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The future of spaceborne systems for Earth Observation is moving towards formations of multiple lightweight spacecrafts, either to enhance the surface temporal coverage or to augment the system performance by increasing the number of simultaneous observation modes. In the case of Synthetic Aperture Radars (SAR), the latter option is of particular relevance and is mainly brought forward by bistatic and multistatic SAR system concepts. In this context, The ESA Earth Explorer 10 Candidate STEREOID (Stereo Thermo-Optically Enhanced Radar for Earth, Ocean, Ice, and land Dynamics) is promoting the launch of 2 sub-500 Kg satellites spacecraft carrying a receive-only radar instrument as main payload, and flying in a re-configurable formation with Sentinel-1 C or D, which will be used as illuminator. From the radar standpoint, the main novelty will be brought by the large orbital (along-track) separation of the 2 companions from the transmitter, with baseline values that exceed 200 km. Such unprecedented line-of-sight diversity, in angular terms, is in fact deemed to significantly boost the sensitivity to 3D motion and deformation of over land, ice and sea surfaces. The system will be designed to operate in two main formation configurations: 1) A ‘stereo’ configuration, where one satellite will fly ahead of Sentinel-1 whereas the other will follow behind, at approximately the same distance; 2) a ‘XTI/ATI’ configuration, where the two satellites fly close to each other, and hence on the same side with respect to Sentinel-1. In the first configuration the system captures the widest angular diversity, in the second configuration the system gains single-pass interferometric capabilities that allow to perform either topography or coherent target velocity retrieval. As the mission acronym suggests, the system carries an additional medium resolution multispectral payload with a VNIR and a TIR component, that shall not be considered less relevant than the microwave one. Although the orbit configuration and the viewing geometry is generally not ideal for an optical payload in isolation, the assimilation of such simultaneous optical and radar observations are indeed considered fundamental for the achievement of several mission goals, such as for the enhanced retrieval and interpretation of the sea surface currents. Several technical and scientific challenges shall be dealt with in the next few years, i.e. during the study phase of the mission. This work will provide an overview of these challenges, focusing in particular on to the coherent and incoherent signal properties of the STEREOID bistatic measurements, on their benefits as well as their negative implications on retrieval performances, and on the possible assimilation strategies for their integration with the optical measurements.
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