Why can satellite signals reach the earth?

An unmanned spacecraft that orbits the earth in space (at least once) is called a satellite. Man-made satellite is the spacecraft with the largest number of launches, the widest use and the fastest development. Satellite launches account for more than 90% of the total number of spacecraft launches. A complete satellite engineering system usually consists of artificial satellites, launch vehicles, spacecraft launch sites, space control and data acquisition networks and user stations (stations and networks). Satellites and subscriber stations (stations and networks) constitute satellite application systems, such as satellite communication system, satellite navigation system and satellite space exploration system. I. Overview of Development1957101On October 4th, the Soviet Union launched the world's first artificial earth satellite. From the late 1950s to the early 1960s, satellites launched by various countries were mainly used to detect the earth's space environment and conduct various satellite technology experiments. In the mid-1960s, artificial satellites began to enter the application stage, and various application satellites were put into use one after another. Since the 1970s, various new special satellites have appeared one after another, and their performance has been continuously improved. By the end of 1984, 3022 artificial satellites had been launched all over the world. The United States launched an artificial earth satellite (Explorer 1) for the first time in February/958, and France and Japan also launched their own satellites in the 1960s and 1970s. China launched the 1 artificial earth satellite "Dongfanghong" on April 24th, 1970, and launched 16 different types of artificial earth satellites in September, 1984. 2. Satellite types Artificial satellites are divided into low-orbit satellites, medium-high orbit satellites, geosynchronous satellites, geostationary satellites, sun-synchronous satellites, large-scale elliptical orbit satellites and polar orbit satellites according to their operating orbits (see the operating orbits of artificial earth satellites). People mostly divide artificial satellites into scientific satellites, applied satellites and technical test satellites according to their uses. 1, a scientific satellite used for scientific exploration and research, mainly including space physics exploration satellites and astronomical satellites. Instruments used in scientific satellites include telescopes, spectrometers, Geiger counters, ionization meters, pressure measuring instruments and magnetometers. With these instruments, we can study the upper atmosphere, the earth's radiation belt, the earth's magnetosphere, cosmic rays, solar radiation and aurora, and observe the sun and other celestial bodies. 2. Technical test satellites are new technology test or application test satellites. New principles, technologies, schemes, instruments, equipment and materials in space technology often need to be tested in orbit before they can be put into practical use. The number of such satellites is small, but the test contents are extensive, such as gravity gradient stability test, electric rocket test, biological adaptability test to space environment, verification test of life support system and return system of manned spacecraft, rendezvous and docking test, transmission test of new radio frequency band, flight test of new remote sensor, on-orbit interception test and so on. 3, the application of satellites directly for the national economy and military service satellite. Among all artificial earth satellites, it has the most types and the most launch times. Application satellites can be divided into communication satellites, meteorological satellites, reconnaissance satellites, navigation satellites, geodesic satellites, earth resources satellites, interception satellites and multi-purpose satellites. According to whether it is specially used for military purposes, it can be divided into military satellites and civilian satellites, and many application satellites are both military and civilian. There are three main uses of applied satellites: ① Radio signal relay: these satellites are developing rapidly, including "international communication satellites", domestic communication satellites, military communication satellites, maritime satellites, broadcasting satellites, tracking and data relay satellites and search and rescue satellites. These satellites are equipped with transponders and antennas, working in different frequency bands, relaying radio signals from satellites on the ground, at sea, in the air and in low orbit, and are used to transmit telephone, telegraph, TV broadcast programs and data communication. Most of these satellites operate in geostationary orbit. Others use large elliptical orbits, such as the Soviet "Lightning" communication satellite. ② Earth observation platform: These satellites include meteorological satellites, earth resources satellites and reconnaissance satellites, which are called Earth observation satellites. These satellites are equipped with remote sensing instruments or other detection instruments with various wavelengths from ultraviolet light to far infrared light, which collect electromagnetic waves in various frequency bands in land, ocean and atmosphere, extract useful information from them, and analyze, judge and identify the nature and state of the measured object. These satellites can directly serve meteorology, agriculture, forestry, geology, water conservancy, surveying and mapping, ocean, environmental pollution and military reconnaissance. Many of these satellites use sun-synchronous orbit, and some use geostationary orbit and other orbits. ③ Navigation and positioning standards: These satellites include navigation satellites and geodesic satellites. These satellites are equipped with optical beacon lights, laser reflectors, wireless beacons and transponders. The space position, distance to the ground and running speed of this satellite can be determined in advance, so it can be used as a benchmark for positioning, navigation and geodesy. Fixed or moving objects on the ground, airplanes in the air and ships at sea can all use this satellite to determine their own coordinates. The orbits of these satellites are mostly polar orbits. The artificial earth satellite basically runs around the earth according to the laws of celestial mechanics. However, the actual motion is much more complicated, mainly influenced by the gravitational field of the aspheric earth, and the low-orbit satellite is also affected by atmospheric resistance; High-orbit satellites, especially geostationary satellites, are also affected by the gravity and light pressure of the sun and the moon (see spacecraft orbit perturbation). The orbit of a satellite depends on its mission. The shape and height of the orbit depend on the speed and direction of the vehicle to the satellite. Three, the composition of the satellite is composed of a number of systems including various instruments and equipment, which can be divided into two categories: dedicated systems and support systems. Dedicated system refers to the system directly related to the tasks performed by satellites, which can be roughly divided into three categories: detection instruments, remote sensing instruments and transponders. Scientific satellites use various detection instruments (such as infrared telescopes, cosmic ray detectors, magnetometers, etc.) to detect the space environment and observe celestial bodies; Communication satellites transmit various radio signals through communication repeaters and communication antennas; Earth observation satellites use various remote sensors (such as visible light cameras, side-looking radars, multispectral cameras, etc.). ) to get all kinds of information about the earth. The supporting system mainly includes structural system, thermal control system, power supply system, radio measurement and control system, attitude control system and track control system. Some satellites are also equipped with computer systems to handle, coordinate and manage the work of each subsystem. The recoverable satellite also has a return landing system, which consists of a braking rocket, a parachute and a beacon. 4. Satellites use artificial satellites to observe celestial bodies without being blocked by the atmosphere. It can receive all electromagnetic radiation from celestial bodies and realize all-band astronomical observation. Satellites fly very fast, circling the earth a few times to a dozen times a day, and can get a lot of information about the earth quickly, which is incomparable to ground survey and aerial photography. Satellites fly at an altitude of more than several hundred kilometers, which is not limited by geographical, airspace, geography and climate conditions, and has a wide field of vision. A satellite photo of earth resources, covering an area of tens of thousands of square kilometers. In geostationary orbit, satellites can "see" 40% of the earth's surface, which is very beneficial to communication and can realize global information transmission and exchange. Satellites can fly over any part of the earth, especially the inaccessible virgin forests, deserts, mountains, oceans and the North and South poles, and observe underground mineral deposits, marine resources and stratigraphic fault zones. Therefore, satellites can be used for astronomical observation, space physical exploration, global communication, television broadcasting, military reconnaissance, meteorological observation, resource investigation, environmental monitoring, geodesy, search and rescue, etc. (Min, He)

Sources of information; China Academy of Launch Vehicles

Science and technology satellite

There are various natural phenomena in the vast space from the earth to Datong. These phenomena all affect human activities on the earth. The sun is most closely related to human activities. It brings sunshine and warmth to the earth, and makes life live and develop. It also often interferes with the earth. The eruption of sunspots will disturb the earth's magnetic field, destroy high-level electricity and weaken or even interrupt the radio communication on the earth. Therefore, it is very important for human beings to study the characteristics of the sun and its activities. At the same time, it is very useful to study the earth's magnetic field and gravity field and the earth's atmospheric structure for understanding the formation of the earth, designing the control system of missiles or spacecraft and making weather forecasts. However, the study of these phenomena can only be carried out on earth before the satellite is launched into the sky. The earth is surrounded by a thick atmosphere. This kind of atmosphere blocks people's vision and prevents us from studying the sun and observing the universe well. Therefore, people have long been looking forward to one day being able to send research instruments into space and "open" the atmosphere to directly observe the universe.

Scientific satellite is such an excellent research tool. It carries all kinds of research instruments, as a pioneer of space science research, goes deep into distant space and reveals the mysteries there.

Over the past 30 years, countries around the world have launched more than 500 scientific satellites, such as Japan's Nova and Britain's Antelope. These scientific satellites have obtained many very valuable scientific data and new discoveries. For example, it is found that there are two radiation zones 600 ~ 40000 kilometers above the ground; It is found that the sun constantly emits plasma (called solar wind)'; It is found that there is a corona composed of atmosphere and helium at a height of about 1000 km on the highland. Besides the sun, many ultraviolet and X-ray radiation sources are observed.

198 1 year, China launched the "Practice II" scientific experimental satellite. The mass of this satellite is 250 kilograms. The main body of the satellite is an octahedral prism with a circumscribed circle diameter of 1.23m and a height of1.1m. There are four solar panels on the side, * * * has 5 188 batteries, and the output power is about 140W.

"Practice II" satellite is a space physical exploration and new technology test satellite. There are 1 1 kinds of detection instruments on board, including magnetometer, semiconductor proton detector, semiconductor electron detector, scintillation counter, infrared radiometer, atmospheric ultraviolet radiometer, solar X-ray detector, thermoelectric barometer, etc. The satellite is used to detect charged particles in space near the Earth and predict solar proton events, which is helpful to improve China's radio communication, navigation, high-altitude atmospheric density measurement and satellite orbit prediction.

meteorological satellite

Meteorology is closely related to human survival. If rainstorm or typhoon is not predicted in time, it will destroy a year's harvest and even endanger people's lives. Sailing ships and planes, without the protection of weather forecast, the consequences are even more unimaginable.

From the practice of production struggle, the working people in our country have long learned the ability to predict future weather changes from observing the sky and the earth. Later, the appearance of balloons and radio detection instruments, especially modern meteorological rockets, sent meteorological instruments to hundreds of kilometers, which made meteorological observation a big step forward. However, whether using balloons, radio equipment or meteorological rockets for meteorological observation has limitations. For example, balloons can only detect low-altitude meteorological conditions; Meteorological rockets can only obtain short-term meteorological data of an area. In addition, meteorological observation with balloons or meteorological rockets is also limited by geographical conditions, and it is difficult to detect the weather in many uninhabited places.

The appearance of meteorological satellites makes up for the shortcomings of these meteorological observation methods. The height of near-earth meteorological satellites from the ground is generally about 800 kilometers. Television cameras are installed on meteorological satellites. It can shoot cloud pictures all over the world. In the past, cloud pictures could only be taken from bottom to top. Because the clouds above are covered by the clouds below, we often can't photograph the clouds above. With meteorological satellites, cloud pictures can be taken from top to bottom.

Meteorological satellites are also equipped with scanning radiometers. The probe of scanning radiometer can sensitively detect electromagnetic radiation in a certain band. When scanning clouds and atmosphere, it can record the radiation intensity of visible light, infrared light and microwave in all bands of clouds and atmosphere, convert them into electrical signals and send them to the ground through radio waves. After being received by the ground station, the cloud shape, cloud top height, atmospheric temperature and humidity, sea surface temperature and hail coverage area can be obtained through computer processing.

After comprehensively analyzing the meteorological data obtained by meteorological satellites and other detection means, the weather can be accurately predicted.

Since 1960, the United States launched the first meteorological satellite "Tyros 1", many types of meteorological satellites have been launched in the world. So far, the United States and the Soviet Union have launched more than 100 meteorological satellites.

In the mid-1970s, according to the "Global Atmospheric Research Plan" formulated by the World Meteorological Organization and the International Association for Science, the United States, Europe, Japan and the Soviet Union launched various meteorological satellites, forming a global meteorological satellite observation network. The observation network consists of five meteorological satellites in geosynchronous orbit and two meteorological satellites in polar orbit, with a height of 800 ~ 900 kilometers from the ground. The position of five geosynchronous orbit meteorological satellites is 0. , 140 and 70 E, 75 W and135 w were launched by the European space agency, Japan and the United States respectively.

The Japanese geostationary meteorological satellite, launched in 1977, is located above the equator of 140 "E. It can observe a wide area from 8 E to160 W, with a latitude of 5 N and 5 S, including the Pacific Ocean, the East Indian Ocean, the East Asian continent and Oceania. China is also within the observation range of the Japanese geostationary meteorological satellite. China has developed a receiver for Japanese stationary satellite cloud picture, which is used for weather forecast in China.

After Japan launched its first geostationary meteorological satellite, it launched three geostationary meteorological satellites, of which two satellites launched by 1984 and 1989 are currently in operation. The masses of the two satellites are 304 kg and 325 kg respectively.

China launched the first experimental meteorological satellite "Fengyun-1" in the sun-synchronous orbit in September, 1988. Fengyun-1 satellite has an orbital altitude of 900km, an orbital inclination of 99, a satellite mass of 750kg, and a box star with an altitude of1.76m.. There is a solar cell wing on each side of the star with a wingspan of 8.6 meters. On the wing of the solar cell, there are 14256 pieces of 2×2 square centimeter silicon solar cells. The satellite is equipped with visible and infrared radiometers, and there are five bands at night, of which 1 band is infrared and the other four bands are in the visible range. The temperature of clouds, land and ocean surfaces can be observed day and night. The ground center resolution of the image obtained by radiometer is 1.l km, and the edge resolution is 4 km. After that, Fengyun-2 meteorological satellite was successfully launched.

earth observing satellite

Earth observation satellites include earth resources satellites, military reconnaissance satellites, ocean satellites and geodesic satellites.

(1) earth resources satellite

Due to the rapid development of industrial production and the increasing population, human demand for various natural resources is increasing day by day. However, due to the limitation of natural conditions, extremely rich natural resources are still sleeping in uninhabited mountains, vast deserts and vast oceans. This urgently requires us to take effective methods to survey these resources. It is an effective method to detect the earth's resources with artificial satellites. We call this kind of satellite Earth Resources Satellite.

The height of the earth resources satellite from the ground is generally about 700 kilometers, which is hundreds of times higher than the flying height of the plane. Only 300 ~ 500 photos are needed to investigate the resources of China with Earth Resources Satellite, while 500 ~ 1 10,000 photos are needed to investigate the resources of China with airplane.

Earth resources satellites can survey pseudo resources in all regions of the earth, and are not limited by natural conditions such as topography. At the same time, the earth resources satellite can also survey the same area repeatedly in different seasons, which is very suitable for observing some crops that change with the seasons.

1In July, 972, the United States launched the first experimental "Earth Resources Satellite No.1", which was later renamed "Dadi No.1". This satellite is modified from the "Rain Cloud" meteorological satellite. Its shape is exactly the same as that of "Rain Cloud". After the satellite entered the orbit, it obtained a lot of very important information. It found many important mineral information in the world, such as confirming that there are two banyan copper mines somewhere in Pakistan; Some geographical parameters have been revised, for example, the original standard of Tak Lake in Gaize County, Tibet, China is 95.8 square kilometers, but it should actually be 495.5 square kilometers; The serious pollution of Niigata Bay in Japan and a river in New York State in the United States was discovered. I also took photos of our capital. In its photos of Beijing, we can clearly see the Forbidden City, Peking University, Dong Jiao Airport, Miyun Reservoir and the Great Wall.

1978, the French government decided to develop the "SPOT" earth resources satellite to investigate natural resources, such as mineral resources, plant resources and crop yields. "Spot 1" has been in service since 1986.

"Spot" satellite has a mass of1.850kg, a length of 2m, a width of 2m and a height of 4.5m.. When the two solar panels are unfolded, the width is 3.5 meters and the output power is 1.800 watts.

There are two high-resolution cameras on the spot satellite. The focal length of the camera is 1 meter and the aperture is f/3.5. They work in visible light and near infrared band and are divided into four spectral bands: 0.50 ~ 0.59 micron, 0.6 1 ~ 0.68 micron, 0.79 ~ 0.89 micron and 0.5 1 ~ 0.73 micron. The ground resolution of the first three bands is 20m, and that of the last band is 10m. "spot" satellite runs in sun-synchronous orbit, with an altitude of 832 kilometers and an inclination of 98.7 degrees. Two cameras working at the same time can cover the whole world in 26 days.

China started to launch a recoverable Earth observation satellite on 1977. The satellite has a mass of about 1800kg, an orbital inclination of 59.5, a perigee of 180km and an apogee of 490km. The satellite consists of an instrument module and a return module. Visible light ground object camera and starry sky camera are installed in the instrument cabin. Other cameras take photos of a predetermined area of China in orbit. The starry camera shoots the starry sky, which is used to analyze the attitude error when the satellite shoots the earth. The return capsule is equipped with a braking rocket, a self-recovery system and a film box of the return capsule. Recently, the shape of capsule is the shape of ball head-cone-cone-bottom.

(2) Military reconnaissance satellites

To win the modern war, it is very important to destroy the enemy's strategic objectives first in military operations. There are two kinds of strategic targets: one is direct military targets, such as missile nuclear weapon bases, naval and air bases, ammunition depots, main command and control centers, etc. The other is military-related economic strength targets, such as important military factories, power plants and transportation hubs.

To destroy the enemy's strategic objectives, we must first understand the situation of these objectives. With the development of modern science and technology, it is very difficult to go deep into the hinterland of enemy lines for reconnaissance. After the appearance of artificial satellites, the Soviet Union and the United States gave priority to the development of military reconnaissance satellites. According to incomplete statistics, for more than 30 years. The Soviet Union launched nearly a thousand military reconnaissance satellites. Now, military reconnaissance satellites have become an indispensable partner of strategic weapons.

According to the different reconnaissance means and tasks, reconnaissance satellites can be divided into photographic reconnaissance, electronic reconnaissance and early warning.

Photographic reconnaissance satellite. This satellite is equipped with various remote sensors such as visible light camera, multispectral camera, multispectral scanner and television camera. According to the different processing methods of satellite photos, photographic reconnaissance satellites are divided into two types: return type and transmission type. The film shot by the recoverable satellite is sent to the recovery cabin of the satellite through the back channel and returns to the ground with the recovery cabin. For example, Discovery used this method to photograph reconnaissance satellites. This method is generally used for visible light photographic reconnaissance. The recoverable photographic reconnaissance satellite must solve the technology of satellite returning from orbit to the ground. The transmission photographic reconnaissance satellite sends photos directly to the ground by radio. Therefore, this reconnaissance satellite can quickly transmit information and immediately report some active military targets to the ground, such as the movement of troops and the course of missile nuclear submarines. This method usually uses TV cameras, multispectral cameras and multispectral scanners as reconnaissance means.

In order to make the camera on the satellite "see clearly" the ground target as much as possible, the orbit requirement of the photographic reconnaissance satellite is not high, generally about 200 kilometers from the ground.

Early reconnaissance satellites equipped with visible light cameras were small in size and light in weight, and carried little film. They don't fly in orbit for long, and usually return to the ground after a few days of flight. With the continuous development of space technology and the improvement of photographic reconnaissance means, the "life" of photographic reconnaissance satellites is getting longer and longer. For example, the service life of American "Big Bird" satellite is close to one year, and the service life of KH- 1 1 satellite has exceeded three years.

Photographic reconnaissance satellites provided many extremely important military information for the Soviet Union and the United States.

Electronic reconnaissance satellite. Electronic reconnaissance satellite is a kind of reconnaissance satellite that uses the radio receiving equipment on the satellite to receive the radio waves sent by enemy early warning radar and military radio stations. By analyzing these radio signals, we can know the pulse frequency used by early warning radar. Pulse width and other important parameters and communication information of military radio stations. In addition, the location of early warning radar and military radio can be determined.

The orbit of electronic reconnaissance satellites is higher than that of photographic reconnaissance satellites, usually about 500 kilometers on the ground.

Electronic reconnaissance satellites have a long life. As long as the radio receiver and antenna on the satellite are not out of order and there is enough power supply, the satellite can work around the clock, usually about 5 years.

Early warning satellite. With the development of strategic nuclear weapons, early warning satellites have appeared. This satellite is a loyal sentry in geosynchronous orbit. The radio radar and infrared detector installed on the early warning satellite monitor the enemy intercontinental ballistic missiles and nuclear submarines day and night. Once the enemy missile takes off, the early warning satellite can find it within one and a half minutes and notify the ground command center so as to take corresponding countermeasures.

(3) Geodetic satellite

Although human beings have lived on the earth for generations, due to various natural conditions, they have not fully understood the true face of the earth. Knowing the true face of the earth is very important for the development of economy, science and military affairs. Landsat is a kind of satellite developed to find out the true face of the earth.

It can accurately measure geographical coordinates. Due to the limitation of past surveying methods or some confidential reasons, many geographical coordinates in the world maps published by various countries are not accurate at present, so geodesic satellites should be used to correct them.

Geodetic satellites can measure the precise distribution of the earth's gravity field. Accurate data of the earth's gravity field are often needed in missile hit accuracy and satellite orbit calculation.

Geodetic satellites can also measure crustal drift. Crustal drift is often related to earthquakes, so measuring crustal drift can provide basis for earthquake prediction.

Before 1975, some countries launched geodesic satellites, and the positioning error of their geographical coordinates was less than 10 meter. The geodesic satellite launched by 1976 adopts advanced laser ranging technology, and can even measure the crustal movement as small as 5cm every year.

Radio relay satellite

A communication satellite is a satellite used for long-distance radio communication.

Before the emergence of communication satellites, there were two ways of communication in two distant places on the earth; One is to use cables, and the other is to use ground radio equipment. Using cables for communication has good confidentiality and stable transmission, but the cost of laying and maintaining cables is expensive. According to the wavelength of radio waves, radio communication can be divided into three types. First, the long wave band (wavelength from10000m to1000m) is used. This kind of wave mainly propagates along the ground. Because of the earth's absorption of radio waves, the intensity of radio waves decreases rapidly with the increase of propagation distance. In order to make up for this attenuation loss, the transmitting power of the transmitter must be as high as several kilowatts, and the antenna must be erected on a tower hundreds of meters high, so the long-wave communication project is huge. In addition, the information capacity of long-wave transmission is very small, which will also cause serious distortion. So long-wave radio communication is rarely used now. Later, people used radio short wave (wavelength from 100 m to 10 m) for communication, and this kind of electric wave spread according to the reflection of ionosphere over the earth. However, the ionosphere varies with day and night, season and geographical location; In addition, the ionosphere is also affected by solar activity, so short-wave communication is very unstable. In recent decades, people began to use radio and microwave widely for communication. Radio microwave (wavelength from 1 m to 1 mm) can transmit a lot of stable information. However, like light, this kind of electric wave can only travel in a straight line within the line of sight ("visible"), and the two places on the earth are far apart, so it is impossible to communicate directly by radio microwave. In order to overcome this weakness, people have come up with a relay mode such as relay race. Set up a relay station every 50 kilometers or so. After receiving the radio signal from the previous station, the relay station amplifies it and sends it to the next station. In this way, information can be transmitted to far away places. However, it will cost a lot of money to set up many relay stations, especially in mountains and vast oceans.

In the late 1950s, when the artificial earth satellite was launched, people soon thought that it could be used for long-distance communication. The United States launched the first such satellite in August 1960. This satellite is 30 meters in diameter and is named "Echo 1". In fact, it is a balloon made of aluminized plastic film. Because the radio waves reflected back to the ground by this satellite are still very weak, to receive such weak radio waves requires the ground receiving station to have a high-sensitivity receiver or the ground transmitting station to have a high-power transmitter. Therefore, it is still very difficult to make long-distance communication with radio waves reflected by satellites. In order to strengthen the radio waves reflected from satellites back to the ground, people regard satellites as microwave relay stations on the ground. After receiving the radio waves from the ground, the satellite amplifies them and sends them to the ground. This method is adopted by all communication satellites currently working.

At first, people could only launch communication satellites thousands of kilometers from the ground. The communication satellite stays above the mine communication station for a short time, and the total communication time for one day and one night is only a few minutes. Later, people launched a communication satellite with a large elliptical orbit, pulling the satellite from apogee to an altitude of more than 30 thousand kilometers above the ground. This kind of satellite can stay over the ground communication station for more than ten hours day and night, but it can't communicate all day. 1963 February, the United States launched the first geosynchronous orbit communication satellite. Geosynchronous orbit satellites can be "fixed" at a certain point above the equator of the earth, and when such satellites are above the ground communication station, they can realize 24-hour uninterrupted communication. Theoretically, if the three communication satellites in geosynchronous orbit are evenly distributed over the equator of the earth, almost global continuous communication can be realized except in polar regions.

Geosynchronous orbit communication satellite is the fastest-growing artificial earth satellite in recent 40 years because of its superior communication conditions, and has become a commercial communication tool. 1In August, 964, the "International Telecommunications Satellite Consortium" with the participation of eight countries was formally established. From April 1965 to the present, the United States has developed and launched nine kinds of international communication satellites with the funds provided by the consortium. More than 100 countries have used international communication satellites.

With the increase of communication services and the development of space technology, many communication satellites with different purposes have been developed in various countries. For example, domestic communication satellites suitable for a certain country or region; National defense communication satellites specially serving the military; Providing maritime communication satellites for ships; Tracking and data relay satellites that provide satellite orbit measurement and data transmission; Broadcasting satellites that provide direct TV broadcasting services for families and so on.

The United States began to develop tracking and data relay satellites (TDRS) from 1976. Appropriate deployment of three tracking and data relay satellites in geostationary orbit is equivalent to moving three TT&C communication stations on the ground into space, which can realize continuous tracking and data communication for all user satellites and load spacecraft and space stations with orbital heights ranging from 200 to 12000 km.

The tracking and data relay satellite has a mass of 2270 kg and a hexahedron, and the width of two solar panels after deployment is 17.4 meters. It can provide 1850 watts of electricity. The satellite is equipped with KU-band and S-band parabolic antennas with a diameter of 4.9 meters. In addition, the satellite is equipped with a phased array antenna working in S-band, which can serve 20 user satellites at the same time.

The radio transmission power of TV broadcasting satellites is much larger than that of communication satellites. The radio transmission power of communication satellites is usually only a few watts to tens of watts, while the radio transmission power of TV broadcasting satellites can reach several hundred watts. Because the TV broadcasting satellite has such a large radio transmission power, the ground receiving station does not need a parabolic receiving antenna with a diameter of tens of meters like the communication satellite, but only a parabolic receiving antenna with a diameter of half a meter or several meters. The TV broadcasting satellite is in the sky, and families with TV can directly receive TV programs sent by the TV broadcasting satellite. Television broadcasting satellites are very suitable for a country with a vast territory and a large population like ours. For example, as long as two broadcasting satellites are launched, CCTV programs can be sent to vast rural areas and mountainous areas, and there is no need for such huge microwave trunk lines and many relay stations. This will greatly enrich our scientific and cultural life.

China launched an experimental communication satellite in 1984, and then successfully launched several geosynchronous practical communication satellites.

China experimental communication satellite has a mass of 900 kg at launch and 420 kg in geostationary orbit. The maximum height from the apogee engine nozzle to the top of the antenna is 3.l meters, and the diameter of the star is 2. 1 meter.

The communication frequency band of China's experimental communication satellite is selected from the frequency band specified by ITU, with uplink frequency of 6225 ~ 6425 MHz and downlink frequency of 4000 ~ 4200 MHz. There are two sets of forwarding pots on board, which can communicate 24 hours a day. The communication repeater consists of 1 1 components. It constitutes a complete receiving, amplifying, frequency conversion and transmitting system. The receiver adopts low noise tunnel diode amplifier, and in order to improve the amplification gain, it adopts intermediate frequency amplification. In order to meet the needs of real-time communication of ships, besides the wide-band channel of relay TV, a narrow-band channel is also set in the repeater, which increases the forwarding gain by 6 dB, and the final power amplifier adopts a traveling wave tube amplifier.

Space clairvoyance-early warning satellite

Some people may not be familiar with the name early warning satellite. To put it bluntly, it is like a sentry, standing in the air, staring at a certain area on the ground at any time, and reporting the situation in time when there is any trouble. Early warning satellites are generally launched into geostationary orbit, and high-precision detectors are installed on the satellites. The detector faces the air and always points to the enemy area. Once the enemy launches a missile, the satellite can detect it in less than a few minutes. At the same time, by calculating the flight trajectory, we can determine its landing point and attack target, and immediately send the information to the headquarters command center to remind it to prepare for counterattack. General intercontinental missiles take tens of minutes to fly, even ordinary medium-range missiles take several minutes to ten minutes. The early warning of the early warning satellite has won us valuable time. Some satellites are also equipped with X-ray detectors, X-ray detectors and other nuclear radiation detectors to monitor nuclear explosions inside and outside the atmosphere. The early warning satellite is a veritable clairvoyance, which can even be called Wan Li Eye. Representative early warning