A radar is an active microwave device that transmits short directional pulses of energy, then operates as a sensitive receiver to measure the returned energy or radar echo. The term radar is an acronym for radio detection and ranging. The oceanographic value of radar is due to its response to different surface conditions. When the pulse interacts with a surface that is strongly reflective, the return is strong or bright; when the surface is non-reflective, the return is weak or dark. The properties of the reflected and scattered pulse are called backscatter. Because of the large variety of oceanic surface phenomena that modulate the backscatter, radars can retrieve wind speed and direction, ocean swell properties and the presence of heavy rain. They can also make precise measurements of distance and observe phenomena such as internal waves, sea ice, oil and biological slicks and human-made structures such as ships and oil platforms.
Two specialized radars discussed in this and following chapters are the scatterometers and imaging radars. By transmitting a pulse, receiving the return, then correcting the return for atmospheric interference and instrument noise, a scatterometer makes quantitative measurements of the backscatter from small surface areas. With these corrections, any radar that measures backscatter can serve as a scatterometer (Ulaby et al., 1981, pp. 9–10). In oceanography, the backscatter return from a scatterometer is used to retrieve vector wind speed. In contrast, the backscatter from an imaging radar consists of the return from a large surface area, which is then binned according to either time delay or Doppler shift. This subdivides the field-of-view (FOV) into many small areas, which, when combined with the radar motion, produces an image of the area covered by the FOV, where the image consists of the relative changes in backscatter.