Objective Accurate, multi-spectral and multi-angle infrared radiation characteristics data of satellite targets are of great significance for the identification of satellite types, functions, key instruments (such as solar wings, antennas, etc.) and the monitoring of satellite working status in orbit. At present, due to the limitation of measurement means and calibration level, the infrared characteristic data obtained by in-orbit measurement generally contains only a few pixels, and there is a lack of high-resolution infrared imaging data of satellites in orbit. Using the opportunity of vacuum thermal test in the satellite development stage, high quality infrared simulation characteristic data measurement of satellite in orbit could be implemented. The traditional method is to place the thermal imager outside the environmental simulation container and to measure the infrared characteristics of the target inside the container through an optical window mounted on the container. Meanwhile, the infrared background of the natural environment and the infrared radiation of the thermal imager will be reflected back to the thermal imager by the optical window. And there will be obvious "ghost shadow" of the natural environmental background and the thermal imager on the optical window. Through the vacuum low-temperature adaptive design, the infrared thermal imager can be placed in the environmental simulation container, and the influence of the optical window can be eliminated from the root, and the accuracy of the infrared radiation characteristics data can be greatly improved.

Methods An oil removal process ensures the normal use of the infrared thermal imager under vacuum conditions better than 5×10⁻⁴ Pa. The active and passive thermal protection are carried out respectively through a thermal control system placed on the shell surface of the thermal imager and a specific shield designed for the thermal imager. Thus, the temperature control requirement of the thermal imager in the range of −10 ℃ and 20 ℃ is satisfied under the environmental condition of −173 ℃. On the other hand, through the thermal control treatment of the thermal imager shield surface, the emissivity and the heating rate of the thermal imager under high temperature environment are reduced, and the long-term use of the thermal imager under the long-term irradiation environment of the solar simulator is ensured. Through the integrated design of infrared thermal imager and PTZ, the horizontal rotation and up and down pitching motion of thermal imager on PTZ are realized.

Results and Discussions The low-temperature blackbody developed by Beijing Institute of Spacecraft Environment Engineering (BISEE) was used as the calibration source of the thermal radiation, and four position points around the target on temperature measurement surface are selected where four thermocouples T1-T4 are pasted on(Fig.5). The temperature measured by the thermocouples was taken as the actual temperature and compared with the temperature data of points P1 to P4 set at the same position in the screen of the thermal imager. The temperature measurement error of the thermal imager is ±3 ℃, and the thermal imaging remains stable with a temperature measurement error close to 0 ℃. At the same time, there is no 'ghost shadow' phenomenon in the infrared image of the calibration source collected by the thermal imager.

Conclusion Using the opportunity of vacuum thermal test in the satellite development stage, clear and accurate satellite in-orbit simulation infrared images can be obtained by developing and transforming infrared acquisition equipment adapted to the vacuum low-temperature environment. These images can support the research of satellite in-orbit status monitoring and function analysis based on infrared characteristic data, and can also support the demonstration and construction of infrared sensor related equipment.