Classification of mobile satellite communication systems The classification of satellite mobile communication systems can be classified according to their applications or technical means adopted. (1) can be classified into maritime satellite mobile system (MMSS), aviation satellite mobile system (AMSS) and land satellite mobile system (LMSS) according to the application. Maritime satellite mobile system is mainly used to improve maritime rescue work, improve the efficiency and management level of ships, and enhance maritime communication services and wireless positioning capabilities. Aviation satellite mobile system is mainly used to improve voice and data communication between aircraft and ground for crew and passengers. The land satellite mobile system is mainly used to provide communication for moving vehicles. (2) According to the classification of orbits, there are two kinds of orbits for communication satellites. One is low orbit or medium-high orbit. The satellite in this orbit moves relative to the ground. It can be used for short-time communication, and the coverage area of satellite antenna is also small, so the ground antenna must also track the satellite at any time. The other orbit is a synchronous fixed-point orbit with a height of 36,000 kilometers, which is a circular orbit in the equatorial plane. The operating period of the satellite is the same as the rotation period of the earth. On the ground, this kind of satellite looks stationary and is called synchronous fixed-point satellite. It is characterized by a large coverage area, and three satellites can cover almost the whole area of the earth, realizing 24-hour all-weather communication. (3) According to frequency classification, satellites are divided into L-band satellites and Ka-band satellites according to the frequency range used by satellites. (4) Classification by service area With the rapid development of space technology, there are more and more kinds of communication satellites. According to the service area, there are global, regional and domestic communication satellites. As the name implies, global communication satellites refer to communication satellites whose service areas are all over the world, and often need multiple satellites to form a network. Regional satellites only serve communication in a certain area. Domestic satellites have a narrow range and are limited to domestic use. In fact, all kinds of classification methods want to reflect some characteristics of satellites more strongly, so that people can better distinguish various satellites. Classification by satellite orbit: the application and development of satellite-based mobile communication can be roughly divided into three situations: (1) the satellite is stationary (synchronous orbit satellite), Inmarsat which has been widely used at present, and mobile communication systems such as AMSC (USA), CELSAT (USA), MSS (Canada) and Mobilesat (Australia) which are actively developing. These systems have realized the communication with cars, ships, planes and other mobile bodies, and the communication with mobile phones is just around the corner. (2) The satellite moves (non-synchronous orbit satellite), and the terminal does not move. It communicates with larger terminals (such as the base station of mobile communication network) through non-synchronous orbit satellites, and then connects to the users of mobile phones. This is usually the case when calling the (American) system. This is basically the case that mobile users communicate through the satellite of the gateway station. (3) When the satellite moves (non-synchronous orbit satellite), the terminal also moves. At present, a large number of low and medium orbit systems (such as iridium satellite system, global satellite system and Audi system) are polarized in this case. Their characteristic is to make the terminal handheld and realize satellite communication to meet the needs of personal mobile communication in the future. Since 1960s, human beings have sent hundreds of communication and broadcasting satellites into high orbit (GEO), and these satellites have been playing a leading role in realizing international long-distance communication and television transmission. However, GEO satellites also have some problems: in (1) free space, the signal strength is inversely proportional to the square of the transmission distance. High-orbit (GEO) satellites are too far away from the Earth, which requires a communication antenna with a larger aperture. (2) Long-distance signal transmission will bring great delay. This delay will obviously make people feel uncomfortable when making a phone call. In data communication, time delay limits the reaction speed. For a 200 1 desktop supercomputer, a delay of half a second means that hundreds of millions of bytes of information are stuck in the buffer. (3) Shortage of track resources. There is only one GEO satellite, and the interval between adjacent satellites should not be too small, because the ability of the earth station antenna to distinguish satellites is limited by the aperture size of the antenna. In Ka-band (17 ~ 30 GHz), the reasonable aperture of the ground station antenna should not be less than 66cm in order to separate satellites with a distance of 2. According to this calculation, GEO satellite can only provide 180 common orbit position. This also includes many jobs with poor practical value and above the ocean. Brief introduction of MEO satellite communication system 1. Overview The characteristics of GEO mobile communication services come from the conditions of using geostationary satellites located at 35800km above the equator to carry out communication services. At this altitude, a satellite can cover almost the whole hemisphere, forming a regional communication system, which can provide services to any place within its satellite coverage. For example, an American satellite can cover continuous parts of the continental United States, such as Alaska, Hawaii and Puerto Rico, hundreds of nautical miles offshore. In GEO satellite system, only one domestic exchange is needed to route the call, and the signaling and dialing methods are relatively simple, so any mobile user can be called without knowing its location. At the same time, mobile phones can land at any convenient place without expensive long-distance connection, and the cost of satellite communication has nothing to do with distance, which is similar to the cost of providing local services by ground systems. When the elevation angle of the satellite to the ground station is large (for example, in the longitude range of the United States, the elevation angle of the satellite to the ground is generally between 20 and 56), the mobile antenna has an upward beam, which can be distinguished from the reflection of the ground, so that the common deep multipath fading in the ground system can be almost completely avoided. Because of its high elevation angle, the satellite signal only passes through the canopy, thus reducing the attenuation caused by branches and leaves to only a few decibels. Second, the characteristics of satellite mobile communication services can provide two common services: one is public satellite relay speaker and the other is dedicated satellite relay speaker. The former needs to be interconnected with the public switched microphone network so that the mobile station can call any fixed microphone in the world, while the latter is only between the mobile station and its scheduler. Both services can transmit caller, paging and location information. These two services can also be combined to form a unique communication capability. (1) public satellite relay telephone service The network includes satellites, mobile stations working in L-band, network operation centers working in K-band and gateway earth stations/switches. It uses radio frequency signals distributed by the network operation center to mobile stations and gateway stations through dedicated signaling channels. In order to establish a call and determine the connection route, the mobile station dials the telephone number of the destination address and gives its own number. The network operation center allocates an L-band RF channel to the mobile station and a corresponding K-band channel to the gateway station near the fixed caller address, where the usual caller signaling is generated to establish the call. The network operation center records the route, caller and call time for calculation. On the other hand, the operation is similar. The importance of gateway stations is noteworthy in providing flexible connection capability for long-distance connections, and hundreds of gateway stations may be needed. Once the call is set up, it can transmit voice in-band data, packet messages, positioning, paging and other business information, and a radio station can complete all these functions. (2) Dedicated satellite mobile phone service The system includes satellites, mobile stations and base stations located outside the user's building. The base station allocates one or several circuits to the system as needed through a simplified gateway (without call routing and long-distance interconnection equipment). It can use a simple "push-to-talk" operation, or it can use a more complicated switching mode, thus allocating the system time to different users for different purposes. Each mobile station can use a single wireless station to complete the transmission of predetermined callers, data at different rates, packet messages, paging and positioning messages. If the radio station can tune to the signaling channel of the public satellite relay transmitter, it can also have the function of a wireless public transmitter. ACeS satellite communication system is a good communication equipment for ships, mountain workers, tourists and outlying islands sailing in Asian waters. GALUDA- 1 synchronous satellite is used, which is located directly above the equator of 123 degrees east longitude and 36,000 kilometers away from the earth. The satellite body of ACeS satellite communication system is manufactured by Lockheed Martin Company of the United States. The mobile phone is made by Ericsson: Ericsson Aces R 190 Yaxing intercom. Features: 1, the phone bill can be charged at any time. Ericsson ACER190 satellite relay telephone is the smallest and lightest telephone in the world at present. 2. The call fee is 0.25 USD per minute (incoming call) and 0.35 USD per minute (outgoing call); 3. Internet access fee, monthly fee and share fee are not required; The phone bill can be recharged at any time. Application fields: villages, forests, mountains, deserts, Gobi, mines, oil wells, oceans, geology, tourism, exploration, railways, water conservancy, electric power, meteorology and scientific investigation far away from cities.