## 2 Methodology
### Experimental Setup  
Since there are no radar satellites operating at GEO yet, the objective is to perform an experimental proof of concept by tracking illuminators of opportunity. The target constellation is Astra 19.2\({}^{\circ}\)E due to its excellent coverage of DVB-S TV signals over Europe and its high number of active channels.  
The proposed interferometric orbit determination relies on the ability of coherent radio receivers to accurately preserve the phase of the downlink satellite signals [34]. The experimental system consists of three main differentiated modules: the front-end receives the channel spectrum from the satellite at the Ku-band (11.70-12.75 GHz) and performs the first downconversion to the intermediate frequency (IF) (1-2 GHz) by means of a commercial Low-Noise Block (LNB). Then, the baseband module performs the second downconversion from the IF to the baseband and digitizes the signal to be processed by the correlator. These downconversions require a set of local oscillators, which must be synchronized. The reference module provides them a clean and stable clock signal in order to preserve the phase coherence of the system.  
This modular approach allows the system to be adapted to any kind of satellite transmission. Instead of receiving Ku-band signals, the front-end can be replaced by another version, which receives a radar signal (at the C-band, for instance) and downconverts it to the IF. Thus, it can track a different satellite only by changing a single module of the reception system, while leaving the rest as they are. Since a radar signal consists of pulses, the correlation method could be optimized in order to only process the useful signal sections. Other than this, pulsed radar signals pose no further problems owing to the relative motion of GEO satellites with respect to the receiver.  
The three receiver antennas are clustered within 1 meter. In this way, the LNB downconverters' local oscillators are synchronized by means of a phase-locked oscillator, which is disciplined to a common main clock. All the local oscillator connections use very short cables in order to minimize thermal phase drifts.  
The measurement sensitivity of the interferometer increases with the separation between antennas (baseline). The secondary antennas are not pointing towards the satellite, but to their corresponding reflectors located about 15 m apart (Figure 3). The reflectors consist of a 1 m\({}^{2}\) flat aluminum plate, which does not introduce any phase noise nor SNR degradation. The performance of the link is excellent since the reflected signal received in the secondary antennas is adequate to watch the satellite TV broadcast. The position of the mirrors with respect to the main antenna must be carefully determined in order to avoid errors. Therefore, the baselines of the system are extended while keeping the antenna connections very short.  
This configuration has been chosen to serve as a proof of concept of the technique. It is an intermediate step before increasing baselines in order to obtain more precise orbit determinations. However, the present baseline benefits from a distance of ambiguity of about 60 km between two possible satellite positions sharing the same phase principal value [35]. Larger interferometric baselines in the order of kilometers present smaller distances of ambiguities in the order of hundreds of meters. For this reason, the short baseline configuration will be maintained throughout the duration of the project, allowing easily selecting the correct orbit from other ambiguous solutions in future large baselines.