Images provided by optical sensors contain information about the surface layer of the imaged objects (i.e. colour), while microwave images provide information about the geometric and dielectric properties of the surface or volume studied (i.e., roughness).
These images were acquired over the city of Bucharest (R), by ERS-1 and Landsat-5 respectively. The city has a star-shaped urban structure, with a historical centre from which the urban area extends along the main road.
In the center of the city, the presidential palace can be seen, and to the right even the main entry is visible.
The main avenues form three concentric rings around the centre. The national airport is located in the north. The Dimbovita river, a tributary of the Danube, crosses the northern part of the city, and has meanders, partly filled by an artificial lake. Other lakes are visible in the center and in the lower part of the image. The circular zone in the South is a forested area, with a large building in the center.
The vegetation visible in the image is an agricultural area, mainly corn fields. In the north, large zones essentially covered by trees can be observed.
The SAR image is a multi-temporal composition of three images (3/07/92, 25/11/92, 30/12/92), the urban area is well imaged and the density of the
built-up area can be assessed by the strength of the backscattered signal.
In contrast to the optical image, highways, large roads and avenues are presented as dark lines. This is also true for the runways on airports, because of the smooth surface. Bridges and railways on the contrary are imaged mostly very brightly due to the dielectric property of metal.
The different colours of the agricultural fields depend on the changes in surface roughness occurred between the acquisition dates.
Data acquired in spring and summer is used to identify the crop type, a methodology similar to the one applied with optical data.
However, it has been reported that also data acquired in the winter is also of great interest, since it reveals the type of field labour performed which is often typical of the preparation of the fields for certain crop. It allows already very early in the year to assess certain crop types and estimate their surfaces.
It is obvious that such data application must be based on good knowledge of the time and type of field preparation. Initially sufficient ground survey needs to be available.

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