Radar Viewing Geometry: Incidence Angles; Slant and Ground Range; Spatial, Azimuth, and Slant Range Resolution; And SAR (Especially for GATE-Geospatial 2022)

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The platform carrying the radar sensor moves along the orbit in flight direction. You can see the ground track of the orbit/flight path on the Earth՚s surface at nadir.

Radar Viewing Geometry

The microwave beam illuminates an area, or swath, on the Earth՚s surface, with an offset from the nadir. The direction along-track is called azimuth, the direction perpendicular (across-track) is called range.

Radar Remote Sensing Geometry

Radar sensors are side-looking instruments. The portion of the image that is closest to the nadir track of the satellite carrying the radar is called near range. The part of the image that is farthest from the nadir is called far range.

Radar Incidence Angle and Local Incidence Angle

The incidence angle of the system is defined as the angle between the radar beam and the local vertical. Moving from near range to far range, the incidence angle increases. It is important to distinguish between the incidence angle of the sensor and the local incidence angle, which differs depending on terrain slope and earth-curvature.

Radar Incidence Angle and Local Incidence Angle

Local incidence angle defined as the angle between the radar beam and the local surface normal. The radar sensor measures the distance between the antenna and the object.

Slant Range and Ground Range and Geometric Distortions in Radar Imagery Due to Terrain Elevations

The line between radar and object is called the slant range. But the true horizontal distance along the ground corresponding to each measurement point in slant range is called ground range. Following figure shows the geometric distortions in radar imagery due to terrain elevations

Geometric Distortions in Radar Imagery

Spatial Resolution

  • In radar remote sensing, the images are created from transmitted and backscattered signals. If every single transmitted pulse forms one element in the image the system is called Real Aperture Radar. The spatial resolutions in slant range and azimuth direction are defined by pulse length and antenna beam width, respectively.
  • Due to the different parameters that determine the spatial resolution in range and azimuth resolution, it is obvious that the spatial resolution in the two directions is different. For radar image processing and interpretation, it is useful to resample the image data to regular pixel spacing in both directions.
  • In the case of ERS-1 SAR, this spacing is maybe 30 m × 30 m or 12.5 m × 12.5 m depending on the parameter setting in the processing software

Slant Range Resolution

  • In slant range, the spatial resolution is defined as the distance that two objects on the ground must be apart to give two different echoes in the return signal.
  • In other words, two objects will be resolved in range direction if they are separated by at least half a pulse length.
  • The slant range resolution is independent of the range. However, in ground range geometry the resolution will depend on the incidence angle.

Azimuth Resolution

  • The spatial resolution in azimuth direction depends on the beam width and the range.
  • The radar beam width is proportional to the wavelength and inversely proportional to the antenna length, i.e.. , aperture; this means the longer the antenna, the narrower the beam and the higher the spatial resolution in the azimuth direction.

Synthetic Aperture Radar (SAR)

  • Obviously, there is a physical limit to the length of the antenna, the aperture that can be carried on an aircraft or satellite. On the other hand, shortening the wavelength has its limitations in penetrating clouds.
  • Therefore, an approach in which the aperture is increased synthetically is applied. Systems using this approach are called Synthetic Aperture Radar (SAR) .
  • The synthesization of the antenna length is achieved by taking advantage of the forward motion of the platform and using several backscattered signals including the same object to simulate a very long antenna. Most airborne and space borne radar systems use this type of radar.

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