Airborne 0.5–2 GHz brightness temperatures were collected along a transect of the Priestley Glacier, Northern Victoria Land, Antarctica that coincides with previously acquired 189–199 MHz depth sounding radar data from NASA’s Operation IceBridge (OIB). The measured brightness temperature spectra evolve from negative spectral gradients (brightness temperature decreases with frequency) over the inland ice sheet and upper reach of the outlet glacier toward positive spectral gradients in the central region of the glacier. The spectra then increase almost linearly with frequency over the floating portion of the glacier terminus. The positive spectral gradients are more similar to sea ice on the ocean as compared to the grounded interior ice sheet. Comparison with modeling studies at Ross Ice Shelf sites where physical temperature has been measured in boreholes shows that positive gradients are to be expected where glacier ice overlies a water base. Calculations of radar reflectivity support the assumption of patchy regions of basal water and help to resolve ambiguities associated with processes such as interface roughness and near-surface firn layering.