EROSS+ Phase A/B1 Guidance, Navigation and Control design for In-Orbit Servicing
Authors:   Thales Alenia Space, Cannes, France, Casu Davide , Dubanchet Vincent , Renault Hervé , Comellini Anthea , Dandré Pierre 
Published: January 2024

The H2020 project “EROSS+ Phase A/B1” standing for “European Robotic Orbital Support Services” has been over 2021-2023 to mature the future robotic servicing missions with a highly-autonomous and coupled Guidance, Navigation and Control (GNC) architecture for both a satellite platform and its embedded robotic arm. This project is built upon the previous developments of the Operational Grants led by the Strategic Research Cluster in Space Robotics funded by the European Commission since 2016. More specifically, EROSS+ project aims at deriving a system design of a robotic Servicer approaching, capturing and servicing a Client satellite. It thus integrates and demonstrates the key European robotic building blocks by demonstrating their performances from Model in the Loop (MIL) tests to Hardware in the Loop (HIL) experiments.

The main use-case of EROSS+ project is to demonstrate the capability of a Servicer spacecraft to perform medium and close-range rendezvous, before capturing and manipulating a Client satellite with a high degree of autonomy. The client satellite is considered collaborative and prepared for servicing operations such as refuelling and payload replacement. EROSS+ timeline is based on four main steps covering the approach with an autonomous visual-based navigation using advanced processing and filtering techniques; the capture using state-of-the-art compliance control techniques to synchronize the robotic arm and its platform; the mating of the two spacecrafts through a dedicated interface for refuelling; and then the robotic exchange of a replacement payload designed with standard interfaces.

The goal of this article is to present the work done in EROSS+ Phase A/B1 aiming at deriving a versatile GNC architecture for multi-orbit purposes LEO and GEO minimizing delta-design impact.

Initially, we will present an overview of the main design drivers and requirements, both in terms of mission constraint (e.g., ground station visibility for communication and monitoring of critical operations, observability constraints) and safety requirements (e.g. exclusion zones, approach corridors, and GO/NO-GO decision at predefined hold points). In fact, the RDV strategy proposed has to be compliant to the ESA Safe Proximity Operation guidelines and the French Space Act (“Lois des Operations Spatiales”). A high level description of the CONcept of OPerationS (CONOPS) will be provided, focusing on the strategy from Phasing, Homing, Closing, Inspection, Approach and Capture. The RDV mission will be divided in a long-range phase with absolute to relative navigation synergy and full autonomous handover, and an autonomous but supervised safe short-range phase.

A description of the mission timeline will follow, taking into account the synchronization of visibility windows for data exchange with Ground (operational procedures, checks and TC GO/NO-GO, monitoring, data downloading/uploading), the execution of maneuvers from main boost to correction maneuvers, and navigation observability constraints for vision, based navigation (e.g., Sun Phase Angle and eclipses). Short range maneuvers from fly-around, inspection and forced motions along predefined corridors will lead the Servicer to the capture-point conditions, where the robotic arm will initiate robotic capture and mating.

Then, we will focus on the On-Board Autonomy layer and high level FDIR strategy, complying with the safety guidelines of uncrewed missions but still considering the high criticality of the RDV operations (with the constraint of avoiding collisions while ensuring a high probability of completing the mission -without executing too many cost impacting Collision Avoidance Maneuvers). A special attention is given on how safety is addressed and ensured in the GNC design, and verified during the Validation & Verification (V&V) process (safety by design is ensured when possible -i.e. passive safe orbits design-), and how early safety and FDIR analyses impact the overall architecture and design choices.

Subsequently, we will detail the GNC operational modes and phases to ensure the In-Orbit Servicing mission, firstly describing the high level architecture, which is oriented on the operational modes and the Hardware Matrix (i.e. classical modes such as Stand-By Mode (SBM), Safe Hold Mode (SHM), Nominal Mode (NOM), Orbit Control Mode (OCM) for absolute navigation, will be presented along with RDV & capture modes and Collision Avoidance Mode), and secondly focusing on the mid-level layer, which consists in GNC phases (e.g., Station-Keeping (SK), 6 Degree-Of-Freedom forced motion, and others).

The GNC architecture will have to take into account the typical challenges of RDV & capture in terms of HW (i.e., the presence of optical sensors for vision based navigation and image processing, the robotic arm for capture, and so on).

Then, an overview of the main functions of Guidance (e.g., computation of several maneuvers from long range to inspection, fly-around, R-Bar and V-bar forced motions), Navigation (with focus on multiple sensors data fusion) and Control (e.g., open loop, 6DOF Robust control, robotic arm and platform coordinated control) will be provided.

Finally, we will present an brief overview of the GNC workflow -from design to Validation and Verification (V&V)- executed following the Agile philosophy to be pursued along the following EROSS In-Orbit Demonstration (IOD) program. The V&V process is allowing fast prototyping from building blocks development to flight software, thanks to the autocode framework and early integration of the GNC software in representative space hardware, to asses required computational budgets and confirm early preliminary algorithms profiling. Early testing of several hardware elements has taken place during the Hardware-In-the-Loop testing (e.g. vision systems capability assessments in robotic test benches) to corroborate Processor-In-the-Loop, Model-In-the-Loop, and Software-In-the-Loop tests, from open-loop simulations to full GNC closed-loop ones. The core GNC software is claimed to be at TRL5 at the end of EROSS+ Phase B1.

EROSS+ project has been co-funded by European Union’s Horizon 2020 research and innovation program under grant agreement N°101004346 and is part of the Strategic Research Cluster on Space Robotics Technologies as Operational Grant n°12. Thales Alenia Space has led this project in collaboration with DLR, GMV, SINTEF AS, and PIAP Space.