Along-Track Radar Interferometry for Maritime Surveillance Using a Tethered Satellite System

The European marine domain faces constant challenges, from illegal tuna and swordfish capture to border control and even piracy or smuggling operations, and the need for a specialized surveillance system is on the rise. We aim to fill that role by providing an all European dedicated architecture, working in support of coastguard airborne and ground-based operations. Our measurements can help discerning both the position and the velocity of the vessels, by carrying out multi-angular synchronous radar interferometry operations, enabled by an Along-Track stabilized tethered satellite system. The strength of the proposed design lies on its high level of adaptability and performance for SAR Interferometry missions. By keeping the tether in tension with electrical propulsion, it is possible to maintain constant baseline and performances. Moreover, tether-torque-based control can be implemented to further stabilize each satellite's attitude during operations; furthermore, by acting on the length of the tether, it is possible to change the spacing of the satellites between observations, tailoring the measurement's sensitivity to specific moving targets or terrains. Finally, the system can switch the observation geometry multiple times during the same mission, for example by aligning and stabilizing the system with the along-track direction for maritime surveillance or with the cross-track one for Digital Elevation Models. This paper presents a study of feasibility for different system configurations and a parametric, multi objective, orbit and payload optimization for maritime surveillance application focused on the Mediterranean sea. The total observed area and observation time window are exploited to build different cost functions according to the use case. To do so, both the orbital elements and the SAR antenna specifications are used as optimization variables to assess the preliminary system design. Final results present the optimized architecture and how it can be integrated within the existing ecosystems and dataset (airborne and ground) to increase the performance for maritime surveillance. The paper will showcase the obtainable performance and highlight the flexibility of the system, outlining how the tethered configuration can be seamlessly and uniquely applied to meet different commercial opportunities during the lifetime of a single mission.