The absence of a dense atmosphere in an Earth orbit eliminates the natural heat transfer though convection between the satellite and its environment. Thereby temperatures ranging from -100°C to +150°C are possible, whereas the electronics in satellites are only certified for operation between -20°C and +80°C.Outside of this temperature range it is likely that electronical connections on circuit boards break or melt.
The objective of the thermal control system is to keep the temperature of the satellite and its components within the allowed temperature ranges. The thermal control system consists of several elements such as temperature sensors and elements to influence the temperature on the satellite.
The temperature inside the satellite is measured via Pt1000 platinum temperature sensors. These sensors increase their electric resistance during rising temperatures and this resistance can be measured. Pt-elements provide a better average accuracy over the entire possible temperature range on the satellite compared to other temperature sensors and they do not require measuring amplifiers.
In order to influence the temperature with passive means the main body of the satellite is covered mostly with MLI (Multi Layer Insulation) to minimize the radiation exchange between the satellite and its environment. MLI consists of multiple stacked layers of film made of e.g. polyimides which are seperate by fine synthetic meshes. By these means each film layer decreases the absorption and emission of radiation by the satellite.
On all surfaces not covered with MLI so called SSMs (Second Surface Mirrors) will be applied, which serve as radiators emitting the satellites heat in the form of infrared radiation while reflecting the sunlight.
The batteries and the star trackers are both equipped with appropriate radiators and are insulated from the rest of the satellite to ensure the narrow temperature ranges of these components.
The satellite is also equipped with electrical heating films, which are activated in case the temperatures are too low. These heaters are primarily controlled by the on-board computer of the satellite. In case of a on-board computer failure the heating films are controlled by thermostats (bimetal-switches).