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Earth observation is among the most prolific space applications. Earth imagery, for instance, has initially been used for defence and then for scientific purposes. It gained high popularity within the NewSpace paradigm as new data products became commercial successes. This was partially because new, very small satellite form factors such as the CubeSats format allowed very low data production cost. Imagery is still the most common Earth observation type. Alternatives are atmosphere occultation methods and radar methods. Latter allows observing the Earth in day and night, and in cloudy conditions. However, classical, so-called mono-static radar applications require high power output for the interrogating primary signal. Thus, those are typically not suitable for CubeSat platforms. Recently, a CubeSat-compatible, novel method has emerged that enables all-lighting and all-weather Earth observation, that is, Global Navigation Satellite System-reflectometry (GNSS-R) method described in Figure 1. It receives GNSS signals reflected off the Earth surface and allows hence determining surface properties. This technique constitutes a bi-static radar and signal-of-opportunity as it makes use of signals available but not designed for this application. A disadvantage of the method lies in the size of the reflection area also known as glistening zone commonly estimated to be 10 km in diameter. Beamforming allows to significantly reduce this size. It requires a very large antenna array or the distribution of antenna array elements over several satellites. Within the High-resolution Reflectometry Solar-aerodynamic Satellite Swarm (HiR3S) concept study, we brainstorm an Earth observation system based on a CubeSat-formation of in total 20 satellites. Those fly in proximity to enable the beamforming at the L-band GNSS frequency band. To maintain the low cost of the satellites, a propellentless method is proposed for orbit control. The moderate requirements onto the formation flight, that is, the moderate need of an accurate relative positioning of the formation members and the absence of an agility requirements permits the use of aerodynamic forces and solar-radiation pressure for formation deployment, maintenance, and reconfiguration. The method is highly cost-efficient as no additional hardware subsystem is needed. Existing deployable solar panels are used as sails controlled by an on-board algorithm. The concept study establishes the main characteristics, main engineering budgets and a conceptual reference design. It consists of 19 satellites in a circular formation with a central satellite no participating in the beamforming but acting as communication relay as illustrated in Figure 2. Our preliminary computations reveal the resolution of the Earth observation method can be improved by an order of magnitude and thus advocates to further study and eventually implement this mission concept.
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Thoemel, J., Drouguet, M., Tabibi, S., Lederer, D., Lambot, S., & Craeye, C. (2023). Enhancing GNSS-reflectometry Earth Observation through a CubeSat Formation. 13th European CubeSat Symposium, Leuven. https://hdl.handle.net/2078.5/240751