Distortions in Radar Images: Scale Distortions, Terrain-Induced Distortions, Foreshortening, Layover, Shadow, Radiometric Distortions, Multi-Look Processing, and Speckle Filters (Especially for GATE-Geospatial 2022)

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Due to the side-looking viewing geometry, radar images suffer from serious geometric and radiometric distortions. In radar imagery, you encounter variations in scale (slant range to ground range conversion) , foreshortening, layover and shadows (terrain elevation) . Interference due to the coherency of the signal causes speckle effects.

Scale Distortions

Radar measures range to objects in slant range rather than true horizontal distances along the ground. Therefore, the image has different scales moving from near to far range. This means that objects in the near range are compressed with respect to objects at far range. For proper interpretation, the image has to be corrected and transformed into ground range geometry.

Terrain-Induced Distortions

Like optical sensors that can operate in an oblique manner (e. g. , SPOT) radar images are subject to relief displacements. In the case of radar, these distortions can be severe. There are three effects that are typical for radar: foreshortening, layover and shadow.

Foreshortening

Radar measures distance in slant range. The slope area is compressed in the image. Depending on the angle that the slope forms in relation to the incidence angle of the radar beam the slope will be shortened more or less. The distortion is at its maximum if the radar beam is almost perpendicular to the slope. Foreshortened areas in the radar image are very bright.

Layover

If the radar beam reaches the top of the slope earlier than the bottom, the slope is imaged upside down, i.e.. , the slope ‘lays over’ . As you can understand from the definition of foreshortening, the layover is an extreme case of foreshortening. Layover areas in the image are very bright.

Shadow

In the case of slopes that are facing away from the sensor, the radar beam cannot illuminate the area. Therefore, there is no energy that can be backscattered to the sensor and those regions remain dark in the image. Radar shadow has to be distinguished from shadow areas in optical images where the radiometric values are altered due to the difference in Sun illumination.

Radiometric Distortions

The above-mentioned geometric distortions also have an influence on the received energy. Since the backscattered energy is collected in slant range the received energy coming from a slope facing the sensor is stored in a reduced area in the image, i.e.. , it is compressed into fewer image pixels that should be the case if obtained in ground range geometry. This results in high digital numbers because the energy collected from different objects is combined. Unfortunately, this effect cannot be corrected. This is why especially layover and shadow areas in radar imagery cannot be used for interpretation. However, they are useful in the sense that they contribute to a three-dimensional look of the image and therefore help the understanding of the terrain structure and topography.

A typical property of radar images is the so-called speckle. It appears as grainy ‘salt and pepper’ effects in the image. Speckle is caused by the interaction of the different microwaves backscattered from the object area. The wave interactions are called interference. Interference causes the return signals to be extinguished or amplified resulting in dark and bright pixels in the image even when the sensor observes a homogenous area. Speckle degrades the quality of the image and makes the interpretation of radar imagery difficult.

Original (A) and Speckle Filtered (B) Radar Image

Original (a) and speckle filtered (b) radar image

Multi-Look Processing

It is possible to reduce speckle by means of multi-look processing or spatial filtering. In the case of multi-look processing, the radar beam is divided into several narrower beams. Each beam provides a look at the object. Using the average of these multiple looks, the final image is obtained. Multi-look processing reduces spatial resolution. If you purchase an ERS SAR scene in PRI-format you will receive a 3-look image.

Speckle Filters

Another way to reduce speckle is to apply spatial filters to the images. Speckle filters are designed to adapt to local image variations in order to smooth the values to reduce speckle but to enhance lines and edges to maintain the sharpness of the imagery.

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