ONION analysed user and stakeholder needs, the state of the art in Distributed Satellite Systems (DSS), architectural elements, related technological gaps, expected trade-offs, key enabling technologies, and identified 10 uses cases with a focus on Marine weather forecast (MWF); Artic Sea ice monitoring; Agricultural hydric stress; and Fishing pressure & Aquaculture, providing high revisit, high access time, and complexity of measurements requirements. The user needs and current technology maturity of the European EO infrastructure were translated into functional requirements.
Upon UAB recommendations, the ONION architecture selection process has been detailed only for the MWF and followed by a Tradespace Exploration, to filter down the possible geometries of constellations (among 104 architectures) and including ONION nodes (a spacecraft or set of spacecraft). Simulation tools were used to assess revisit time and latency parameters.
A space model-based optimisation framework (OASF) has been implemented and tailored for assessing functional characteristics and ONION mission performances, including costs, use-case requirements and high-level architectural attributes. The best ranked architectures for the MWF use case have been deeply analysed to evaluate key parameters (revisit time, data latency, on board memory evolution, power budget of each satellite, ground station contact, Inter-satellite links...). The selection process has been extended to the Agriculture Hydric stress use case.
Due to the types of instruments embarked and the nature of the potential Federated missions (Metop, Cryosat-2, Swot-Karin, etc.) the selected ONION architecture has been configured in a Walker delta constellation orbiting at 807 km, in SSO orbit, and composed of 16 nodes, distributed in 8 planes. Four of these planes encompass heavy platforms with both SAR-X and optical imager, while the other four allocated small nodes (6U type of CubeSat) with GNSS-R instruments. These two types of planes are alternatively distributed in the constellation.
Some analysis shown that the data flow and on-board data handling is not a critical point, and many satellites are in visibility of one ground station at the same time. To improve revisit time below few hours, more instruments are required, on the same orbital plane and on more orbital planes. The most critical aspect is the power budget.
The technological gap analysis presented how far is the current space market, from the ONION system defined and identified technologies needing further evolution to reach ONION requirements. The results have been presented to stakeholders, experts and wide public in workshops, with a video and public reports on the website.
