Characterization of Full-Spectrum GPS L1, Galileo E1, and BeiDou-3 B1 Signals for Interferometric GNSS-R Ocean Altimetry

Using reflected signals from global navigation satellite systems (GNSS) to measure sea surface height is an important application of the GNSS-reflectometry (GNSS-R) technique. The interferometric GNSS-R (iGNSS-R) utilizes the full spectrum of the navigation signal for ocean altimetry, improving precision due to the increased bandwidth compared to conventional GNSS-R (cGNSS-R). The implementation of iGNSS-R relies on the autocorrelation function (ACF) of the full-spectrum navigation signals. However, the unknown power distribution of each signal component complicates the acquisition of the theoretical ACF. In this article, we obtain the ACFs of GPS IIR-M/IIF/III L1, Galileo E1, and BeiDou-3 B1 signals by processing the intermediate-frequency (IF) data from a high-gain directional antenna. These measured ACFs are then applied to spaceborne iGNSS-R simulation to analyze their impact on ocean altimetry, including altimetry sensitivity, precision, and delay correction in altimetric waveform retracking. The simulation results indicate that iGNSS-R achieves submeter-level altimetry precision, with an improvement of 1.31x - 7.15x compared to cGNSS-R. Moreover, the ACF of the composite GNSS signal can also affect the altimetric waveform retracking. The deviation of the ACFs can induce a significant systematic effect in reflected signal delay estimation, which may cause centimeter- to decimeter-level bias in iGNSS-R altimetry results for different retracking methods. In addition, the reflected waveform varies with changes in the incidence angle and wind speed. While the wind speed and incidence angle have minimal effects on DER, the delay difference of HALF caused by waveform variations increases at higher incidence angles.