## 6 Conclusions and Future Research  
Autonomous orbit maintenance paves the way for Trusted Autonomous Satellite Operations (TASO) to become a reality in Distributed Satellite Systems (DSS). Here it is shown that TASO is attainable with low-thrust electric propulsion for two main objectives: achieve and maintain the satellite orbit on the constellation, using absolute orbit navigation, and on the formation, using a more precise relative orbit navigation. In this way, there is a dedicated navigation type for each Autonomous Orbit Control objective. As a case study, we considered a DSS mission for Maritime Domain Awareness (MDA) using Distributed Synthetic Aperture Radar (SAR) instruments. We showed a formation geometry capable of tracking ship movements using single-pass Along-Track SAR Interferometry (AT-InSAR). In particular, we have proposed a single pass and multi-baseline implementation using a safe three-satellite formation, which allows us to avoid temporal decorrelation and to have different velocity scales to track simultaneously. In order to improve the repeatability of these SAR products, we have proposed a constellation of these formations. As we demonstrated, this can be solved by autonomous orbit control using low thrusts compatible with electric propulsion. A particular formation mass distribution was analyzed on which there is a chief whose mass is equal or greater than the mass of the followers by certain common factor. It was shown that for the combined formation/constellation control, the relative importance of the control authority (in terms of maneuver total Delta V) of the chief decreases in relation to the equivalent figure for the followers, as this mass ratio increases. A _Constellation of Formations_ approach was proposed as a way to model the problem, and the solution's concept has been determined. The approach is based on the concept of a system of particles to describe each of the formations in the constellation in such a way that the relative control within the formation determines the Formation Flying, while there is a separate constellation control objective stated in terms of the center of mass of each formation, i.e., a constellation of formation's center of masses. Both objectives were solved with the same feedback control law structure using relative orbital elements obtained from the mean orbit elements of each of the spacecrafts. This requires an inter-satellite communication link between the satellites on the same formation for the Formation Flying feedback computation and the knowledge of the constellation objective in terms of mean orbital elements for the constellation feedback computation, which may be obtained onboard by the desired orbit propagation. Finally, notice that with the recent evolution of inter-satellite communications, it is possible to augment this Inter Satellite Link (ISL) capacity in order to share also the SAR data information in order to generate the interferogram onboard and, therefore, deliver it in near real-time to the user. In this way, as distributed satellite system solutions become more readily available, a concept of operation with onboard single pass multibaseline interferometry computation will make it possible to deliver high-quality SAR data products faster to the user for effective maritime monitoring. Additional work must be addressed to perform more realistic simulations, including hardware in the loop to test Global Navigation Satellite System (GNSS) navigation hardware, control nonlinearities, and possible inter-satellite links topologies, in order to complete the mission concept at the flight segment system level. Conceptualization, K.T. and P.S. ; methodology, K.T. and P.S. ; formal analysis, K.T. and P.S. ; investigation, K.T. and P.S. ; validation, K.T. and P.S. ; writing--original draft preparation, K.T. and P.S. ; writing--review and editing, D.S., H.F., K.T., M.E., P.M., P.S., R.S. and S.H.C. ; visualization, K.T. and P.S. ; supervision, D.S., H.F., M.E., P.M., R.S. and S.H.C.