Imaging radar is an active radar system that sends out pulses and records the echoes scattered back by the objects (scatterers) to the sensor. Imaging radars are independent of weather conditions and can operate day or night. It uses microwave wavelengths, radar bands from X- to P- or VHF-band, in four polarisations to illuminate an area on the ground. Normally only the horizontal (H) or vertical (V) linear polarizations are used. The radar system is characterized by combination of polarization of transmitted and received pulse: HH, HV, VH or VV. A typical radar system measures the strength and roundtrip time of the microwave signals that are emitted by a radar antenna and reflected off a target area. An imaging radar is therefore both and imaging and a ranging system. The illuminated objects are mapped in the radar depending on their backscatter intensity and respective range to the sensor.
Imaging radar can be mounted on aircraft or satellite. It operates in a side-looking configuration, left or right with reference to the flight direction. 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. The radar sensor operates not in the real aperture of the radar antenna, i.e., real spatial width, radar (RAR) mode but in the synthetic aperture radar (SAR) mode. Synthetic aperture is possible to set up through the forward motion of the spacecraft, which enables to “extend” the real size of the radar antenna. With a SAR, each object on the ground is sampled at several antenna positions along the flight path, i.e., as long as the antenna beam is illuminating it.
Imaging radar can be used for a different of land and water applications.