The principle of satellite communication

Since the launch of the first man-made Earth satellite in the Soviet Union in 1957, satellites have been widely used in communications, radio, television and other fields. In 1965, the first commercial resounds communication satellite was sent to orbit over the Atlantic Ocean, and commercial communications using geostationary satellites began.

The satellite communication system is composed of two parts, satellites and earth stations. Satellite in the air as a relay station, that is, the Earth station sent up to the electromagnetic wave to return to another Earth stations. The earth station is the interface between the satellite system and the ground public network, and the ground users form the link through the earth station to access the satellite system. Because the geostationary satellite is 3600Km above the equator, its orbit around the Earth coincides with the Earth's rotation for one week (23 hours, 56 minutes and 4 seconds), from the ground to look as static. Three of the 120 ° health can cover the equatorial circumference. Therefore, satellite communication is easy to achieve transatlantic and intercontinental communications. The most suitable frequency for satellite communications is the 1~10ghz band. In order to meet the increasing demand, research and application of new frequency such as 12ghz,14ghz,20ghz and 30GHz have been started.

The main features of satellite communications are as follows: Advantages (1) The communication range is large, as long as the range of satellite beam coverage can be transmitted. (2) Not easily affected by land disasters. (3) The construction speed is fast. (4) Easy to realize broadcast and multiple access communication. (5) The circuit and traffic volume can be flexibly adjusted. (6) The same communication can be used in different directions and different regions. Disadvantages: (1) because the two Earth station to the electromagnetic wave transmission distance has 72000Km, the signal arrives has the delay. (2) The frequency of the above 10GHz is affected by precipitation. (3) The antenna is affected by the solar noise.

In the microwave band, the working band of the communication satellite is about 500MHz wide, and a number of transponders are generally set up for global amplification, emission and reduction of modulation interference. The working band width of each forwarder is 36MHz or 72MHz. The current satellite communication uses frequency division multiple access technology, different Earth stations occupy different frequencies, that is, the use of different carrier. It is more suitable for point-to-point and large capacity communication. In recent years, Time division multiple Access technology has been gradually adopted, that is, each Earth station occupies the same frequency band, but occupy different time slots, it has a series of advantages over frequency division Multiple access, such as not to generate intermodulation interference, do not need to go up and down the frequency of the ball station signal separate, suitable for digital communication, according to the changes in the industry power on demand can use the digital voice interpolation and so on new technology, causes the capacity to increase 5 times times. Another kind of multiple access technology is Code Division multiple Access (CDMA), that is, different Earth stations occupy the same frequency and the same time, but different random codes are used to distinguish different addresses. It uses the extended spectrum communication technology, has the anti-interference ability, has the good secret communication ability, can flexibly dispatch the channel and so on the advantage. The disadvantage is that the spectrum utilization is low. It is suitable for small capacity, wide distribution, with a certain confidentiality requirements of the system.

In recent years, the development of satellite communication technology has been endless. For example, the very small aperture antenna earth station (VSAT) system, the low memoirs of the Mobile Satellite communication system, etc. have been widely concerned and applied. Satellite communication is also an important part of the future global information superhighway.