Retrieving 3D microphysical properties of shallow clouds with nanosatellites flying in formation

Formation flying enables innovative Earth observation missions. Synchronised images from several viewpoints can be captured with a fleet of satellites in close formation. CloudCT is implementing such a mission to observe fields of warm, small clouds. These images are post-processed to retrieve their 3D micro-physical properties using computer tomography approaches considering multi-scattering and the sun as an uncontrolled light source. The retrieved data will be used as validation data to high resolution Cloud Resolving Models (CRM) dedicated to improving cloud parametrisations to reduce uncertainty in Global Climate Models (GCM). The retrieval method introduces requirements on both, the single satellite as well as on the whole system. The payload selection is driven by maximising the information gained through the interactions of the recorded electromagnetic radiation and the clouds' droplets. Relationships exist between the camera's spatial resolution and the orbit geometry. Both spatial resolution and overlapping area requirements affect Attitude Determination and Control (ADC) and due to the inter-dependency of requirements and constraints in this mission, several trade-off studies were necessary to consolidate a feasible system design. These trade-off studies were performed by application of classical system engineering methods in combination with computer aided approaches in simulation. Three computer simulation frameworks were utilised to analyse the effect of different parameters on the overall system: In a radiative transfer simulation we analysed the effect of wavelengths, bandwidth and ground sampling distance on the accuracy of the retrieved physical variables. Simulations on formation acquisition and maintenance have been implemented to analyse the impact on low altitude operations and establish a preliminary ΔV budget. Moreover, satellite pointing was simulated to identify the impact of pointing errors on the overlapping area. The trade-off studies have shown, that the mission can be implemented using a 3U design as originally planned. Analysis of a String-of-Pearls formation has shown, that a 500km altitude orbit baseline is a good compromise on mission lifetime for the available thrusters and achievable ground resolution given all risk mitigation criteria.