Very high resolution scanning radiometer on FY-1 meteorological satellite and its operation performance in orbit

Very high resolution scanning radiometer on FY-1 meteorological satellite and its operation performance in orbit

Kuang Dingbo, Gong Huixing, Zheng Qinbo
Shanghai Institute of Physics, Academia Sinica

Abstract
This paper describes the operation principle of the very high resolution scanning radiometer on "FY-1" meteorological satellite, the characteristics of its output imagery, the quality of the imagery, acquired after entering the orbit and measuring results of the operation performance in orbit.

Purpose of the mission
The "FY-1" meteorological satellite is the first satellite for environment remote sensing ever launched by China. Its main purpose is to acquire global cloud picture day and night and carry out ocean color remote sensing experiments. The satellite entered accurately the sun synchronous orbit on Setp. 7th, 1988 with a high of 901 km, inclined angle 99.10 and period 102.8 min.

The very high resolution scanning radiometer is the main detecting instrument of the satellite and Fig. 1 shows its out appearance. With five channels the scanning radiometer can obtain simultaneously five spectral bands data of objects.

Spectral band	Detected object
Ch1 : 0.58~0.63mm	Visible cloud, vegetation, oceansilt, snow cover
Ch2: 0.725~1.1mm	Visible cloud, vegetation, demarcation between water and land, snow cover
Ch3: 0.48~0.53mm	Chlorophyll at sea.
Ch4: o.53~0.58mm	Chlorophyll at sea and silt
Ch5: 10.5~12.5mm	IR cloud, target temperature

In which Ch1, Ch2, Ch5 are used to take pictures of visible and infrared clouds for weather forecast. The data measured from Ch1 and Ch2 vegetation parameter and differentiate cloud and snow. Ch4 and Ch5 are used for observation of ocean cloud to obtain distribution of chlorophyll with medium and high concentration at sea. The thermal IR channel of the scanning radiometer has the ability of radiation response calibration in flight and can measure quantitatively equivalent blackbody temperatures of targets (such as sea surface, cloud top etc) the image data can also be used for monitoring piled snow, sea ice and vast area of floods and other disasters.

The imagery information obtained by the scanning radiometer is transmitted in real time to the areas covered by the satellite in two forms. The HRPT image signals containing five channel's digital image signals with ground resolution of 1.08 km at nadir are modulated and transmitted at a frequency of 1700 MHz. The APT image signals containing two channel's analog image signals after having geometric calibration and resolution reduction proceeding, are modulated and transmitted at a frequency of 137 MHz. The image signal format of both HRPT and APT is the same as that of American NOAA and the various ground receiving stations for NOAA are available for FY-1 satellite.

With a width of 3235 KM. of HRPT and APT images, the satellite circles the earth 14 times per day and the visible / near IR channel covers the whole globe every 254 hours and the coverage period of the thermal IR channels is 12 hours.

Fig. 1 Outer apperance of the very high resolution scanning radiometer

Operating principle
The very high resolution scanning radiometer is an imaging remote sensing instrument by means of opto-mechanical scan. Fig.2 is its optical configuration. The 45 mirror conducts line scan which enables the field of view cross over the flight trace and the movement of the satellite around the earth pushes scan line forward, forming two dimensional image of the earth. The primary optical system consists of co-axial and co-focal paraboloids with a diameter of the primary mirror of 200mm. the beam splitters of the visible/IR channels divide the incident beam into infrared, near IR and visible beams, which are projected on the HgCdTe IR detector, single element Si detector and three element-Si detector after being defined in operation spectral bands by filters, forming five detecting channels with an IFOV of 1.2 mrad. The IR detector is cooled by a radiant cooler operation at 105 k with D* = 8.6* 109Cm Hx½ W-1.

The radiant cooler is a passive cooler, which is mounted on one side of the satellite back to the sun light and thermal exchange of radiation between it and 4K cold background of the space is made use of to enable the second patch to reach below 105K. the HgCdTe IR Detector has about 1.2%/K change in relative responsibility at about 105km, therefore, the patch heating circuit of the radiant cooler is used for temperature control with heating power less then 15MW.

Fig. 2 Optical configuration of the scanning radiometer

The optical scanner is composed of an elliptical Beryllium based plane mirror and a driving motor. The mirror is driven directly by the motor with the rotating axis having 450 angle with the mirror and the rotating speed is 360 revolution/min. In normal case, when the synchronous torque is greater then 350 . cm there will be moment redundance greater then a factor of five and the rotating stability better then 1;5*10-4. The rotating shaft is lubricated and sealed by means of labyrinthian packing specially so as to meet the requirement of long life operation in super high vacuum environment.

The signals form the five detectors are amplified by five independent low noise pre amplifiers, then preprocessed by the main amplifier before enter the A/D converter, the and width of the signal is 0.04Hz - 15.,7KHz and the conversion period of A/D converter is 25ms. The detecting dynamic range and detection sensitivity of the target are set during radiation calibration and measured respectively, with the requirements as follows :

Channel No.	Level range	Detecting dynamic range	Sensitivity (NED_r)
Ch1	0-6V	0-85% target albedo	£0.20%
Ch2	0-6V	0-85% ditto	£0.15%
Ch3	0-6V	0-30% ditto	£0.25%
Ch4	0-6V	0-3% ditto	£0.25%
Ch5	6-0V	4-320K target blackbody	NEDT£0.8K

Under the control of the system logic circuit, when the scan mirror rotates to 72.70 nadir (observing 4K background radiation of the space), the main amplifier conducts DC level restoration, establishing radiation level corresponding to zero radiation. For the IR chancel, there is a reference blackbody at the top of the optical scanner. Its temperature is measured by four uniformly distributed Pt resistors. The temperature voltage is transmitted together with image signals, providing another radiation calibration reference for the IR channel, which realizes radiation signal calibration of various at every scan period.

Processing of HRPT and APT imagery information
Fig.3 is a information processing diagram of the scanning radiometer. At an orbit height of 901 k. the satellite angle subtended by the earth is 122.4. Since the earth curvature and the distance from the satellite to the observed point vary with nadir anglea, the ground resolution DL of the image in the directing of scanning is

R-the earth radius; H-orbit height

The actual output HRPT and APT imagery information from the scanning radiometer after seriously distored parts on eh edge are removed is 55.4 both side at nadir. The arc length on the earth surface corresponding to the scan line is 3235 KM, which ensures linking up of the images two neighbouring orbits. The variation range of the ground resolution of images is 10.08-6.8 Km, and sampling output of each scan lien from each channel is 2048 pixels.

In order to expand the imagery information which only occupies 1/3 of each scan line on the time axis so as to fill the whole scan period, it is necessary to have real time rate buffering processing on the high resolution HRPT imagery information. Under the control of the time sequence circuit, five 8 bits A/D converters convert simultaneously the five analog signals at a periods of 25 micros second. Then the signals are stored into the RAM in the order of channel 1-5. The data in the RAM are buffered in the manner of 3 site in and 1 read out. The earth view signals at ± 55.40 on the time axis are expanded to 332.40. after parallel serial conversion 10 bits/word serial code si thus formed. For the rest 27.60 time synchronous code, time code attitude deviation of the satellite and radiation calibration data are inserted by a multiplex data selector, forming HRPT imagery information flux which output at a code rate of 665.4 Kbps. The APT imagery information processor divides the observing angle of ± 55.40 in to nine segments and samplings of various segment are made at different frequencies. The signals are stored in the RAM after A/D conversion, then, linear read out is conducted, thus correction of scan geometric distortion of images is accomplished. The average ground resolution of images is about 4KM.

Fig.3 Information processing daiagram of the scanning radiometer

Fig.4 Ground resolution of APT images in the scan direction

Operation performance of orbit
There are two sets of scanning radiometers mounted on the satellite which operate alternately under the ground command.

The FY-1 meteorological satellite entered accurately the sun synchronous orbit at 5:42 summer time on Sept, 7 1988 and the optoelectronic system of the scanning radiometer successes in undergoing mechanical vibration during the launching of the Long March No, 4 carrying rocket and shock caused by separation of the satellite and the rocket. The ground station received HRPT imagery signals output three minutes later after the satellite entered the orbit. During the effective 385 day's operation, the scanning radiometer obtained a lot of imagery data. After measurements the images of the four visible and near IR channels clearly display the Fuzhou gulf of 1KM wide between the south end of Liaodong Peninsula and changxin Island with the ground resolution reaching the designed specification of 1.1KM and the detecting sensitivity NED_r) better then 0.15%, which also met the designed specification. The image quality has received good comment.

Shortly after the satellite entered the orbit the radiant cooler performed heating and out gassing. Eleven days later the anti contamination cover unlocked reliably and internal heating stopped, as a result the radiant cooler started to cool down and reached the reset temperature of 105K within 36 hours. The signals from the IR channel indicted that the HgCdTe detector operated normally. The amplitude of the IR signals, however tested by the internal reference blackbody, indicated a decrease of a factor of 2.85 to the preset value. After enhancement processing the IR images were clear with a detecting sensitivity 0.5K and ground resolution of the image reaching 1.1KM, which was the designed specification. The measured temperature values of sea surface by the IR channel of the scanning radiometer were the same as those of NOAA satellite operating in orbit. Afterwords, the signals of the IR channel deteriorated rapidly and became ineffective. Analysis of the deterioration in the signals of the IR channel holds that it was caused by the water vapor carried form the ground which condensed on the optical parts at low temperature in the radiant cooler.

Analysis indicates that the visible and IR channels of the scanning radiometer have good observing performance of the clouds and underlying surface. Owing to the supplement of two high gain ocean color channels the FY-1 satellite has been enhanced in the ability of observing land targets and ocean color, eg. having contained clear fluctuation of dune in the Ladan Jilin desert, the water color layers of Lake Baikal details of mixing of the fresh water at Changjiang River mouth, coastal water and water at continental, distribution of silt at Bohai sea and distribution of high concentration chlorophyll etc.

Reference

Gong Huixing, Infrared Research, 9 (1990), 2:81-90
Zheng Qinbo, Infrared Research, 9 (1990) 2:91-98
Wong Cuijun, Li Zhenchang, Zhou Yanmo, Infrared Research 9 (1990), 2:99-107

Fig.5 Flood scene of Nenjiang River taken on Sept. 9, 1988(Ch1, Ch2)

Fig.6 IR picture of Yellow sea taken on Sept. 20, 1988(Ch5)