Speaker
Dr
Thomas Nagler
(ENVEO IT GmbH)
Description
The Sentinel-1 satellites of the European Copernicus Programme, equipped with a C-band SAR sensor, are providing improved, long-term observation capabilities for important bio- and geo-physical parameters of the Earth system. We developed, implemented and tested a procedure for retrieving maps of snowmelt area. The retrieval algorithm uses a similar change detection method as applied for snow mapping with SAR data of ERS and Envisat. We use Level 1 (swath based) single look complex (SLC) SAR products acquired in Interferometric Wide swath (IW) mode in dual (VV, VH) polarization. The IW mode is the nominal operation mode over land surfaces, with 250 km swath width and 5 m x 20 m nominal ground resolution. The reduced backscattering coefficient of snowmelt areas is detected by computing the ratio of backscatter intensity of the SAR image with melting snow versus reference images from the same satellite track acquired under snow-free conditions. As first processing step multi-channel speckle filters are applied to the precisely co-registered snow image and reference images. The ratio of the snow image versus the reference image in the VV and VH channels is combined to obtain an optimum feature space for snow segmentation. Terrain corrected geocoding and segmentation is performed for the combined ratio image, using precise orbit data and a DEM. Post-processing employs a land cover map in order to exclude water surfaces and dense forests. We generated a sequence of snowmelt area maps with 100 m spatial resolution for April to June 2015 covering the whole Alpine area, and maps with 50 m resolution for regional studies. For quality assessment we compare the S1 snow maps with snow extent derived from Landsat images and from high resolution TerraSAR-X images. The snow maps of the different sensors show good overall agreement. Along the snow boundaries there are some differences between SAR and optical sensor products, with a trend for underestimation of snow extent by SAR in particular in areas of broken snow cover due to high backscatter signals of snow-free patches. The combination of co- and cross-polarization data in the snow retrieval procedure yields improvements over single channel data. Differences in angular dependence of backscatter in the two channels are exploited to improve the backscatter contrast between snow-covered and snow-free surfaces over a wide range of local incidence angles. The retrieval algorithm enables the regular operational production of high resolution snow maps over extended areas. Sentinel-1 IW mode data provide complete repeat coverage in mid latitudes within 6 days with a single satellite, and within 3 days with the two satellite constellation, being an excellent basis for matching the snow mapping requirements in many fields of application.
Primary author
Dr
Thomas Nagler
(ENVEO IT GmbH)
Co-authors
Ms
Elisabeth Ripper
(ENVEO IT GmbH)
Mr
Helmut Rott
(ENVEO IT)
