The Role Of The Instrument Control Unit Within The Ariel Payload And Its Current Design

ARIEL, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey mission1{3 was selected in early 2018 by the European Space Agency (ESA) as the fourth medium-class mission (M4) launch opportunity of the Cosmic Vision Program, with an expected launch in late 2028. It is the first mission dedicated to the analysis of the chemical composition and thermal structures of up to a thousand transiting exoplanets atmospheres, which will expand planetary science far beyond the limits of our current knowledge.The ARIEL Payload (P/L)(4-6) is designed to carry out transit spectroscopy during both primary and secondary planetary eclipses, which in the end will form a wide picture on the nature of exoplanets atmospheres and their interiors, by determining the key factors that influence the formation and evolution of these planetary systems.(7, 8) ARIEL will not only observe warm and hot transiting gas giants, but also super-Earths and Neptunes around a large range of host star types, however it will target planets hotter than 600 degrees K to take advantage of their well-mixed atmospheres. The mission will exploit the spectral range between 1.10 and 7.80 mu m, and broadband photometry in the optical (0.50 - 0.80 mu m) and Near IR (0.80 - 1.10 mu m).The Fine Guidance System (FGS) is one of the instruments of the Payload and it contains three photometric channels (two of which are used for guiding as well as science) between 0.5-1.1 mu m and a low resolution NIR spectrometer for the 1.1-1.95 mu m range. An IR Spectrometer (AIRS)(9) is also foreseen along with the FGS, as it will provide low resolution spectroscopy in two IR channels: Channel 0 (CH0) for the 1.95 to 3.90 mu m band and Channel 1 (CH1) for the 3.90 to 7.80 mu m range. At last, an Active Cooler System (ACS) that includes a Ne JouleThomson cooler is adopted to give active cooling capability to the AIRS detectors, which will work at cryogenic temperatures. AIRS is meant to be located at the intermediate focal plane of the telescope and common optical system and it will host two HgCdTe-based hybrid IR detectors and two cold front-end electronics (CFEE) for detectors control and readout. Each CFEE is driven by a Detector Control Unit (DCU), which is part of AIRS but is hosted within and managed by the Instrument Control Unit (ICU)(10). ICU is a warm unit that is located inside the spacecraft Service Module (SVM) and it is based on a cold redundant configuration with the Power Supply Unit (PSU) and the Commanding and Data Processing Unit (CDPU) boards; both DCUs are instead cross-strapped and can be managed by the nominal or the redundant (PSU+CDPU) chain. ICU is in charge of AIRS management, collecting scientific and housekeeping (HK) telemetries from the spectrometer and HK from the telescope (temperatures readings), the P/L Optical Bench (OB) and other Subsystems (SS), because of a warm slave unit (TCU, Telescope Control Unit) interfaced to the ICU. After being collected, Science and HK telemetries are then forwarded to the S/C for temporary storage, before finally sending them to Ground. In this short paper we describe the status of the ICU design at the end of B1 Phase, just after the Mission Adoption Review (MAR) by ESA, and explain some still open architectural choices to be addressed and finalised once the ICU industrial Prime contractor is selected.