| Speckle.
SAR is a coherent imaging technology, recording both the amplitude and
the phase of the back-scattered radiation. Because of this, it suffers
from a noise-like phenomenon known as speckle. Each resolution cell of the
system contains many scatterers; the phases of the return signals from
these scatterers are randomly distributed and speckle is caused by the
resulting interference. This gives the images a grainy appearance.
This imposes a significant limitation on the accuracy of the
measurements that can be made: the brightness of a pixel is determined not
only by properties of the scatterers in the resolution cell, but also by
the phase relationships between the returns from those scatterers. In
single-look images, the uncertainty is equal to the expected value. This
problem is overcome to some degree by averaging to produce multi-look
images (ERS-1 data is 3-look).
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