Improving the temporal resolution of GNSS-IR water level monitoring using single-cycle periodogram

GNSS-IR represents an innovative technique for monitoring water levels. By analyzing the frequency of interference patterns between the direct GNSS signals and the signals reflected off the water surface, GNSS-IR offers a robust alternative to traditional tide gauges. GNSS-IR provides various advantages, including cost savings, convenient implementation, and accurate separation of vertical land motion. Recently, commercial companies started to adopt GNSS-IR for operational water level monitoring campaigns. The Lomb-Scargle Periodogram (LSP) is widely used to determine the frequency of interference patterns in the GNSS signal-to-noise ratio (SNR) data. Subsequently, the frequency/period can be converted into reflector height and water level. The LSP retrieves only one dominant frequency for each satellite and each channel, ascending or descending, over a time period longer than 20 min. Consequently, the temporal resolution of GNSS-IR water level measurements with LSP is lower compared to traditional tide gauges. High-temporal-resolution water level data would be valuable for applications like coastal hydrodynamics and hurricane studies. To address the temporal resolution, we developed a Single-Cycle Periodogram (SCP) analysis. The SCP analysis uses the LSP retrieval as a priori value and determines the period for each SNR cycle by tracking the maximum/minimum point corresponding to constructive/destructive interferences. Due to the reduced data span, the SCP suffers from noise. To improve the data quality of the interference patterns, we installed a GNSS antenna 90 degree tilted, facing the horizon, taking advantage of the antenna gain characterises. Such an experimental installation exists at the Onsala Space Observatory, with a relative small reflector height of approximately 3 m. Usually a small reflector height GNSS-IR installation results in low temporal resolution due to few interference fringes. However, using the proposed SCP analysis, preliminary results from 26 days of data indicate a significant increase in the number of water level retrievals. The LSP method yields approximately 200 unevenly distributed results per day, with occasional gaps exceeding 30 min. The SCP method gives approximately 10 times more retrivals. Furthermore, using the nearby traditional tide gauge (in the Swedish observation network of sea level) as a reference, the SCP retrievals, averaged over 6 min, provide a higher accuracy compared to the unevenly distributed LSP results.