Very high resolution scanning radiometer on FY-1 meteorological satellite and its operation
performance in orbit
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 (NEDr) |
| 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.
