L-Band Polarimetric InSAR Observations of Greenland Ice Sheets using ALOS

The ALOS PALSAR instrument has acquired L-band (23.6 cm wavelength) fully polarimetric synthetic aperture radar (SAR) observations of Greenland with 10 meter single-look resolution. We examine images from a strip in northern Greenland extending from latitudes of 75 degrees N to 80 degrees N, which covers the dry snow, percolation, and wet snow zones of the Greenland ice sheet, as well as the rocky coastal area. Images for repeat-pass interferometry with a 350 meter baseline were acquired at a 46 day interval in March and April 2007. The images from the two dates are coregistered by cross-correlating the HH observations, and we observe fringes in all polarizations in the dry snow, percolation, and wet snow zones, and also in the stable parts of the rocky coastal area. In the dry snow zone of inner Greenland, we observe significantly higher coherence in the HH-HH interferograms (around 0.7) compared to the HV-HV interferograms (around 0.4), and similarly higher coherence in the VV-VV interferogram compared to the VH-VH interferogram. These differences between co-polarized and cross-polarized signals result from volume scattering and lower SNR in the cross-polarized channels. They indicate that scalar models do not fully describe L-band microwave scattering from firn. On each observation date, the phase difference between the HH and the HV returns is almost constant over the dry snow zone of the interior of Greenland. However, there is significant variability in the phase difference between HH and HV returns closer to the coast. The phase difference between the VV and VH returns shows similar behavior, again indicating a difference between co-polarized and cross-polarized scattering mechanisms. We derive polarization signatures for the various scattering regions in the Greenland ice sheets to better understand the scattering mechanisms involved. We model the firn in the dry snow zone as a layered medium with rough interfaces between the layers, and we use the polarimetric InSAR observations to constrain the parameters of this model. Firn structure is directly influenced by accumulation rate in the dry snow zone, and is thus important for understanding the mass balance of the ice sheet.