Meteorological satellite
program of China Zhang Wenjian i
National Satellite Meteorological Center (NSMC) China Meteorological Administration (CMA) Beijing 100081, P. R. China Keywords: Meteorological satellite, program,
application system, international collaboration. Abstract China has been making her unremitting efforts on pushing her meteorological satellite program and building up the meteorological satellite as well as data application systems. The meteorological satellite program of China consists of two major systems: Polar-orbiting system and geostationary system. The main objectives of the program are to establish, with combination of polar and geostationary orbits, a comprehensive operational meteorological satellite system as well as data application systems around the turn of the century, in order to meet the needs on various aspects in China, and enhance the capability of participating international collaboration. 1. Introduction Meteorological satellites have become an irreplaceable weather and ocean observing tool in China. These satellites are monitoring major natural disasters and improving the efficiency of many sectors of our national economy. Therefore, meteorological satellite has been regarded as a kind of applied satellite with notable social and economic benefit among man-made satellites. It is not feasible nowadays to ignore the space derived data in the field of meteorology, hydrology, agriculture as well as disaster monitoring in China, such a big agricultural country. For this reason, China is making her unremitting efforts on building up the meteorological satellite system and data application system. The meteorological satellite program of China consists of two major systems: Polar orbiting and geostationary series. The main objectives of the program are to establish, with combination of polar and geostationary orbits, a comprehensive operational meteorological satellite system as well as the ground monitoring and application data system around turn of the century, in order to meet the needs on various aspects in China, and enhance the ability to participate international collaboration. The Ministry of Aerospace of China takes the responsibility for the space segment, while China Meteorological Administration is in charge of the ground segment. In China, meteorological satellites are named simply as Feng-Yun series, abbreviated as FY-series. The Chinese words Feng-Yun in English standards for "Winds and Clouds". We use the FY- odd number ,i.e. FY-1,FY-3, etc. to name the polar orbiting satellite series, and FY-even number, i.e.FY-2, FY-4, etc. to the geostationary series. 2. China's first generation of polar orbiting meteorological satellites: FY-1 According to the present plan, China's first generation of polar orbiting meteorological satellite system, FY-1, will consist of four satellites, as well as the corresponding ground data acquisition, processing amd application systems. 2.1 The FY-1A and FY-1B With the increasing awareness of the importance and notable benefits of meteorological satellites in 1960’s, China has initiated her first polar orbiting meteorological satellite project FY-1 series in 1977. The FY-1A and FY-1B were designed and manufactured by the Ministry of Aerospace of China according to the requirement specified by China Meteorological Administration (CMA). The satellite is a hexahedron of 1.4×1.4×1.2(in height) and the weight is 750 kg. The two solar cell arrays mounted on both sides of the main body make the total length of the satellite 8.6 meters. The attitude control of the satellite is three-axis stabilized with a precision of no less than 1 degree in all three axis. FY-1A and FY-1B operated in a sun-synchronous orbit at an altitude of 901 km with an orbital period of 102.86 minutes. The inclination angle is 98.9° and the eccentricity is less than 0.005. Table 1 gives the main orbit parameters of the FY-1A and FY-1B satellites
The main meteorological payload on board FY-1A and FY-1B satellites are five channel Visible and Infrared Radiometers, similar to AVHRR instrument but with different channel wavelengths. Table 2 gives the channel characteristics of the radiometer on board FY-1A and FY-1B satellites
The FY-1 observation data are disseminated in three modes: High Resolution Picture Transmission (HRPT), Automatic Picture Transmission (APT), and Delayed Picture Transmission (DPT). For HRPT and APT, direct readout services are provided during satellite operations with data format compatible with NOAA satellites. The DPT data are cloud images for selected areas over the world. The ground segment of FY-1 consists of three ground stations located in Beijing, Guanzhou and Urumuqi respectively and a Data Processing Center (DPC) at National Satellite Meteorological Center in Beijing. The data received at the ground stations are relayed in real time to the Beijing DPC for processing, distributing, and archiving. Derived products from the DPC include cloud image masaics in a variety of projections, meteorological parameters such as sea surface temperature, clout top temperature and total water vapor; regional environmental parameters such as vegetation index, snow cover, sea ice, land cover, etc. All raw data and products are archived on digital tapes. Images are broadcast via TV to the public. 2.2 The Status of FY-1A and FY-1B FY-1A was launched successfully on September 7, 1988 by launch Vehicle of Long March-4. The spacecraft went into the predicted orbit accurately and started to make observations at the same day. The pictures from the visible channels were quite good, and the signal to noise ratios was higher than the designed ones. Unfortunately, it worked only for 39 days and did not fulfill its expected lifetime because the satellite attitude was out of control. However, during the flight of FY-1A the performance and specifications of the payload and the satellite subsystems were examined. There were two problems with FY-1A: satellite attitude was out of control after 39 days; and water vapor contamination on the infrared detectors, which was the main reason of the IR signal loss. The second spacecraft, FY-1B was launched after two years by Lang March-4 rocket successfully into predicted orbit on September 3, 1990. Although the overall design of FY-1B is almost the same as FY-1A, the quality of FY-1B was really an improvement on FY-1A's, both on accuracy and stability of satellite attitude control and on scan radiometers. This satellite made successive observations for half a year. However, the same problem, the attitude out of control appeared again. After rescuing by ground commanding, the satellite was brought back and made observations again until late 1992 when the rescuing was give up. This indicated that there are some problems on reliability remained to be improved further. 2.3 The FY-1C and FY-1D China will continue the development of FY-1 series and to launch FY-1C and FY-1D in the year of 1999 and 2001 respectively. Now the satellites are developed on the basis of FY-1A and FY-1B. Besides the efforts to improve the reliability of the satellites there are some changes on imaging instruments and data transmission as follows:
On May 10, 1999, FY-1C was launched successfully and now the satellite is operating well. Many new images and products are produced based on the FY-1C data. A separated paper will describe the FY-1C products in more detail. 3. Geostationary meteorological satellite program of China 3.1 General China has launch its first geostationary meteorological satellite FY-2A with the Long March-3 rocket from the Xi Chang Satellite Launching Center on 10, June 1997. The satellite is located in the equator of 105°E. FY-2 satellite data is open for international users, therefore the satellite data can be shared with other countries. User stations covered by FY-2 can receive S-VISSR high resolution digital data and WEFAX low resolution analog data. 3.2 Specifications of FY-2 satellite and the radiometer 3.2.1 Functions of the satellite FY-2 meteorological satellite has the following functions:
3.2.2 Visible and infrared spin scan radiometer The major payload of FY-2 meteorological satellite is Visible and Infrared Spin Scan Radiometer (VISSR) . The characteristics of the instrument are shown in Table 5.
The VISSR performs Earth and cloud observations from space. Visible, infrared and water vapor images of he Earth and its clouds are derived from the VISSR. During a scanning, the optical telescope collects visible, infrared and water energies from the Earth and clouds, and then focuses them on the focal plane with primary and secondary mirrors. Visible fiber optics and infrared relay optics system relay energies from the telescope focal plane to visible, infrared and water vapor detectors. Si detectors convert visible light into visible analog signals and HgCdTe detectors cooled by radiation coolers convert the Earth’s radiation into infrared analog signals. The S-VISSR outputs are fed to a VISSR Digital Multiplexer (VDM) unit with redundancy.
3.3 FY-2 ground application facilities The FY-2 program ground system consists of the following: A Command and Data Acquisition Station (CDAS), a Data Process Center (DPC), a Satellite Operation Control Center (SOCC), Ranging Stations (one primary station, three secondary stations including one in Australia), widely dispersed Data Collection Platforms (DCP), Medium-scale Data Utilization Stations (MDUS) and Small-scale Data Utilization Stations (SDUS), and a Ground Communication system etc. The tasks of FY-2 ground system are as follows:
One of the major functions of FY-2 system is to broadcast data including S-VISSR, WEFAX and S-FAX data via FY-2 satellite. The S-VISSR data are transmitted to Medium-scale data Utilization Station (MDUS) through the FY-2 during the VISSR observation. WEFAX and S-FAX data are retransmitted to Small-scale Data Utilization Station (SDUS). The S-FAX of FY-2 is only for domestic users. 3.4.1 Transmission characteristics of FY-2 S-VISSR The S-VISSR data are the digital image data originated by VISSR on board and the stretched on CDAS in time. Therefore, the transmission rate is reduced. The S-VISSR data are retransmitted to MDUS via the FY-2 during the VISSR observation. The signal characteristics of FY-2 S-VISSR data are as follows:
Since the signal characteristics of FY-2 S-VISSR data are as the same as GMS S-VISSR data except frequency, the user stations now receiving GMS S-VISSR data can receive FY-2 S-VISSR data by changing the antenna pointing and frequency of receiver local oscillator. 3.4.2 Transmission of the FY-2 WEFAX The WEFAX is disseminated to SDUS users via FY-2 satellite. The WEFAX transmission is in the format which is completely compatible with those of other geostationary meteorological satellites. The WEFAX is composed of gray scales, marks, annotation and earth image. The annotation signal is inserted at the head of the picture, so as to recognize the image information automatically. The earth image contain latitude-longitude grids and coastline bases of the prediction of the satellite’s orbit and attitude. 3.5 The FY-2 data collection system There are 133 data Collection Platform(DCP) channels in FY-2 system, including 100 regional DCP channels and 33 international DCP channels, which can collect data from a wide variety of platforms. The regional DCPs are stationary DCPs that installed on buoys, isolated islands, rivers, mountains or ships for meteorology, oceanography, hydrology and other purposes. The collected data are edited at the NSMC and distributed to the user via GTS. 4. Considerations on development of FY-3 series FY-3 series, the second generation of Chinese polar orbiting meteorological satellites, will be launched from 2002-2016 according to present plan. The main mission objectives for FY-3 are:
To achieve above-mentioned objectives, a meteorological core payload with ten instruments as well as complementary instruments are considered as follows:
The Chinese meteorological satellite program is considered to be the contribution of China to the global environmental satellite system. China is willing to share her satellite data with the international remote sensing community, and thus to make contribution to the people worldwide. References
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