China’s satellite butler ＂Mu Xing Ren＂ is the satellite in orbit ＂hanging wire to feel the pulse＂

"hanging wire for pulse diagnosis" of satellites in orbit

■ Lu Binghong Zhang Mi Liberation Army Daily reporter Wang Lingshuo

On the eve of the Spring Festival this year, Xi ‘an Satellite TT&C Center successfully completed the health status assessment of 52 Beidou navigation satellites. Through the "physical examination" of the Beidou navigation satellites in orbit, the scientific and technical personnel ensure that the key technical indicators of these satellites meet the requirements of providing various services normally and continue to maintain good working conditions in orbit.

The reason for the "physical examination" of satellites is that satellites in the past days will sometimes have "minor ailments and pains" or even "incurable diseases", which will affect business applications and lead to the failure of the whole satellite. China’s space science and technology workers are deeply involved in the on-orbit diagnosis and maintenance technology of spacecraft to realize early detection, prevention, diagnosis and treatment of satellite faults.

Schematic diagram of injecting maintenance instructions on the ground to satellites in orbit

Satellites in orbit are so delicate.

When it comes to space flight, people often think of the rocket launch that attracts people’s attention and rises in flames. In fact, this is only the prelude to the space mission, and the work really begins when the satellite enters the scheduled orbit and is put into business application in space.

Satellites are the largest number of spacecraft launched by human beings, accounting for more than 90% of the total number of spacecraft launched, and have considerable uses in meteorological monitoring, communication and navigation, disaster prevention and mitigation and other fields. As a complex system composed of thousands of components, satellite has more and more functions and greater value with the continuous development of technology. The times of detection, diagnosis, calculation and treatment, control and disposal, and the telemetry parameters that need to be downloaded and monitored are also increasing. This puts forward higher requirements for the accuracy and stability of the ground control system. However, satellite technology is complex and dense, and it is in a complex space environment, so the probability of failure exists objectively and there are various types.

In 2018, shortly after the latest model of meteorological satellite GOES-17 in the United States was put into orbit, the cooling system failed, which made the imaging system of this satellite unable to provide satellite images in near infrared and infrared channels. Satellites can’t get infrared cloud pictures taken at night in time and effectively, and they become "blind" at night.

Coincidentally. In July, 2019, the European Galileo satellite navigation system lost all 24 satellites due to the timing error of the precise timing facilities located in Italy, and the whole system was once paralyzed. It was not until a week later that the system fully resumed service.

At the same time, satellites operate in a vacuum, weightlessness, ultra-low temperature, strong radiation and full of unknown factors for a long time, and space junk and solar activities will bring trouble and even fatal damage to satellites. Therefore, it is inevitable that there will be some technical problems during the life of the satellite.

What’s more, the cost of modern satellites is high, and with the launch cost, the cost of a satellite is often calculated in hundreds of millions of yuan. If the technical failure of the satellite is not eliminated in time or cannot be eliminated, huge economic losses will be inevitable. Therefore, once the running satellite fails, all countries will spare no effort to rescue the satellite and minimize the loss.

Troubleshooting with "Hanging Wire Pulse Diagnosis"

When there is a problem with the machine on the ground, people can locate and eliminate the fault through various methods. For satellites in orbit, it is the only way for technicians to master the attitude and working state of satellites by means of electromagnetic wave transmission.

The so-called satellite fault diagnosis and maintenance means that when the satellite fails, in order to avoid the paralysis or functional failure of the whole system caused by the failure of some components, the state detection, fault diagnosis, health assessment and other work are carried out quickly through telemetry data, the current state and future "illness" trend of the satellite are analyzed, the satellite fault level is defined, the fault location and cause are determined, and the treatment is carried out in time. This is just like the plot of "hanging the wire to feel the pulse" in Chinese classical novels and traditional operas. With this core technology, ground measurement and control personnel can detect tiny faults hidden inside the satellite as soon as possible, and after fault identification and fault isolation, take corresponding maintenance support or fault degradation treatment, so as to avoid catastrophic failures of the satellite and ensure the safe operation of the satellite.

The diagnosis and maintenance technology of spacecraft is developed with the realistic requirements of spacecraft mission expansion, control accuracy improvement and working life extension. The traditional space powers, represented by the United States and Russia, started earlier and have done a lot of research and practice in this field.

In the 1960s, with mankind gradually entering the space age, the spacecraft diagnosis and maintenance technology was also launched. Due to the limitation of technical conditions, component quality and design level at that time, people can only rely on simple instruments to monitor some key parameters of the satellite with single signal threshold state in this period to judge whether there is a fault. This method provides a certain guarantee for the on-orbit operation of early human spacecraft. At this stage, the average life of spacecraft is generally short. In manned space mission, the safety guarantee system of the system is composed of state monitoring, ground expert consultation and astronauts’ direct participation, and the autonomy, real-time and reliability of the spacecraft fault detection system are also poor.

Since 1970s, with the development of technology and the improvement of component quality, design level and process level, the diagnosis and maintenance technology based on hardware redundancy has been widely used in the field of spacecraft. The diagnosis method has also developed from simple threshold judgment to making full use of various dynamic testing technologies, monitoring technologies and information processing technologies, which has greatly extended the average life of spacecraft.

During the Apollo mission to the moon, the United States first established a flight anomaly analysis and evaluation system. The system includes flight fault monitoring and processing, flight state evaluation, flight data standardization, fault ground simulation, etc., and all emergency life-saving parameters are equipped with triple repetitive sensors and dominant voting logic circuits, which obviously enhances the autonomy and reliability of the spacecraft fault detection system.

In the 1980s, artificial intelligence research flourished, and knowledge-based diagnosis and maintenance technology, especially expert system technology, began to be widely used in the aerospace field. Among them, several intelligent fault diagnosis technologies based on expert system and testability model data-driven are the most representative. At the same time, the appearance of highly complex and reusable spacecraft such as the space shuttle makes the fault diagnosis and maintenance system not only have the distribution of modular functions, but also have the integrity of coordination. With the gradual construction and on-orbit operation of the International Space Station, its fault diagnosis and maintenance system has further greater variability, better adaptability and stronger robustness, and gradually develops in the direction of self-management, so that the failed space station can continue to complete its scheduled tasks without affecting its performance.

"Star Shepherd" guards "China Star"

The management level of long-term in-orbit satellites largely determines the realization of satellite efficiency. Since China launched the first artificial earth satellite in 1970s, the spacecraft on-orbit management and control technology has made great progress through the unremitting efforts of several generations of scientific and technological personnel.

As early as 2007, relying on Xi ‘an Satellite Monitoring and Control Center, China began to build a national-level "satellite general hospital"-spacecraft on-orbit diagnosis and maintenance center, and immediately carried out major scientific research projects in the field of spacecraft on-orbit fault diagnosis and maintenance.

With the increasing number of satellites in orbit, China urgently needs to further improve its on-orbit diagnosis and maintenance capabilities. In 2014, Xi ‘an Satellite TT&C Center established China’s first on-orbit fault diagnosis and maintenance laboratory. Through the research on early identification and location technology, on-orbit fault simulation and maintenance technology, on-orbit reliability growth and life extension technology, the independent innovation ability of on-orbit fault diagnosis and maintenance of spacecraft in China was further improved, which promoted the sustainable development of China’s aerospace engineering and became a solid foundation support for the continuous increase of the number of on-orbit satellites in China.

By collecting and sorting out thousands of fault cases at home and abroad and summarizing nearly one million pieces of diagnosis knowledge, the center independently developed an expert system for spacecraft fault diagnosis, which effectively solved the problems of late discovery, slow analysis and difficult positioning of spacecraft micro-faults.

In addition, starting from Beidou satellite, they independently developed a satellite health assessment system, established a relatively complete three-level health assessment index system of "sub-system, key single machine and key performance", put forward a meta-knowledge construction method of health assessment for single satellite in orbit, and built a detailed "physical examination file" for each satellite. The evaluation conclusions of the satellite system put forward targeted countermeasures and suggestions for the follow-up satellite on-orbit management from the aspects of key components’ operation state, abnormal prevention and response, space environment change and load efficiency guarantee, which effectively ensured the safe and stable operation of Beidou system in orbit.

As the largest long-term management center of spacecraft in China, Xi ‘an Satellite TT&C Center has been managing China’s satellites in orbit for a long time, taking on the on-orbit management tasks such as daily telemetry monitoring, orbit control, fault diagnosis and maintenance, and it is a veritable "big housekeeper" of China satellites. Although the "China Stars" are in space, they are not alone, but are always under the care of the "Star Shepherd".

In the future, Xi ‘an Satellite TT&C Center will also deeply and extensively apply artificial intelligence technology, comprehensively improve the level of diagnostic and maintenance capabilities, and lay a solid technical foundation for the on-orbit management of the "giant constellation" in the future.