Abstract
Spaceborne remote sensing can track ecosystems changes thanks to continuous and systematic coverage at short revisit intervals. Active remote sensing from synthetic aperture radar (SAR) sensors allows day and night imaging as they are not affected by cloud cover and solar illumination and can capture unique information about its targets. However, SAR observations are affected by the coupled effect of viewing geometry and terrain topography. The study aims to assess the impact of global digital elevation models (DEMs) on the normalization of Sentinel-1 backscattered intensity and interferometric coherence. For each DEM, we analyzed the difference between orbit tracks, the difference with results obtained with a high-resolution local DEM, and the impact on land cover classification. Tests were carried out at two sites located in mountainous regions in Romania and Spain using the SRTM (Shuttle Radar Topography Mission, 30 m), AW3D (ALOS (Advanced Land Observation Satellite) World 3D, 30 m), TanDEM-X (12.5, 30, 90 m), and Spain national ALS (aerial laser scanning) based DEM (5 m resolution). The TanDEM-X DEM was the global DEM most suitable for topographic normalization, since it provided the smallest differences between orbital tracks, up to 3.5 dB smaller than with other DEMs for peak landform, and 1.4–1.9 dB for pit and valley landforms.
Highlights
Synthetic aperture radar (SAR) is an active imaging system with several advantages over optic sensors, such as Landsat OLI (Operational Land Imager) or Sentinel-2 MSI (Multi-Spectral Imager)
For each digital elevation models (DEMs), we analyzed the difference between orbit tracks, the difference with results obtained with a high-resolution local DEM, and the impact on land cover classification
The objective of this study was to investigate the impact of global DEMs on the normalization of synthetic aperture radar (SAR) backscatter and coherence observations by Sentinel-1 at two sites characterized by complex topography
Summary
Synthetic aperture radar (SAR) is an active imaging system with several advantages over optic sensors, such as Landsat OLI (Operational Land Imager) or Sentinel-2 MSI (Multi-Spectral Imager). The SAR technique uses the return time to convert a table of recorded echoes into an image (focusing). These times are shortened in sensor-facing steep slopes, causing echoes to overlap and slopes to appear shortened in the focused image (a pixel represents more area). Both effects can be compensated for by using the acquisition geometry parameters and a digital elevation model (DEM). We refer to this compensation as topographic normalization. Normalization results are heavily dependent on the DEM characteristics and quality [2,3] as they can be generated using different data sources, including remote sensing (optic, SAR, airborne laser scanning—ALS), and processing techniques (ALS point cloud, stereography, interferometry, and radargrammetry)
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