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Proactive disclosure Print version   | |  Radar Imagery Versus VIR Imagery
5.8 Radar Imagery Versus VIR Imagery How does radar imagery differ from VIR imagery? A quick glance comparing radar imagery with aerial photography or VIR satellite imagery will reveal obvious differences. Imaging geometry and electromagnetic wave properties together produce the very different appearances of a radar image, an aerial photograph or a VIR satellite image. The following sections demonstrate the effects of imaging geometry and microwave properties on a typical radar image.
Relief Displacement Vertical structures on radar images, and aerial photographs or VIR satellite images appear to be very different. The most obvious difference is that relief displacement is in opposite directions. On aerial photographs and other optical sensor images, relief displacement falls away from the nadir point because the top is imaged further from nadir than the base of a structure. In radar images the top of a structure may be imaged before the base. Thus, the relief displacement falls towards the nadir. Relief displacement will be greater in slant range than ground range due to the fact that the image is more compressed in a slant range presentation. Relief displacement is also most pronounced at near range.
Range direction distortions on radar images are comparable to those encountered in oblique photographs. But, as can be seen from the previous figure, relief displacements occur in opposite directions for optical and SAR sensors. The radar perspective represented on an image is portrayed as being orthogonal to the radar direction. Consequently, a viewing angle "
A = relief displacement in the direction away from the optical sensor B = relief displacement toward the radar sensor
Keep in mind this relationship. It will help you in understanding the impact of geometry on SAR imagery. For example, it is important when considering stereo configuration and the positioning of SAR stereo image pairs. The four characteristics resulting from the geometric relationship between the sensor and the terrain that are unique to radar imagery are foreshortening, pseudo-shadowing, layover, and shadowing. Foreshortening (A'B') is the effect by which the foreslopes of hills and mountains appear to be compressed. The image of foreslopes will therefore appear brighter than other features on the same image. The greatest amount of foreshortening occurs where the slope is perpendicular to the incoming radar beam. The base, slope and top of the mountain will be imaged at the same time and will be superimposed on the image. Foreshortening can be minimized by using a less sharp incidence angle. However, lower incidence angles allow for more shadowing to occur on the image.
Pseudo-shadowing is an effect by which the backslope of hills and mountains appear to be expanded. It is the result of return signals spread out over a larger distance (A', B') than the actual horizontal distance (A, B). This dispersed return is not always detectable (Leonardo, 1983).
Layover is the effect where the image of an object appears to lean toward the direction of the radar antenna. It is the result of the tops of objects or slopes being imaged before their bases. Layover effects are most severe on the near range side of images.
Shadow is very useful to image interpreters interested in terrain relief. As discussed in the first chapter, shadowing is one of the psychological cues used for depth perception. Radar shadows produce a 3-D effect without the use of a stereoscope.
When terrain slopes are greater than the depression angle, true radar shadows mask down range features. In this case, slopes facing away from the radar antenna will return very weak signals if any. This results in dark or black areas on the image. In areas of high relief, as the depression angle becomes shallower, shadow length increases with range. The shallower the depression angle is on such terrain, the more information will be lost. Microwave properties Objects on the Earth's surface react differently with electromagnetic energy. The strength of reflected energy, which is recorded in order to produce a radar image, depends on factors such as:
It is important to note that microwave reflections from the Earth's surface are not related to their counterparts in the VIR section of the EM spectrum. Surfaces that return a strong signal and are bright in a radar image may return a weak signal in the VIR range of the spectrum and appear dark on a photograph, Landsat or SPOT image. Surface Roughness Surface roughness is determined with respect to radar wavelength and incidence angle. A surface will appear to be smooth if its height variations are smaller than one-eigth of the radar wavelength. In general, a rough surface is defined as having a height variation greater than half the radar wavelength. Surfaces will appear to have a greater or lesser degree of roughness, depending on which designated radar bandwidth is used for imaging. In terms of a single wavelength, a surface appears smoother as the incidence angle increases. On radar images, rough surfaces will appear brighter than smoother surfaces composed of the same material.
Surface roughness influences the reflectivity of microwave energy. Horizontal smooth surfaces that reflect nearly all incidence energy away from the radar are called specular reflectors. These surfaces, such as calm water or paved roads appear dark on radar images.
Rough surfaces scatter incident microwave energy in many directions. This is known as diffuse reflectance. Vegetated surfaces cause diffuse reflectance and result in a brighter tone on radar images. Moisture Content The complex dielectric constant describes the ability of materials to absorb, reflect and transmit microwave energy. The dielectric constant increases with the presence of moisture in a material. Moisture content changes the electrical properties of a material, which in turn affects how the material will appear on a radar image. The reflectivity, and therefore image brightness of most natural vegetation and surfaces increases with greater moisture content. Consequently, soil moisture maps can be derived from radar backscatter.
Test site at Outlook, Saskatchewan showing potato fields at pre-emergence stage;
Microwaves may penetrate very dry materials such as desert sand. Both surface and subsurface properties affect the resulting scatter. In general, the longer the radar wavelength, the deeper it will penetrate dry material. Fading and Speckle Fading and speckle look like grains of salt and pepper randomly distributed on an image. Fading and speckle are noise-like processes inherent in coherent imaging systems. Radar imagery is created with coherent radar waves which causes random constructive and destructive interference producing random bright and dark spots in radar imagery. Fading and speckle can be reduced by averaging adjacent pixels on radar images, or by designing the antenna to use a multiple look imaging technique.
Fading and speckle can be reduced by averaging adjacent pixels on radar images, or by designing the antenna/processor to use a multiple look imaging technique. 
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" of less than 90°, usually employed in SAR systems, has approximately the same effect as an equivalent angle of "90°-
= incidence angle
= radar wavelength.
