Feasibility assessment of a Fine Guidance Sensor in the Infrared band
Authors:   Thales Alenia Space In Italy, Turin, Italy, Bosco Andrea , Nicolas Deslaef Ing. , Spagna Alessandro 
Published: January 2024

The former ESA M5 candidate SPICA mission, required an unprecedented attitude estimation performance for an infrared mission, with requirements close to the feasibility limit even using a Fine Guidance Sensor (FGS) accommodated inside the payload. To have a first assessment of feasibility for an FGS in the infrared band ESA founded on 2020 a dedicated study and contracted Thales Alenia Space in Italy for exploring the architecture, the available detectors and the infrared star catalogues. This activity was successful, demonstrating that the attitude estimation performance can be met over the specified sky coverage and identified a preliminary design for the Fine Guidance Sensor.
The demonstration included several trade-offs and sensitivity analyses on the FGS design with the detector selection and its accommodation, the sensitivity analysis on the size of the FOV, and the sensitivity of the performance in terms of sky coverage and FGS Attitude Knowledge Error (AKE) to the integration time, while considering all the design constraints in terms of environment, operations and accommodation.
In parallel INAF Astrophysical Observatory of Turin collaborated to the assessment of existing IR star catalogues to identify the most suitable one for the SPICA infrared band in terms of sky coverage and astrometric accuracy. The studied source catalogues range from Near IR up to Mid-Far IR and the North Galactic Pole was used as study case being a worst case in terms of number of available source in FoV.
The trade-offs were supported by Signal-to-Noise analysis and dedicated simulation of the PSF detection with relevant centroid computation leading to estimate the expected measurement performance at single star level. It was performed an apportionment of the requested attitude knowledge performance (in the order of 0.15 arcsec across the Line of Sight and 40 arcsec along the LoS @ 99.7% confidence level in static pointing conditions) between bias, field-dependent-bias and time random contributors. A preliminary performance assessment using the sky scene of the North Galactic Pole and the baseline of using SMI/CAM instrument of SPICA provided positive results.
It was then derived a plan for the design, development and verification of an FGS equipment sensitive to the Far IR wavelength based on baseline SMI/CAM detector and architecture guided by the challenging aim to reach a TRL level 6. Following one of the main concern indicated by the development plan a second study is started by Thales Alenia Space in Italy with the collaboration of INAF in 2022 to consolidate two key aspects for the FGS AKE performance in the infrared domain: the sky coverage around the Galactic Centre region with the indication of possible critical sky portions, and the star magnitudes prediction in the Far IR based on existing photometry.
The paper will present the main results derived in the two studies with particular attention to the FGS architecture, the sky coverage in the infrared band and the attitude measurement performances.