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[PP2-3-8] Terrain reflectivity and geometric distortions

Synthetic Aperture Radar (SAR) backscatter is determined both by dieletric and geometric properties of the illuminated target. While the water content of the target plays an important role, its surface roughness determines the scattering mechanisms and the amount of incoming signal sent back to the sensor. Depending on its characteristics but also on the considered wavelength, a surface appears more or less rough. On smooth surfaces, specular reflection occurs, meaning that most of the incoming signal will be reflected away from the sensor. For rough surfaces, diffuse reflection occurs, meaning that part of the signal is scattered back to the sensor, the amount of it depending on different surface roughness parameters. Depending of the observed target and surface, single or multiple scattering mechanisms occur. A particularly important scattering mechanism is the double bounce, which occurs generally at two perpendicular surfaces (e.g. ground and building wall). Through two successive specular reflections, the whole signal comes back to the sensor. Due to the side-looking geometry of SAR systems and the range dependent image representation, specific additional effects occur and affect the backscatter intensity. Whereas a flat terrain only appears more compressed in near range and more stretched in far range, larger geometric distortions appear for terrain with more topography (e.g. mountains) or high objects (e.g. trees, buildings). This relief displacement is caused by the target’s elevation. A high elevated object is closer to the sensor than the ground below it. Due to the image formation in range direction depending on the distance between sensor and targets, its signal comes back sooner to the sensor and it is represented in the SAR image in nearer range than the ground below it. High objects in the SAR image are therefore displaced horizontally toward the radar antenna. This horizontal displacements contrast with the radial displacement observed in optical imagery due to central projection. Furthermore, such objects hide part of the ground below them, which do not receive any signal and cannot scatter information back. Three particular geometric distortions exist: foreshortening, layover and shadows. Depending on the illuminated target, different scattering mechanisms occur in combination with geometric distortions, which makes the interpretation of the SAR image challenging. A good example are buildings, where layover, shadow and single- and double-bounce occur.

Introduction

Comment: Divide into two concepts

External resources

  • Dubois, C., Thiele, A., & Hinz, S. (2016). Building detection and building parameter retrieval in InSAR phase images. ISPRS Journal of Photogrammetry and Remote Sensing, 114, 228-241.
  • Henderson, F. M. & Lewis, A. J. (ed.) (1998). Principles & Applications of Imaging RADAR. Manual of Remote Sensing. Third Edition, Volume 2. John Wiley & Sons, USA.
  • Jensen, J. R. (2000). Remote Sensing of the Environment An Earth Resource Perspective Prentice Hall. Upper Saddle River (NJ), USA.
  • Rees, W. G. (2010). Physical Principles of Remote Sensing. Second Edition. Cambridge University Press, pp. 343.
  • Soergel, U. (2010). Review of radar remote sensing on urban areas. In Radar remote sensing of urban areas (pp. 1-47). Springer, Dordrecht.
  • Thiele, A. (2014). 3d Building Reconstruction from High-resolution Multi-aspect Lnterferometric Synthetic Aperture Radar Data. Bayerische Akademie der Wissenschaften.
  • Woodhouse, I. H. (2017). Introduction to microwave remote sensing. CRC press.

Learning outcomes

Self assessment

Completed

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