Beidou satellite successfully networked, time accuracy is the life gate of satellite navigation. The more synchronized the time between heaven and earth and the smaller the error, the higher the accuracy of positioning. The on-board atomic clock, known as the "heart of the navigation satellite", plays a role in providing a time benchmark. So, what is an atomic clock? Why is it important? What achievements has China made in the research and development of atomic clocks? In 2018, Xie Jun, deputy chief designer of the "Beidou" No. 3 project and chief designer of the satellite, introduced us to the difficulties and technical research encountered in the development of atomic clocks in the Beidou three-step project in an exclusive interview with Wu Xiaoli.
What is an atomic clock?
At the heart of navigation is time measurement. In the constellation coordinate system built by navigation satellites, each constellation has its own coordinate information, and by measuring the distance between us and the constellation, we can solve our position in that coordinate system. With a known speed of signal propagation (the speed of light), the more precise the time measurement, the more precise the position solving.
In a satellite navigation system, a time error of 1 nanosecond (parts per billionth of a second) will result in a distance error of 0.3 meters. To achieve such a precise time measurement, only an atomic clock can do it. The atomic clock is a tiny quantum component of the atom, which realizes the "ticking" sound of ordinary clocks. The difference is that this "tick" is more uniform, manifesting as a form of electromagnetic wave, usually inaudible, and can only be output through microwave signal processing techniques.
If we compare atomic quantum components to large ropes, when we inject an electromagnetic wave into it so that the electromagnetic wave resonates with the atomic binary, the oscillator that produces the electromagnetic wave can be locked onto the atomic binary transition frequency. The oscillator outputs a high-precision frequency signal for precise time measurements. At present, hydrogen, rubidium and cesium atomic clocks commonly used in navigation satellites are based on this working principle.
China's atomic clock
The accuracy of the new generation of domestic atomic clocks configured on the Beidou-3 satellite is an order of magnitude higher than that of the previous atomic clocks on Beidou-2, and it is said that there is only an error of 1 second in 3 million years.
Wu Xiaoli: Above the navigation satellite, the atomic clock is also a life gate.
Xie Jun: Yes, atomic clocks are very, very critical. If the atomic clock is not accurate, the entire space-time benchmark cannot be discussed. The atomic clock we use on The Big Dipper one is bought from the United States. Then at Beidou-2, we started talking to Europe, and Europe's own atomic clocks were not in orbit and had not been verified.
Wu Xiaoli: Did we take it back and use it later? What are the results of the use?
Xie Jun: I know the matter of technological disclosure myself, so you don't want to talk about it at all. Including when we hope that it can give me some particularly key indicators, he said that I can only reach this indicator, and no matter how high I am, I will block your technology, so basically as a technology transfer or something, don't think about it. The finished product will not work for us either.
From today's point of view, even if we want a higher index product, in fact, it did not have it at that time, and it could not be made. When we sent our first navigation satellite in 2007, we actually used four rubidium atomic clocks, all of which were our own. But when we launched the navigation satellite in 2010, we used part of its atomic clock.
Xie Jun: But at the same time, we have also launched our own technology to develop atomic clocks. Since the "Eighth Five-Year Plan" period, our country has actually had some good foundations for ground atomic clocks. But the atomic clock on the ground is to be moved to the stars, and there are many, very many problems: one is that the volume and weight of the atomic clock must not be particularly large; the other, it requires no lower indicators than the ground. This irradiation, thermal environment, and mechanical environment of the etheric space are very complex. When the Beidou-2 project was first established, based on the basis of our country's on-board atomic clock, we still decided to buy a part of the foreign atomic clock and independently develop a part of the atomic clock.
Wu Xiaoli: What about Beidou No. 3?
Xie Jun: Since the Beidou No. 3, we have all used our own rubidium atomic clocks. Based on our understanding of the company's products in Europe, when the annual meeting was held in Xi'an in 2015, the boss of this company joked with me, you buy my atomic clock later, I said I will not buy yours, unless your atomic clock is two orders of magnitude better than my atomic clock, and the price is twice as low as mine.
We are in Beidou-3, based on the development of the entire satellite navigation technology, and the level of hydrogen atomic clock technology in our country, we are now also using hydrogen atomic clocks. Hydrogen atomic clock is relative to rubidium atomic clock, its size is larger, but its electrical performance index is higher and better.
In order to improve the accuracy of the service and the reliability of the satellite in orbit service, the Beidou-3 satellite has also taken a number of reliability measures to make the design life of the satellite reach 12 years, reaching the advanced level of international navigation satellites, which provides a basic guarantee for the continuity and stability of the Beidou system service.
For more excitement, pay attention to the special program of "Q&A Shenzhou"
"Into the Radiant Big Dipper"
Q&A Beidou-3 project deputy chief designer, satellite chief designer Xie Jun
Choreographer: Jenny Sue
Editors: Meng Han, Li Wei