The performance of a compact baseline interferometer for orbit observation of SAR missions is assessed. The proof of concept is carried out by tracking the non-cooperative ASTRA 1M satellite. A nominal slave-master baseline provides orbit observables in terms of phase measurements. A second baseline has been added. The reflector of the second slave has been removed in order to test how the system performs with an important SNR degradation. Experimental results show how, despite the module of the correlation drops dramatically due to the SNR degradation, the quality of the phase measurements remains unaltered. This demonstrates the robustness of the interferometer and allows to work with weak signals.