Application system and data description of the China Seismo‐Electromagnetic Satellite

The China Seismo‐Electromagnetic Satellite, launched into orbit from Jiuquan Satellite Launch Centre on February 2nd, 2018 , is China's first space satellite dedicated to geophysical exporation. The satellite carries eight scientific payloads including high‐precision magnetometers to detect electromagnetic changes in space, in particular changes associated with global earthquake disasters. In order to encourage and facilitate use by geophysical scientists of data from the satellite's payloads, this paper introduces the application systems developed for the China Seismo‐Electromagnetic Satellite by the Institute of Crustal Dynamics, China Earthquake Administration; these include platform construction, data classification, data storage, data format, and data access and acquisition.


Introduction
The China Seismo-Electromagnetic Satellite (CSES, also called ZH-1) is China's first satellite dedicated to geophysics; it is also the first component of the space-based Chinese earthquake stereoscopic monitoring system (Shen XH et al., 2018). Its scientific goal is to preliminarily explore the characteristics and mechanisms of changes in ionospheric response before and after earthquakes, based on real-time monitoring of changes in the state of the space electromagnetic environment. In addition, it helps to study the earth system, especially the interaction and effects of the ionosphere with other related Earth's spheres. There are 8 payloads on the satellite (Figure 1), which are divided into 3 categories, including electromagnetic type of high-precision magnetometers (Cheng BJ et al., 2018), search-coil magnetometers (Cao JB et al., 2018), and electric field detector (Huang JP et al., 2018), insitu detection type as plasma analyzer and Langmuir probe (Liu C et al., 2018), and high energy particle detection package, structural type of GNSS occultation receivers (Cheng Y et al., 2018; and tri-band beacon transmitters . The satellite is also equipped with another high-energy particle detector (Ambrosi et al., 2018) from the Italian Space Agency. The satellite has a flying height of 507 km, inclination angle of 97.4°, a descending local time node of 14:00, and a revisiting period of 5 days.
In accordance with the engineering requirements and construction design of the ZH-1 satellite, the project is divided into six parts: a satellite system, a launch vehicle system, a launch site system, a measurement and control system, a ground system, and an application system. The satellite system is responsible for the development of the satellite platform and payloads; the launch vehicle system is responsible for the development of the rocket; the launch site system is responsible for launching the satellite into the target orbit; the measurement and control system is responsible for monitoring and controlling satellite operations, receiving the telemetry data transmitted by the satellite and transmitting it to the ground system; the ground system is responsible for coding the satellite work instructions, with the ground station receiving scientific data and conducting primary processing; and the application system is responsible for satellite mission planning, data processing data service, and scientific applications. The satellite transmits the scientific data through the X-band at the downlink rate of 120 Mbps; the telemetry data is transmitted through the S-band (Yuan SG et al., 2018). Data links among the measurement and control systems and the ground system, and the ground system and application systems, are transmitted through the fiber-optic line.
The structure of this paper is as follows: Chapter 2 explains the overall construction of the application system. Chapters 3 and 4 respectively introduce data classification criteria and data processing procedures. Chapters 5 and 6 introduce data storage solutions and data service solutions. Chapter 7 suggests desirable future optimizations.

The Overall Construction of the Application System
The main task of the application system is to develop a work plan for the satellite and payloads, and receive scientific data and telemetry data transmitted from the ground system. Based on the above work products, it then produces standardized advanced products and scientific application products in order to provide data in formats suited to the needs of users. Therefore, the construction design of the entire application system is divided into an operation management subsystem, a data management subsystem, a data processing subsystem, a product quality evaluation subsystem, and a data service subsystem. For ahe block diagram of the structure, see Figure 2.
The operation management subsystem is further divided into two principal subsystems, dedicated to (1) mission planning and (2) task operation and control. The mission planning subsystem is responsible for formulating and pushing the work plan of areas with the burst mode and satellite (and payloads) to the ground system ( Figure 3). The satellite and load operations are monitored according to telemetry data pushed by the ground system. In addition, satellite orbit calculation, simulation, and prediction are carried out instantaneously through the orbit, pushed by the ground system. The task operation and control subsystem is responsible for sending tasks of data processing, archiving, and management of the entire system, and for monitoring the operating state between task flow, the subsystems, the application system, and the external interface.
The data management subsystem is further subdivided into a data access subsystem and a data archiving and query subsystem. The data access subsystem is responsible for receiving scientific data and telemetry data at 0-2 levels, pushed by the ground system, and auxiliary data (including earthquake catalogue, space weather index, etc.) required by scientific applications. The data archiving and query subsystem is responsible for archiving received data and products at all levels in the form of a file directory, and querying, displaying and downloading data in various forms according to daily needs.
The data processing and analysis subsystem is divided into eight payload processing subsystems and one integrated processing subsystem in order to realize single processing and comprehensive product display of each level for all data from each payload. At present, the format and procedure for the Italian high-energy particle payload have not been fully confirmed; these specifications will be provided to China for integration into the overall plan.
Through a web interface, the data service subsystem provides users, based on user levels and rights, with these functions: data registration, browsing, query and download.

The Classification of Scientific Data
ZH-1 satellite data are divided into scientific data, telemetry data, and auxiliary data. Principal telemetry data include satellite position and speed, payload telemetry parameters, and other information. Examples of auxiliary data are calibration parameters for data processing, seismic catalogs, and space weather indices.
The scientific data and products are divided into the following categories and levels according to their content and processing actions.
Raw data: Raw data are data received by the ground receiving station, including the data from the satellite, and data from each station received by the tri-band beacon-receiving station of a particular application system. Level 0 data product: Level 0 data include time-aligned scientific data and engineering parameters from each payload after frame synchronization, descrambling, error correction, and de-duplication, and observation data received by the tri-band beacon ground station.
Level 1 product: Time-aligned physical quantity data obtained after format conversion and calibration processing of Level 0 data are termed Level 1.
Level 2 product: Physical quantity data with information of geographic and geomagnetic coordinate system, time, position and attitude, generated after coordinate transformation and inversion of Level 1 data, are termed Level 2. Level 2A product: Level 2A data include (1) electric field observation data generated after removing the influence of the V s ×B electric field based on the electric field waveform data of the Level 2 payload coordinate system; (2) GNSS Radio occultation observation data obtained after precise orbit determination and inversion of Level 1 data.
Level 3 product: Time series data of the revisited orbits generated by resampling based on Level 2 and 2A data, or the ionospheric and atmospheric 2D structure data generated by inversion based on 2 and 2A level data, are termed Level 3.
Level 4 product: Spatial evolution data associated with a particular region generated after spatial interpolation processing, based on Level 2 and 2A data.

Data Storage Solution
The designed work scope of ZH-1 is the area between 65° north latitude and 65° south latitude, and the designed working modes of the payloads are burst and survey. According to the timeliness of data downlink, the data are divided into real-time data and revisit data. Therefore, in order to facilitate data storage, reading, and effective identification, ZH-1 data files are stored according to the orbit number, payload, and sub-probe. The stored content include the data file and relevant image product, the processing report, and the configuration file.

File naming
The data file naming rules are as follows in Figure 4.
(2) Satellite number (2 digits): starting from 01 and increasing in order; 01 for ZH-1. (4) Payload serial number (1 digit): used to distinguish multiple similar detector data generated by one payload, starting from 1 and increasing in sequence. If there is no necessity to distinguish the probe, it is represented by 0. If necessary, the digital correspondence is shown in the table.

Earth and Planetary Physics
doi: 10.26464/epp2018042 447 Classification and Code" (draft for review). For Level 0 data, the observation object is coded as 00; for Level 1-4 of data, A1 represents satellite electric field observation, A2 represents satellite magnetic field observation, A3 represents satellite plasma in situ observation, A4 represents satellite high-energy particle observation, and A5 represents satellite ionospheric observation. The details are shown in Appendix Table 1A .
(7) Orbit number (5 digits): used to organize data files by track, starting from 00001, accumulating in turn. The data products that cannot be marked with a track number are represented by "00000".
(8) Descending/Ascending mark (1 digit): the descending is 1 (satellite flying from north to south) and the ascending is 0 (south to north).
(9) Data starting time, expressed by 14 digits: year (4 digits), month (2 digits), day (2 digits), hour (2 digits), minute (2 digits), and second (2 digits). For the TBB receiving station, this is the starting time received by each station. For the station chain data, it is the starting time of the chain combined by several stations along the same orbit.
(10) The data ending time: format is the same as (9). For the TBB receiving station, it is the ending time received by each station; for the station chain data, it is the ending time of the chain.
(11) Receiving station code (3 digits): refers to the information of earth station or GNSS satellite or TBB receiving station. The high 0 represents the data from the earth receiving station, and the code is recursed from 001. When the high bit stays at least 1, it indicates that the data is received from the TBB station. The highest bit refers to the link number, starting from 1, and the lower two bits refer to the station number, starting from 01. For example, the station code in chain 1 is 101-199 and the station code in chain 2 is 201-299. When the lower two bits are 00, it represents all the station data of a certain link. For example, 100 represents all the station data of the first link. For the GNSS occultation receiver, it represents the satellite signal of the received signal source, with the first digit representing GPS or BeiDou (G or B); the last two digits represent the satellite number.
(13) The expansion code of the observation object consists of two characters. The first digit is used to distinguish different regions: 1 for China, 2 for the global area, and 0 for no distinction of area; the second digit is used to distinguish among multiple images of the same payload, starting from 1 and increasing sequentially.
Because the Level 4 image is a spatial image, independent of the orbit, (7) (in Figure 5) is set at zero. (9) represents the year, month and day of the first day of the last 5 days; the hour, minute, and second are set at 0. (11) represents the year, month and day of the fifth day, that is, the current day; the hour, minute, and second are set at 0.

File content
Standard scientific products are in hdf format, and the contents include interpretation of file attribute and data. Taking the ULF frequency band of the electric field as an example, the content of the second-level product is listed in Table 2 and Table 3.
The number of A111 in Table 3 is the data item code, which is  Table 3. Description of ULF Level-2 data of the electric field detector in tableau format Number Table name  Table content  Table type  Table size  Table attribute

Data Service
According to the requirements of the "China National Space Administration and China Earthquake Administration's Notice on Strengthening Electromagnetic Monitoring and Test Satellite Data Management" issued by China National Space Administration and the China Earthquake Administration in 2018, the application system will be responsible for the updating, service, release and maintenance of ZH-1 satellite data. At present, the primary data service webpage has been built (www.leos.ac.cn) to provide related functions such as user registration and data download.

User Registration
On the website of the Center for Satellite Application in Earthquake Science of the China Earthquake Administration, there are the column of "Data Service" and the function of user login/user registration. New users must click the user registration and fill in relevant information online. After the platform confirming relevant information, users can click the data download in the data service section to select the required data based on the data selec-tion method provided on the page.

Sharing Products
Data products of the ZH-1 Satellite will be shared for scientific application objectives; data products to be shared are shown in Table 4.

Share Permissions
After registering, users will gain corresponding levels of ZH-1 satellite data according to the rights agreement. The corresponding relationship is as presented in Table 5.
According to the "China National Space Administration and China Earthquake Administration's Notice on Strengthening Electromagnetic Monitoring and Test Satellite Data Management", payload R＆D users refer to companies or individuals involved in specific research and development of satellite engineering, especially in payload research and development. The project cooperation users refer to users who signed the agreement on the ZH-1 satellite data with data management administrations of the ZH-1 satellite, such as China National Space Administration, China Earthquake Administration, and Institute of Crustal Dynamics, China Earthquake Administration; such users can obtain relevant data according to the agreement. General professional users refer to scientific and technological personnel with professional tech- nical backgrounds, such as seismic science and space physics science investigators, who plan to conduct public welfare scientific research based on ZH-1 satellite data.

Conclusion
The ZH-1 Satellite has been operating for more than 8 months to complete commissioning tests. The sub-systems of the application system are in regular operation, and preliminary scientific data products are ready to be officially released. The current working status shows that the operational control, data processing, data management, and data service of the application system of ZH-1 satellite have achieved designed functions and performance metrics. Since the satellite was launched into orbit, the commissioning tests have shown that the payload records contain not only information in the field of the space geophysics but also other information, including from the satellite platform itself. Future tasks include additional optimization of data processing algorithms. Dynamic and timely updating of the data content and data processing plan are needed for the application system to ensure the data quality of the ZH-1 satellite, improve the application efficiency of the data, and to exert greater scientific and social value.

Acknowledgments
This work was supported by the Civil Space Research project (ZH-1 data validation: Ionospheric observatory theory) and NFSC grant 41574139 and 41874174. This work made use of data from the ZH-1 mission, a project funded by China National Space Administration (CNSA) and China Earthquake Administration (CEA).  Three-dimensional structure of electron density Structural changes in electron density with height, latitude, and height A513 Four-dimensional structure of electron density Structural changes in electron density with height, latitude, height, and local time

A514
Ionospheric scintillation index S4 Ionospheric scintillation refers to the rapid and random fluctuation of signal intensity and phase caused by the irregular structure of the ionosphere when radio waves cross the ionosphere. The S4 index is a standard deviation of mean normalization that represents the intensity of the ionospheric scintillation information and indicates signal strength.

A52
Total electron concentration (TEC) The integral of the electron density per unit area along the transmitreceive path A521 Absolute TEC Total concentration of ionospheric electron concentration (TEC), also known as the ionospheric electron concentration column content, integral content, etc.. It is the integral of the electron concentration per unit area with the height.

A522
Relative TEC Relative change in the total content of integrated electrons