## 4 Conclusions  
Geosynchronous SAR (GEOSAR) missions require unprecedented orbit determination precision in the scale of meters. A precise orbit determination technique based on interferometry is proposed, and the first experimental proof of concept is presented. The results conclude that interferometric orbital tracking of SAR missions can be successfully performed making use of static low-gain antennas, avoiding the use of complex dynamic pointing. The interferometric system would be able to track SAR satellites making use of signals such as radar, telemetry, or pilot beacon transmissions, even in adverse SNR conditions such as radar transmission sidelobes. In comparison with other techniques, interferometry presents the advantage of tracking the radar antenna phase center, which is the information required in SAR processing, hence avoiding the need to compensate for satellite body lever arms and attitude-induced phase changes. Figure 8: Resulting trajectory of the Astra 1M satellite represented in the Earth-Centered Earth-Fixed (ECEF) coordinate system. Each figure represents three orbit arcs of 25 h each along the interferometric observation period. The \(Y\) axis points to eastern longitudes and \({}^{\star}\) indicates an orbital maneuver. (**a**) The satellite slows down its movement towards the west from 9 to 12 April, when it starts naturally drifting towards the east. (**b**) The satellite keeps drifting towards the east from 17 to 21 April, when a station-keeping maneuver is performed, causing it to start moving towards the west. Solar radiation has an important effect on the interferometric phase measurements. Therefore, the system must be designed in order to minimize the impact of temperature drifts, in particular minimizing the length and solar exposure of radiofrequency cabling. A compact interferometric baseline of 10-15 m was conceived as a validation prototype due to its convenience of operation and testing. The experimental system setup allowed acquiring continuous interferometric signals of a communications satellite along several weeks involving several cycles of orbital drift and station-keeping maneuvers. They were used to retrieve the trajectory of the satellite by means of a least-squares estimation technique, a dynamical model tailored for geosynchronous orbit, and a Runge-Kutta-Fehlberg 7(8) integrator. The estimated trajectory was used to compute the estimated observations. The good fit between the measured and the estimated data demonstrated that interferometric orbit observation is a valid technique for tracking satellites at GEO, even in the presence of station-keeping maneuvers. Furthermore, the retrieved trajectory of the satellite, represented in the ECEF coordinate system, presents the expected elliptical motion with a daily period, as well as a long-term cycle corresponding to the natural longitude drift towards the east and the subsequent station-keeping maneuver towards the west. Since the interferometric observation system is able to work with weak satellite signals, it only requires to be in the line-of-sight of the target satellite. Therefore, a single interferometer is suitable to track spacecraft in any low-inclination geosynchronous orbit. However, high-inclination geosynchronous missions, such as the ones proposed by China, with a large \"figure 8\" shape nadir-point trajectory, would require extra interferometric receivers in order to keep track of the spacecraft along all latitudes. Future work will provide observables obtained with longer interferometric baselines in order to assess the degree of precision the technique can achieve. Conceptualization, J.N.-A., X.C.-M. and A.B. ; methodology, J.N.-A., X.C.-M. and A.B. ; software, J.N.-A., E.F. and J.R.-T.; validation, J.N.-A., X.C.-M., A.A. and A.B. ; formal analysis, [PERSON]-A., E.F. and [PERSON]; investigation, [PERSON]-A., [PERSON]-M. and [PERSON]-G.; resources, A.A. and A.B. ; data curation, [PERSON]-A. and [PERSON]-M.; writing-original draft preparation, J.N.-A. ; writing-review and editing, J.N.-A. and A.B.