Characterization of the Greenland Ice Sheet Basal and Surface Layer Microwave Angular Scattering Characteristics

The use of microwave backscatter signatures to characterize the physical attributes of surfaces and layers, such as roughness and dielectric constant, has been used for many geophysical applications. For ice sheet studies, the characterization of these parameters for the basal and surface layers could aid in the identification of the presence of liquid water, bed roughness, and surface snow layer characteristics. To date, the main source of basal layer scattering data have been low-frequency microwave sounders, such as the various systems deployed through the years by the University of Kansas. The conventional use of these data has been to process them to retrieve the backscatter signature from the nadir direction alone, which has yielded extremely useful data for the characterization of ice sheet thickness and attenuation. However, due to the uncertainties in the basal layer attenuation, it is problematic to use this data for basal layer backscatter characterization. In this study, we report on a novel technique, which we call Squinted Unfocused SAR Interferometry (SUSI), which has promise in retrieving the angular characteristics of the microwave backscatter signature from the basal and surface layers for incidence angles in the range from 0 to ~15 degrees. It is well known that the angular decay of the radar cross section in the near nadir regime is an indicator of the interface root mean squared slope (i.e., surface roughness), independent of the absolute magnitude of the return. Therefore, these angular signatures provide a unique tool for characterizing the basal layer unavailable from other measurements. We present the theoretical basis for the SUSI technique, and then apply it to data collected over the Greenland ice sheet by the University of Kansas radar system during May 2006, September 2007, and July 2008. These data were collected at 150 MHz and 450 MHz, allowing the retrieval of basal characteristics at two different scales of roughness. The SUSI technique can also be used to retrieve the scattering characteristics of the surface layer, which yields insights about the angular characteristics of surface clutter. This characterization has implications about the feasibility of spaceborne radar sounders for Earth or planetary applications