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SAR basics

SAR basics

Synthetic aperture radar (SAR) is an active side-looking sensor for the Earth remote sensing, which is payload for the airborne and spaceborne platforms, for example, unmanned aircrafts, airplanes or satellites. Because the radar is an active sensor, it illuminates an underlaying surface in order to acquire background responces on his antenna. Radar has its own energy source and, therefore, can operate during day or night independently of sun lighting. Radar wavelength allows to image ground surface through cloud cover. As an side-looking system, SAR moves along its path, illuminates ground targets several times and accumulates data. In this way, continuous strips of the ground surface are illuminated parallel and to one side of the flight direction.

Remote sensing survellence begins when SAR sends pulse toward terrestrial surface. Pulse interacts with the terrestrial surface, partially it's absorbed, and part of the dropped energy is reflected backward to the sensor. On the input of receiving device (SAR antenna) a returned pulse comes with certain delay. Then the returned signal passes throughout SAR receiver and comes to storage device as stream of complex data or so called raw signal data. For this records of raw signal data, special processing (SAR processing) is needed to produce radar image. After SAR processing radar image contains two main parameters - amplitude (brightness) and phase (directly connected with a signal time delay and a wavelength of a sensor). The phase may be used in further for measurement of height with accuracy up to several meters and surface displacement with accuracy up to centimeters and even millimeters.

The length of the SAR antenna determines the finest of theoretically possible spatial resolution in azimuth direction: the more longly the antenna the better is the resolution. In difference from a usual side-looking radar, SAR achieves the high spatial resolution in the azimuth direction with help of the synthesized aperture formation. The principle of the synthesized aperture formation is based on reception of a signal from the same point of surface during enough long time of SAR platform flight. At such way of a signal reception the artificial increase of antenna size due to platform movement occurs.

What is the interferometry?

Interferometry in general is a measurement method which uses effect of an electromagnetic waves interference. For interference at least two waves is necessary. Result of waves interaction depends on many reasons and under specific conditions contains useful information. The interferometric SAR data processing technique assumes reception of several coherent measurements of the same surface area with shift in space of the radar reception antenna. The fact that SAR is the active sensor makes possible reception of required measurements independently of time of day and weather conditions. Thus, we receive the perfect tool for digital elevation models building and for surface displacement monitoring.

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