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Radar interferometry provides a novel way to study the subsurface of glaciers: interferometric correlation. Since the two complex images that comprise the interferogram have slightly different incidence angles at each point on the ground, a decorrelating phase noise, with statistics related to the scattering medium, is present in the interferogram. The amount of "surface" decorrelation is increased by volume scatter. The larger the vertical extent of the scatterers contributing to the radar echo, the greater the decorrelation will be. By modeling this effect, the authors can estimate radio wave penetration depths within the upper layers of the glacier or ice sheet. Observations of the Greenland Ice Sheet using ERS data yield penetration depths (one-way, 1/e point for power) that range from 12 to 35 m. Due to the contribution of volume scatter, the critical interferometer baseline is decreased, and the authors find for the Greenland data, the baseline must be restricted to be less than 300 m. The authors also compare penetration depths measured within the dry snow zone with those found in the percolation zone and coastal areas. They find that as expected, the rocky coastal areas evidence minimal penetration. Interestingly, the penetration depths that the authors measure in the percolation zone, ∼23 m, indicate a large degree of volume scatter, which is contrary to earlier results that found the scattering in the percolation zone dominated by structures in the first few meters. This discrepancy may be due to unmodeled scattering behavior, or the radar return may indeed include significant contributions from scatterers far beneath the surface.