FY-2 Geostationary Satellite Program

China begins its geostationary meteorological satellite FY-2 program in 1980. Through hard work for more than ten years, the first FY- 2 satellite was launched on June 10, 1997 and located in the geostationary orbit at an altitude of 35800 kilometers over 105 degrees E. The satellite is a cylinder of 2.1m by 1.6m. The attitude of the satellite is spin stabilized with a speed of 100 rotation/min.  

FY-2 main functions are:

. To obtain hourly full-disc image. The full-disc image includes three channels of visible (0.55-1.05 micrometer), infrared (10.5-12.5micrometer) and water vapor (6.2-7.6 micrometer). The sub-point resolution of visible channel is 1.25 Km while infrared channel and water vapour channel are about 5 Km. The reflective radiation of cloud and Earth surface on daytime can be obtained by visible channel; the infrared radiation of diurnal cloud and Earth surface can be obtained from the infrared channel, and the water vapour content of middle and upper atmosphere can be obtained by the water vapour channel. By using these raw cloud image data, various kinds of image and meteorological parameter can be processed for the users.

. To collect and transmit in situ observation data from the DCP for the meteorology, oceanography and hydrology.

. To broadcast the stretched digital image, low resolution cloud image, synoptic map and processed products for users.

. To monitor space environment.

The Transmission Characteristics of FY-2A

Introduction

FY-2A is the first geostationary meteorological satellite in China. It is a spin-stabilized satellite. The main function of FY-2A is observation. It takes visible, infrared and water vapor disk images of the Earth hourly.

FY-2A was launched on 10, June 1997 with Long March-3 launch vehicle from Xi Chang Satellite Launching Center. The satellite was located at the equator of 105oE on 17, June 1997 and acquired the first visible image on 21, June 1997. The first infrared and water vapor images were received on 13, July 1997. The system check out showed good performance of the satellite.

FY-2A satellite data is open for international users. User stations covered by FY-2A can receive S-VISSR high resolution digital data and WEFAX low resolution analogue data from FY-2A.

VISSR AND S-VISSR

The main payload of FY-2A is a Visible and Infrared Spin Scan Radiometer (VISSR). The characteristics of the VISSR are shown in Table 1.

Table 1. Major Characteristics of FY-2A VISSR

  

Visible

Infrared

Water Vapor

Wavelength

0.55-1.05 micrometer

10.5-12.5micrometer

6.2-7.6micrometer

Resolution

1.25 Km

5 Km

5 Km

FOV

35 microrad

140microrad

140microrad

Scan Line

2500_4

2500

2500

Detector

Si-photo-diode

HgCdTe

HgCdTe

Noise Performance

S/N=6.5 (albedo=2.5%)

S/N=43 (albedo=95%)

NEDT=0.5-0.65 K

(300 K)

NEDT=1 K

(300 K)

Quantification Scale

6 bits

8 bits

8 bits

Scan step angle

140 microrad (N-S scanning)

     
Frame time

30 minutes

     

  

The VISSR takes the Earth and cloud images from the space. A complete 20o x 20o scan covering the full Earth disk can be accomplished every 30 minutes by means of combination of satellite spin motion (100 rpm from the west to east) and step action of the scan mirror (2500 steps from north to south). The S-VISSR data are retransmitted to user stations via FY-2A during the VISSR observation.

The stretched VISSR (S-VISSR) data are the digital image data originated by VISSR on board and stretched on the Command and Data Acquisition Station (CDAS) in time. After stretching, the transmission rate of S-VISSR data is reduced and can be easily received by the users.

S-VISSR SIGNAL CHARACTERISTICS

The signal characteristics of FY-2A S-VISSR data are as follows:

Frequency

1687.5MHz

Modulation

PCM/BPSK, NRZ-M

Bit rate

xed)

EIRP

57+1 dBm

       Polarization

Linear

Bandwidth

2 MHz

       Data volume

329.872 bits/line (including SYNC code)

       Data sequence
  1. data coding

Most significant Bit (MSB) first

Byte complimenting and PN scrambling

  

S-VISSR data is modulated by two stages of coding in order to distribute more equally the R.F. signal spectrum and to maintain sync-lock of users demodulator.

The first stage of coding is started at the beginning of the information sectors. The contents of every other eight bits (even bytes) are complemented and this process continues up to just before the beginning of SYNC code. Note the SYNC code is not complemented.

The second stage of coding is PN scrambling. The bit stream from the byte complementing process and the output of the PN code generator enter an exclusive OR gate. The resulting output is transmitted through the S-VISSR PSK modulator at the CDA station for relay by FY-2A.

Since the signal characteristics of FY-2A S-VISSR data are very close to those of GMS S-VISSR data except frequency, the user stations now receive GMS S-VISSR data can receive FY-2A S-VISSR data by changing the antenna pointing and frequency of receiver local oscillator.

DATA FORMAT OF FY-2A S-VISSR

FY-2A S-VISSR data of each frame consists of 2500 lines. The effective sector of image is 2291 lines. Calculating from the center of image (located near the equator), there are 1145 lines on the north and south of the Earth. The first line of the image is 0.5o distant from the northern edge of the Earth.

The S-VISSR data consists of synchronous code, information sectors and dummy data. The data format of S-VISSR is shown in Fig. 1 [sic].

1. Synchronous Code

The synchronous (SYNC) code is transmitted to allow bit and frame synchronization at user stations. The code consists of 20000 bits of PN code of Maximum Length Sequence generated by a 15 digit serial shift register. The PN sequence begins with 010001001100001 at a timing of every satellite spin, and the last 15 bits are fixed-logic ones.

2. Information Sector

The information sector consists of 8 sectors: documentation (DOC) sector, infrared image (IR) data sector, water vapor image (WV) data sector, IR reservation image (RES) data sector and 4 visible image (VIS) data sectors.

The DOC sector, IR sector, WV sector and RES sector contain 2293 eight bit words and each VIS sector contains 9166 six bit words. Besides every sector has also 16 bits of Cyclic Redundancy Check (CRC) code and 2048 bits filled with logic zeros (Filler).

2.1 Documentation Sector

This sector contains eight data blocks, which consists of 2293 eight-bit words. The block format is shown in Fig. 2. The details of every word are attached.

(a) Sector ID:

This block, containing 2 words (16 bits) of all logic zeros, is used to identify the documentation sector.

(b) Satellite and CDAS status block:

This block contains 126 words (1008 bits) and the information is repeated in every line with the same content for the process of S-VISSR data.

(c) Constant parameters for simplified mapping:

This block consists of 64 words (512 bits) and contains parameters to be used for simplified mapping together with the data in the simplified mapping block for geographical location. The content of information in this block are the same in every line.

(d) Sub-Commutation ID block:

The data volume of the parameters for simplified mapping block, orbit and attitude data block and MANAM block is too large to be transmitted in only one line. Thus, the whole text is divided into 25 sets and transmitted at different places of the data stream. To reduce transmission error, each group repeats 8 times. In other words, in consecutive 8 lines, every line contains the same content. To complete the transmission of whole text, 25 sets 8 lines/set = 200 lines are needed.

In order to make a difference between transmission number and repetition time for simplified mapping, orbit and attitude data, operational schedule, Sub-Commutation ID block is set.

This block consists of 4 words (32 bits) divided into two 16 bit binary counters (Word 193, 194, 195 and 196). The first counter (Word 194) is the repeat counter indicating the subcommutation ID and increments from 0 to 24 for the 25 documentation text groups. The second counter (Word 196) is also the repeat counter and increments from 0 to 7 for each repeated line of a group. The most significant word of each counter (Word 193 and 195) are always zero.

(e) Parameters for simplified mapping:

The block consists of 2500 words (25 sets, 100 words/set) and contains mapping information (longitude range 45oE - 165oE latitude range 60oE - 60oE, line and pixel number for each 5o longitude 5o latitude.

(f) Orbit and attitude data block:

This block contains the information of the orbit and attitude parameters of the satellite. It consists of 3200 words (25 sets, 128 words/set).

(g) Satellite operational schedule block (MANAM block):

This information is provided to notify users of the FY-2A operational schedule. This block consists of 10200 words (25 sets, 408 words/set).

Data in MANAM block is coded as ASCII characters and a set of characters consists of 82 alpha-numeric characters including CR and LF. There are 100 lines in every MANAN and 82 words in every line.

(h) Spare block:

This block contains 1461 words filling with some meteorological satellite products data to be used by user stations. The content of 1461 words are repeated in the consecutive 8 lines to reduce transmission error.

2.2 Sensor Data

(a) Infrared image data sector:

The infrared data sector consists of two words (16 bits) of sector ID code, 2291 words of infrared image data, 16 bits of CRC code and 2048 bits of filler.

ID word for infrared image data is: 00010001, 00010001.

(b) Water vapour image data sector:

The water vapor data sector is similar to the infrared data sector. It consist of two words of sector ID code, 2291 words of water vapor image data, 16 bits of CRC code and 2048 bits of filler.

ID word for water vapor image data is: 00100010, 00100010

(c) Reservation sector:

The rest are reserved for future use and filled with logic zeros, reservation sector is similar to the infrared. It consists of two words of sector ID code, 2291 words of reservation information, 16 bits of CRC code and 2048 bits of filler.

ID word for reservation sector is: 01000100, 01000100.

(d) Visible image data sectors:

Each visible image data sector consists of two words (12 bits) of sector ID code, 9164 words of visible image data, 16 bits of CRC code and 2048 bits of filler.

The four visible sectors contain the observed image data of the four visible detector (visible 1,2,3,4) from one VISSR scan.

ID word for visible image data is:

Visible 1 011011, 011011

Visible 2 101101, 101101

Visible 3 110110, 110110

Visible 4 111111, 111111

The filler of the sensor data sectors are also the same. The filler containing 2048 bits is a series of dummy bits filling with logic zeros. It works to provide a buffering time of approximately three milliseconds for data processing in computer.

3 Dummy Data

Dummy data is filled with all logic zero bits.

  


NOTE: This page was copied and simplified from the CMA site http://nsmc.cma.gov.cn/fy2e.html in October 2004, to assure availability.