Foreshortening

Probably the most striking feature in SAR images is the "strange" geometry in range direction.
This effect is caused by the SAR imaging principle: measuring signal travel time and not angles as optical systems do.
The time delay between the radar echoes received from two different points determines their relative distance in the image.
Let us consider the mountain as sketched in the figure. Points A, B and C are equally spaced when vertically projected on the ground (as it is done in conventional cartography). However, the distance between A'' and B'' is considerably shortened compared to B''-C'', because the top of the mountain is relatively close to the SAR sensor .
Foreshortening is a dominant effect in SAR images of mountainous areas. Especially in the case of steep-looking spaceborne sensors, the across-track slant-range differences between two points located on foreslopes of mountains are smaller than they would be in flat areas.
This effect results in an across-track compression of the radiometric information backscattered from foreslope areas (see example) which may be compensated during the geocoding process if a terrain model is available.
Foreshortening is obvious in mountaineous areas ( top left corner ), where the mountains seem to "lean" towards the sensor.
It is worth noting that shortening effects are still present on ellipsoid corrected data.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry