Radar with synthesis of opening

A Radar with synthesis of opening ( RSO ) is a radar imagor which carries out a data processing received in order to improve the azimuth resolution. The treatment carried out makes it possible to refine the opening of the antenna. One thus speaks about synthesis of opening. From where the name of this type of system.

The radars with synthesis of opening are thus to oppose to the " radars with opening réelle" (RAR or real aperture radar in English) for which the azimuth resolution is simply obtained by using a sending antenna/reception having a narrow lobe of antenna in the azimuth direction.

The abbreviation anglo-saxone SAR ( Synthetic Aperture Radar ) is frequently used to indicate this type of radar.

One distinguishes two big families from RSO:

static mono RSO for which only one antenna is used in emission and reception
the RSO Bi or multi static for which different antennas are used in emission and reception

Principle

The antenna of the radar is fixed on a side face of a carrier (plane or satellite). It has a rather large opening azimuth (several degrees) in the direction of the movement and laterally it can go from the horizon to the vertical what gives a rather weak resolution. The time of return of the echoes being carried out to various times according to their distance to the radar, one can thus obtain a coarse image of the ground if one probes only in one fixed direction.

As the radar moves, the same point is however illuminated several times, one obtains a set of data for each point under the radar. By combining the variation of amplitude and phase of these returns, the treatment of synthesis of opening makes it possible to obtain images of the zones observed as if a broad antenna with very high resolution were used. Like the targets met (ground, foliage, buildings, etc) have different polarizing properties, the intensity coming from the various waves will vary with the type of targets met (materials, forms, mechanisms of " rebonds"). One then studies the differences of intensity and phases between the images generated starting from these various polarizations to deduce some from the descriptive parameters of the picturesque scene. One can thus raise contrasts of certain nonvisible details on traditional images (nonpolarimetric), or deduce from the properties of the target such as the type of vegetation.

Interferometry

One uses simultaneously two radars with synthesis of opening, or the same radar is used at different moments. One then studies the differences in point-to-point phase of the images generated to find the vertical dimension of the ground. One speaks then about interferometric SAR or InSAR.

This method makes it possible to generate digital models of rise, or, by withdrawing a digital model of ground, to measure centimetric displacements in the zones where the signal remains coherent (Interferometry radar differential) the coherence of the zones depends on the geometry of acquision of the images radars, but also of the nature of the zone: in band C (Envisat, RadarSat) the urban areas are generally adapted to the InSAR treatment while the covered zones of vegetations are incoherent.

Radargrammetry

Based on the principle of the stereoscopy, the radargrammetry consists in rebuilding the relief starting from two images radar of the same zone, acquired with different angles of sight. Less precise than interferometry, this method is however less constraining concerning the conditions of acquisitions.

See too

Radar with side aiming

External bonds

Natural resources Canada
Exemple of cartography marinades by RSO
Exemple of measurement of current

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