Behind the improvement of this picture is the breakthrough of space communication relay in China. This is how Tianlian satellite was born. Its "by-product" became the hero of Chang 'e IV, and it is still in service!
People who have seen satellite launches and TT&C documentaries will definitely find a problem. It takes time to inject TT&C signal into the satellite, and it must be completed from a certain moment to a certain moment, otherwise it will be lost because the satellite fails to inject TT&C data in time.
Why is this happening? The reason is not difficult to understand at all. This satellite revolves around the earth. For example, the flight time of China's Tiangong Space Station is about 92.2 minutes, and the optimal elevation range of the antenna of the measurement and control station corresponds to the orbit of Tiangong Space Station of 400 kilometers, exceeding 1000 kilometers!
Assuming that the optimal elevation angle of TT&C antenna is 30 degrees and the orbital height of the space station is 400 kilometers, the optimal TT&C range is only about 1400 kilometers, and flying out of this range at a speed of 7.8 kilometers per second is less than 180 seconds, leaving TT&C only 3 minutes. ..
Many friends should say that the next lap will not fly overhead again, right? What's the hurry? It only takes 90 minutes to turn around, just wait for another turn. Here comes the problem. The earth makes a circle every 24 hours and 92 minutes, and one day is 15.6. The distance on the equator will be 2560 kilometers every 92 minutes.
The TT&C area is a circle with the zenith of TT&C antenna as the center and a radius of 700 kilometers (assuming the optimal elevation angle is 30 degrees and the orbital height of the space station is 400 kilometers). 2560 kilometers has already exceeded the TT&C range, so wait for tomorrow!
So if there is no dead angle for 24-hour monitoring, you need monitoring antennas all over the world. Obviously, this is impossible. In order to carry out the Apollo mission, the United States spent 600 million dollars (1.960 years) to build more than 20 ground stations around the world, but even under the most favorable circumstances, it can only cover less than 30% of the earth's orbit.
How can we solve the problem of global measurement and control?
The early system composed of ground station and communication network was the earliest "global TT&C system" established by NASA, but obviously it could only provide TT&C within 90 minutes of orbital time 15 minutes, which still could not meet the demand, so NASA decided to establish MSFN network. Later, due to the birth of the space shuttle program in1970s, higher requirements were put forward for TT&C, and NASA merged the former two into STADAN.
However, STDN needs monitoring stations all over the world, and NASA changed the system, adding TDE and TDW (for the east and west) and an on-orbit standby satellite, which eventually became TDRS.
The above figure shows the current TDRS configuration as of March 20 19, in which 10 satellites are in orbit (four first generation, three second generation and three third generation satellites).
The orbit is roughly located in the geostationary satellite orbit of 36,000 kilometers. Theoretically, only three satellites in this orbit can "see" all the low-orbit satellites in the world. Combined with the configuration of the ground station, the satellite can be monitored and managed without dead angle for 24 hours.
Everyone knows the benefits of TDRS, but there are two problems. The first is the technical problem of establishing this relay satellite, the first is relay communication technology, and the second is antenna alignment. You can't always follow the satellite with the mechanical antenna of that year. What if you want to track multiple satellites at the same time, so the spaceborne phased array antenna is also a problem?
Of course, the most troublesome thing happened a long time ago. Skylink satellite is a measurement and control satellite, and it needs measurement and control itself. Then the problem comes. Before the Skylink program, China's satellite and space program started long ago. How to solve it?
Ground monitoring station and voyage series
The longest distance between east and west in China is more than 4,000 kilometers, with a difference of 60 longitudes, accounting for 360 1/6 of the earth. However, all spacecraft launched into orbit are facing southeast or southwest, and will soon fly out of China's measurement and control area. What should we do?
To build ground stations, NASA can build ground stations all over the world, and they can negotiate as long as they want, but we can't. We can only build Roy series TT&C ships. Yuan Hang 1 was built and launched in Jiangnan Shipyard on August 3, 1977. It has conducted 44 scientific expeditions, sailed more than 440,000 nautical miles in three oceans, completed 57 major scientific research and experimental tasks, and carried out monitoring and control tasks in Shenzhou 5.
From Roy 1 to Roy No.7, and then to Roy No.21and No.22 series TT&C ships, they were all built for TT&C in the world. The Roy series ships are classic and flexible in deployment, but the cost is high, and they need the cooperation of ground stations. 1996 10 10 On October 6th, TT&C Station in Kiribati, China was completed, and the cost of TT&C satellites launched by China for many times was much lower than that of TT&C ships in Roy series.
On June 7, 2003, with the operation of the United States, Kiribati established diplomatic relations with Taiwan Province Province of China, which led to the loss of China's satellite monitoring station in Kiribati. Although this has not affected the development of China's space industry, it is also a setback in the construction of overseas monitoring and control stations.
Since 1996, the construction of overseas monitoring and control stations in China has also entered a good trend. Up to now, there are 10 overseas monitoring stations, as shown in the following table:
TT&C: Skylink Satellite
As early as 1970s, China put forward the idea of developing relay satellites. However, due to the need for technology and funds to match the space program, it was not until June 65438+ 10, 2003 that the Tianlian-1 relay satellite system project was formally established.
Tianlian-1 satellite was launched into geostationary orbit on April 25th, 2008. After the launch, TT&C coverage of China spacecraft in low and medium orbit increased from 12% to about 50%, completing the data relay of Shenzhou VII. On July 6th, 20021year, the launch of the 05-satellite antenna chain No.1 was completed, and the total number of 6-satellite antenna chain series was * * *, covering 100% of the earth orbit measurement and control.
As early as the morning of June 20th, 20 13, the lecture of Shenzhou-10 in space has already made everyone know the role of satellites. During the whole class of 5 1 minute, Shenzhou-10 flew over half the world. If the ground station relays, it must be intermittent, but with satellites, there will be no problem. In this process, the switching of signal relay did not stop.
At present, the number of Skylink satellites has increased to six, and the bandwidth has been greatly improved compared with 20 13 years. The downlink rate of communication has reached 1.2G, which is similar to or even better than the communication quality of the 5G line you use. The data will land at the Beijing Flight Control Center, and then be transmitted to all parts of the world from there.
As you have noticed, there is a wireless router in the core cabin of Tiangong, and astronauts can freely use TCP/IP-based communication devices such as mobile phones, just as convenient as using them at home. Behind this is the support of Tianlian satellite.
In addition, you should also notice that the Queqiao relay satellite prepared for Chang 'e IV is also a breakthrough application of relay satellite technology in China.
Do you still need to build a ground station and an ocean TT&C ship to use Tianlian satellite?
Of course, it is necessary, but relatively speaking, the dependence on ground stations and long-distance series of measurement and control ships will be less, and these satellites are all in the sky, and it is useless for the United States to block them again.
But there is one thing that Tianlian satellite can't solve, such as deep space measurement and control. When China's Zhu Rong lunar rover on Mars transmits data to Earth through its orbit, or transmits measurement and control signals from Earth to Mars, there is no need for Tianlian-1 at all.
Not because their positions or angles are wrong, but because the antennas are not big enough, because transmitting signals to Mars requires large-caliber measurement and control antennas, at least tens of meters, and satellites in near-earth orbit can't reach this level for the time being, so our measurement and control stations and telescope series are indispensable.
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