The radar moves along a flight path and the area illuminated by the radar, sometimes called the footprint, is moved along the surface in a swath, building the radar image as it moves. Radar images are composed of many dots, or picture elements called pixels. If a feature is bright, that means a large fraction of the radar energy is reflected back to the radar. Scientists often can then measure the size of the object and also the nature of the surface. For example, a rough object is brighter than a flat one.
Vegetation is moderately rough but roads are flat.
The angle of the reflection helps identify objects also by giving shades of gray which enable identification of specific objects. Wet objects are bright, so water or wet objects can be identified by their degree of brightness. Electrical properties and polarization are revealed also by radar signals.