How does imaging radar work

SAR imagery is considered a non-literal imagery type because it does not look like an optical image which is generally intuitive to humans.

These aspects must be understood for accurate image interpretation to be performed. Shadowing Shadowing is caused for the same reasons that shadows are formed in optical imagery: an object blocks the path of direct radiation—visible light in the case of optical imaging and the radar beam in the case of SAR.

However, unlike optical imagery in which objects in shadows can be seen due to atmospheric scattering, there is no information in a SAR shadow because there is no return signal.

Foreshortening Because SAR is a side-looking, ranging instrument, the backscattered returns will be arranged in the image based on how far the target is from the antenna along the slant plane radar-image plane. This causes some interesting geometrical distortions in the imagery, such as foreshortening. As seen in Figure 4, the slope A-B is compressed in the slant plane because the radar signal reaches point B shortly after reaching point A in time. Layover Layover is an extreme example of foreshortening where the object is so tall that the radar signal reaches point B before it reaches point A.

This causes the returns from point B to be placed on the image closer to the sensor near range and obscure point A, as if the top has been overlaid on the foot of the mountain. A larger look angle increases the effect of shadows lengthening the shadow , while minimizing the effect of layover less layover.

A smaller look angle has the opposite effect. Figure 6 gives examples of these effects on rugged terrain. Figure 7, meanwhile, shows an example of how buildings in an urban setting are distorted by the same effects. All tall buildings appear laid out horizontally because of layover.

While a radar image can look like a monochrome optical image, this impression is deceptive. The intensity of pixels in a radar image are not indicative of the color of the object as in a color photograph. Instead, the intensity depends on the amount of energy the SAR sensor transmitted like the brightness of the illumination source , the material properties of the object, the physical shape of the object, and the angle from which the object is viewed.

Sensor Parameters Design and operation parameters allow engineers to have control over the reflected return signal called backscatter. Engineers design and model the system and operating parameters to maximize radar returns and therefore the information collected against specific targets.

These fixed sensor parameters dictate at some level the resulting brightness of a pixel in a particular image. The wavelength affects the azimuth resolution but it also has important implications for penetration, see Figure 8. In general, radar penetration increases with wavelength. The look angle affects layover and shadow as described above but can also have an effect on pixel brightness because it changes how the radar beam interacts with the object.

The polarization on transmit and on receive also affect the pixel brightness as described in the following section. Implementing all of these improvements, however, did require making difficult choices.

The satellite also doubled in size, increasing from 48 kg to kg. Yet despite these choices, we are thrilled with the outcome— a world-class SAR satellite that delivers what our customers need and expect. Surface Parameters The surface parameters that affect pixel brightness are the surface roughness of the material, with respect to the system wavelength and the scattering material the dielectric constant of the object.

If the surface roughness of the material is smooth with respect to the system wavelength, the radar beam is reflected Figure 9 according to the law of reflection. This is called specular reflection. If the surface is rough with respect to the system wavelength, the radar beam is scattered in all directions. This is called diffuse scattering. Posted by Thomas Ager All Posts. Previous Geotagging Cities. This website uses cookies, including from third parties, to improve your user experience.

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