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Abstract

Ground-based global navigation satellite system reflectometry (GNSS-R) has demonstrated strong potential for absolute sea level retrieval within a global reference frame at a low cost with a high resolution. In this paper, a combination of single-frequency code and a carrier phase (code-minus-carrier phase), referred to as CMC observable, is used to estimate sea level changes for the first time. The CMC method is free of geometric delays, while the ionospheric terms can be mitigated by adopting the moving average filtering method. The relationship between the antenna height and the spectral peak frequency of the CMC time series is described as a linear model. In addition, we combine CMC and SNR measurements from four constellations (GPS/GLONASS/Galileo/BDS) by adopting the robust regression strategy with iteratively reweighted least squares. GNSS datasets collected from three sites with different tidal amplitudes were used to monitor sea level changes and compare them with tide gauge records. The research findings show that the root mean squared errors (RMSEs) of the CMC method across three study sites are between 6.95 and 11.39 cm, and the corresponding correlation coefficients exceed 0.99, which is comparable to that of the current SNR method. The sea level retrieval accuracy can be further improved by combining the CMC and SNR observations with RMSE between 3.78 and 9.06 cm. We also compare the performance of GNSS-R retrieval with different dynamic models. Because vertical acceleration has been considered, the 2nd-order dynamic model reports a better performance than the 1st-order dynamic model.