Deformation measurement millimeter level! A Star of Group 01 of Land Exploration No.1 is another weapon for geological disaster monitoring in China.

On January 26, the Land Exploration Group 01 A star developed by the Eighth Academy of Aerospace Science and Technology Group was successfully put into the scheduled orbit by the Long March 4C carrier rocket, and the launch mission was a complete success.

This satellite will operate in a network with the subsequent launch of the B satellite to carry out surface deformation monitoring and topographic mapping, provide support for my country's autonomous satellite detection and prevention of geological disasters, maintain national resources, and protect ecological security.

Double stars fill the gap

Li Ruixiang, commander-in-chief of the 01 group of land exploration, said that this group of satellites is an important part of the medium and long-term development plan of the national civil space infrastructure (2015-2025). The mission of the satellite system is to simultaneously develop two L-band differential interferometric SAR (synthetic aperture radar) satellites of a unified model, the Land Exploration One 01 Group A and B, and to provide in-orbit technical support. The satellite not only has on-orbit mission planning, which requires the acquisition of SAR observation data, but also has corresponding auxiliary collection capabilities, coverage characteristics, and storage characteristics to meet the mission planning and requirements of multiple users.

Li Ruixiang introduced that by carrying L-band multi-polarization SAR, the satellite uses differential interferometric deformation measurement and interferometric altimetry technology to carry out surface deformation monitoring and topographic mapping of designated areas. It has the characteristics of all-weather, all-weather, multi-mode, multi-polarization, etc., and can be widely used in natural resource investigation and monitoring, emergency management, forestry and grassland monitoring and other fields. It is mainly used by the Ministry of Natural Resources, the Ministry of Emergency Management, the State Forestry and Grassland Administration, China Earthquake Administration and other industry departments provide public welfare data services.

"The double star fills the gap of L band land exploration satellite in China." Wei Chun, deputy chief designer and executive chief engineer of Group 01 satellite of Land Exploration 1, said that L-band signals have the characteristics of long wavelength and good penetration, and can penetrate these vegetation-covered areas for plane measurement. Compared with other bands, it is easier to generate interferograms that meet the requirements of deformation measurement. "For example, when observing the forest surface, the L-band signal can pass through a 20-meter-high tree to accurately measure the ground height, instead of only measuring the forest height."

Wei Chun introduced that the Land Exploration 1 satellite is the first satellite to apply the new Beidou 3 system signal, realizing the on-orbit application of high-precision Beidou.

In fact, most of the international L-band SAR in orbit, the satellite navigation system is not working. Because navigation satellites are all L-band satellites, using the same frequency band will cause co-frequency interference. In this regard, the team of the Land Exploration No. 1 satellite took the lead in proposing a pulse technology to solve the problem of high-precision orbit determination under the same frequency interference. Using this technology, the satellite radar can also receive high-precision Beidou signals during the opening period to meet the needs of orbit determination.

According to Zhang Bingliang, the overall chief designer of the satellite, as the first L-band land exploration satellite in China, the dual-star development work is new and difficult, and it also leads a number of key technologies and core components to achieve self-control from scratch. For example, in view of the load characteristics, the satellite uses an air suspension deployment device, which is the first in China. In the development stage, the team made a lot of tooling equipment to carry out test verification, and it took half a year to overcome the difficulties and specially developed the air suspension device.

Can "each side" and "dance ballroom"

Wei Chun introduced that after the two-star network, Star A and Star B can be the "leading role" and "supporting role" of each other, working in the two on-orbit flight modes of following and orbiting, with the surface deformation measurement accuracy reaching millimeter level.

Among them, the follow-up mode refers to two satellites flying 180 degrees apart in circular orbits. In the follow-up mode, the binary can achieve high-frequency sub-Earth observations. "For example, Beijing was originally seen once in 8 days, but now it can be seen once in 4 days." It can continue to conduct encrypted observations to better monitor surface deformation.

When following the flight, the double star can also realize large-scale, millimeter-level high-precision surface deformation measurement, which can serve the emergency investigation of major geological disasters such as earthquake disasters, provide data support for the exploration and prevention of land subsidence, landslide, ground collapse, earthquake deformation and other disasters, and is a "sharp weapon" for early identification of major geological disasters in complex areas ".

Compared with other SAR satellites, the Land Exploration Group 01 satellites have achieved mixed-mode polarization for the first time in China using the L-band. This enables the satellite to have advanced polarization measurement modes such as circular polarization and simplified polarization, which can enhance the satellite's quantitative remote sensing capability and better provide data protection for land use classification and other services.

Double stars also have a fly-around mode. Wei Chun vividly compared the double-star flying mode to "social dancing"-a satellite is flying in orbit, the "dance partner" is flying on its left or right, and the two satellites look in the same direction together.

In the fly-around mode, the two satellites are closely spaced. The fly-around model is mainly used for height measurement, which can observe places that are difficult for geologists to reach, such as forests and steep mountains, and is mostly used in the forestry field, which can provide data protection for vegetation biomass estimation and other businesses. In the fly-around mode, double stars can realize on-board interferometry and multi-mode polarization system design. This is also the first satellite in the world with single-track multi-mode polarization interference technology.

"During the period of orbit, the two modes of follow-up and around-the-fly of double stars can be flexibly switched according to user needs." Wei Chun said, "Compared with Star A, Star B is also equipped with on-board SAR real-time processing equipment, which completes data processing on the star and directly returns images, providing strong support for emergency disaster reduction."

"Five years to develop 3.5 stars"

Satellite from October 20, 2016 project, to the end of last year to complete the development, development lasted 5 years. In five years, the team has developed one initial sample platform, two initial sample payloads and two positive sample satellites, "equivalent to 3.5 satellites developed in five years".

According to the development characteristics of the double star, the satellite command line system innovates the management mode, and reasonably optimizes the post allocation of personnel in the process of satellite assembly and testing according to the idea of "one satellite development team and one set of ground equipment". In a short period of five years, the development task of 3.5 satellites has been successfully completed.

"From the initial sample to the original sample, the team did every test solidly. For one month, they did 20 consecutive days of uninterrupted electrical testing. Among the designers, 60% were young people, and these children worked overtime with us. They had no complaints as long as the development needed." Wei Chun said.

Zhang Bingliang recalled that when the prototype was developed, the SAR antenna had a particularly large area of over 33 square meters. The assembly plant and vacuum thermal test tank could only accommodate one satellite for testing and vacuum thermal test, and the two satellites could not work in parallel. In view of this situation, the development team planned in advance and continuously optimized the development process of the double star according to the actual situation. The team adopts a series-parallel approach to minimize bottlenecks and single points in the process and ensure that the pace of dual-star development matches.

"On the one hand, everyone thinks in one place and works hard in one place, and no matter how big the difficulties are, they can be overcome. On the other hand, the team thinks ahead in terms of technology and planning, plans carefully, and faces and solves problems with a strict and careful attitude." Talking about the experience gained in the development and launch preparations, Zhang Bingliang said.

For example, after entering the launch site, under the condition of only one team, one factory building, one set of test tooling and one set of ground test equipment, the test team, under the careful organization of the project office and the close cooperation of the satellite as a whole and various units, basically completed all the work of the two satellites in the technical area in just over 40 days, during which no safety or quality problems occurred.

In order to launch this time, the Eighth Academy organized the project office and the satellite general assembly unit to conduct field research, demonstration and coordination at the launch site more than a year in advance. These coordination and demonstration work laid a good foundation for the smooth entry of the test team to carry out its work. "Now everyone is very motivated, because the work has been completed relatively smoothly, and they are looking forward to carrying out the task as soon as possible and watching the products they have developed fly into space." Zhang Bingliang said.

This article is authorized to be reproduced from "China Aerospace News", the original title "deformation measurement millimeter level! Land Exploration One 01 group A star for China's geological disaster monitoring to add a weapon", reporter | Song Haowei, map, video | Aerospace Science and Technology Group Eighth Academy