SAR is also like a camera that captures images. Both SAR and optical camera have different penetrating capability. This penetrating capability depends upon the capturing phenomenon. In order to create an image, the optical camera depends on the light while SAR relies on radar signals which it transmits. This significant difference of SAR allows capturing the images in pure darkness and also permits to see through clouds, rain, fog, and snow. Creating a SAR image requires a heap of data and terrific calculating power.
In general, the basic working principle of radar i.e. RAR is to calculate the time taken by the radio wave to travel from an antenna to object and back to the antenna. Radar computes this traveling distance from an antenna to object and back to the antenna using time interval but this kind of radar can only trace an object and measure its speed. To create an image, a significantly greater heap of data and terrific calculating power is needed. It needs an exceptionally elongated antenna. But in SAR, it uses a smart way of synthetically creating a long antenna by moving a small antenna to cover a long distance. After receiving all data, it saves it and starts processing it.
As the airplane flies and moves forward, the SAR antenna transmits the high-frequency radio waves also called radar waves in the direction of the ground. In the middle of the pulses, the antenna collects backscattered high-frequency radio waves that have bounced off objects on the ground. These backscatter pulses contain information that includes the total distance traveled by the pulses to make the round trip to and from the plane. It also contains the information about the movement of the SAR whether it is moving in the direction of or against from the object on the ground. The backscattered pulses are recorded and if the pulses are recorded to be very close to each other, then the plane with the SAR antenna is moving towards the ground and if the pulses are recorded to be far from each other the plane is moving away from the ground.
From the huge amount of backscattered pulse data collected, there are certain pulses that show a specific pattern, this specific pattern denotes that those pulses are bounced off from the same location. During this process the information related to distance is known from the exact location where the plane was when it sent and received the pulses that fit the pattern, it's a simple matter to plot the point of the object on the image it's creating. The point on the image is plotted, but the brightness of the point is still not known. SAR determines the brightness by the intensity of the signal it receives. An object such as a tree absorbs some of the radar energy and so it appears gray. A metal object-oriented toward the SAR antenna reflects a lot of the energy back, so it appears bright.
In SAR image processing, there are a thousand calculations needs to be performed for each and every pixel and such image is shown below of Washington, D.C., which is made up of several million pixels in Figure 1.
Note: If you are taking help from the above work and also incorporating the above content in your research work, Kindly make sure you are citing the below papers.
- https://www.sciencedirect.com/science/article/pii/S2215098617314003
- https://www.sciencedirect.com/science/article/pii/S1319157817301106
- https://www.sciencedirect.com/science/article/pii/S1319157817303865
- https://link.springer.com/article/10.1007/s11831-021-09548-z
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February 03, 2022
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