ESA GNC Conference Papers Repository

Refactoring the AOCS/GNC Sub-System from HERA to COMET-INTERCEPTOR - a Lesson in Minimizing Validation Efforts Using Small-Body Heritage
Simon Chalkley, Claire McCrorie, Francisco Da Silva Pais Cabral, Pablo Colmenarejo, José Francisco Briz, Pedro Cachim, Maciej Quoos, OleksanNaronin , Julia Wajoras, João Pedro Rei, Jorge Arnedo, Carlos Corral Van Damme, Manuel Sanchez Gestido, Andrew Hysl
Presented at:
Sopot 2023
Full paper:

In the context of the parallel system studies for the European Space Agency's Comet Interceptor Mission, during its phase A/B, GMV performed the design of the AOCS + GNC (AOGNC) sub-system. This is a cost efficient mission by ESA that is run under the “design-to-cost” approach – meet the system requirements and maximize scientific return on an available budget. GMV’s answer has been to adapt HERA’s GNC sub-system with focus on reducing architectural and philosophy strategies to minimize the validations efforts, one of the main cost drivers of a sub-system development. In this paper GMV will present the changes that performed to the HERA S/S to answer to the COMET-I need under a methodology that reduced developments effort but, more importantly, reduced validation effort in a potential ensuing phase C/D. The approach to this refactoring was methodic and followed a protocol: 1.Analysis of the COMET-I mission and comparison with the HERA mission at the level of environment, requirements, objectives, and design drivers. 2.Identification of the required AOGNC modifications at the functional and performance algorithmic levels. 3.Identification of different solutions at the architectural and algorithmic levels and that answered to the required modification identified in step 2. 4.Identification of the required facilities, or modification from existing ones, to test and validate the AOGNC S/S for each solution. 5.Trade-Off and selection of the optimal design based on requirements compliance and effort/cost. 6.Development of the required facilities to test the selected 7.Development of the selected approach at the different levels (architecture, algorithmic) and validation in the FES (Functional Engineering Simulator) and by the run of a HIL (Hardware-In-the-Loop) campaign. COMET-I presents a novel challenge to the sub-system designer, rare in previous missions. This is a F-class mission, launched in piggyback with the ARIEL mission, expectedly in 2028, to the Sun-Earth L2. From here, the mission will wait for up to three years until a long-period comet (LPC) is identified; if no suitable target is identified, the mission will proceed to visit a known backup target. After identification, the spacecraft will transfer back to Earth to perform a Moon swing-by, and transfer to the identified target. This allows the mission to visit targets within a broad range of characteristics (activity level, size, shape, brightness) in a wide range of fly-by geometries (speeds, solar angles, Sun and Earth distances) – this poses challenges for the design and, especially, for the validation of the AOGNC S/S and its units. The following key COMET-I challenges are identified as design drivers for the AOGNC S/S: •A priori unknown target with little information available on this type of comets, LPCs, with only the Giotto Mission going to a short period comet in 1986 resulting in very little flight data to build assumptions or expectations. •High fly-by velocities anywhere between 10 and 70 km/s. As a result, a short autonomous phase will not allow recovery of significant failures, hence the AOGNC shall be designed to provide added robustness. •Fly-by Solar aspect angles between 45 and 135 degrees; unknown target size, shape or activity level. This means that the image processing must be able to cope with a wide range of conditions. Validation facilities must be able to simulate this wide range of conditions. •“All-or-nothing” scenario. A fly-by scenario presents a single opportunity to collect the scientific data that will make the mission successful. This drives the need of the AOGNC and its Fault Detection and Isolation component (AOGNC-FDI) to prioritize robustness over performance. •Key scientific payloads to be commanded with rotating mechanisms that may require architectural changes from previous designs (in specific HERA). •F-class mission requiring a cost efficient and fast development solution. In COMET-I, during the fly-by, the AOGNC S/S is responsible for determining the spacecraft’s attitude and relative position and velocity and to command the spacecraft’s attitude to ensure its safety and to point its scientific instruments to the target comet. From the analysis of the scenario that derived these challenges and the system requirements analysis, different solutions were assessed for the AOGNC architectural and algorithmic designs. Modifications were minimized from the previous HERA to the extent possible, especially at the architecture level, but some modifications were identified as essential from the HERA mission: •On-Board Navigation Filter – the need for a more robust approach led to a solution based on the use of an UDU and information matrix formulations for added numerical stability with on-line scaled measurement bias estimation; this is particularly suitable for scenarios where observability can change very fast. •Image Processing – the appearance of a high-activity level comet is significantly different from an asteroid; moreover, the wide ranges of characteristics means that its shape (of body, coma, and jets) can have different sizes, shapes, and gradients. An algorithm that provided an acceptable performance robust to a wide range of possible appearances had to be designed. •Payload Commanding – in COMET-I the AOGNC is responsible for commanding the rotating mechanisms that house the scientific payloads. This requires architectural changes to compute and provide this information to the payloads in a robust way. This paper provides the following novel contributions to the AOCS/GNC literature: •Methodology to re-factor an AOCS/GNC S/S between missions; •Summarized description of specific algorithms for fast fly-by missions: oA robust Image Processing to an unknown dust harsh environment; oA comet image generation tool (CIG) to allow the validation of the AOGNC from the FES to the PFM levels; oA failure robust filter design approach that takes into account possible losses and/or malfunction of either HW or SW due to the impossibility of recovery before closest-approach passage. •Summarized description of a HIL setup approach for comet missions based on the developed CIG.