1. What is it?
Optical images are images acquired using optical photography systems on photosensitive film, typically referring to image data acquired by sensors in the visible light and some infrared bands.
SAR images are generated by SAR (Synthetic Aperture Radar) systems, an active Earth observation system that can be mounted on aircraft, satellites, spacecraft, and other flying platforms to conduct all-weather, 24/7 Earth observations and has a certain degree of surface penetration capability.
2. Differences (Information, Resolution, Imaging Mechanism):
In terms of information contained: Optical images typically contain grayscale information from multiple bands to facilitate target identification and classification. SAR images, on the other hand, record only the echo information of one band, recorded in binary complex form; however, based on the complex data of each pixel, corresponding amplitude and phase information can be extracted.
In terms of resolution: SAR images have relatively low resolution and low signal-to-noise ratio, so the amplitude information contained in SAR images is far less than that of optical images; however, its unique phase information is unattainable by other sensors, and phase-based interferometric modeling is a major application of SAR.
Differences in Imaging Mechanisms: Optical images typically acquire data using central projection area imaging or pushbroom scanning; while SAR, due to signal processing requirements (synthetic aperture process, which will not be discussed here), cannot use a vertically downward illumination method and can only transmit and receive area radar waves through active observation 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 has virtually no statistical characteristics; only amplitude information can be used for applications such as target identification and classification. Amplitude information is heavily affected by noise, coupled with the unique geometric distortions of SAR images (overlay, perspective contraction, multipath false targets, etc.). Optical images are easier to segment in terms of information content and statistics.
Although optical and SAR images differ in imaging mechanisms, information content, and application areas, they are also complementary. In practical applications, optical and SAR images can be used in combination to obtain more comprehensive information and more accurate analysis results. For example, in target recognition and classification, the high spatial resolution and color information of optical images can be used for initial identification, and then the all-weather and temporal resolution of SAR images can be used to monitor the movement and changes of the target.
Below are optical 0.1m and X-band image data acquired during a flight test over a city in a certain region using our series of products equipped with an optoelectronic pod. These images can be mounted on large and medium-sized multi-rotor or fixed-wing UAVs and have the capability to simultaneously image the same scene using multiple sensors, generating multiple images from a single flight.
Figure 1 Optical image
Figure 2 SAR image
From the above information, we can conclude that optical images and SAR images, as two important remote sensing imaging methods, each possess unique imaging mechanisms, information content, and application areas. In practical applications, the appropriate image type can be selected based on specific needs and scenarios, or both can be used in combination to obtain more comprehensive information and more accurate analysis results!
