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At 5:42 on December 6 (Beijing time), the ascender of Chang'e-5 probe successfully rendezvoused and docked with the orbiter-returner combination. The sample container was safely transferred from the ascender to the returner at 6:12. This is the first time a Chinese spacecraft has carried out rendezvous and docking in lunar orbit.
From the moment the ascender entered the orbit around the moon, the orbiter-returner combination gradually approached the ascender through long-distance guidance and short-range autonomous control, and captured the ascender with holding claws to complete rendezvous and docking.
And then the orbiter-returner combination of Chang'e-5 probe successfully separated from the ascender, and waited for the right time to return to Earth.
Chang'e-5 successfully accomplished the first automatic rendezvous and docking in lunar orbit, and brought back lunar samples. It is the rendezvous and docking microwave radar developed by CASIC that helps China realize the first rendezvous and docking in lunar orbit. The probe cross 380,000 kilometers to realize perfect "hand in space", playing a key role in the first lunar sample return mission of China. The stable and reliable "inertial measurement group", consisting of high-precision accelerometer unit, quartz flexible accelerometer and I/F conversion circuit developed by CASIC, provides high-precision acceleration information for Chang'e-5 and stabilizes the speed during rendezvous and docking.
The lunar sample return mission of Chang'e-5 is the most complicated and challenging mission in China's aerospace field so far. The rendezvous and docking microwave radar of Chang'e-5, developed by CASIC and the probe's only instrument for long-distance measurement in lunar orbit, successfully guided the first unmanned rendezvous and docking operation in lunar orbit. The docking technology is one of the "four key technologies" in the Chang'e-5 mission.
Be More Accurate with "Eyes"
The lunar orbit microwave radar is a pair of products, composed of radar host and transponder, which are respectively installed on the orbiter and the ascender of Chang'e-5 probe. When the orbiter and the ascender was about 100 km apart, the microwave radar started to work, constantly providing the navigation control subsystem with relative motion parameters between the two spacecraft and conducting two-way air-to-air communication. The two spacecraft adjusted their flight attitude according to the signals provided by the radar until the docking mechanism on the orbiter captured and locked the ascender. Subsequently, the lunar samples and container in the ascender were transferred.
According to Sun Wu, Chief Engineer of the rendezvous and docking microwave radar from No. 25 Institute of the Second Academy of CASIC, in the previous manned space engineering missions, CASIC's microwave radars have been applied to the rendezvous and docking by Chinese spacecraft in near-Earth orbit. The five victories in the five missions are suffice to show that China has successfully mastered the rendezvous and docking technology. However, this rendezvous and docking in lunar orbit, around 380,000 km away, is much more difficult.
"Compared with low-Earth orbit, the lunar orbit has no service resources like satellite navigation, and microwave communication is the only means for medium and long distances. To overcome the influence of lunar gravity in the complex lunar orbit environment, automatic rendezvous and docking places have extremely demanding requirements on microwave radar. To this end, the microwave radar team has overcome key technologies such as phase interferometer angle measurement and wide-angle measurement". Sun stressed that the microwave radar should cope with new changes of the lunar orbit environment.
The team led by Sun has made great efforts to accurate wide-angle measurement.
This rendezvous and docking between the orbiter and the ascender of Chang'e-5 is a complex stressing process of "chasing after the small" with huge difference in mass. For this reason, holding claws are used in the docking mechanism to weaken the impact, but this requires a higher degree of precision from the microwave radar in terms of angle measurement.
"We used an innovative error compensation algorithm to further improve the angle measurement accuracy of microwave radar, which greatly enhanced the chances of a successful precise docking", said He Zhongqin, Chief Designer of the microwave radar project.
In addition, the ascender equipped with a transponder for docking will inevitably form invisible lunar dust when the probe lands on the moon, causing interference and seriously reducing the accuracy of angle measurement.
To ensure a safe trip to the moon, a dust cover made of special materials is installed on the transponder. "It's like putting on "goggles" for the "forward-looking eyes" of the Chang'e probe, protecting the "forward-looking eyes" against myopia or even complete blindness", Ji Bo, a young designer, proudly and excitedly said. This is the second time he has participated in rendezvous and docking mission.
Be More Reliable with "Mouth"
Sun Wu said, "We have created "forward-looking eyes" and "attentive ears" for this rendezvous and docking mission. The upgraded radar is smaller, powerful and more reliable".
While ensuring the microwave radar's "duty" (i.e. rendezvous and docking measurement), we have also upgraded its "second job", i.e. two-way air-to-air communication between spacecraft. In other words, we have upgraded the transmission mode of "asking and answering interchangeably" between radar and transponder to the mode of "communication and dialogue" between orbiter and ascender, realizing two-way transmission of telecommand and telemetry parameters.
"It used to be like a radar sent a message and a transponder gave a reply accordingly, just like a teacher calling the roll in the class. Now, radar and transponder not only need to complete communication by themselves, but also to transmit information between the ascender and the orbiter", He Zhongqin said.
The microwave radars applied to the rendezvous and docking in the "Tianzhou" and "Tiangong" missions has achieved weight reduction by half. On this basis, the lightweight design has been further improved this time.
"The weight lost by the rendezvous and docking radar is higher than the weight of the lunar samples. Weight reduction even by one gram is of enormous significance to the lunar sampling task", Sun Wu said with emotion.
The microwave radar performed perfectly without suspense or error in the first docking in extraterrestrial orbit, as it did in the previous five manned space engineering missions. This contributes to the insistence of Sun Wu's team on "zero defect" in product quality and is an interpretation of "get it right the first time".
The growth record of each step and the photo of each frame guarantee 100% process traceability. The radar team repeatedly tested and upgraded their equipment to ensure infallible function and performance. "There is no shortcut. The most effective way is to strengthen the work at the beginning and the end and strictly control the process", Sun Wu said, "We will never stop thinking ahead and back, as long as the launch mission has not been completed".
"Through full verification including seven individual tests, three subsystem tests and five system tests, we and the radar are fully prepared for the mission". He Zhongqin was very excited when the mission was coming. She said, "For the special application in lunar orbit, compared with the previous missions, the radar have additionally experienced lunar dust resistance test, anti-interference test and rendezvous and docking wireless compatibility test verification, and performed well in these tests". To He Zhongqin, microwave radar is like her own child. Before the radar went out to battle, she was full of confidence and expectation.
"We would never send any doubts or defects into space". This is really a reliable product with zero defects.
CASIC's rendezvous and docking microwave radar in lunar orbit performed well in this mission and witnessed the implementation of China's first unmanned rendezvous and docking technology on an extraterrestrial object. As Sun Wu said, "We are ready to go to the moon, go further into deep space, and march forward to a wider field!"
Chang'e-5 successfully accomplished the first automatic rendezvous and docking in lunar orbit, and brought back lunar samples. The stable and reliable "inertial measurement group", consisting of high-precision accelerometers, quartz flexible accelerometer and I/F conversion circuit developed by CASIC, provides high-precision acceleration information for Chang'e-5, and stabilizes the speed during rendezvous and docking.
CASIC's experts said: "To realize automatic rendezvous and docking, very precise control is required to ensure that the relative velocity between the orbiter-returner combination and the ascender is controlled as finely as possible, and the acceleration can reflect speed change. The acceleration during rendezvous and docking is much smaller than (about one in ten million) the maximum acceleration during the lift-off of a probe, and the high-precision accelerometers can realize the precise measurement of tiny acceleration".
In other words, without this "inertial measurement group", we will "pass by" this "hand in space" around 380,000 kilometers away.
The "Leader" Goes on a Journey with "Cushion"
During rendezvous and docking in this mission, the high-precision accelerometers mainly measure acceleration and act as the "leader" of the "inertial measurement group". This is the third time they have accompanied Chang'e to the moon. To provide high-precision and high-reliability measurement performance for Chang'e, the accelerometers are of unique redundant design, ensuring a 0.999983 mission reliability and having once contributed to the successful completion of Chang'e-3 and Chang'e-4 missions with perfect performance.
"In this mission, the accelerometers mainly implement the translational acceleration measurement of the probe in the stages of earth-moon transfer, lunar flight, rendezvous and docking in lunar orbit and moon-earth transfer, to achieve smooth and accurate maneuvering control over the probe during the orbital flight and the rendezvous and docking", said Yu Huanan, Chief Designer of the high-precision accelerometers from No. 33 Institute of the Third Academy of CASIC.
For this "journey", the development team specially added a new component — internal shock absorber. To meet the higher requirements of this flight for the shock resistance of the accelerometers, the team equipped the accelerometers with internal shock absorbers, just like a set of vibration absorption and isolation "cushions" for the accelerometers. With these "cushions", the accelerometers can still work normally in a relatively stable environment even if there are stronger impact and vibration in the external environment.
However, the design and installation of shock absorber is not as simple as "cushions". Fu Jibo, a designer from No. 33 Institute of the Third Academy, said: "The shock absorbers are installed in different positions of the accelerometers, just like cushions at four corners of a cabinet. Any impact can give rise to a slight difference in the deformation of the shock absorbers, just like that the "cushions" are of different heights. As a result, the accelerometers will rotate, greatly affecting the measurement accuracy, no matter how slight such rotation is". To eliminate the rotational deformation as much as possible, the team conducted impact tests repeatedly in more than two months, and thoroughly figured out the deformation of the shock absorbers in different positions, different impact conditions and different installation methods, and finally controlled the deformation at the angular second level, ensuring the measurement accuracy of the accelerometers not affected.
"Gold Partner" Guarantees Acceleration in the Whole Journey
The quartz flexible accelerometer and the I/F conversion circuit in the "inertial measurement group" are a pair of "golden partners". In this mission, the "golden partners" undertook all acceleration measurement tasks at various key stages in the whole journey of Chang'e-5 to the moon. They were not only used in the high-precision accelerometers, but also played a key role in the inertial measurement units (IMUs) of the orbiter-returner combination and the returner.
As the core devices for acceleration measurement, this pair of partners can be described as "meritorious artifacts", and have once successively contributed to the successful completion of the 11 Shenzhou missions as well as the "Tianzhou", "Chang'e-3" and "Chang'e-4" missions. With high-reliability and high-precision measurement signal conversion capability, the I/F conversion circuits have once been successfully applied in the Long March-4 and Long March-6 launch vehicles and the new-generation manned spacecraft and a cargo return capsule for test.
The quartz flexible accelerometer is a key sensor that is sensitive to acceleration, while the I/F conversion circuit can convert the current signals output by the accelerometer into digital pulse signals suitable for computer processing. Considering different tasks undertaken by different parts of the probe, the development team equipped the accelerometers with I/F conversion circuits with different ranges. "The process of signal conversion is like "weighing" signals with scales and weights. If current signals vary greatly with the acceleration, large "weights" are required", said Zhao Hongli, Chief Designer of I/F conversion circuit.
Considering a little acceleration change of the orbiter during rendezvous and docking and orbit control, the team used a small-range conversion circuit, which could provide acceleration measurements with higher precision under the same conditions, to achieve more accurate speed control of the probe. In the lunar landing and reentry stages with great acceleration change, the team adopted a large-range conversion circuit with large weight to ensure full coverage of acceleration information and ensure the speed change under control in the whole flight process of the Chang'e probe.
