Pearls embedded in the air
1990 During the Gulf War, TV viewers all over the world witnessed the thrilling scene of modern war. In the live broadcast, communication satellites played a central role.
Speaking of artificial satellites, students will not feel strange. At present, people generally use satellites for communication, reconnaissance, navigation and meteorological observation. With the continuous development of satellite and modern communication technology, satellite communication has actually entered the family. Maybe one day you will pick up your wireless phone and exchange greetings with friends and relatives on the other side of the ocean via satellite.
The idea of satellite communication was inspired by the moon. Everyone knows that the moon itself does not shine, but it can reflect sunlight. So can it reflect radio waves and realize long-distance communication? Of course. But now we use the "artificial moon"-artificial satellite, and we don't rely on its "reflected" radio waves for communication. We let it receive the radio waves from the ground first, amplify them and then send them back to the ground. In this way, the radio waves received on the ground are more powerful and the communication is more reliable. This practice is like riding a horse for a long run, changing horses before running. Satellites are like "relay stations" in the air, enabling people to realize long-distance communication between two or more places. This way of realizing communication by satellite forwarding signals is called satellite communication.
Because communication satellites live in the vast space of thousands and tens of thousands of kilometers, overlooking the endless vast land, they have unparalleled advantages compared with other communication methods.
The microwave signals from satellites hanging in the sky can cover an area of several million to tens of millions of square kilometers. In this area, whether it's a flying plane, a ship braving the wind and waves, a high-speed Mercedes-Benz car, or even a pedestrian walking on the street, you can receive the signal sent by the satellite by wireless phone. It has been calculated that a satellite covers an area equivalent to more than 200 microwave relay stations erected on the ground. Moreover, satellite communication can be realized at any two or more points in the satellite coverage area, whether it is across the ocean or mountains and seas.
Thousands of mobile phone communications and many TV communications can be carried out by satellite. In addition, it can exchange data, characters and images. If you have a portable fax machine and want to inquire about a book in the Library of Congress, you can get a fax copy in a few minutes by remote wireless online retrieval via satellite and computer.
If a communication satellite is "embedded" over the Pacific Ocean, Atlantic Ocean and Indian Ocean, the radio waves emitted by these three satellites can cover the whole world. The maritime communication satellite system used by people now is such a group of satellites. People call it "Pegasus". Thousands of ships sailing on the ocean communicate through it.
Radio waves emitted by ground stations can be transmitted to ships, planes, vehicles and other ground stations by satellite. Similarly, radio waves emitted by another ground station can also be relayed by satellite. Many mobile communication stations use the same satellite to establish a multi-directional communication channel, which is called multiple access connection.
Multiple access connection is an important advantage of satellite mobile communication. It enables satellite communication to serve many unrelated users at the same time.
If we want to make a wireless call to our friends in Detroit, USA, we can directly dial in the country code, city code and the other party's phone number. The radio wave sent by the wireless telephone, carrying our dialing pulse, will be received by the base station, and the signal will be transmitted to the satellite over the Pacific Ocean through the satellite ground station of the mobile wireless exchange bureau, and then forwarded to a satellite ground station near Detroit, USA, and then transmitted to the local wired telephone office through the microwave circuit, and then transmitted to the other party's telephone. Then, a mobile phone was connected through satellite and ground station equipment.
Some people also call "Iridium" a cellular mobile phone system upside down in the air. As we have learned before, in a cellular network, the "hive" is set on the ground and the cellular service area is fixed. The "honeycomb" structure of the "iridium" system is set in space. If someone carries a wireless phone and "roams" between adjacent "cells", the Iridium system will transfer the signal from one satellite to another. Data processing and exchange are all done in space orbit.
In the near future, with the implementation of the Iridium project, people will realize communication within the four seas. Even in remote areas where there is no electricity supply or telephone lines are blocked, people can communicate with all corners of the world immediately by using solar iridium telephone booths or a mobile phone similar to "Big Brother".
At present, people call "Iridium" system a revolution of communication family. It indicates that mobile communication will get rid of a large number of complex ground equipment and install base stations and exchange offices on satellites in space, thus realizing the "three-dimensional" communication of personal mobile phones. This pioneering work will herald the arrival of the era of personal satellite communication, and the stars embedded in the air will be more brilliant.