SARAL provides higher along track resolution, data is available every 175m, as compared to Envisat that provides data at 350 m along track interval. For comparison a master track over Greenland was selected with minimum altimeter track loss. For SARAL, along this master track for every 40 Hz point, a rectangle was selected with 100m in latitudinal direction and 1000m in longitudinal direction, only same pass number was considered at the time for prevent overlapping. For Envisat along track distance in latitudinal direction selected was 200m. Ku/Ka band waveforms were classified and compared for different accumulation zones and geographical regions. Waveform shapes and derived product was compared within this smaller domain for inter-satellite comparison and seasonal variability. NSIDC produced 1km resolution DEM using ICESAT/ GLAS altimeter was used as reference. Delaunay Triangulation based technique is used for minimizing the interpolation produced errors. Envisat ICE-2 retracker work best with highest correlation coefficient of 0.99 and lowest RMSE of 175m. After that Sea-Ice retracker works better with overall r2 0.97 and RMSE 220m. For SARAL the maximum correlation coefficient found was only 0.89 that also for Sea-Ice retracker. For SARAL Sea-Ice retracker work best with lowest RMSE of 460m. After that Ice-2 retracker works better with overall r2 0.88 and RMSE 520m. ICE-1 and Ocean retracker were also studied but accuracy was comparatively poor. Penetration depth of Ka band waveform was found to be 0.1m-0.4m as compared to 2m-8m for Ku band waveforms. Over all accuracy for Envisat was far better than SARAL as most of the recent retrackers are optimized for the Ku band waveform. But for Ka band refinement is required in the retracker for taking into account the sharp trailing edge, millimeter scale scattering and less penetration over the ice.
Keywords: Waveform retracking, ICE-1 (OCOG), Sea-Ice (Threshold), ICE-2, dry snow zone, percolation zone, wet snow zone, ablation zones

Shard Chander