In contrary to a real aperture, a synthetic aperture results from an aperture “synthesis”. Synthetic aperture were built in order to overcome the limitation of real aperture and therefore enhance the resolution in azimuth direction. It uses the subsequent positions of a real aperture sensor during its forward motion along the azimuth direction to create a synthetic longer antenna. Via the analysis of the Doppler shift induced by the different echoes of the illuminated objects in the different positions of the real aperture, the azimuth resolution can be improved.