Monitoring Alpine glacier activity by a combined use of TerraSAR-X images and continuous GPS measurements – the Argentière glacier experiment Emmanuel Trouvé, Ivan Pétillot, Philippe Bolon, LISTIC, Université de Savoie, Annecy, France Michel Gay, Lionel Bombrun, GIPSA-lab, INP Grenoble, France Jean-Marie Nicolas, Florence Tupin, Télécom Paris-Tech, France Andrea Walpersdorf, Nathalie Cotte, LGIT, Grenoble, France Irena Hajnsek, Martin Keller, DLR Microwaves and Radar Institute, Wessling, Germany Abstract The monitoring of Alpine glacier activity is one of the applications which require the combined use of ground measurements and SAR data. It should become feasible thanks to the new generation SAR satellites (ALOS, TerraSAR-X, Cosmo-SkyMed and RadarSAT-2) and the high precision GPS measurements. This paper presents the early results of an experiment which started in 2007 on Argentière glacier (Mont-Blanc area) and includes a moving corner reflector (CR) oriented for TerraSAR-X acquisitions, SAR images where this CR is visible and 3 continuous GPS stations providing the CR position and information on the tropospheric path delay corrections. 1 Introduction The monitoring of temperate glacier fast evolution is an important issue for economical and security rea- sons and as an indicator of the local effects of global climate change. Compared to sparse terrestrial ground measurements, remote sensing is expected to allow regular observations of glacier activity and to provide dense measurements of physical parameters which are necessary to detect significant changes and to con- strain glacier flow models. High resolution optical images can be used to measure the glacier topography and summer displacement [1]. However, snow makes optical correlation most of the year impossible, and especially at glacier accumulation areas around the year. SAR data are a complementary information source which can be used in the cold season where reduced surface changes make SAR interferometry feasible [2]. The high spatial resolution images (3 m in range) and the relatively short repeat cycle of Ter- raSAR-X (11 days) will allow new investigations over moving glacier surfaces made of ice, firm, snow and rocks. Surface motion analysis will benefit from the higher resolution or the dual polarization for fea- ture tracking approaches and from the increased inter- ferometric potential to obtain displacement fields by differential SAR Interferometry (D-InSAR). Nevertheless, several difficulties have to be solved in order to obtain reliable displacement measurements over the fast moving Alpine glaciers. The three main limitations which affect the D-InSAR approach are the coherence preservation, the atmospheric artefacts and the unknown phase unwrapping offsets when the moving surface cannot be connected to a non-moving area. In order to investigate these issues on Alpine glaciers, series of TerraSAR-X images are expected to be acquired over the Chamonix Mont-Blanc test-site which includes well-known glaciers such as the Mer- de-Glace glacier (the second European largest glacier complex) and the Argentière glacier (AG) which has been monitored and instrumented for several decades by different scientific teams. Moreover, a corner re- flector (CR) provided by the DLR and a continuous GPS recording station (see Figure 1) have been fixed on the ice on the upper part of the glacier during the second E-SAR campaign performed over this test-site in February 2007 [3]. Two other non-moving con- tinuous GPS stations have been installed, one near the glacier at 2700 m above msl height and one down in the Chamonix valley. Despite the difficulties to main- tain this equipment on the moving and changing gla- cier surface, it allows to perform several experiments based on the detection of the CR in the SAR images and the knowledge of its position by the GPS data. Figure 1 Corner reflector and continuous GPS mov- ing with the Argentière glacier (Feb. 2007).