What is the function of military spacecraft?

There is a difference between civilian satellites and military satellites. According to statistics in the early 1990s, there were 3,824 spacecraft successfully launched in the world over the past 30 years, including 2,566 US-Soviet military satellites. Of course, satellites have a lifetime, the longest is a few years, and the shortest is only a few days. So there won't be hundreds of satellites in the sky at the same time. Many civilian and scientific satellites also serve some military purposes.

For example, meteorological satellites and communication satellites, it is difficult to distinguish between civilian and military. Because it is very useful for studying the climate and communicating with people, it is also needed in the military. The peaceful use of space and the use of space for war are not as clear as black and white. Therefore, if such satellites are added, about 3/4 of the spacecraft launched in the world are used for military purposes.

The structure, temperature control, attitude, program, antenna and launching means of each satellite are almost the same, but the difference is the special system of the satellite. Different tasks, satellite design and equipment are different.

There are many kinds of military satellites, including:

Military communication satellites can provide reliable means of communication for land bases, sea warships and air planes to ensure smooth command;

Military navigation satellites can guide surface ships and underwater submarines, and also provide accurate positioning data for high-speed aircraft, missiles and ground forces;

Military geodesic satellites can accurately measure the geographical positions of various targets that need to be attacked and improve the hit rate of missiles and other weapons;

Military meteorological satellites provide more accurate meteorological data to improve the accuracy of global or local weather forecasts, provide a basis for making operational plans, and so on.

The most important military satellites are all kinds of military reconnaissance satellites.

There is a saying in China's ancient art of war that "know yourself and know yourself, and you will win every battle". It means that if you are familiar with yourself and everything about the enemy, you will win every battle. Therefore, relying on reconnaissance to obtain information is an important part of war preparation.

In order to get all the information of the enemy, military leaders and their staff departments at all times and all over the world really racked their brains and exhausted all means. Commonly used is to send, snare spies, steal and eavesdrop on all kinds of military secrets, and return them by means of employing people, homing pigeons and wireless telegrams.

However, it is not enough to rely solely on such reconnaissance means.

Do you know where the enemy missile silo is? Do you know when and where the enemy troops assembled? Do you know where the enemy planes and tanks are hidden and where they have been transported? Not everyone can go anywhere, and there are not so many spies to send. China wrote a "clairvoyance" in his well-known and somewhat absurd novel "The List of Gods", looking at what happened thousands of miles away, just like in front of his eyes. I also wrote a "native grandson", which can be drilled into the soil from here and unearthed there. If you have such a pair of clairvoyance and the ability to drill the earth, you can see wherever you want and go wherever you want!

Contemporary military reconnaissance satellites can be called a pair of real clairvoyance.

It's fast. How fast? If it is a reconnaissance satellite in low-earth orbit, it can fly about seven or eight kilometers per second and circle the earth in an hour and a half. This kind of reconnaissance satellite is hundreds of times faster than trains and cars, and ten or twenty times faster than supersonic aircraft. For example. It takes only half a minute from Beijing to Tianjin, three minutes to Shanghai and five minutes to Lhasa. It can not only timely reconnaissance, but also ensure continuity. Long-life reconnaissance satellites can generally stay in the air for more than two years, during which continuous changes of targets can be detected.

② Wide range. Compared with satellites, airplanes also have a 20 angle of view. From the plane at an altitude of 3000 meters, you can see the ground area of 1 square kilometer; From the satellite at an altitude of 300 kilometers, we can see the ground area of 1000 square kilometers, which is more than 10,000 times different. It has been calculated that it takes 1 10,000 photos and 10 years to photograph China on a high-altitude plane. If you shoot by satellite, you only need to take more than 800 photos, which will take a few days.

③ Less restrictions. If you shoot a military target on the other side's ground, the other side will definitely arrest you and ask you to confess. You can't take pictures in my air. If the sovereignty of airspace is violated, the plane will be destroyed. Who can care about the satellites in the sky? It has the freedom to cross the border without the trouble of violating the airspace. Mountains, seas, desert Gobi, dense forests and inaccessible places can't stop satellite reconnaissance.

According to this, there are no defects and deficiencies in military reconnaissance satellites? Of course not. Nothing in the world is perfect, and so are satellites.

"Space clairvoyance"

Military reconnaissance satellites, because of their different heights and orbits, seem to have two States from the earth: moving and stationary. Most reconnaissance satellites are in the low orbit of 120~200 km and the medium orbit of 350~ 1500 km. When they go around the earth, the earth itself keeps spinning, so they don't stay at one point all the time. These satellites circle the earth once, just like tying the earth together with a broadband, but the next circle is not at the same latitude of the earth, and there will be an unobserved blank area between the two circles, which is called a "blind zone".

There's a way to make it up. One is that when designing the satellite orbit, the relationship between the satellite orbit period and the earth rotation period is intentionally different, or more or less. The staggered results can detect all the places to be detected without missing the detection. Another method is to put more satellites into the same orbit and connect them to the power grid. Just like the difference turntable used in TV broadcast, one is set at a certain distance, so you are not afraid of leaking places.

If the reconnaissance satellite is launched at an altitude of 35,800 kilometers above the earth's surface, at this time, the time for it to circle the earth is exactly the same as that for the earth to rotate once. Seen from the ground, it seems motionless. This kind of satellite is called geosynchronous orbit satellite, also called geostationary satellite. It can stand tall, see widely, stay for a long time and work continuously for 24 hours. With three such satellites, the whole earth is under its surveillance.

In terms of performance, there are four kinds of military reconnaissance satellites.

① Photographic reconnaissance satellite. Its equipment is mainly visible light camera, including panoramic scanning camera for general survey and frame camera for detailed investigation. Some photos taken by the satellite are sealed and put into the recovery cabin, and sent directly to the ground for washing and interpretation. Only three countries in the world have mastered this technology. In order to prevent the recovery cabin from falling into the hands of others, a self-explosion device will be installed to explode after a specified time. Another method is radio transmission, that is, the photos taken on the satellite are automatically developed, and then the target image is converted into electrical signals, which are sent to the ground in the form of radio waves and then restored to the target image. Some photographic reconnaissance satellites are equipped with TV cameras, which can send information to the ground command center while reconnaissance of ground targets, which is faster and simpler.

Visible light camera has high resolution, but it is greatly affected by the weather, so it is not suitable for rainy days, cloudy days and nights. Only 8-9 am and 2-3 pm work best during the day. So people developed infrared photography, multi-spectral photography, microwave photography and other equipment.

However, these photographic equipment also have their own advantages and disadvantages. The advantage of infrared photography is that it can reveal some camouflage on the surface. The advantage of multispectral photography is that it is easy to identify more targets, but it is still affected by fog, rain and snow. The advantage of microwave photography is that it is not affected by the weather, can work day and night, and has the ability to penetrate the ground, ice and forests. So ordinary covert camouflage can't escape its eyes. Its disadvantage is that when it encounters radio interference, all the functions that can be played will fail. Wouldn't it be better if these photography methods were used together and complemented each other? However, in this way, the burden on satellites has increased.

The resolution and interpretation of satellite photos is another key technology. What can the satellite see? Is the photo taken the same as the portrait taken in the photo studio, or is it the same as the scenery and landscape photos taken by our stupid machine? There used to be a legend that the number on the license plate of a car parked on the Beijing Military museum square can be seen in satellite photos. Is it true?/You don't say. To be sure, modern high technology can't make the photos taken on satellites look like the photos we take every day, showing people with noses and eyes and houses with bricks and tiles.

The most advanced photographic reconnaissance satellite, taking photos with a camera with a focal length of 2.44 at an altitude of 160 km, can distinguish objects 0.3 meters long and wide on the ground. But this object is still just a small point in the photo, and it needs to be combined with other target information in the surrounding environment to identify what this point is after interpretation. If the length and width of the target are greater than 1.5~2 meters, it is possible to clearly see some detailed features in the photos, such as the bow and stern of the warship.

② Electronic reconnaissance satellite. Equipped with reconnaissance receiver and tape recorder, it is mainly used to detect the position and frequency of radar and military radio, and sometimes intercept the telemetry signal sent out during missile test, so as to grasp the development of the other side's strategic weapons. Electronic reconnaissance satellites are superior and safer than other electronic reconnaissance means. The weakness is that when there are too many ground radars or radio stations and the signals are too dense and mixed, it is difficult to screen out really useful information and it is easy to be deceived and interfered by false signals. If the ground radar and radio station are temporarily shut down, it can also escape its reconnaissance.

③ Missile early warning satellite. Used to defend against missile attacks. Intercontinental missiles have fast flight speed, high trajectory and long range, and can hit the target of 8000~ 13000 km about 30 minutes after launch.

Because the surface of the earth is oval rather than flat, the most advanced early warning radar can only find the missile after it flies halfway. The early warning satellite can find the missile more than one minute after it leaves the launcher/kloc-0, and the whole process takes only three or four minutes from discovery to automatic transmission of information to the ground station and then from the ground station to the command center. Intercontinental missiles launched from land have a warning time of 25 minutes, and intercontinental missiles launched from submarines have at least 15 minutes. The problem of this kind of missile early warning satellite is that it can't track the inertial flight of the missile after shutdown, and the satellite can't tell whether it is a real missile or a fake missile.

(4) Marine surveillance satellites. It is specially used to monitor the activities of surface ships and underwater submarines. This kind of satellite equipped with electronic reconnaissance equipment is no different from electronic reconnaissance satellite; Equipped with radar equipment, you can actively search for the target, and you don't need to rely on the other side to transmit signals to find the target. Foreign countries are experimenting with marine surveillance satellites equipped with infrared detectors. It is assumed that a nuclear submarine 60 meters deep underwater can be detected from hundreds of miles high.