Imaging RADAR (RAdio Detection And Ranging) is an active remote sensing system which bounces microwave energy from a target and records the energy that returns to the sensor. The radar antenna alternately transmits and receives pulses at particular microwave wavelengths (in the range 1 cm to 1 m, which corresponds to a frequency range of about 300 MHz to 30 GHz) and polarizations (waves polarized in a single vertical or horizontal plane).
Microwave energy pulses are emitted at regular intervals and focused by the antenna into a radar beam directed downwards and to the side. The radar beam illuminates the surface obliquely at a right angle to the motion of the platform. Objects on the ground reflect the microwave energy depending on factors such as roughness and attitude. The antenna receives this reflected (or backscattered) energy.
By measuring the time delay between the transmission of a pulse and the reception of the backscattered "echo" from different targets, their distance from the radar and thus their location can be determined. As the sensor platform moves forward, recording and processing of the backscattered signals builds up a two-dimensional image of the surface.
Unlike aerial photographs and satellite images which are passive remote sensing systems, in active systems such as radar, the brightness or darkness of the image is dependent on the portion of the transmitted energy that is returned back to the radar from targets on the surface. Bright areas are produced by strong radar response and darker areas are from weak radar responses., while the response to radar energy by the target is primarily dependent on the three factors (1) Surface roughness of the target, (2) Radar viewing and surface geometry relationship, and (3) Moisture content and electrical properties of the target.