## 3 Results
### SNR Degradation Experiment  
This experiment was carried out in order to test how the interferometric system performs under adverse conditions. It consisted of using two east-west horizontal baselines with almost an identical length of 3.5 m, sharing a common main antenna (Figure 4). On the one hand, the first baseline (\(R_{1M}\)) was designed to work in nominal conditions. The Secondary 1 receiver presents the same parameters as the main antenna, and it works with moderate (10 dB) SNR levels. On the other hand, the second baseline was intended to work in unfavorable conditions, i.e., a very low SNR. The parabolic reflector of the Secondary 2 receiver was removed in order to intentionally cause an important SNR degradation, resulting in a useful signal at \(-15\) dB below noise. The measurements acquired by the two interferometric baselines during September and October 2019 are shown in Figure 5. They correspond to the orbit observations of the geostationary Astra 1M satellite while transmitting DVB-S TV broadcasting signals from a \(19.2^{\circ}\)E longitude station. The left vertical axis represents the interferometric phase measurements. They match the movement of the satellite with respect to Earth. The right vertical axis shows the correlation magnitude, which is defined in Equation (8) and would be one for perfectly correlated noiseless signals. The magnitude of the first correlation, \(R_{1M}\), presents nominal values between 0.7 and 0.8, which is consistent with the channels' limited SNR. In contrast, the magnitude of the second correlation, \(R_{2M}\), drops dramatically due to the SNR degradation caused by the reflector withdrawal. However, due to the large processing gain of the correlator (Equation (1)), the quality of the phase measurements between nominal and degraded conditions is comparable. The Root-Mean-Squared Deviation (RMSD) between the measurements of both baselines presents a residual of 0.18 rad, which is small in front of the interferometric phase evolution of \(\approx\)5 rad per day. Figure 3: Air-view layout of the interferometric system. The satellite signal (blue dashed lines) is directly received by the main antenna and the mirror reflectors. The secondary antennas point to their corresponding mirrors, which reflect the satellite signal (green and red dashed lines). The reflectors allow increasing the baseline while keeping the receiver antennas very close, so they can share a common reference frequency with minimal phase drift error induced by temperature changes. This test demonstrates the robustness of the interferometric satellite tracking system. It is capable of working with very low (\(-15\) dB) SNR levels. Therefore, it would be able to track satellite signals such as radar, telemetry, or pilot beacon transmissions, even in adverse SNR conditions such as radar transmission sidelobes. Hence, it only requires being in the line-of-sight of the target satellite, making a single interferometer 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. Furthermore, the results prove that the noise of the observations is not due to internal noise or a systematic uncertainty of the measurements. Even though the SNR at the Secondary 2 receiver is negative, the large processing gain of the correlator is able to recover the useful signal. Moreover, the GEO dynamical model acts as a very narrow filter since it rejects the observations, which are not compatible with the dynamics of the GEO orbit. On the other hand, the hourly pattern of phase disturbance with higher  
Figure 4: **(a)** The validation interferometer consists of two baselines. The correlation between the secondary receivers is performed with respect to the common main antenna. The first \(R_{1M}\) baseline operates in nominal conditions.