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7. Radar principles
Radar sensors are usually divided into two groups according to their modes of operation. Active sensors are those that provide their own illumination and therefore contain a transmitter and a receiver, while passive sensors are simply receivers that measure the radiation emanating from the scene under observation.
- radar imaging systems (Radar = RAdio Detection And Ranging)
- microwave radiometers
We are interested in radar imaging systems. The basic principle of a radar is transmission and reception of pulses. Short (microsecond) high energy pulses are emitted and the returning echoes recorded, providing information on:
- time interval between pulse emission and return from the object
- Doppler frequency
The same antenna is often used for transmission and reception. This animation presents the basic elements of an imaging radar system.
The two types of imaging radars most commonly used are:
- RAR --> Real Aperture Radar;
- SAR --> Synthetic Aperture Radar
Real Aperture radars are often called SLAR (Side Looking Airborne Radar). Both Real Aperture and Synthetic Aperture Radar are side-looking systems with an illumination direction usually perpendicular to the flight line.
The difference lies in the resolution of the along-track, or azimuth direction. Real Aperture Radars have azimuth resolution determined by the antenna beamwidth, so that it is proportional to the distance between the radar and the target (slant-range).
Synthetic Aperture Radar uses signal processing to synthesise an aperture that is hundreds of times longer than the actual antenna by operating on a sequence of signals recorded in the system memory.
These systems have azimuth resolution (along-track resolution) that is independent of the distance between the antenna and the target.
The nominal azimuth resolution for a SAR is half of the real antenna size, although larger resolution may be selected so that other aspects of image quality may be improved.
Generally, depending on the processing, resolutions achieved are of the order of 1-2 metres for airborne radars and 5-50 metres for spaceborne radars.