Coastal subsidence monitoring typically employs Global Navigation Satellite System (GNSS) positioning technology. This method provides information only about subsidence below the station base. Sediments in coastal areas tend to accumulate quickly, and subsidence can change significantly due to compaction and alluvium. Therefore, monitoring subsidence above the base is essential to obtain overall coastal subsidence. A new technology called GNSS-Interferometric Reflectometry (GNSS-IR) has been recently developed, which can utilize multipath effects to monitor reflector height. Since the base of the GNSS station is deep and the base length remains constant, the height changes measured by the GNSS-IR technology can reflect subsidence above the base. Accordingly, this paper employs GNSS-IR technology to measure subsidence changes above the base. Additionally, GNSS positioning technology is used to obtain subsidence changes below the base, and the overall subsidence change is then calculated using both GNSS-IR and GNSS positioning technology. The Mississippi River Delta, known for its significant sediment thickness, was selected as the study area, and data from FSHS, GRIS, and MSIN stations was analyzed. The results demonstrate that GNSS-IR can be used to measure the subsidence rate above the base, and the corrected overall subsidence rate is equivalent to the relative sea level rise rate.

The Global Navigation Satellite Systems (GNSS) broadcast radio signals are continuously at two or more frequencies in the L-band, and the multipath signals from sea surface recorded by off-the-shelf geodetic receivers have been demonstrated they can be used to estimate sea level, using a technology called GNSS multipath reflectometry (GNSS-MR). Before proceeding to estimate reflection parameters, the azimuth range and elevation angle range are needed to be defined, as only with suitable azimuths and elevation angles the sensing zones can be guaranteed on water. So, this study presents an angle dependence analysis method to jointly select the azimuth range and elevation angle range based on wavelet analysis which can describe the non-stationary power of different sinusoidal oscillations changed with elevation angle. The key of this method is to use one grid model to screen the spectrum power of multipath oscillation on different elevation angles and azimuths in this work. Then the elevation angles and the azimuths can be determined by searching grids with greater power. The GPS and GLONASS data of two Multi-GNSS Experiment (MGEX) stations named BRST and MAYG was analyzed and used to retrieve. Firstly, the angle dependence analysis was carried out to determine the elevation range and azimuth range. Secondly, the sea levels were retrieved from individual signals. Finally, the retrievals of individual signals are combined to form a 10-min sea level retrieval series. The RMSEs of the combined retrievals are both less than 15 cm. The results show the effectiveness of the selection of angle range based on the angle dependence analysis method.

The expansion of research and applications of Global Navigation Satellite Systems (GNSS) has revealed the information of reflecting surface in inherent multipath errors. GNSS signals, usually used to measure position, have been demonstrated that they can be used to retrieve water properties including water level, soil moisture, snow depth, and vegetation water content, which are important for climate analysis and water resources monitoring. Reflected GNSS signals with different azimuths can carry information of the corresponding reflecting zone, which means every reflected signal has distinct “signal-to-noise ratio (SNR) characteristics” influenced by specific reflecting zones—and the parameter named “Reflector Height (RH)” deduced from SNR frequency is focused on in this study. Thus, after interpolation of a series of reflector height by coordinates of the footprint, products describing highly detailed terrain over a reflecting footprint can be produced. Data of three GNSS sites in EarthScope Plate Boundary Observatory, named P025, P351 and P101, was used to evaluate the terrain after calculating the terrain slopes and correcting the footprint following the slopes. A comparison of the results with a digital elevation model (DEM) showed that it is possible to retrieve terrain by GNSS-Interferometric Reflectometry (GNSS-IR); and the comparison with terrain slopes from DEMs in previous research also validated its potential.