Terrain Effects on Envisat and Sentinel-1 SAR Coverage and Relevance for Tropical Forest Monitoring in Southeast Asia
Rueß, Dominik¹; Pflugmacher, Dirk²; Reulke, Ralf³; Hostert, Patrick^2
1DLR, GERMANY; ²Humboldt-Universität zu Berlin, Geography Department, GERMANY; ³Humboldt-Universität zu Berlin, Department of Computer Science, GERMANY

Remote sensing of forest cover and land-use change in tropical regions is challenging because of persistent cloud cover and topographic effects in mountainous terrain. SAR sensors have the ability to penetrate clouds and the backscatter is related to forest structural characteristics such as canopy density, crown and leaf shapes. However, radar shadows, foreshortening and layover effects limit data usability in steep terrain. Most radar studies in tropical regions have been conducted for small, selected areas over flat terrain. So far, little is known about the potential and limitations of SAR for monitoring large tropical areas. The aim of this study was to explore the utility of space-borne C-band SAR sensors for mapping of forest cover and other land-uses in Southeast Asia by examining topographic effects on radar visibility in this region. To obtain information on radar coverage for such a large area, we used data on sensor orbits and viewing geometry from Sentinel-1 and Envisat ASAR. Based on these data and a 90-m resolution DEM from the Shuttle Radar Topography Mission, we then projected the area influenced by radar shadow and layover. Our results showed significant layover effects in mountainous regions for low incidence angle swaths (e.g. Envisat ASAR IS-2 or Sentinel-1 S1). Using higher swath numbers (IS-7 or S3-6) significantly reduced the area affected by layover without significantly increasing shadow regions, despite the larger side looking angles. The combined use of ascending and descending orbits provides a chance to obtain higher slant range or ground range resolutions as compared to flat regions. Also, layover effects and shadow areas decreased even further. While Sentinel-1 provides the better resolution, this comes at the cost of overall coverage. With Envisat ASAR complete coverage was achieved using a single image mode and swath width, with Sentinel-1 full coverage was achieved only by combining several image modes (e.g. S3-S6). With ERS-1/2 data and ENVISAT ASAR there are now, for the first time, long radar time series available (since 1990 more than 20 years). The long history of acquisitions and ensured data continuity through future missions (Sentinel-1) makes these sensors particularly attractive for operational monitoring of tropical forests.