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The Influence of Celestial Objects on Satellite Communication

Communication satellites, the Earth, the moon and the sun are in their respective orbits around the clock. When they are in a specific relative position, it will constitute an impact on satellite communications, which is commonly referred to as "celestial impact". In a terrestrial eclipse, when the earth is in a line between the sun and the satellite, due to the shadow of the earth, the solar cell of the satellite cannot supply power normally. Solar transit is another celestial phenomenon that affects satellite communications, and its cause is similar to that of a terrestrial eclipse.

It is also around the vernal and autumnal equinoxes every year when the sun crosses the plane of the earth's equator. At the subsatellite point of the satellite, in the afternoon local time, the satellite is on the connection line between the earth and the sun. At this time, the antenna of the earth station pointing to the satellite is just aimed at the sun, and the strong solar noise causes the communication to be interrupted, that is, the earth station cannot receive the satellite signal. This phenomenon is known as the Sun Interruption phenomenon. The interruption lasted for about 6 days, with a few minutes each time, accounting for about 0.02% of the annual communication time.

1. Satellite communication system

The satellite communication system mainly includes modulation mode and multiple access mode. Modulation methods include FM, PSK and other modulation methods. The multiple access technology is used to solve the problem of sharing communication resources without interfering with each other when users access the network. In general satellite communications, multiple access technologies such as FDMA, TDMA, and CDMA are used.

2. Synchronous satellite communication

In 1945, the British physicist Clarke published an article entitled "Relay beyond the Earth" in the "Radio World" magazine, predicting that humans can use satellites operating in synchronous orbits over the equator to achieve long-distance communication. Today, this has become a reality.

In the area covered by the beam of a communication satellite antenna, each earth station can relay signals through satellite relays to realize communication. For example, when an earth station in a certain place wants to communicate with an earth station in another area, the station transmits a radio signal to a satellite, and the satellite amplifies and frequency converts the received signal and then forwards it to another earth station. A communication link between the station and another earth station is then established.

A synchronous communication satellite that is 36,000km away from the earth's surface (42,000km from the earth's center) has an antenna beam coverage area (ie, the view area to the ground) that exceeds 42.4% of the earth's surface. In the sky, global communication can be achieved except for the North and South Pole.

Synchronous satellite communication has the following advantages: the communication distance is long, which can be reached by "one hop" of satellite, and the farthest communication distance can reach 13000km. Communication is not limited by geographical conditions (such as the barrier of mountains, rivers and oceans), nor is it affected by natural disasters (such as floods, earthquakes, etc.) and man-made events. Communication quality is high.

This is first because the information can be transmitted to the other party through a single hop of the satellite, unlike microwave relay communication which introduces noise; secondly, radio waves mainly propagate in outer space, and are less affected by atmospheric refraction and ground reflection, and have better performance. propagation characteristics. The communication capacity is large, and more than 60,000 people can make calls at the same time. Applicable to broadcast-type and user-type services.

Using satellite communication, TV and radio programs can be transmitted point-to-multipoint in a large area, which is difficult to replace with other communication means. Using satellite communication, it can also realize the interactive data transmission directly to the user's office building, which is also suitable for user-type business, and the communication is flexible and flexible. Satellite communication can provide small and medium-capacity voice and data channels between two points can realize multiple access transmission, and can also provide maritime and air mobile satellite services.

Related Product:

Maritime Satellite Terminals

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