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Why does the top of a mountain appear bright on a SAR (radar) image. I know that the peak of a mountain is subject to a layover effect, thus appearing closer on the radar image than the bottom part of the mountain. And, that the back part of a mountain is dark (shadow) because it is not reached by radar's radiations.

The only reason I see that makes the top of a mountain look brighter than the bottom part is because the slope of the mountain is narrower on slant range compared to its real dimensions on ground range. This implies that backscatter return from many objects is returned inside the same cell (more intensity). But I'm not sure if that is really the exact reason.

See Sentinel-1 SAR image of mountain (to understand what I'm talking about):

<code>Sentinel-1</code> <code>SAR</code> image of sahara mountains

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    I think you answered your own question " slopes facing the radar will be foreshortened and ones facing away from it will be lengthened from their horizontal(map) dimensions. The former will therefore be brightened and the latter dimmed. " en.wikipedia.org/wiki/Synthetic_aperture_radar#Image_appearance
    – Mapperz
    Jul 28, 2015 at 17:56
  • @Mapperz Thanks for your answer. So it will have a higher intensity because the backscatter return is compressed in a small area (slope) on the radar image, right?
    – Hakim
    Jul 28, 2015 at 18:06
  • @Mapperz The multiple bounce between ground and mountain slope could also cause an increase of the backscatter return. Don't really know if this could be the reason though.
    – Hakim
    Jul 28, 2015 at 18:17
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    Since you are looking at intensity images you allready answered your own question. If the signal hits a slope that is almost perpendicular to it more of it will be scattered back to the satellite therefore increasing the intensity of the measured return signal.
    – Kersten
    Jul 28, 2015 at 19:58
  • @Kersten Thanks. But I thought that layover is an extreme example of the foreshorting at least according to what I'm reading on this book Ulaby - Microwave Radar and Radiometric Remote Sensing p726. Foreshorting cause the fore slope of a mountain to have a length shorter than it's real length, and layover cause the peak to appear closer to the radar.
    – Hakim
    Jul 28, 2015 at 20:21

1 Answer 1

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You are probably also getting more backscatter because there is bare rock at the summit instead of vegetation. Vegetation holds moisture, which would absorb microwaves rather than backscattering.

Besides, the foreshortening which affects the fore slope increases its intensity (having more backscatter returned into the same cell yield more intensity.

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  • I know that wet vegetation appears brighter than dry vegetation and that dry soil usually appear black on radar images, so why would rock have a higher backscatter than vegetation? Is it because it has a rough surface? Besides, this image was taken in the sahara (arid climate); no vegetation exist there, and rock will definitely be dry too.
    – Hakim
    Jul 28, 2015 at 18:11
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    Waves penetrate less solid/hard surfaces easier than more solid. (Trees & Vegetation vs. Rocks). Less penetration equals more reflection, which results in more backscatter. Jul 28, 2015 at 19:14
  • You're right, I think I should just add to the answer above (@Will McInnes) the effect of foreshortening and layover on the increase of the intensity (having more backscatter returned into the same cell yield more intensity).
    – Hakim
    Jul 28, 2015 at 20:26

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