Editor's note: This year marks the 75th anniversary of the founding of the People's Republic of China. Three-quarters of a century has witnessed a large country in the East with a large population and a poor and white country, from the "backward era" to "catching up with the times" and then to "leading the times", standing tall among the nations of the world.
From now on, Jiefang Daily· Shangguan News and the Bureau of Veteran Cadres of the Municipal Party Committee will jointly launch a series of oral history interviews to chat with many veteran comrades in various fields and fronts in Shanghai about the extraordinary history they have experienced. Through their memories, we are more and more convinced that "China's yesterday has been written in the annals of human history, China's today is being created in the hands of hundreds of millions of people, and China's tomorrow will be even better."
"I saw that you asked a lot of questions in the interview outline, and some of them were not very accurate." After Qian Zhihan sat down, the first thing he said was "scientist".
He explained with a smile: "A lot of my work at the observatory you mentioned, I didn't do it personally, but everyone cooperated. ”
The 89-year-old man in front of him is a radio astronomer and an expert in lunar and deep space exploration VLBI (Very Long Baseline Interferometry) orbit. In 1976, he joined the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, where he studied VLBI technology in the Radio Astronomy Laboratory.
On June 25 this year, Chang'e-6 successfully returned with 1,935.3 grams of lunar samples, achieving the feat of the first human sample return from the lunar back. Behind this, VLBI real-time tracking track measurement plays an important role.
Sun Jiadong, the first chief designer of the lunar exploration project and academician of the Chinese Academy of Sciences, once commented that the addition of VLBI has advanced the establishment of the mainland lunar exploration project by at least five years. If VLBI is not added, to solve the problem of short-arc high-precision orbit determination, several more large-scale TT&C stations need to be built, which will take at least 5 years.
From participating in the compilation of the overall technical scheme of China's VLBI survey network, to conducting the mainland's first VLBI international joint test, serving as the chief designer of the overall technical subsystem of the VLBI orbit measurement subsystem of the Chang'e-I project, to the Chang'e-2 flight around the moon, Chang'e-3 made a soft landing on the lunar surface, and Chang'e-4 achieved the first soft landing on the far side of the moon...... Qian Zhihan has experienced too many important moments and witnessed the innovative development of VLBI technology in China.
After a lifetime of scientific research, he has been retiring and maintaining exchanges with his counterparts at home and abroad, and cooperating with colleagues at the Shanghai Astronomical Observatory to publish a new professional book of 500,000 words next year.
Looking back on the nearly 50 years of work at the Shanghai Observatory, Qian Zhihan told the story.
Biography: Qian Zhihan, radio astronomer, VLBI orbit survey expert of lunar and deep space exploration engineering. Born in Shanghai in 1935, he entered the Department of Aerial Photogrammetry of the Institute of Surveying and Mapping of the Chinese People's Liberation Army in Beijing in 1953, and later stayed on to teach. Since January 1976, he has been working at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, engaged in VLBI research, and is one of the pioneers in the construction of this field in mainland China and its application to astronomy and geodynamics and deep space exploration.
Pursuit of the ultimate resolution
Shangguan: When you joined the Shanghai Astronomical Observatory of the Chinese Academy of Sciences in 1976, what was the research environment like?
Chin Chi Han: When I was in high school at Gezhi Middle School, I was greatly influenced by the spirit of "learning from things", and I aspired to become an engineer in the aviation industry. Later, I was admitted to the Department of Aerial Photogrammetry of the People's Liberation Army Institute of Surveying and Mapping, and enrolled and joined the army in October 1953.
You asked me why I studied and worked at the PLA Institute of Surveying and Mapping, so why I went to study radio astronomy. In fact, when we do military surveying and mapping, although it seems that it is to measure ground coordinates and draw military maps, we also need to rely on stellar surveying to determine the location.
When I came to the Shanghai Astronomical Observatory in January 1976, my first job was to participate in the overall design and demonstration of the construction of China's VLBI measurement network. I was 41 years old at the time, and at that time, there was a general confusion and confusion in society. I haven't thought about my future and direction completely, but from the perspective of my interests and personality, I really hope to calm down and do some scientific research. The scientific research atmosphere at the Shanghai Astronomical Observatory is very pure, and I have devoted myself to my work as soon as I arrived.
Shangguan: How did you come up with the idea to start VLBI research?
Astronomy mainly pursues two aspects in technology, one is sensitivity, that is, it can see farther, and the other is called resolution and positioning accuracy, that is, it is to be able to observe extremely small celestial details and precise positioning. Celestial bodies in the universe are extremely far away from us, and celestial bodies such as quasars and radio galaxies can only see one bright spot in early observations. In order to study the internal structure and brightness distribution of these objects in depth, there is an urgent need to improve the resolution of observations.
Radio astronomy is all about the ultimate resolution. The larger the aperture of a radio telescope, the higher the resolution, but there is always a limit to the aperture. If two or more radio telescopes with independent atomic frequency scales are combined into a "giant telescope" for observation, this is VLBI technology. The distance between the two telescopes that are the farthest apart is the equivalent aperture of this "giant telescope". The distance between two stations, also known as the baseline. The longer the baseline, the clearer it can be seen and the higher the positioning accuracy. So when we say "very long baseline", we mean the baseline length, very long.
Since its advent in the late 60s, VLBI technology has been the highest spatial resolution of all astronomical observation techniques, hundreds of times higher than that of the Hubble Space Telescope. At present, VLBI technology has achieved micro-arcsecond resolution for observing black holes and other celestial objects. The first image of the M87 black hole was obtained by astronomers with 8 VLBI radio telescopes around the world, and the baseline they formed was up to 12,000 kilometers, close to the diameter of the Earth, which is also the longest baseline that we humans can obtain on the Earth.
The development of VLBI in China was first proposed by Mr. Ye Shuhua (academician) of the Shanghai Astronomical Observatory in the early seventies. At that time, as soon as Mr. Ye resumed his work, he immersed himself in the library to consult international scientific and technological literature. She found that new technologies such as Very Long Baseline Interferometry (VLBI) and Laser Human Health Ranging (SLR) have emerged in the field of international astronomy, which are ten or even more than 100 times more accurate than classical techniques. At that time, our country still used the old technology of classic small optical telescopes to measure the position of stars and measure the world time. Mr. Ye judged that this old technology would be obsolete, and if it did not catch up, the mainland would lag far behind the international level in this field, so she put forward a proposal to build a VLBI measurement system in China.
Under Mr. Ye's leadership, our project team conducted in-depth research on the necessity and feasibility of the project, mainly including: construction and principle verification test of experimental VLBI system, international VLBI test observation, and feasibility investigation of equipment development or introduction. After 5 years of pre-research work, we finally completed the overall scheme design and scheme demonstration report of the project in 1978.
Shangguan: It took 5 years to pre-research, is it more difficult to officially promote it?
Qian Zhihan: Yes, at that time, there was a big gap between our scientific and technological level and the international advanced level, and the national economic situation was not very good, and the VLBI measurement system was a high-tech project, and it was very technically difficult to build a VLBI measurement system for three observation stations equipped with large-aperture radio telescopes, and the cost was tens of millions of yuan. This is a huge expense, so it has also been questioned by some people. A leading expert privately asked me, this high-tech cost is so high, are our current conditions suitable for construction?
Under the leadership of Mr. Ye, we fully discussed and demonstrated the overall technical plan for the construction of VLBI, and proposed to build observation stations in Shanghai, Kunming and Urumqi according to the national conditions of the mainland to form China's VLBI network. This triangle basically covers the land area of the mainland, and the longest baseline is about 3,200 kilometers from Shanghai to Urumqi. I am responsible for the writing and reporting of the main report of the overall plan demonstration meeting, and I still remember very clearly that in late December 1978, we passed the overall plan review meeting jointly held by the Chinese Academy of Sciences and the former Fourth Machinery Department, and the Chinese Academy of Sciences officially approved the project and started construction after the review meeting.
In order to compile the overall plan for the construction of VLBI in China, I read a lot of foreign literature and materials, and understood that VLBI will not only make a major breakthrough in astronomy and geodynamics research, but also play an important role in aerospace engineering, and hope that one day, China's VLBI equipment will also be used in the mainland's aerospace project. In October 1987, the Sheshan 25-meter aperture radio telescope in Shanghai was finally completed, and we were really excited to stand under such a large telescope. It was the largest radio telescope in China at the time, and we were able to compete with other VLBI observatories in the world in the very long baseline field.
Shangguan: What are the specific challenges in the process of construction?
This is the first time that we have engaged in such a big thing, VLBI technology is completely blank on the mainland, there is no domestic experience to learn from, and some countries in the world have also imposed restrictions and embargoes on the mainland on certain key technologies and equipment, so it is very difficult to develop them. In 1979, the exchange rate of the US dollar against the renminbi was about 1:1.8, and then it rose all the way to 1:8. However, our projects are budgeted in RMB, so the funds are always insufficient when importing equipment, and the money is not enough to do it. Due to financial reasons, the construction of the Kunming Observatory was put on hold. We have found a lot of manufacturers and thought of a lot of ways, such as buying fewer parts, arranging special people to study and develop themselves, etc., saving a lot of money.
In the late 80s of the 20th century, the Shanghai Astronomical Observatory cooperated with the former Urumqi Astronomical Station (now the Xinjiang Astronomical Observatory) to build China's second 25-meter radio telescope in Nanshan, Urumqi. In order to reduce the influence of temperature, the main surface measurement of the large antenna must be carried out at night, when it was autumn, the altitude of Nanshan Station was 2000 meters, and the temperature at night had dropped to nearly zero degrees, and the conditions were still relatively difficult.
The Nanshan station was completed in 1994, and China's VLBI network finally had its first baseline. This is just the first step, the next thing is to use. When we first started participating in the international joint test, we were under a lot of pressure, because the technology was very transparent, and we could see the details of each station in the data processing report of the joint observation, which country was good and which country was bad, and no one could hide it at a glance.
During the early operation of the 25-meter VLBI observatory in Sheshan, Shanghai, I organized observations almost every time in the field. A commonly used observation mode is 24 hours a day, the operator takes shifts to carry out the observation, and I and the equipment maintenance personnel carry out inspections and solve problems in time.
Not far from the radio telescope, there is a small building for living purposes, where the staff eats and rests. The noise of the 200-ton 25-meter antenna is relatively loud, and I am often so noisy that I can't sleep when I am resting, but if I really can't hear the sound suddenly, I will wake up when I sleep: why is there no sound? Is the antenna or other equipment malfunctioning? Hurry up and run to see it with a flashlight. So I can't sleep even if it's noisy, and I can't sleep without sound, so I'm afraid that there will be a problem. If the observations are not successful or the quality of the data is not good, this is not a personal problem, it will affect the reputation of the country, so it is necessary to do everything possible to get it right.
Qian Zhihan explaining VLBI (photo by Liu Xueyan)
It's ingenious, it's bold
Shangguan: When it comes to lunar exploration, is there more pressure, especially since the "Chang'e Phase I Project" is the first lunar exploration in the mainland, how to ensure that the surveying and mapping is "foolproof"?
Qian Zhihan: It's been a lot of pressure. At the end of the 20th century, the state officially organized the demonstration of the lunar exploration project. At that time, the measurement and control of lunar exploration spacecraft was a major problem. At that time, the mainland's spacecraft tracking and control equipment was designed and built according to the earth's satellite, and the maximum distance was about 80,000 kilometers, but after the Chang'e-1 satellite entered the orbit around the moon, the maximum distance was 400,000 kilometers. Obviously, the original measurement and control equipment cannot meet the needs of lunar exploration, and it will take many years to upgrade and transform.
How to break through this technical bottleneck? The Shanghai Astronomical Observatory put forward a proposal to apply VLBI technology to real-time tracking and orbit measurement of lunar exploration satellites, and combined it with the existing aerospace tracking and control equipment (appropriate modification) in the mainland to jointly complete high-precision orbit determination and positioning. The original ranging and velocity measurement method of the original TT&C equipment has the advantage of radial measurement, while the advantage of VLBI is transverse measurement, and the combination of the two is "one plus one is greater than two", which can determine the instantaneous three-dimensional position of the spacecraft and realize the precise orbit determination of the short arc.
The proposal is ingenious, but also bold. United States is the first country in the world to use VLBI for spaceflight, and has used radio astronomical interferometry to measure the route of the lunar rover in the Apollo manned lunar landing program, and the mainland is the first country to use VLBI for real-time orbit measurement of lunar exploration spacecraft. At that time, when Mr. Ye gave a report at an international conference and talked about the mainland's use of VLBI technology to carry out real-time orbit measurement of lunar satellites, with a lag time of no more than 6 minutes, everyone was stunned, because the astronomical community did not do so.
For a long time, astronomical observation data has had enough time to be processed and analyzed, and it is not too late to spend days or weeks; If the observation fails today, it can be observed tomorrow as long as it is not a transient target. However, when used in the lunar exploration project, the orbit observation data must be used for positioning or orbit determination of the spacecraft in a short time, and no errors or observation failures are allowed in the observation data. Because orbit measurement errors may lead to the failure of space missions, it is necessary to meet the requirements of high reliability, high real-time performance and high accuracy of aerospace engineering.
The data processing center receives the data sent by the four observation stations in real time, with a total amount of hundreds of megabits per second, and the cross-correlation processing of 6 baselines and multiple channels is carried out, and then the VLBI delay and delay rate observation measurement of each baseline are extracted, and then various error corrections and satellite angular position calculations are carried out.
At that time, at the discussion meeting on the overall technical plan of the first lunar exploration project, some people questioned, are you completely sure? We have done a lot of verification and promised that the VLBI orbit measurement data provided to the Beijing Aerospace Flight Control Center will not lag more than 10 minutes.
In fact, in 2007, we built China's VLBI orbit survey network with "four stations and one center", covering radio telescopes in Sheshan in Shanghai, Urumqi in Xinjiang, Miyun in Beijing and Kunming in Yunnan. At the same time, the VLBI data processing center was built at the Shanghai Astronomical Observatory, and a high-speed data communication line was established in cooperation with the telecommunications department, which could effectively ensure the real-time transmission of observation data. After the launch of Chang'e-1 this year, the VLBI orbit measurement data was sent to the space control center within 6 minutes, and the orbit measurement task was completed beyond the target.
Kamikan: Isn't that time shorter now?
Qian Zhihan: It's faster now, and it can be done in less than a minute. This is our specialty, and it is currently the highest level in the world. From Chang'e-1 to Chang'e-6, the accuracy of VLBI delay measurement has increased by more than 5 times, which is the result of a combination of many improvements.
In 2012, the completion of the Shanghai Tianma 65-meter antenna radio telescope greatly improved the sensitivity and measurement accuracy of the VLBI measurement network in China. In addition, in the Chang'e-3 project, we also measured the relative position of the lunar rover and the lander with an accuracy of 1 meter, which is also the highest level in the world.
Look up at the stars and keep your feet on the ground
Kami: At the same time as the construction of the very long baseline, you were also involved in the observation mission of Halley's Comet?
Qian Zhihan: Yes, in 1984, during the return of Halley's Comet, our research group cooperated with the Astrophysics Teaching and Research Group of Peking University, and at that time, the Shanghai 25-meter antenna was still being debugged at the Meixian test site in Shaanxi Province, so we went there to observe the OH spectral line of Halley's Comet.
Halley's Comet OH spectral intensity is only a few flow units, our antenna has a high system noise temperature in the 18 cm band, the spectral line receiver is also the first development, the performance is relatively poor, and the working conditions of the test site are also poor, so it is very difficult to use a 25-meter antenna to observe Halley's Comet OH spectral line under such conditions.
We then stayed there for almost a week, making continuous observations in visual time, and finally detected Halley's Comet OH spectral lines. This is the first successful observation of comet spectral lines on the mainland, and it is a good start for the radio spectral line observation of celestial objects on the continent in the future.
Qian Zhihan talks about international cooperation (photo by Liu Xueyan)
Shangguan: You mentioned that VLBI is a project with close international cooperation, how did you communicate with international counterparts during the research process?
Qian Zhihan: VLBI technology is a high-precision technology, and when we first started researching, there was a complete blank in China, and everyone only knew what to do, but they didn't know how to do it. In 1978, when Mr. Ye received Guo Zongfen, a Chinese-American scientist who had returned to China for a visit, he introduced the work, and the other party was very surprised: Can you do such a difficult thing by yourself behind closed doors?
With his help, the Shanghai Astronomical Observatory organized a delegation to visit the United States for the first time. When they learned that China was preparing to develop VLBI technology, their United States counterparts were taken aback, because United States has only been developing this technology for about a decade and is not mature. But this experience accelerated our R&D progress and opened the door for the Shanghai Observatory to "come in and go out".
Later, the former director of the Radio Astronomy Laboratory and I went to the United States for the first time to attend the VLBI academic conference and reported on the construction process of China's first VLBI observatory. At that time, foreign countries still felt that China was quite backward, and they were very surprised that we were able to do this, and they were willing to cooperate with us.
At present, the technical capability of VLBI in mainland China has been continuously enhanced, and it has reached the international advanced level, and has formed a distinctive Chinese character. In addition to the VLBI orbit survey tasks of the lunar and deep space exploration projects on the mainland, the VLBI stations of Tianma, Sheshan, Urumqi and Kunming also undertake the VLBI or single-antenna observation tasks of domestic astronomical research. At the same time, it is also a member of the East Asian VLBI network, the European VLBI network, and the international geodesy/astrometric VLBI network, and undertakes international VLBI observation tasks.
Shangguan: Chang'e has won six victories in six lunar explorations, and the first Mars exploration mission in 2020 has also been successful, and VLBI orbit measurement has played an important role, how can it be so successful every time?
Qian Zhihan: The common people think that what our country has put in the sky will definitely succeed, but in fact, every time they are on the scene, everyone is very nervous. There are so many factors that affect it, even if you carefully consider 100 factors, you are still worried about whether there will be a 101st accident. The moon is close, and after a successful capture, the ground knows the message almost in real time. Mars is 400 million kilometers away from the Earth, and the longest time for the signal to be successfully transmitted back to the Earth is 22 minutes later, which is also a great psychological test for scientific researchers. Before the implementation of each task, full preparation is carried out, each equipment is carefully inspected, and a trial run is carried out; In the implementation of observation, the division of tasks is clear, and the operation is careful to ensure that there are no errors.
Now that deep space exploration is developing rapidly, the previous observation target of the VLBI network is once a few years, but now it is several a year, and even two target detectors in different celestial regions need to be measured at the same time in a mission, and the existing "four stations and one center" can no longer meet this demand. The Shanghai Astronomical Observatory is building two 40-meter aperture radio telescopes in Shigatse, Tibet, and Changbai Mountain, Jilin. After completion, China's VLBI network will form "six stations and one center", the longest baseline will be extended to 3,800 kilometers, and the comprehensive measurement capacity will be doubled.
In August this year, Qian Zhihan took a photo at Sheshan 65-meter station
Shangguan: You work overtime almost every day, how do you maintain such enthusiasm?
Qian Zhihan: At work, Mr. Ye is my role model. In August 2018, the 30th Congress of the International Astronomical Union (IAU) was held in Vienna, Austria, and Mr. Ye made a special trip to Vienna to recommend the "Space Low Frequency Radio Astronomy Observatory" project to her international peers and to understand the new progress in international astronomy.
She was 91 years old that year, and I accompanied her to the conference. The IAU conference is held every three years, and this conference lasts for 12 days and is very large, with two or three thousand people attending. There are both plenary meetings and various professional chapters, and the daily schedule is full. During the conference, Mr. Ye always rushed to the venue early every morning to participate in a full day of meetings, and sometimes even attended and communicated in the evening. At such an advanced age, she still does not know how to work tiringly, and this spirit of "living to old age, learning to old age, and working to old age" is admirable.
There are many similar examples, and it is very profound for my education, and I practice it as well. I believe that a person must contribute to society and the country in order to be valuable. In a limited life, doing as much as possible is equivalent to prolonging life, if you only do a few things, in layman's terms, you are living in vain.
Editor-in-chief: Zhang Jun
Source: Author: Liu Xueyan