Interpretation of SAR Images

Each pixel in a SAR image represents the back-scattered radiation from an area in the imaged scene. A large pixel value (bright) represents a strong received signal. The strength of the received signal depends on many things. These include:

• Radar wavelength and the relative size of the scatterers.
  Smooth, flat surfaces usually reflect very little radiation back to the antenna, so roads and still water will appear dark. Objects much smaller than the wavelength of the radar are invisible to the detector, so for a low frequency (L-band) radar, fields may also appear smooth. At shorter wavelengths, fields or woodland often give the image a textured appearance.

• Polarisation of the pulses.
  Radar pulses can be polarised into a vertical or horizontal plane. For a linear feature on the ground, the reflected signal is much stronger when the polarisation agrees with the orientation of the scatterer. For example, a pylon will appear much brighter in a VVimage than in an HH image. The same is true for other objects with a strong vertical component such as walls, and waves.

  For larger surface features, the changes are unpredictable. However, the relative intensities in a patchwork of fields can be quite different under different polarisations, allowing field boundaries invisible in one image to be detected in another; or providing the means to distinguish between various classes of ground cover much more reliably than if only unpolarised data is available.

• Topography of the ground.
  Ground which is sloping towards the antenna will appear brighter than ground sloping away. This effect is known as layover. It is also possible to get radar shadows -- dark areas in an image that were not illuminated.

• Water content of the imaged area.
  The scattering of radar pulses depends on the dielectric properties of the scatterers. These are affected by water: drier objects appear darker than wetter ones (except that, as noted previously, still surface water appears dark).

• Incidence angle.
  The incidence angle is the angle between the radar beam and a perpendicular on the imaged area. A small incidence angle gives high backscatter; larger angles give weaker signals but larger shadows, so increasing the texture in the image. Images taken at large incidence angles are also less susceptible to layover.

• Direction of the flight-path.
  The appearance of linear features such as walls, hedges and ocean waves will depend on their orientation relative to the radar beam. If buildings are aligned to the beam, it is possible to get a `double bounce' of the radar -- once off the building, and once off the ground. This produces very bright pixels in the image. To a lesser degree, this also happens from tree-trunks along the edge of a wood.

  These effects are also present in satellite SAR imagery.