In the past half a century, humans have achieved global coverage of communication networks. Today, there are basically places where humans are frequently active, and there are basically wireless communication base stations. With these communication base stations, we can make phone calls and access the Internet at any time.
However, China's spacecraft faces a problem. There is a window period for China's Shenzhou spacecraft and Tiangong Space Station to communicate directly with the ground.
In 2003, when China's first astronaut, Yang Liwei, took off, China had several ground measurement and control stations such as Kashgar , Xi'an, Qingdao , and several Yuanwang measurement and control ships.
At that time, the Shenzhou V spacecraft that Yang Liwei was riding had an orbital altitude of about 340 kilometers. The time for one week to fly around the earth was about 90 minutes. The entire process of entering the Kashgar measurement and control station and leaving the Qingdao measurement and control station was only about 10 minutes. At that time, the Beijing Space Flight Control Center wanted to talk to Yang Liwei, so it could only use these 10 minutes.
, and the communication bandwidth at that time was very narrow. The audio and video data collected on the spacecraft needed to be compressed 180 times before it could be uploaded to Beijing. At that time, the flight control center saw Yang Liwei's picture, and basically could only re-compile frames by blinking. It was the real PPTh frame rate .
In that era, astronauts wanted to say something to the ground or say hello were very luxurious. During the time outside the window period, the ground flight control center is completely unaware of the operating conditions of the spacecraft and astronauts.
There are only two ways to solve this problem. One is to build a measurement and control station around the world.
For example, in South America, China rented a 200 hectares of land in a place called Neuken in , Argentina, for a term of 50 years. It built a giant 35-meter-diameter antenna, which is commonly known as the "pot".
For example, in Australia in the southern hemisphere, it is the best place for Oceania to establish measurement and control stations. There are more than 10 measurement and control stations established in China, the United States and Europe.
Of course, opportunities like Argentina are not common. More importantly, the measurement and control stations of countries around the world realize networked shared communications and measurement and control services. When foreign spacecraft and satellite pass over China, we provide communications and measurement and control services. When Chinese spacecraft or space station flew over foreign countries, foreign measurement and control stations provide data to Beijing flight control center.
At present, in addition to NASA, China and many countries have established international networks of measurement and control stations, such as France, Chile and Swedish . At the same time, China's measurement and control stations and measurement and control ships also provide foreign countries with over-measurement and control services, such as Australia's communication satellite and early Iridium .
However, the limitations of the ground measurement and control station are too strong. In addition to the influence of international political factors, they are also affected by geographical factors and weather.
A more complete method is to build a space-based measurement and control network in space and cooperate with the ground measurement and control network.
The United States launched space-based measurement and control network satellites in the 1970s. So far, about 12 satellites have been launched, and 10 are currently in operation.
International Space Station now performs 4K video live broadcast 24 hours a day, which is achieved through these 10 relay satellites.
After the launch of the Shenzhou V spacecraft, China also began to establish our own space-based measurement and control network, named it Tianlian Satellite. Tianlian Satellite is actually a communication satellite in the Ka/S band. There is no difficulty in building such a satellite itself, and the difficulty lies in launching and launching.
Domestic measurement and control satellites operate in geostationary orbits. The geostationary orbit is a very special orbit, which specifically refers to the geosynchronous orbit directly above the Earth's equator. The satellites on this orbit are relatively stationary and there is only one. If you want to deploy satellites on this orbit, you must report them in ITU. After approval, the satellite must be launched within the specified time limit, otherwise it will not be outdated.
Moreover, the orbit is 35,000 kilometers, which is quite far from the earth. This requires the rocket's orbital accuracy to be very high. Although it is easy to enter orbit in China, the United States and Europe, it is still very difficult for many small space countries to launch satellites even if they can pass the ITU approval. For example, India, India failed to launch its first geostationary orbit satellite in 2020, and by now, there is no chance to fill this satellite in 2022.
China launched three sky chain satellites in 2008, 2011 and 2012, one stayed over Africa, one over the Indian Ocean, and one over the Pacific Ocean. When three satellites were networking, China became one of the three countries with global coverage of space-based measurement and control, and surpassed Russia. Currently, only China and the United States have the ability to connect seamlessly in real-time globally.
At present, China's space communication bandwidth is between 4G and 5G. The lifespan of the sky chain satellite is about 5 to 8 years. In the future, we will also launch larger and more advanced sky chain satellites.
