Communication satellites, the earth, the moon and the sun are constantly in their respective orbits day and night. When they are in a specific relative position, they will affect the satellite communication, which is commonly referred to as "astronomical influence".
During a terrestrial eclipse, when the earth is between the sun and the satellite and forms a line, due to the shadow of the earth, the solar cells of the satellite cannot supply power normally. Sun transit is another astronomical phenomenon that affects satellite communications, and its cause is similar to that of the eclipse of the earth and the stars.
It is also around the vernal equinox and autumnal equinox every year that the sun crosses the earth's equatorial plane and is at the point below the satellite. In the afternoon local time, the satellite is on the connecting 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 communication interruption, that is, the earth station cannot receive the satellite signal. This phenomenon is the phenomenon of solar transit interruption. The interruption of Riling lasts about 6 days, and each time is a few minutes, accounting for about 0.02% of the annual communication time.
Satellite communication (satcom) system mainly includes modulation mode and multiple access mode. Modulation methods include FM, PSK and other 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.
In the area covered by the beam of a communication satellite antenna, all earth stations can transmit 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 the satellite, and the satellite amplifies the received signal, converts the frequency, etc., and then forwards it to another earth station. A communication link between the station and another earth station is then established. A geostationary satellite communication is 36,000km away from the earth's surface (42,000km from the earth's center), and its antenna beam coverage area (that is, the field of view to the ground) exceeds 42.4% of the earth's surface. In the sky, global communication can be realized except for the North and South Poles.
Synchronous satellite communication (satcom) has the following advantages: the communication distance is long, and it can be reached by "one hop" via the satellite, and the farthest communication distance can reach 13000km. Communication is not restricted by geographical conditions (such as mountains, rivers and oceans), and is not affected by natural disasters (such as floods, earthquakes, etc.) and man-made events.
Communication quality is high. This is firstly because the information can usually be transmitted to the other party with one hop through the satellite, unlike the noise introduced by microwave relay communication; secondly, radio waves mainly propagate in outer space, and are less affected by atmospheric refraction and ground reflection, and have better propagation characteristics.
The communication capacity is large, and more than 60,000 people can make calls at the same time. Applicable to broadcast and user services. Satellite communication (satcom) can be used to transmit TV and radio programs point-to-multipoint in a wide range, which is difficult to be replaced by other means of communication.
The use of satcom communication can also realize the interactive data transmission directly to the user's office building, that is, it is also suitable for user-oriented business, and the communication is flexible and maneuverable.
Satellite communication (satcom) can provide small and medium-capacity voice and data channels between two points, can realize multiple address transmission, and can also provide sea and air mobile communication services.
