The SAR and the interferometry
With the term synthetic SAR or aperture radar it is defined a radar antenna mounted on a mobile platform. The operating principle of a SAR system is the same for all radar systems (Radio Detection And Ranging). A transmitter (antenna) radiates the surrounding space with an electromagnetic wave which affects the objects that meets undergoing a phenomenon of reflection. A part of the wave back to the antenna where it is measured. In this way the radar is able to detect objects (detection) and by measuring the delay between the moment of transmission and the receipt, it is possible to measure the distance (slant-range) of these objects from the antenna (Fig. 1.). This measure is however only in the direction of illumination of the antenna, so, it is not possible to distinguish two objects placed at the same distance from the antenna.
Fig.1: SAR resolution cell
More specifically, every radar antenna is characterized by a directivity value, which characterizes the width of the illuminating beam in the surrounding space, as bigger is the antenna, as narrower is its beam and therefore its footprint and consequently, the target is localized better, however this reduces the extension of the illuminated area.
In this way the width of a beam antenna determines the resolution in azimuth of a radar image, but to have a resolution of the order of the meter would be necessary to use an antenna of kilometric sizes, which is clearly unrealizable.
The underlying idea of the SAR allows us to avoid this limitation, reaching a compromise between resolution and extending of the observed area. With The term synthetic aperture and radar, we refer to a technique used to “synthesize” an antenna of kilometric size, combining the radar echoes received by a real antenna to different positions (Fig. 2). Particularly in a SAR system every time a radar antenna emits a pulse, because the motion of the platform on which is mounted, receives back its echo in a different location. In this way, the echoes collected at different positions can be understood as being derived from different portions of a single “synthetic” antenna of large dimension (Fig. 3.).
Fig.2: Geometry of SAR acquisition.
Fig.3: Comparison between synthetic antenna aperture and real antenna aperture.
To distinguish more objects with one beam pointing angle of a SAR antenna it shall not be oriented perpendicular to the ground but at an angle of view ?.
This angle creates some perspective deformations due to the topography of the land. Ensuing three different types of deformation (Fig. 4), according to the slope of the land:
- foreshortening: occurs when the slope of the land tends to be perpendicular to the line joining the target and the sensor in such cases the contribution of several points is concentrated in a few cells producing very bright pixels in the image of size.
- layover: occurs when the slope of the land is greater than the angle ?, this produces a strong distortion of the image that prevents the proper interpretation of the signal and any quantitative analysis.
- shadowing: occurs when some areas may not be illuminated by the radar impulse because screens by other objects; then are produced in the image size some y dark areas (shaded).
Fig.4: Perspective deformation due to land
A SAR image is formed by an array of pixels each of which is associated with a resolution cell (Fig. 5). Also a SAR image is said to be coherent, when contains both information of intensity, or amplitude, and the phase information (related to the distance between target and radar).
Fig.5: Description of a SAR image.
The amplitude identifies the portion of the incident electromagnetic field reflected towards the sensor. In the radar images of amplitude are particularly reflective metal structures (trellis, antennas, railway lines), while basins and roads are usually much less reflective: they are, in fact, characterized by phenomena of specular reflection in which the share of the field reflected towards the satellite is very subdued.