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SAR Geometry

Due to its side looking geometry, a radar sensor measures the distance from the satellite to a point on the ground. This distance is referred to as "slant range". In contrast to this perspective, the SAR image can also be projected such as maps or optical images. In this case the distances are in "ground range". This fundamental difference between these two geometries is reflected in a number of effects. In an optical image, the front face of a mountain takes up a large angle, and is larger, while the back face, if visible at all, is smaller. In SAR, all points on the front face of a mountain lie at nearly the same distance away from the satellite, and hence pile up into just a small portion of the image, while the back face of a mountain slopes away and takes up a large part of the image. These "leaning of the mountain" effects are called foreshortening and layover). In SAR, objects do not become smaller with distance!

The following images compare the two viewing methods. Each shows a satellite, complete with solar panels, as it looks toward the horizon to view a mountain. For clarity, only a horizontal cross-section of the image is shown, through the central mountain. The optical image has been rotated by 90 degrees clockwise to line up with the standard SAR view.


Difference in geometry between optical and radar images

In the SAR image, the front side of the mountain appears smaller than the back side, while in the optical image the front side is the only one visible. In the optical image, the scene is lit by the sun; while SAR shines its own illumination. Thus the SAR image shows the front of the mountain as much brighter than the back side.

Also, several features are present in the optical image which are missing from the SAR image:

  • Color-- SAR satellites like ERS-1, ERS-2, JERS, and RADARSAT can only detect a single wavelength and polarization of SAR. Hence they see a grey-scale image.
  • Clouds-- microwave radiation penetrates even thick cloud cover and fog. In addition, since they carry their own radiation source, they can be used day and night.
  • Horizon-- although objects do not get smaller with distance, over the course of a SAR image the earth curves significantly. However, since this is like a hill in the middle of the scene, the near-to-satellite portion of the image actually looks smaller than the far-from-satellite portion of the image.
Since SAR images are based around distance, the "near range" side of the image is the side closest to the satellite, whereas the "far range" side of the image is the side farthest from the satellite.