With an error of one second, this clock directly affects the navigation and positioning accuracy of Beidou satellite

The atomic clock is the “heart” of Beidou navigation satellite, which directly determines the accuracy of navigation and positioning. If the satellite has a time error of one billionth of a second (1 nanosecond), a ranging error of 0.3 meters is generated. As one of the three key technologies of the new generation beidou navigation satellite, the passive airborne hydrogen atomic clock developed by the Shanghai Observatory of the Chinese Academy of Sciences has only been awarded the first prize of Scientific and Technological Progress in Shanghai in 2019.

With an error of one second, this clock directly affects the navigation and positioning accuracy of Beidou satellite

Developed and completed multi-Taipei Doo-group network satellite hydrogen clock

According to the principle of quantum mechanics, atoms have discontinuous energy values, and when atoms jump from one energy level to another, the frequency of electromagnetic waves they absorb or release is fixed, and the atomic clock is a tool for timing electromagnetic waves that use atomic leaps to produce fixed frequencies. The satellites of the four global navigation systems, GPS, Glonas, Beidou and Galileo, are equipped with high-performance atomic clocks, including the xenon clock, the xenon clock and the hydrogen clock. Among them, hydrogen clock has the characteristics of good frequency stability and low drift rate, which is very beneficial to the improvement of navigation signal accuracy.

How high is the accuracy of a hydrogen clock as a precision timer? “A mechanical watch has a one-second error a day, a quartz watch has a 0.1-second error a day, and a hydrogen clock has a 1-second error for millions or even 10 million years. “The head of the Beidou 3 satellite on-board hydrogen clock project, The Shanghai Observatory Shuai Tao researcher, told the Liberation Daily.

At present, only the European Galileo satellite is equipped with the onboard hydrogen clock, compared with China’s onboard hydrogen clock, the two ground test performance is comparable, but from the comprehensive performance in orbit, China’s onboard hydrogen clock to achieve a smaller user ranging error.

With an error of one second, this clock directly affects the navigation and positioning accuracy of Beidou satellite

Shanghai Observatory’s onboard hydrogen clock development team

Shanghai Observatory has a good research foundation in the field of time frequency, since the 1960s to undertake China’s world time-granting work, the 70’s developed China’s first ground-based active hydrogen atom clock. For the application of Beidou navigation satellite, the Shanghai Observatory launched the development of China’s first on-board hydrogen clock in 2002. In 2010, the On-board Hydrogen Clock Project Group, with the support of the Chinese Academy of Sciences and Beidou, jointly carried out the engineering research and development of the on-board hydrogen clock in conjunction with the Shanghai Institute of Aerospace Electronics Technology and the Shanghai Institute of Technical Physics of the Chinese Academy of Sciences. In September 2015, China’s first onboard hydrogen clock, developed by the Shanghai Observatory, was applied to the sky with a new generation of Beidou navigation satellite. In the construction of Beidou 3 global system, the Shanghai Observatory has developed and completed several network satellite on-board hydrogen clocks, the long-term forecast accuracy is higher than the star-borne clock a magnitude level, greatly improving the time-benchmark accuracy of Beidou navigation satellite system.

All the core components are domesticated

China’s first on-board hydrogen clock pioneered and broke through a number of key technologies, the core components are all domestic, the realization of the navigation satellite “heart” completely autonomous and controllable.

Hydrogen clock in the working environment in space there are certain temperature fluctuations, and circuit parameters are sensitive to temperature. In order to realize the long-term stability of hydrogen clock, researchers at the Shanghai Observatory pioneered the hydrogen clock time dual-frequency modulation technology, which effectively reduced the sensitivity of the hydrogen clock output frequency to the change of the correction signal phase, so that the temperature coefficient index reached the international advanced level.

For the first time, the research team applied the original electrode microwave cavity on the hydrogen clock, which was 10% lighter than the foreign magnetron microwave cavity, and the signal gain of the physical system was higher, which laid the foundation for the realization of the hydrogen clock high stability index.

What if the hydrogen clock suddenly fails in the air? “Don’t worry, the satellite is equipped with a time-frequency generation and hold system, you can achieve a seamless switch between the main atomic clock and the backup atomic clock, switching before and after the satellite time change is less than 20 picoseconds (1 pilya equals one trillionth of a second), the corresponding user ranging error is less than 1 cm.” That means that if the satellite switches the atomic clock on the star while driving, the user is completely unaware that the navigation altrudesis has changed, Shuai Tao told the Liberation Daily.

In order to meet the needs of the next generation of high-integration navigation satellite development, the research team, while ensuring performance indicators, reduced the weight of the hydrogen clock by about half. The first generation of onboard hydrogen clock 24 kg, just completed this year only 13 kg, power consumption is also reduced by about 10%. In the future, in addition to navigating satellites, hydrogen clocks will also be used in scientific experiments such as very long baseline interferometry in space.

From the launch of the onboard hydrogen clock in 2002 to the first on-board hydrogen clock in 2015, the Shanghai Observatory development team has come a long way. Especially in the early star-borne hydrogen clock identification products and decent product development that period, working overtime 2 or 3 hours a day is “home-cooked”, simulation analysis, test verification, improvement and promotion, iterative verification, time after time modification, time after time perfect. “There will be times of irritability, but as long as there is a little progress, everyone will be excited for a few days, encourage each other, continue to move forward.” “