# Experimental Validation of GNSS Interferometric Radio Occultation
## 5 Conclusions  
In this work we have presented experimental results of the interferometric Radio Occultations technique (conventional and with reference signal acquired at Zenith) to measure the Doppler frequencies of GNSS signals, and compared the results with the ones obtained with other processing techniques, such as closed-loop and open-loop tracking using the clean replica.  
We have seen that the conventional iRO technique is easy to implement in terms of signal processing needs, but suffers from inter-satellite interference due to the relatively large antenna beam width. In our experiment the interference was from a SBAS satellite at GEO orbit with nearly constant amplitude and Doppler, which we were able to estimate and remove from the correlation outputs in post processing. After the inter-satellite interference mitigation, the Doppler of the received signal has been estimated showing good agreement with its prediction. In terms of system architecture, implementing a receiver that employs the iRO technique requires highly directive, and thus large, antennas.  
The precision of the iRO Doppler estimation depends on the SNR at the output of the cross-correlator. The relationship between the clean replica SNR and the interferometric SNR is analyzed and validated with the measured data. This analysis enables to model analytically the performance of the iRO technique in future space-borne GNSS remote sensing missions under different orbital and instrumental configurations.  
In the demonstration of the zenith iRO technique, the reference signal was obtained when the SNR was high enough and the received signal did not suffer serious interference from the sea surface reflection. Moreover, the reference signal was averaged coherently before the cross-correlation to improve SNR, which can be also used in the space-borne scenario in case the antenna gain is not high enough. Also the effect of the local oscillator frequency offset on the observed Doppler estimation has been modeled analytically and successfully applied to the experimental data. After the comparison with the results obtained with the conventional clean replica open-loop processing, it is proved that the zenith iRO technique can achieve similar performance as the conventional approach. These results are relevant to potential future missions implementing interferometric GNSS reflectometry, such as the ESA PRETTY mission, enabling complementary iRO measurements using the same architecture.  
Conceptualization, S.R., W.L., E.C., F.F., A.R. and M.M.-N.; Data curation, S.R.; Formal analysis, S.R. and W.L.; Funding acquisition, S.R., E.C., A.R. and M.M.-N.; Investigation, S.R. and R.P.; Methodology, S.R. and W.L.; Project administration, S.R. and M.M.-N.; Resources, S.R.; Software, W.L.; Supervision, S.R., A.R. and M.M.-N.; Validation, W.L.; Visualization, W.L.; Writing--original draft, S.R. and W.L.; Writing--review & editing, S.R., W.L., E.C., F.F. and R.P.  
Figure 15: Doppler observation after the correction of the local oscillator frequency offset.  
**Funding:** This research was funded by the Spanish research projects: ESP2015-70014-C2-2-R (MINECO/FEDER) and RTI2018-099008-B-C22 (MCIU/AEI/FEDER, UE), and ESA contract: RFQ/3-12747/09/NL/JD-CCN5.  
**Acknowledgments:** The authors would like to thank [PERSON] and [PERSON] from GFZ for lending us the JAVAD Delta Quatto-G3D GPS receiver that we have used in this work, and the _Museu del Suro de Palafrugell_ for allowing us to carry out the experiment from the top of the medieval tower at Cap de Sant Sebastia.  
**Conflicts of Interest:** The authors declare no conflict of interest.