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Radiometer for (Fy-2) Synchronous Meteorological Satellite

Guilin Chen, Yuntian Pei
(Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, 200083)


1.Introduction
On June 10th 1997 China launched (FY-2) Geostationary Meteorological Satellite into earth-synchronous orbit with a Long March-3 rocket at Xichang Satellite Launch Center. The satellite is positioned at E105o,35800 Km above the equator. Onboard the satellite thee is a multi-channel scanning radiometer (MCSR) which can acquire simultaneously earth images from visible, IR and water vapor cannels. The MCSR I the main effective payload onboard the satellite with the following functions:
  1. Taking visible cloud images at daytime, IR images and water vapor profile day and night above the earth and providing realtime meteorological information.
  2. Obtaining upper and lower wind information from the movement of tracing cloud, forecasting timely typhoon, cold-air outbreak, whirlwind and other disastrous weather.
  3. Providing temperature at ocean surface, analyzed cloud image and temperature profile of cloud top after data processing.
The MCSR was developed by the Shanghai Institute of Technical Physics, Chinese Academy of Sciences. This paper mainly introduces the performances, operation principle, basic structure and in-orbit operation of the instrument.

2 MCSR
  1. The specifications of MCSR are listed in the following tables.
Table 1 Main System Specification
Spin rate 100 rpm
Image size 20° x20°
Image rate 30 min
Scan step distance 0.14 mr
Aperture 400 mm
Lifetime 3 years
Weight 75 Kg
Size 1.3 x 0.8 x 0.6m

Table 2 Technical requirements of the instrument
Parameter Vis-channel IR-channel WV-channel
Number of chan. 4(+4back-up) 1(+1back-up) 1(+1back-up)
Wave band 0.5~1.05mm 10.5~12.5mm 6.3~7.6mm
IFOV 0.04 mr 0.16mr 0.16mr
Number of line/pic. 10000 2500 2500
Detector S photodiode HgCdTe HgCdTe
Band width of circuit 130.8 KHz 32.7KHz 32.7KHz
Dynamic range 0.5~95%
(albedo a)		 180~320K 200~300K
S/N or NEDT ³43(a =95%)
³1.5(a =95%)		 £0.5(300K) £1K(260K)
Calibration on- Board satellite The sun, cold space, electronic Balckbody, cold space, electronic Blackbody, cold space electronic
Sampling rate 437.5K samp./s 109.4K samp./s 109.4Ksamp./s
Quantized level 6 bit 8bit 8 bit
Transmission rate 14 Mbps 14 Mbps 14 Mbps

  1. Operation principle of MCSR When the satellite rotates at the rate of 100rpm, the MSCR accomplishes the scan of the earth from west to east. When the field of view of the MCSR scans out of the edge of the earth, the telescope on the MCSR starts to step at a step angle of 0.14 mr. Thus, the MCSR conducts stepping of the earth 2500 steps from north to south step by step, obtaining 10000 visible scan lines, 2500 lines for IR and water vapor respectively. Normally, it takes 25 minutes to accomplish a full-frame scan and takes 2.5 minutes for the telescope to scan back, then, 2.5 minutes for the satellite to get stabilized, so, it takes altogether 30 minutes to accomplish one frame of an image. The MCSR can also conduct, according to remote controlled command, regional scan and single line scan, adjusting for channel gain, adjustment of focal plane of visible, IR and water vapor channels.
  2. Composition of MCSR
    It mainly consists of
    1. Optical system: Primary optical system, rear optical system and calibration optical system.
    2. Mechanical structure: main structure, scanning mechanism, telescope, focusing mechanism and calibration mechanism .
    3. Electronics : information processing for visible, IR and water vapor channels, video digital multiplexer (VDM), controller, telemetry and power.
    4. Radiant cooler
    5. Detector PIN Si photodiode for visible channel, HgCdTe detectors for IR and water Vapor channels.
3. Testing of MCSR
A lot of testing on accuracy and function must be conducted before the MCSR was launched together with the satellite.
  1. Testing of the stepping accuracy of the two scan mechanisms and the testing of accuracy of the combined stepping.
  2. Testing of the optical instantaneous field of view of the visible, IR and water vapor channels and transfer function.
  3. The sun and standard albedo plates are used for visible calibration of MCSR and the S/N ratio of the visible channel is also tested.
  4. IR calibration of the MCSR is conducted with IR calibration facilities in low temperature vacuum chamber to obtain several T-V curves at different environment temperature and NE ?T of IR and water vapor channel is also tested.
  5. Mount the MCSR on a rotating platform and when the rotating platform rotates at the rate of 100 rpm, the MCSR takes image of scene.
  6. Testing of other accuracy and functions.
4. In-orbit operation of MCSR
The (FY-2) Geostationary Mateorological Satellite was successfully launched on June 10th, 1997, positioned at 105oE. On June 21, the ground station received that first visible cloud image, which is clear. The targets with a dimension of about 1 Km, such as river, island, cloud line, land and sea breeze, can be clearly resolved. On July 5th the jettisonalble cover of the radiant cooler was successfully thrown away. The first IR cloud image and water vapor profile images were sent back to the ground on July 13. From the cloud images sent back we can see that the resolution of the image, dynamic range, S/N ratio, temperature sensitivity and etc. all reached the designed specifications. Particularly, the IR and water vapor images have great contrast. The various functions of the MCSR operate normally and onboard visible and IR calibration reached predicted results. The instrument can perform various scan ways, such as normal full frame scan, regional scan, single line scan, etc. Since the images from the three channels are clear, visible and IR focusing has not been conducted yet. One point worth mentioning is that the MCSR has been operating for more than two years, the S/N ratio for IR and water vapor channels is very high. Up to now no phenomenon of obvious contamination of water vapor on low temperature optical has ever occurred.

At present the images and data obtained from MCSR are open to the whole would and serve the global weather monitoring and forecasting.