Avalanche and Snowfall Monitoring with a Ground-based Synthetic Aperture Radar

Alberto Martinez-Vazquez(1) and Joaquim Fortuny-Guasch(1)

(1) DG-JRC, Via Enrico Fermi, 1, I-21027 Ispra (Va), Italy

Abstract

In the past few years, ground based synthetic aperture radar interferometry has proved its ability to monitor large man-made structures such as buildings, dams and bridges as well as natural hazards like landslides. The work presented in this paper demonstrates the feasibility of monitoring the snow cover stability with techniques derived from spaceborne SAR imagery, exploiting the advantages of ground based systems.

During the winters 2003-2004 and 2004-2005, a field campaign has been carried out with the LISA (LInear SAR) Ground-Based Synthetic Aperture Radar (GB-SAR) in the Swiss Alps. More than 20000 radar images have been obtained and used for the study of structural changes and stability of the snow cover, corresponding to 250 days of data acquisition.

The system used to monitor the snow cover, LISA, is a ground based linear SAR fully developed and built at the Joint Research Centre (Ispra, Italy). A sled carrying a vector network analyzer, a power amplifier and the antennas is slid along a rail 5 m long in order to synthesize a linear aperture. A personal computer operates all the instruments and is used for data archiving and processing, and for remote controlling through the public GSM network the automatic measurements. The radar was operated in vertical polarization with a central frequency of 5.83 GHz (C-band) and a bandwidth of 60 MHz, sampled in 1601 points. The aperture synthesized along the x-axis was 3.5 m long in 251 steps.

The test-site, located in the Sionne valley -Canton Valais-, is managed by the Swiss Federal Institute for Snow and Avalanche Research (SLF, Davos). It consists of a concave shaped channel of 1200 m vertical drop and an average slope of 27 degrees. The avalanche path length is 2.5 km, starting from the highest elevation level at 2650 m a.s.l. and finishing at the level of 1450 m a.s.l. The LISA system is located on the mountain opposite to the avalanche corridor at an approximate level of 1800 m a.s.l. In this way the instrument has direct visibility of the scene, covering an area of nearly 2 km by 2 km.

The system has successfully monitored 99 natural avalanches, 5 artificially triggered avalanches and other natural phenomena produced in the snow cover like snowfall or snow drift by the wind. With an acquisition rate of one image every 10 minutes and a pixel resolution of approximately 3.6×7 m (range × cross-range) in its full aperture, the LISA instrument has shown its ability to monitor localized changes of the snow cover. Furthermore, using the sliding subaperture technique, the temporal resolution has been improved up to 1 image every 2.6 seconds, being able to monitor the dynamics of avalanches.

A further study of these images, together with the ground truth data, may lead to the discrimination of avalanches with respect to heavy snowfall, or snow drift because of the weather conditions, and to the identification of some new precursors of avalanches based on ground-based SAR imagery.

 

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