Sub-surface glacial structure over Nordaustlandet using multi-frequency Pol-InSAR

Abstract

In recent years there has been increased interest in using SAR (Synthetic Aperture Radar) to study and monitor glaciers and ice sheets for glaciological and climate change research. SAR is a powerful remote sensing tool due to its high spatial resolution and wide coverage, and its ability to penetrate beneath the ice's surface to observe sub-surface structures. However, SAR backscattering from ice and the inversion of geophysical parameters from SAR ice data remain poorly understood. The relative importance of scattering from the air/snow and snow/ice interfaces, from internal layers and structures in the firn, as well as the dependence of the land ice radar signatures on frequency and glacier facie, is unclear. Few models address the interferometric coherence as an observable, and the greatest focus to date has been on modelling single-polarisation backscattering coefficients. This paper augments the use of SAR backscattering coefficients with polarisation and baseline diversity (Polarimetric Interferometric SAR or Pol-InSAR) to provide additional information on the underlying glacial structure and more thorough validation criteria for evaluating the applicability of electromagnetic models. The backscatter and interferometric coherence from the ice is modelled as a coherent sum of slightly rough annual layers as well as volume scattering from ice crystals between the layers. Validation is performed using a unique Pol-InSAR data set acquired over the Nordaustlandet sheet in Svalbard, Norway. The data were collected using DLR's airborne E-SAR (Experimental SAR) system as part of the SVALEX (April 2005) and ICESAR (March/April 2007) campaigns. Fully-polarised multi-baseline data at L- (1.3 GHz) and P-band (0.35 GHz) frequencies and single-pass VV data at X-band (9.6 GHz) were obtained. For calibration purposes and to determine the penetration of the radar signals, corner reflectors and an approximately 200-m-long wire were deployed directly on the surface of the ice. Backscatter and coherence at different frequencies and thus different depths of penetration, and at different polarisations and thus potentially different scattering mechanisms, are examined as a function of incidence angle, spatial baseline, and temporal baseline (a two year temporal baseline is available). Modelled results are compared with the quad-pol observed backscatter and complex coherences, providing insight on the electromagnetic interaction between the radar signal and the polythermal ice. The use of interferometry and surface reflectors enables measurement of the phase centre depth of the penetrating radar signal. Electromagnetic modelling of longer-wavelength Pol-InSAR observables over glaciers and ice sheets has relevance for future spaceborne SAR concepts. Potential satellite missions including a P-band sounder and the BIOMASS earth observation proposal would benefit from an increased understanding of long-wavelength SAR observables over land ice.