Analysis of tidal river height monitoring using low-cost GNSS interferometric reflectometry

Global Navigation Satellite System (GNSS) signal exploration is advancing rapidly, and one of the latest techniques is GNSS interferometric reflectometry (GNSS-IR). This method uses multipath signals to measure the vertical distance from the reflecting surface to a geodetic antenna or receiver. Traditional scientific-grade GNSS instruments are costly, limiting their widespread use in environmental monitoring. To address this issue, we employed low-cost GNSS devices to monitor water levels at PT Garam Pamekasan using the GNSS-IR method. In this study, we tested two antenna configurations: vertically (zenith-looking) and obliquely toward the river (horizon-looking). By leveraging signal-to-noise ratio (SNR) observations, we were able to estimate water surface height with increased precision. The comparison of multipath signal characteristics revealed that the oblique antenna setup amplified the SNR amplitude, improving data quality and accuracy compared to conventional tide gauge observations. The oblique antenna showed a higher average Peak-to-Noise Ratio (PNR) of 4.52 compared to 3.95 for the upright antenna, indicating a higher quality signal. Comparisons with measurements taken using conventional tide gauges reveal that the oblique antenna produced a lower Root Mean Square Error (RMSE) and Mean Absolute Error (MAE), measuring 8.201 cm and 6.691 cm, respectively, compared to 13.894 cm and 11.438 cm for the upright antenna. The oblique antenna also demonstrated a stronger correlation with the tide gauge data, with a correlation coefficient (ρ) of 0.9837, surpassing the upright antenna’s value of 0.9615. The Van de Casteele test revealed fewer residuals and increased accuracy for the oblique antenna. Tide predictions showed 30% higher accuracy for the oblique antenna, with an RMSE of 10.130 cm compared to 14.064 cm for the upright antenna. Tidal predictions showed better performance with the oblique antenna, correlating with higher data quality and accuracy. Thus, our findings suggest that low-cost GNSS devices, particularly with an oblique antenna setup, are promising tools for GNSS-IR monitoring in narrow water bodies, providing reliable and accurate environmental data.