Formation operations and navigation concept overview for the IRASSI space interferometer

IRASSI is an interferometry-based mission concept, composed of five free-flying telescopes orbiting the Sun-Earth/Moon second Lagrange point, L2. IRASSI aims to observe particular regions of the sky, where Earth-like planets are likely to form, such as stellar disks and cold dust clouds. In these regions, important chemical and physical phenomena observable in the far-infrared frequencies (1 to 6 THz) can be recorded, enabling scientists to study star formation and evolution processes, as well as early planetary origins. Such phenomena must be resolved finely by the telescopes, with the angular resolution goal set to 0.1 arcsec or lower. To achieve such granular resolution magnitudes, the interferometer relies on dynamically changing baselines (i.e., physical separation) during the scientific observations. This is coupled with heterodyne detection, which collects and correlates detected signals of a celestial object from all telescopes simultaneously. With such a setup, the telescopes can combine their detected signals at different locations, improving the obtained angular resolution of the observations. In order to precisely correlate the detected far-infrared signals, the baseline vectors between the telescope reference points must be determined with an accuracy of 5 μm. This requirement poses a challenge to existing navigation technology. To overcome this challenge, a ranging system is employed to provide the distance measurements at micrometer-level accuracy between ranging sensors located at each satellite. A further important task is the determination of the lever arms (between the ranging sensor and the telescope reference point) and its orientation in space. Since the position estimation at L2 cannot be provided in absolute coordinates with such high accuracy, the navigation concept consists of two interacting components: (I) the absolute position estimation, with respect to Earth; and (II) the relative position estimation, determining satellite's positions, with respect to each other. For relative positioning, a geometric approach is currently under development, because it is capable of real-time processing. In future work, a semi-kinematic approach is aimed for, i.e., process dynamics is included, in order to increase robustness in case of measurement outliers or missing measurements. For absolute positioning, the Earth-based navigation method has been selected due to its potential to satisfy high accuracy expectations for the initialization of the relative position algorithms. The paper begins with a brief description of the IRASSI mission, where the issue of baseline accuracy determination is introduced. The Nominal Operations phase of the mission is presented in detail, with a focus on formation dynamics. A description of the navigation concepts, including the absolute and relative position estimation, is thereafter provided. The absolute and relative position estimation is analyzed in terms of their dynamic models, observation models and estimation methods. Furthermore, their performance relative to the requirements and overcoming challenges are discussed and compared in terms of their response time and accuracy. The key challenges in the frame of baseline measurement accuracy is then highlighted. Finally, the content of the paper is summarized and the scope of future work concludes the manuscript.