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The satellite system consists of three main circuits, including the satellite, receiver, and transmitter. Satellites are increasingly being used for disaster risk management, weather forecasting, remote sensing, geographic positioning, and navigation.
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Satellite system
Satellite communication system is actually a type of microwave communication, which uses satellites as relay stations to forward microwave signals and communicate between multiple ground stations. The main purpose of satellite communication is to achieve "seamless" coverage of the ground. As satellites operate in orbits of hundreds, thousands, or even tens of thousands of kilometers, their coverage range is much larger than that of general mobile communication systems. However, satellite communication requires ground equipment with high transmission power, making it difficult to popularize and use.
The satellite communication system consists of three parts: satellite end, ground end, and client end. The satellite serves as a relay station in the air, amplifying the electromagnetic waves emitted by the ground station and sending them back to another ground station. The satellite body consists of two subsystems: onboard equipment and satellite body. The ground station is the interface between the satellite system and the ground public network. Ground users can also enter and exit the satellite system through the ground station to form a connection. The ground station also includes the ground satellite control center, as well as its tracking, telemetry, and command stations. The client refers to various user terminals.
In the microwave frequency band, the working frequency band of the entire communication satellite is about 500MHz wide. In order to facilitate amplification and transmission and reduce modulation interference, several transponders are generally set up on the satellite. Each repeater is assigned a certain operating frequency band. At present, satellite communication mostly adopts frequency division multiple access technology, where different earth stations occupy different frequencies and use different carriers. More suitable for point-to-point high-capacity communication. In recent years, time-division multiple access technology has also been widely applied in satellite communication, where multiple earth stations occupy the same frequency band but different time slots. Compared with frequency division multiple access pipelines, time division multiple access technology does not generate intermodulation interference, does not require up and down conversion to separate signals from various earth stations, is suitable for digital communication, and can allocate transmission bandwidth according to changes in business volume, greatly increasing actual capacity. Another type of multiple access technology is code division multiple access (CDMA), where different earth stations occupy the same frequency and time, but use different random codes to encode information to distinguish different addresses. CDMA adopts spread spectrum communication technology, which has the advantages of strong anti-interference ability, good confidentiality communication ability, and flexible scheduling of transmission resources. It is more suitable for systems with small capacity, wide distribution, and certain confidentiality requirements.
According to the working orbit, satellite communication systems are generally divided into the following three categories
1. Low Earth Orbit Satellite Communication System (LEO)
At a distance of 500-2000km from the ground, the transmission delay and power consumption are relatively small, but the coverage area of each star is also relatively small. A typical system is Motorola's Iridium system. The low orbit satellite communication system can support multiple communications due to the low satellite orbit and short signal propagation delay; Its connection loss is small, which can reduce the requirements for satellites and user terminals. Micro/small satellites and handheld user terminals can be used. However, low orbit satellite systems also come at a significant cost for these advantages: due to their low orbit, each satellite has a relatively small coverage area, requiring dozens of satellites to form a global system. For example, the Iridium system has 66 satellites, Globalstar has 48 satellites, and Teledisc has 288 satellites. Meanwhile, due to the fast movement speed of low orbit satellites, for a single user, the time it takes for the satellite to rise from the horizon to fall below it again is relatively short, resulting in frequent switching between satellites or carriers. Therefore, the system composition and control of the low orbit system are complex, with high technological risks and relatively high construction costs.
2. Medium Earth Orbit Satellite Communication System (MEO)
At a distance of 2000-20000Km from the ground, the transmission delay is greater than that of low orbit satellites, but the coverage area is also larger. The typical system is the International Maritime Satellite System. The medium orbit satellite communication system can be said to be a compromise between synchronous satellite system and low orbit satellite system. The medium orbit satellite system combines the advantages of both schemes, while also overcoming the shortcomings of both schemes to a certain extent. The connection loss and propagation delay of medium orbit satellites are relatively small, and simple small satellites can still be used. If both medium orbit and low orbit satellite systems use interstellar connections, when users communicate over long distances, the delay of information in the medium orbit system through the satellite interstellar connection subnet will be lower than that in the low orbit system. Moreover, due to its orbit being much higher than that of low orbit satellite systems, each satellite can cover a much larger range. When the orbit altitude is 10000 km, each satellite can cover 23.5% of the Earth's surface, so only a few satellites are needed to cover the whole world. If there are more than ten satellites, it can provide dual coverage to most parts of the world, which can use diversity reception to improve the reliability of the system, and the system investment is lower than that of low orbit systems. Therefore, in a certain sense, the medium orbit system may be a superior solution for establishing a global or regional satellite mobile communication system. Of course, if broadband services are needed for ground terminals, there will be certain difficulties for medium orbit systems, and the performance of using low orbit satellite systems as high-speed multimedia satellite communication systems is superior to that of medium orbit satellite systems.
3. High Earth Orbit Satellite Communication System (GEO)
35800km above the ground, in synchronous geostationary orbit. In theory, global coverage can be achieved with three high orbit satellites. The technology of traditional synchronous orbit satellite communication systems is the most mature. Since synchronous satellites have been used for communication services, using synchronous satellites to establish global satellite communication systems has become the traditional mode of establishing satellite communication systems. However, synchronous satellites face an insurmountable obstacle of long propagation delays and significant connection losses, which seriously affect their applications in certain communication fields, especially in satellite mobile communication. Firstly, synchronous satellites have high orbits and high connection losses, which require high performance from user terminal receivers. This system is difficult to support direct communication between handheld devices through satellites, or requires the use of satellite antennas (L-band) with a resolution of 12m or more, which places high demands on the payload of satellite communication and is not conducive to the use of small satellite technology in mobile communication. Secondly, due to the long connection distance and large propagation delay, the propagation delay of a single hop can reach several hundred milliseconds. In addition, the processing time of speech encoders and other devices will further increase the single hop delay. When mobile users engage in dual hop communication through satellites, the delay can even reach seconds, which is unbearable for users, especially voice communication users. In order to avoid this double hop communication, it is necessary to use on-board processing to enable the satellite to have switching function, but this will inevitably increase the complexity of the satellite, not only increase system costs, but also have certain technological risks.
Currently, there are four major satellite navigation systems being widely used by many people
1. Global Positioning System (GPS) in the United States
The first thing mentioned is definitely the GPS system, which was developed by the United States in the 1970s, took 20 years and cost $20 billion, and was fully completed in 1994. It is a new generation of satellite navigation and positioning system with comprehensive real-time three-dimensional navigation and positioning functions in sea, land, and air.
Nowadays, it is widely used. It used to be used in China for a considerable period of time, but now it has developed very maturely and is widely used in engineering surveying, civil automobiles, military and many other fields.
2. The Russian GLONASS system
At the beginning, it was researched and developed by the Soviet Union during the Cold War. After the dissolution of the Soviet Union, Russia began to independently research and establish it. The plan was launched in 1993 and it was put into operation around 2007. It has been more than ten years since then.
Due to the huge size of Russia, at the beginning, it could not cover the global scale. After two years of research and development, the technology team expanded its service scope to the whole world in 2009.
The main service content of this system includes determining the coordinates and motion speed information of land, sea, and air targets. At present, the system has 24 satellites with an accuracy of about 9.5 meters, which is still quite poor compared to GPS's 0.1 meters.
3. The European Galileo system
The system was developed by the European Union and has not yet been fully established, as the initial design was to launch 30 satellites at an orbital altitude of over 2000 meters, including 27 working satellites and 3 backup satellites.
However, later on, the speed was relatively slow and I did not have complete operational capabilities, but now the accuracy is also very impressive, with an error of basically within 1 meter.
4. China's Beidou system
There is no doubt that China has independently developed the Beidou Satellite Navigation System (BDS), which is divided into a three-step strategy. It was first planned and deployed in 1994. If we start counting from Beidou-1, a total of 59 Beidou satellites have actually been launched. (Beidou-1 has four satellites; Beidou-2 has 14 networking satellites and 6 backup satellites; Beidou-3 has 30 networking satellites and 5 experimental satellites.)
In 2020, with the official launch of the Beidou-3 global satellite navigation system, our autonomous system also made its debut in the world. Although the accuracy is currently around 10 meters, it already has many advantages that other systems cannot compare with. With China's continuous updates and iterations, the accuracy will gradually approach the world's top level.