What is an optical image? What is a SAR image? What's the difference between them? What is the difference in imaging mechanism? What's the difference between image segmentation?

Optical image is an image obtained by optical photography system with photographic film as the medium, which usually refers to the image data obtained by visible light and some infrared band sensors.

SAR images are generated by SAR (Synthetic Aperture Radar) system, which is an active earth observation system. It can be installed on flight platforms such as airplanes, satellites and spaceships. It can observe the earth all day and all day, and has certain surface penetration ability.

2. Differences (information, resolution, imaging mechanism):

Including information: optical images usually contain gray information of multiple bands, which is convenient for target recognition and classification extraction. However, SAR images only record the echo information of one band and record it in the form of binary complex numbers. However, based on the complex data of each pixel, the corresponding amplitude and phase information can be extracted through transformation.

Resolution: The resolution of SAR image is relatively low and the signal-to-noise ratio is low, so the amplitude information contained in SAR image is far from the imaging level of the same optical image. But its unique phase information cannot be obtained by other sensors, and phase-based interferometric modeling is also the main application direction of SAR.

The difference of imaging mechanism: optical images usually use central projection area imaging or broom scanning to obtain data; For the need of signal processing (synthetic aperture process is not discussed here), SAR can only transmit and receive regional radar waves through active imaging, and then synthesize complex pixels corresponding to ground targets through signal processing (focusing, compression, filtering, etc.). ).

3. Differences in image segmentation:

The phase information of a single SAR image basically has no statistical characteristics, and only the amplitude information can be used for target recognition and classification. Amplitude information is deeply influenced by noise, coupled with unique geometric distortions (overlap, perspective contraction, multipath false targets, etc.). ) the characteristics of SAR images. Optical images are easier to segment in terms of information and statistics.

Extended data:

SAR technology:

Synthetic aperture radar (SAR) uses the principle of synthetic aperture to realize high-resolution microwave imaging, which has the characteristics of all-weather, all-weather, high resolution and wide range. Initially, SAR was mainly used for airborne and spaceborne platforms. With the development of science and technology, SAR carried on various platforms has appeared, such as missile-borne SAR, ground-based SAR, unmanned aerial vehicle SAR, near-space platform SAR and handheld devices, which are widely used in military and civilian fields.

Synthetic aperture radar emits electromagnetic waves in turn, and radar antenna collects, digitizes and stores the reflected echoes for later processing. Since sending and receiving occur at different times, they are mapped to different locations. The ordered combination of received signals constitutes a virtual aperture which is much longer than the length of the physical antenna. This is why it is called "synthetic aperture", which gives it the properties of imaging radar.

References: Baidu Encyclopedia-Optical Image, Baidu Encyclopedia -SAR.