Definitive interpretation of ice-sheet basal conditions from radar-sounding data beneath outlet-glacier grounding zones and shear margins can be problematic due to poorly constrained and spatially variable englacial attenuation rates and losses from propagation through a rough ice surface. To correct for spatially variable attenuation rates, we developed a novel radar analysis approach that provided improved empirical attenuation correction by fitting linearly variable attenuation rates along radar-sounding profiles from the ice-sheet interior to the grounding zone. We also corrected for ice-surface propagation losses by using surface echo amplitude distributions to constrain the loss of coherent power for surface reflections and two-way propagation through a rough ice surface. By applying this approach to airborne radar-sounding observations of the Thwaites Glacier catchment in West Antarctica, we produced relative reflectivity profiles, which show grounding-zone basal conditions varying across the Amundsen Sea Embayment. Additionally, these techniques provided improved characterization of basal conditions across shear margins, showing that — contrary to previous interpretations — the eastern shear margin of Thwaites Glacier corresponded to a change in basal conditions consistent with a transition from frozen to thawed bed.