Characterisation of oriented volumes in glacier ice and extinction inversion with Pol-InSAR
Jayanti Sharma(1), Irena Hajnsek(1) and Kostas Papathanassiou(1)
(1) DLR (German Aerospace Center), Münchner Str. 20, 82234 Weßling, Germany
In recent years there has been increased interest in using SAR to study and monitor glaciers and ice sheets for glaciological and climate change research. 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 radar signatures on frequency and glacier facie, remain unclear.
The objective of this paper is to isolate the volume response to enable estimation of the extinction of the ice volume. Extinction is a relevant parameter for glaciologists since it contains information on the density and internal structure of the ice. This paper describes a new technique of ice extinction estimation through the modelling of Pol-InSAR observables and coherences as a combination of contributions from a Bragg surface (at the air-ice or snow-ice interface) and a volume response from an oriented particle cloud. Since ice volumes may often be expected to have a preferential orientation of scatterers, particularly horizontal stratification and layering, such an extension is important for attempting quantitative remote sensing inversion.
Separation of the ground and volume contributions is achieved through a novel decomposition of the polarimetric covariance matrix in which the volume is permitted to have a preferential orientation. Polarimetric-dependent transmission coefficients of the signal into the ice volume are also integrated into the decomposition. Ground-to-volume scattering ratios derived from the polarimetric decomposition are used in conjunction with Pol-InSAR interferometric coherences and an infinite, uniform-volume-under-ground model to invert ice extinctions. Multiple baselines offer additional estimates which may be combined for a more robust extinction estimation. Simulations are conducted examining the sensitivity of extinction estimation to potential shortcomings in the coherence model and to errors in the ground-to-volume ratio as a function of interferometric baseline and incidence angle.
Validation is performed with airborne Pol-InSAR data at L- and P-band collected using DLR's E-SAR system over the Austfonna ice cap in Svalbard, Norway in March and April 2007 as part of the ICESAR campaign. Inverted extinctions are compared with GPR measurements collected at a comparable frequency and within one month of SAR data acquistion.
Estimation of glacier volume parameters using longer-wavelength Pol-InSAR observables is important for future spaceborne concepts. Potential satellite missions including the BIOMASS earth observation proposal would benefit from an increased understanding of SAR observables over land ice.