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Abstract: Tiangong-2 Space Laboratory was successfully launched on September 15, 2016. It carries three earth observation loads, including Wide-band Imaging Spectrometer, Three-dimensional Imaging Microwave Altimeter and Multi-band Ultraviolet Edge Imaging Spectrometer. This data set contains earth observation data products obtained by Tiangong-2 from September 2016 to December 2017, and preprocessed by the ground data processing system. These data products are released through the Space Application Data Promoting Service Platform for China Manned Space Engineering (web site: http://www.msadc.cn), a platform providing data services for scientific research and data utilization organizations and institutions at home and abroad. These products have been filtered and quality-controlled before release. They could be used in the monitoring of large-scale targets in atmosphere, oceans and land, such as land cover classification, ecological environment assessment, coastline change monitoring, ocean dynamics monitoring, inland water quality monitoring, glacier and topography monitoring, upper and middle atmospheric conditions and disturbances monitoring, ozone forecasting, and so forth.
Keywords: Tiangong-2 Space Laboratory; earth observation data; Wide-band Imaging Spectrometer; Three-dimensional Imaging Microwave Altimeter; Multi-band Ultraviolet Edge Imaging Spectrometer
|Chinese Title||2016～2017 年天宫二号空间实验室对地观测数据产品集|
|English Title||Earth observation data of the Tiangong-2 Space Laboratory (2016 – 2017)|
|Data authors||Li Shengyang, Zhang Wanfeng, Liu Zhiwen, Tan Hong, Qin Bangyong, Hao Zhongweng, Yu Haijun, Shao Yuyang, Liu Yunfei, Qin Man, Liu Kang, Hei Baoqin, Li Xuan|
|Data corresponding author||Qin Bangyong (firstname.lastname@example.org)|
|Time range||From September 2016 to October 2017|
|Data volume||1.22 TB||Data format||*.tif,*.png,*.xml, *.jpg, *.h5|
|Geographical scope||(1) Data products of Wide-band Imaging Spectrometer cover 86 percent of the land and coastal areas in China, and parts of other countries and regions within 43 degrees north and south latitude.|
(2) Data products of Three-dimensional Imaging Microwave Altimeter cover 30 percent of the land and coastal areas in China, and parts of other countries and regions within 43 degrees north and south latitude.
(3) Data products of Multi-band Ultraviolet Edge Imaging Spectrometer cover an altitude range of 10 km –60 km of the earth's atmosphere.
|Spatial resolution||(1) Wide-band Imaging Spectrometer: visible-near infrared spectrum (100 m), short-wave infrared spectrum (200 m), thermal infrared spectrum (400 m).|
(2) Three-dimensional Imaging Microwave Altimeter:regular resolution image (100 m), high resolution image (30 m).
(3) Multi-band Ultraviolet Edge Imaging Spectrometer: ultraviolet forward spectrum data (3 km), ultraviolet ring image data (3 km).
|Data service system||Space Application Data Promoting Service Platform for China Manned Space Engineering (http://www.msadc.cn);|
Science Data Bank (http://www.sciencedb.cn/dataSet/handle/522)
|Sources of funding||The National R&D Infrastructure and Facility Development Program of China – "Fundamental Science Data Sharing Platform" (DKA2017-12-02-23)|
|Database composition||The dataset consists of three parts of data:|
(1) Data products of Wide-band Imaging Spectrometer, which include visible-near infrared images, short-wave infrared images and thermal infrared images;
(2) Data products of Three-dimensional Imaging Microwave Altimeter, which include regular- and high-resolution images;
(3) Data products of Multi-band Ultraviolet Edge Imaging Spectrometer, which include ultraviolet forward spectrum data and ultraviolet ring image data.
As the first space laboratory in China, Tiangong-2 is to implement space science experiments and space technology tests. Equipped with a series of new equipment, it has carried out more than ten experiments and tests across multiple fields, such as microgravity basic physics, microgravity fluid physics, space materials science, space life science, space astronomy, space geosciences and application.1–2 Tiangong-2 carries three earth observation loads: Wide-band Imaging Spectrometer, Three-dimensional Imaging Microwave Altimeter (Microwave Altimeter) and Multi-band Ultraviolet Edge Imaging Spectrometer (Ultraviolet Spectrometer).
Wide-band Imaging Spectrometer is an optical remote sensor with a push-broom mode enabling a large observation swath (300 km) and a wide spectral range. It is able to cover visible-near infrared (0.4 – 1 micron), short-wave infrared (1 – 1.7 microns) and thermal infrared (8 – 10 microns). The three spectral segments have a spatial resolution of 100 meters, 200 meters and 400 meters respectively. Compared with MODIS3 and MERIS,4 Wide-band Imaging Spectrometer has a relatively higher spatial resolution, thus expanding the medium-resolution data sources obtainable by previous imaging spectrometers, especially along the scale of 100-meter resolution.
As the world's first three-dimensional imaging microwave altimeter, Microwave Altimeter uses small-angle interferometric technology and synthetic aperture technology in Ku band to accurately obtain sea level height.5 With a wide swath of 30 kilometers, it fills in the blank of imaging altimeter data at home and abroad, and is of high value in scientific research and application.
Ultraviolet Spectrometer is the world's first large-field atmospheric detector which quasi-synchronously detects the characteristics of ultraviolet limb radiation of the middle atmosphere. It provides a means to obtain the ozone and atmospheric density, together with the vertical structure and three-dimensional dynamic distribution of the trace gases in the middle and low heat layers. Ultraviolet Spectrometer enriches methods for detecting the limb atmosphere. When used in combination with other atmospheric exploration instruments,6 it provides data support for atmospheric scientific research and atmospheric environmental monitoring.
This data set mainly contains data products of Wide-band Imaging Spectrometer, Microwave Altimeter and Ultraviolet Spectrometer. The earth observation loads of Tiangong-2 acquire data according to specific observation requirements. The ground system carries out a series of data preprocessing, such as data format conversion, radiometric correction, geometric correction and so on, to produce standard data products based on the observed data. These data products are then distributed and shared through the Space Application Data Promoting Service Platform for China Manned Space Engineering (http://www.msadc.cn), where authorized users can search, browse or download the data products.
2.1 Data products of Wide-band Imaging Spectrometer
With a push-broom imaging mode, the Wide-band Imaging Spectrometer has three fields of view along the visible-near infrared spectrum range, two fields of view along the short-wave infrared spectrum range, and two fields of view along the thermal infrared spectrum range. Its data products follow the following processing flow: firstly, the downlink data undergo format conversion to produce the raw data of each spectrum band. Then, the raw data of multiple fields of view are combined for uniformity correction to produce a wide-swath image. After that, the wide swath image is divided into a plurality of standard-size images. Finally, the standard-size images undergo relative and absolute radiometric calibration and geometric correction respectively to produce the radiance image products with geographical coordinates.
2.2 Data products of Microwave Altimeter
Microwave Altimeter has two observation modes: ordinary and high-resolution, which is adopted according to specific needs. Its data products follow a similar processing flow. The downlink data firstly undergo format conversion to produce the raw data of Microwave Altimeter. Then, the raw data are used for image reconstruction to produce two-dimensional microwave image. After that, the image data undergo antenna pattern correction and distance attenuation correction to produce radiation correction images. Finally, after ground resolution correction and geometric correction, image data with geographic coordinates are generated.
2.3 Data products of Ultraviolet Spectrometer
Ultraviolet Spectrometer consists of ultraviolet ring imager and ultraviolet forward spectrometer. Their data products follow distinctly different processing methods. The ultraviolet ring data contains three channels – 265 nanometers, 295 nanometers and 360 nanometers. The downlink data of the ultraviolet ring imager firstly undergo format conversion to produce the raw data of each channel. Then the raw data of each channel undergo dark background removal, smear effect correction and radiation normalization to obtain image data. After that, image brightness correction is performed based on calibration parameters, followed by geometric positioning to generate image data products with geographic position information. The downlink data of the ultraviolet forward spectrometer firstly undergo format conversion to obtain the raw data. Then, the raw data undergo dark background removal, smear effect correction, nonlinear correction, stray light correction and radiometric normalization to generate normalized spectrum data. Finally, after radiance correction and geometric positioning, forward spectral data products are generated.
The earth observation data products of Tiangong-2 mainly contain the spatial and attribute information of ground objects in numerical format. This data set uses standard data formats of the industry and contains complete metadata information to facilitate data reading and reuse.
3.1 Data products of Wide-band Imaging Spectrometer and Microwave Altimeter
Data products of Wide-band Imaging Spectrometer and Microwave Altimeter adopt the following naming format:
s1 designates vehicle code. For example, "T2" means "Tiangong-2 Space Laboratory";
s2 designates load name. For example, "MWI" represents Wide-band Imaging Spectrometer, and "IALT" represents Microwave Altimeter;
s3 designates product identification code. For Wide-band Imaging Spectrometer data products, for instance, "VNI", "SWI" and "INF" denote visible-near infrared spectrum, short-wave infrared spectrum and thermal infrared spectrum, respectively. For Microwave Altimeter data products, "RIMG" and "HIMG" represent ordinary and high-resolution two-dimensional modes, respectively;
s4 designates data type. For example, "IMG" represents image data, and "SCI" represents scientific data;
t1 designates the start time (Beijing time) of data collection. For example, "20171012094001" indicates that data collection started at 9:40:01 Beijing time on October 12, 2017;
t2 designates the end time of data collection, which follows the same format as t1;
s5 designates product level. For example, "L1" represents level-one data products, and "L2" represents level-two data products;
s6 designates segment number. For Wide-band Imaging Spectrometer data products, "000" indicates that the whole orbit image is not segmented, and "001" indicates the first segment. For Microwave Altimeter data products, "6" represents the sixth segment of the whole orbit image.
s7 designates scene number. For Wide-band Imaging Spectrometer data products, "6" represents the sixth scene in the current image segment. For Microwave Altimeter data products, "15" represents the fifteenth scene in the current segment of the image.
t3 designates product generation time, which follows the same format as t1.
s8 designates version number, used to identify different versions of data products, such as "V111", "V112", "V211" and so on.
s9 is file suffix, used to identify different types of data products, such as "tif", "png", "jpg", "xml" and so on. "tif" designates an image data product in Geotiff format, which could be opened with remote-sensing image processing software, such as ENVI, Erdas and ArcGIS. "png" designates a preview image. "jpg" designates a thumbnail image. "xml" designates the metadata of data products.
An example of Wide-band Imager Spectrometer data products is shown in Figure 1.
An example of Microwave Altimeter data products is shown in Figure 2.
As can be seen, the latitude and longitude coordinates and the image resolution can be obtained from the xml file of the data products.
3.2 Data products of Ultraviolet Spectrometer
Data products of Ultraviolet Spectrometer adopt the following naming format:
s1 designates vehicle code. For example, "T2" means "Tiangong-2 Space Laboratory";
s2 designates load name. For example, "ZW" represents Ultraviolet Spectrometer;
s3 designates product identification code. "ZWF" denotes the ultraviolet forward spectrometer. "ZW265", "ZW295" and "ZW360" represent the 265-, 295- and 360-nanometer channels of the ultraviolet ring imager, respectively;
s4 designates data type. For example, "IMG340", "IMG170" and "IMG1024" represent image data with a pixel size of 340 * 340, 170 * 170, and 1024 * 1024, respectively;
t1 designates the start time of data collection. For example, "20171013133212" indicates that data collection started at 13:32:12 Beijing time on October 13, 2017;
t1 designates the end time of data collection, which follows the same format as t1;
s5 designates product level. For example, "L1" represents level-one data products;
t3 designates product generation time, which follows the same format as t1;
s6 designates version number, used to identify different versions of data products, such as "V101", " V102";
s7 is file suffix, used to identify different types of data products. For example, "h5" designates a data product in hdf5 format, which could be opened with HDFView software.
An example of ultraviolet ring imager data products is shown in Figure 3.
An example of ultraviolet forward spectrometer data products is shown in Figure 4.
On the left side of Figure 3 and Figure 4 is the structure of the level-one data products of ultraviolet ring imager and ultraviolet forward spectrometer, respectively, including file head, auxiliary information, and observation data.
The Wide-band Imaging Spectrometer, Microwave Altimeter and Ultraviolet Spectrometer of Tiangong-2 have been strictly calibrated before launch, and all the functional performances have met the design requirements. An orbit test shows that, the visible-near infrared and short-wave infrared spectra of Wide-band Imager had a relative radiometric deviation of less than 7%; the thermal infrared spectrum had an absolute radiometric deviation of less than 1.5 K; and the backscatter coefficient of Microwave Altimeter was less than 2 dB.
The ground data processing and service system of Tiangong-2 contains a data quality analysis and evaluation system, which is to regularly evaluate the quality of earth observation data from the aspects of radiation quality, geometric accuracy, among others. According to the results of quality evaluation, data products were further screened and optimized to ensure they met quality and precision requirements.
After quality assessment and control, Wide-band Imaging Spectrometer data products had an average SNR greater than 40 dB, and the deviation of image geometric positioning was within 3 pixels. Microwave Altimeter data products has an average SNR not less than 20 dB, and the deviation of image geometric positioning was within 10 pixels. Ultraviolet ring imager data products had an average SNR greater than 50 dB, and ultraviolet forward spectrometer data products had an average SNR larger than 20 dB.
The data products which met our release requirements were then distributed and shared through the Space Application Data Promoting Service Platform.
Wide-band Imaging Spectrometer is an earth observation optical imaging system with moderate spatial resolutions, programmable channels and a three-spectral-segment push-broom mode. It has wide application prospects for monitoring large- and medium-scale objects in ocean, land and atmosphere. In ocean applications, these data products can be used to monitor ocean chlorophyll, suspended matter concentration, dissolved organic matter, and sea surface temperature.7 They can also be used in red tide monitoring, plankton biomass and primary productivity estimation, coastal mapping, among others. In land applications, the visible-near infrared data products provide statistical support for land cover classification, forestry applications, city environmental monitoring, and so on.8 They can also be used for comprehensive monitoring of large water bodies, ecological environment evaluation, and disaster monitoring. In addition, the short-wave infrared data products can be used for soil moisture monitoring, crop growth monitoring, and cloud monitoring. The thermal infrared data products can be used to monitor land surface temperature9 and fire.
Microwave Altimeter is an observation system with a wide swath, a high precision, and abilities in two-dimensional imaging and sea/land observation interference. It can be used for marine dynamics (including sea level height, sea waves and currents) monitoring, marine forecast,10–11 land topographic survey,12–13 glacier and water monitoring, among others. As products of the world’s first microwave imaging altimeter, these data are of important scientific value.
Ultraviolet Spectrometer data products can be used to monitor the status and disturbance of the upper middle atmosphere, and to study global atmospheric density and ozone distribution14 and the vertical structure and three-dimensional distribution of aerosol components. It can be used in combination with SCIAMACHY15 (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) data of European Space Agency or OMPS16 (Ozone Mapping and Profiler Suite) data of the United States to provide data support for atmospheric scientific research and atmospheric environmental monitoring.
Thank China Manned Space Engineering for providing space science and application data products of Wide-band Imaging Spectrometer, Three-dimensional Imaging Microwave Altimeter, and Multi-band Ultraviolet Edge Imaging Spectrometer.
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1. Qin B, Li S, Hao Z et al. Earth observation data of the Tiangong-2 Space Laboratory (2016 – 2017). Science Data Bank. DOI: 10.11922/sciencedb.522
How to cite this article
Qin B, Li S, Hao Z et al. Earth observation data of the Tiangong-2 Space Laboratory (2016 – 2017). China Scientific Data 3 (2018), DOI: 10.11922/csdata.2017.0003.zh