Slant Range/Ground Range

Concept:

We refer to the way a SAR sees, caring only about distance from the satellite, as "slant range". Humans, on the other hand, prefer images of the Earth which, like maps, are based along the ground. We call an image lying along the ground "ground range".

This fundamental difference creates a number of very strange 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 get smaller with distance!

Example

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.

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, we speak of the "near range" side of the image (the side closest to the satellite) and the "far range" side of the image (the side farthest from the satellite).

Application

Since SAR sees in a fashion humans find difficult to interpret, we often try to correct the slant range nature of SAR.

The simplest conversion is to compensate for Earth's curvature, but leave the mountains leaning over. This is usually called "slant range to ground range conversion". Most SAR images are converted to ground range.
A better conversion requires an accurate DEM, but compensates for Earth's curvature as well as mountain layover. This is called "geometric terrain correction" or "orthorectification".
The best conversion is to compensate for Earth's curvature, mountain layover, and SAR brightness. This is called "normalized incidence angle terrain corrected SAR". This results in an accurate, ground-range image of the SAR brightness which is totally free of terrain effects. The ASF-STEP terrain correction and interferometry packages can both perform this correction.

History

I believe the term "slant range" comes from the fact that the SAR does not look straight down, nor out to the side; but in a slanted fashion. Since SAR images are based on this "slanted distance", and RADAR engineers like to talk about the "range to target", the term "slant range" evolved.

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Last Updated: September 1, 1998
If you have any questions, please feel free to email olawlor@acm.org