Just as the so-called "knowing oneself and knowing the other will never perish in a hundred battles," China's ancient military book "Sun Tzu's Art of War" not only talks about the importance of "knowing oneself and knowing the other," but also gives the method of "knowing oneself and knowing the other," and summarizes these methods into three kinds, namely, the use of spies, observation of the enemy, and enemy reconnaissance.
It can be seen that since ancient times, military experts have attached great importance to intelligence. Compared with ancient times, modern intelligence acquisition methods are much more efficient, as early as before World War II, many countries began to use spy aircraft to detect enemy situations, and later with the development of space technology, spy satellites appeared. Compared with aircraft, satellites operating in low Earth orbit undoubtedly have more reconnaissance advantages, and the scope of detection by spy satellites is unmatched by traditional reconnaissance methods. Satellites can see widely, but why can satellites take pictures so clearly?
What do spy satellites use to take pictures? Of course, it is the camera, so the problem is that this shooting distance is too far.
Spy satellites are usually located at an altitude of about 100-200 kilometers above the surface, which is also the advantage of spy satellites over traditional aircraft reconnaissance. The use of aircraft to recontroll other States is likely to infringe on the territorial airspace of other States, and the use of spy satellites is completely without such a risk. Because airspace usually refers to the airspace above a country's territorial land and territorial water, and the highest height of the airspace is about 35 kilometers, no one can control shooting at an altitude of hundreds of kilometers. But this also raises a new question, which is how to take pictures from hundreds of kilometers away? Although spy satellites use cameras to shoot, they are still very different from the cameras we use every day.
The camera on board the spy satellite is called the "airborne camera", and it has a resolution that is unmatched by ordinary cameras, so that it can clearly illuminate a house or a car on the ground hundreds of kilometers away.
In fact, there is nothing surprising about the Hubble telescope operating in the orbit of the Earth, but it can record even the stars that are several light years away. What is the relationship between space telescopes and spy satellites? The relationship is still quite large, because the Hubble telescope was originally a keyhole spy satellite eliminated by the military, and even the lenses were military. If you don't believe it, you can take a look at the latest generation of keyhole spy satellites in the United States, which look similar to the Hubble telescope from the outside alone. Since even the deep space of the universe tens of light-years away can be seen, it is naturally not a matter of taking surface images hundreds of kilometers away.
Shooting distance only needs to increase the resolution and some other parameters, so how to solve the speed problem?
People who often take pictures know that moving objects are the most difficult to shoot, and the faster the object moves, the blurrier the image taken. Of course, this movement is relative, even if the object does not move, and we ourselves sit on a high-speed train, and we can't shoot anything clearly. For people like me with shaky hands, taking pictures is even more difficult. Of course, it is easy to solve the problem of hand shaking, only a handheld stabilizer is needed, and if you want to shoot moving objects, you can also use a high-speed shutter, but the satellite is obviously moving too fast.
For a satellite to orbit the Earth, it must be kept at a speed between the first cosmic velocity and the second cosmic velocity, which is between 7.9 kilometers per second and 11.2 kilometers per second.
With such a high speed of movement, isn't it all a blur to shoot? Don't worry, at this speed of motion, if you shoot the object in front of you, it will indeed be a blur, but what the spy satellite is going to shoot is the surface located hundreds of kilometers away. The farther away an object is from us, the slower its relative movement speed, such as the cruising speed of a civil airliner is about 200 meters per second, but in our view on the ground, it also moves 2 centimeters a second. Spy satellites are the same thing, although the speed of operation is fast, but fortunately the distance is far enough, so the relative motion speed of surface objects is not very fast, and the camera with slightly better performance can take a clear picture of the ground.
The problem of how to take a clear photo is solved, and now there is a new problem, that is, how to transmit the captured image back to the earth?
Wouldn't it be nice to transfer remotely? That's now, and spy satellites are a product of the 1960s, when on-board cameras still used film. Therefore, when the satellite shoots a roll of film full, it will directly throw it back to the earth, of course, the outside of the film is protected by an insulating layer. However, the film collectors will not wait until the film falls to the ground before recycling, because there may be a risk of leaking secrets, after all, the film is taken in the confidential image, so the recipient will use the plane to recover the film in the air. Now that it is no longer necessary to go to such trouble, and the images taken by spy satellites are becoming clearer, as it continues, perhaps in the future, each of our every move will be recorded by the eyes of space.