The Synthetic Aperture Radar (SAR) sensor is usually mounted on an aircraft or satellite. The instrument altitude above a reference surface stays constant over time, a condition that is easier to achieve for satellite sensors that stay on the same orbit than for aircrafts that are subject to atmospheric conditions. The sensor moves on a straight flight path, which is called the azimuth direction. It corresponds to the flight direction.
SAR systems acquire information in oblique view, the antenna pointing sideways down to the ground. Most satellite systems use an antenna looking to the right side of the instrument. The ground area illuminated by the radar beam is called antenna footprint. As the sensor moves along the azimuth direction (along-track), the continuous strip of the ground area represented by the successive antenna footprints is called swath.
The looking direction of the SAR antenna is called range direction. It is often perpendicular to the azimuth direction (i.e. across-track), but can also present slightly differences depending on the acquisition mode. The angle between the nadir view and the range direction is called incidence angle.
The original SAR image is displayed in what is called slant-range geometry, i.e., it is based on the actual distance from the radar to each of the respective features in the scene. In the slant range direction, each point target’s backscatter is represented as a function of the time delay between the transmission of the electromagnetic pulse and its reception back at the sensor. This range depending representation induces geometric distortions in the SAR image. One distinguishes between near and far range: targets situated in near range are closer to the nadir direction and closer to the sensor than targets situated in far range. The image representation of targets is also more compressed in near range than in far range.
The slant-range representation can be converted in ground range representation, by projecting the image features orthogonally to a ground reference, allowing a proper planimetric position of the targets relative to one another.
This acquisition geometry allows the distinct mapping of scatterers corresponding to their respective distance to the sensor. It causes also geometric distortions in the radar image, i.e., relief displacement (foreshortening and layover) and shadow.