slide 25

SAR applications focus on using amplitude information, but the resulting images can appear grainy due to the speckle effect. The speckle effect is caused by the sum of loads from all scatters within a SAR pixel, resulting in varying amplitudes even in neighboring pixels. Unlike noise, the speckle effect remains consistent when obtaining SAR images multiple times with the same sensor and environmental conditions. As I said previously, SAR applications focus on using amplitude information. However, if you feel the amplitude of the SAR image, you may wonder why the image is grainy. This is not like an orbital image. The data looks noisy. The left figure is the intensity of the SAR image. You can see the data is grainy. We call it the speckle effect. There is an important content to better understand what causes the speckle effect. In a SAR pixel, backscatter results from all scatters within the pixel. The amplitude and phase are the sum of loads from all scatters in the pixel. We can see the right figure. We can express the scattering event in the compressed sector form. The red arrow is the final compressed value in the SAR pixel. The black arrows are the compressed values from each scatter. The scatters in neighboring pixels are different. Even if the neighboring pixels have similar properties, the sum of the amplitudes still can be different. And it causes amplitude noise in the space. This is the main reason why the speckle effect happens. But the speckle effect is not like noise. If we use the same sensor with the same geometry and the same environmental conditions to obtain a SAR image many times, the speckle effect is the same. However, noise will be different at every time.