Architectural design of the ARIEL FGS software

The Atmospheric Remote-sensing Infrared Exoplanet Large-survey was selected as M4 mission in the ESA Cosmic Vision programme. This mission will study the chemical composition of exoplanetary atmospheres via high resolution, multi-wavelength spectroscopy with high photometric precision. These tasks demand highly stable pointing during operation, which is provided by a dedicated Fine Guiding Sensor (FGS). The FGS uses two MCT detectors operating in 0.6-1.95μm range. The instrument provides target identification and centroid measurements to the spacecraft forming a closed loop in the guiding. In addition, the FGS detectors are also used for science including photometric and spectral windows. Our instrument contains it own Data Processing Unit (DPU). This is a dual core LEON-based computer running the Instrument Application Software (IASW). The software implements a large number of ECSS services to fulfill the various operating needs. The mission-specific modes cover target acquisition and tracking tasks, processing of the photometric and spectral windows as well as various calibration modes. Aside from that, the thermal control is also handled by the FGS software. The science data need to be compressed in a lossless manner. In this respect we build upon our experiences gathered in our contributions to the ESA missions Herschel and Cheops. While the science data processing has only soft real-timing needs, the centroiding is critical to run and provide results as fast as possible. We present the architectural design of the software particularly highlighting the low-level software adaptations needed to support the high demands from the centroid timing. The presented overview will cover the current development status of the IASW with a detailed look at the design and expected performance of the algorithms. Furthermore, we will present our development and testing workflow, which is built upon our own EGSE software.