# Experimental Validation of GNSS Interferometric Radio Occultation  
[PERSON]  
1 Earth Observation Group, Institute of Space Sciences (ICE, CSIC), 08193 Barcelona, Catalonia, Spain; [EMAIL_ADDRESS] (W.L.); [EMAIL_ADDRESS] (E.C.); [EMAIL_ADDRESS] (F.F.); [EMAIL_ADDRESS] (R.P.);[EMAIL_ADDRESS] (A.R.)12 Institut d'Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Catalonia, Spain 2  
[PERSON]  
1 Earth Observation Group, Institute of Space Sciences (ICE, CSIC), 08193 Barcelona, Catalonia, Spain; [EMAIL_ADDRESS] (W.L.); [EMAIL_ADDRESS] (E.C.); [EMAIL_ADDRESS] (F.F.); [EMAIL_ADDRESS] (R.P.);[EMAIL_ADDRESS] (A.R.)12 Institut d'Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Catalonia, Spain 2  
[PERSON]  
1 Earth Observation Group, Institute of Space Sciences (ICE, CSIC), 08193 Barcelona, Catalonia, Spain; [EMAIL_ADDRESS] (W.L.); [EMAIL_ADDRESS] (E.C.); [EMAIL_ADDRESS] (F.F.); [EMAIL_ADDRESS] (R.P.);[EMAIL_ADDRESS] (A.R.)12 Institut d'Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Catalonia, Spain 2  
[PERSON]  
1 Earth Observation Group, Institute of Space Sciences (ICE, CSIC), 08193 Barcelona, Catalonia, Spain; [EMAIL_ADDRESS] (W.L.); [EMAIL_ADDRESS] (E.C.); [EMAIL_ADDRESS] (F.F.); [EMAIL_ADDRESS] (R.P.);[EMAIL_ADDRESS] (A.R.)12 Institut d'Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Catalonia, Spain 2  
[PERSON]  
1 Earth Observation Group, Institute of Space Sciences (ICE, CSIC), 08193 Barcelona, Catalonia, Spain; [EMAIL_ADDRESS] (W.L.); [EMAIL_ADDRESS] (E.C.); [EMAIL_ADDRESS] (F.F.); [EMAIL_ADDRESS] (R.P.);[EMAIL_ADDRESS] (A.R.)12 Institut d'Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Catalonia, Spain 2  
[PERSON]  
3 European Space and Technology Center, European Space Agency, 2200 AG Noordwijk, The Netherlands; [PERSON]@esa.  
The inherent properties of the transmitted signals, which were designed to allow precise delay measurements, are used in GPS RO. The received signal is cross-correlated with a local replica (generated at the receiving satellite) of the expected signal. This implies that the receiver has to know the actual transmitted signal in order to be able to generate a local replica of it. In addition, it needs to know its current position and velocity, and the one of the transmitter to be able to generate this local replica with enough accuracy. The phase of the resulting complex cross-correlation value provides the phase estimate with respect to the applied open-loop model, and it is thus the observable that carries information with physical meaning of the medium through which the signal has traveled.  
A new signal processing method was proposed in [8], presenting the interferometric RO (iRO) in the frame of the proposed GEROS-ISS mission, whose primary objective is the GNSS reflectometry (GNSS-R) to obtain ocean altimetry measurements. The instrumental architecture of GEROS-ISS is particularly conceived for high precision altimetry, using only open-loop delay and Doppler compensated cross-correlation between the received direct and reflected signals. Ref. [8] proposed a processing technique which reusing the hardware of the GEROS-ISS instrument enables phase delay measurements of GNSS signals. In this case the technique uses the periodic properties of the GNSS signals, and does not require to know the actual transmitted signal. The instrument requires also high gain antennas. In this paper, we present experimental work demonstrating the capabilities and limitations of this technique. Currently, ESA is developing PRETTY, an interferometric reflectometry GNSS mission with the antenna pointing to the limb of the Earth. Therefore, the iRO technique tested in this study can be relevant to the PRETTY mission [9].  
In the next section, we review the processing concept and express it mathematically, later (Section 3) we describe the set-up for the experimental demonstration concept, we continue (Section 4) with the description of the obtained results and complete the article with our conclusions (Section 5).