ESA Earth Home Missions Data Products Resources Applications
   
EO Data Access
How to Apply
How to Access
Services
Site Map
Frequently asked questions
Glossary
Credits
Terms of use
Contact us
Search


 
 
 

 

Feasibility of snow water equivalent retrieval by means of ground-based and spaceborne SAR interferometry

Thomas Nagler(1), Helmut Rott(1), Markus Heidinger(1), Guido Luzi(2), Macaluso Giovanni(2), Daniele Mecatti(2), Linhsia Noferini(2), Rudolf Sailer(3), Philipp Joerg(3), Andreas Schaffhauser(3) and Reinhard Fromm(3)

(1) ENVEO, Technikerstrasse 21a, 6020 Innsbruck, Austria
(2) University of Florence, via S. Marta 3, 50139 Florence, Italy
(3) BFW, Hofburg-Rennweg 1, 6020 Innsbruck, Austria

Abstract

The accumulated mass of snow (the snow water equivalent, SWE) is a key variable for hydrology of mountain basins, avalanche hazard assessment, and climate research. In mountainous terrain SWE shows high spatial variability, so that remote methods are needed to obtain accurate, spatially detailed maps of snow accumulation. Radar signals in the L- to C-band range are able to penetrate dry seasonal snow with little interaction, so that the dominant part of the backscatter signal originates from the ground surface below the snow cover. The phase delay of the radar signal in snow depends on the mass of snow and the propagation path length. Therefore phase changes in time series of coherent SAR images can be used to detect the accumulation of dry snow. In the EC project GALAHAD we investigated the feasibility for applying this method to map snow accumulation with ground-based (GB) and satellite-borne (SB) SAR data. The GB-SAR, operating at C- band (5.95 GHz) and S-band (2.15 GHz), was installed below an avalanche prone slope at the high alpine site Wattener Lizum near Innsbruck, Austria. Time series of interferometric GB-SAR measurements were acquired through several weeks of the winters 2005 / 2006 and 2006 / 2007. In addition, detailed observations of physical snow parameters were carried out and meteorological date were recorded. The SB-SAR data base for this study includes C-band (ERS SAR and ENVISAT ASAR Image Mode) and L-band (ALOS PALSAR Polarimetric Mode) repeat pass data of the Eastern Alps. Though in the L- to C-band range the backscatter contribution of the snow pack itself is small, the interferometric signal is affected by decorrelation due to snow fall, wind erosion and wind deposition even if the snowpack is dry. The main reasons are differential phase shifts within the SAR resolution element, as theoretical calculations show. This effect decreases with increasing wavelength. Nevertheless, temporal decorrelation is the main limiting factor for application of the method. This problem can be overcome by GB-SAR selecting short repeat observation intervals. Whereas at low incidence angles the interferometric phase shifts can be directly related to SWE (snow depth times density), at high incidence angles (e.g. for GB-SAR) information on snow density is needed to account for refraction angle. We developed a method for estimating SWE from time series of phase measurements by modelling the temporal evolution of the density of the accumulated snow. This enables accurate mapping of SWE. GB-SAR and SB-SAR are complementary tools for SWE mapping, by enabling detailed continuous acquisitions at local scale on one hand, and regional coverage at longer observation intervals on the other hand. However, due to the comparatively long repeat cycles of the present C-and L-band SAR systems, the applicability of SB-SAR is often hampered by decorrelation. Based on theory and on SWE retrievals from GB-SAR and SB-SAR data the capabilities and limitations of the interferometic method for SWE mapping are presented.

 

Workshop presentation

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry