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Thermal Subsystems

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2.3.3 Thermal Subsystems

Radiator A and Active Thermal Control

The OBM needs to be operated in orbit at a constant temperature to preserve validity and accuracy of the on-ground calibration & characterization. Additionally, a low temperature level is required to keep the thermal radiation of the instrument itself at a minimum in order not to enhance the background in the SWIR channels 7 & 8. Therefore a dedicated radiator, RAD A, is used to cool the OBM and the detector module electronics to between -17.6 and -18.2 °C. Its location on the -X side of the instrument avoids direct solar illumination. Heat pipes are used to transfer heat from the OBM and the detector module electronics to the radiator.

While the RAD A provides cooling capacity, thermal stability of the OBM needs to be established via a closed loop Active Thermal Control (ATC) system. It consists of three control loops with heater circuits and thermistors. The heating is controlled by the Power Mechanism & Thermal Control Unit (PMTC) based on measurements by the thermistors. Once ATC settings have been selected, the system maintains the OBM temperature to high precision at the specified temperature.

Thermal Bus and Radiant Cooler Assembly

In-orbit operating temperatures of the detectors lie well below ambient. The detectors are cooled via the Radiant Reflector Unit of the Radiant Cooler Assembly. It is the task of the Thermal Bus to connect the detector modules thermally with the reflector. Heat from detectors 1-6 is transported via an aluminium thermal conductor, from detectors 7 & 8 via two methane filled cryogenic heat pipes. The heat pipes provide an efficient heat transfer in the temperature range 100-160 K. Since the cooling efficiency of the Radiant Cooler is designed to be adequate until the end of the mission, a Thermal Control (TC) system is part of the Thermal Bus. It prevents the detector modules from becoming too cold by counter heating using three trim heaters. The TC system uses open loop heater control. Whenever drifting temperatures of the detectors reach their limits, the power settings of the trim heaters are adjusted by ground command so that the thermal response brings temperatures back within the specified range.

SCIAMACHY’s SRC dissipates heat generated in the detector modules to deep space to permit cooling of the detector arrays to in-orbit operating temperatures.. As for RAD A, the RRU points in the -X direction away from the sun. Earthshine and sunshine are blocked from the radiating surface of the SRC to gain maximum cooling efficiency. Cold temperatures are obtained using a two stage process. An intermediate stage in the Radiator Unit lowers temperatures of detectors 1-6, while the cold stage, fitted with a parabolic reflector, yields temperatures around 150 K for detectors 7 & 8. Due to its low temperature, the RRU surface is expected to attract contaminants from the in-orbit environment, particularly from ENVISAT itself. This would degrade the performance of the Radiant Cooler leading to reduced cooling efficiency. In order to clean the Radiant Cooler, the cold stage and the reflector are equipped with decontamination heaters. Turning the decontamination heaters on raises the temperatures of the RRU, contaminating substances are removed through evaporation and the cooling performance is re-established. Whether contaminants begin to degrade the SRC performance can be determined from the power settings of the TC trim heaters. Degraded cooling efficiency is equivalent to higher radiator temperatures, i.e. higher detector temperatures. Consequently the trim heaters require less power when the SRC efficiency degrades because of contamination.

Because of the necessity to heat up the detectors as much as possible to effectively get rid of the ice layers deposited on channels 7 & 8, the original decontamination procedure was re-defined to form a Non-Nominal Decontamination (NNDEC) to be used during routine operations. During a NNDEC not only the SRC decontamination heaters provide energy to the optical subsystem but also ATC and TC heaters are switched to their maximum power. In the warm-up phase of NNDEC, channels 7 & 8 reach temperatures of 267 K and the OBM approaches a temperature of -3 °C. Also the duration of the warm-up phase was extended by up to 15 days. Measurements continue throughout warm-up and cool-down. Data analysis still permits retrieval of – somewhat degraded – information from the UV-VIS channels even at elevated temperatures such that no long data gaps occur.

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