Test for the deployment of a GNSS-R Station in the Trieste Region: Advances in Ocean Tide measurement in the Adriatic

The enhancement of geodetic observation networks is crucial for advancing the understanding of environmental and geophysical processes. This study presents a recent development in the Trieste region: the test for a multifunctional GNSS station for Interference Reflectometry (GNSS-R) and precise positioning conducted in the Northern Adriatic close to Trieste, in Sistiana. GNSS Interference Reflectometry (GNSS-R) is an innovative technique that uses reflected GNSS signals to measure environmental parameters, such as sea level fluctuations. By analyzing both direct and reflected GNSS signals, this method provides a non-invasive means of observing dynamic changes in the environment. The GNSS-R technique is particularly valuable for studying sea-level variations, as the reflected signals carry information about the distance to the reflecting surface, enabling precise estimations. This approach allows for continuous monitoring over time, offering a reliable and cost-effective alternative to traditional measurement techniques, such as tide gauges. The method was first introduced by Larson and colleagues (2013) as a tool for environmental monitoring and has since evolved into a key technique for measuring sea level, soil moisture, and other geophysical parameters. The GNSS-R test involved the deployment of a temporary GNSS station in the Bay of Sistiana, as a preliminary step toward establishing a permanent station. The station was designed to be multifunctional, serving both GNSS-R for the estimation of sea level variations and conventional continuous GNSS for precise positioning. The GNSS-R data were processed using the open-source GNSSRefl software developed by Larson (2024), which analyzes the signal-to-noise ration to estimate the distance between the receiver and the reflecting surface. With a Trimble NetRS receiver, a simpleANT3B-CAL antenna mounted on a geodetic tripod, and supporting instrumentation, the station enabled estimation of the distance between the receiver and the reflecting surface by analyzing both direct and reflected signals. The GNSS system also provided highly accurate positioning data, which contributes to long-term geodetic monitoring of the area. Figure 1 represents sea level variations measured during the three days of data acquisition. The results are compared with tide gauge data from Trieste and Monfalcone, showing a correlation of 96% and 91%, respectively. The mean has been subtracted from all the analyzed data. The red curve, representing the sea surface calculated using reflectometry, is generated by interpolating the observed GNSS-IR data, which depends on the number of satellites tracked by the receiver. An average discrepancy of approximately 5 cm is observed between GNSS and the Trieste tide gauge. The tidal pattern at the Sistiana station closely resembles the one measured by the Trieste tide gauge, with a time lag of 1–2 minutes, while it leads the Monfalcone tide gauge by approximately 7 minutes. Even though the survey lasted for a short period, and there are a few parameters that decrease the number of observations, the test demonstrated its potential as a non-invasive, reliable method for monitoring tidal dynamics, and the outcomes will guide the installation of a permanent, multifunctional station optimized for both reflectometry and geodetic applications. The improved knowledge of the ocean tides will be beneficial to the geodetic observations in the Grotta Gigante cave, where presently there is the couple of geodetic pendulums measuring tilt and one continuous ZLS spring gravimeter, measuring time variations of gravity. Both instruments offer a high-precision tool measuring the ocean tidal loading (OTL) estimation, the gravimetric ocean tide, next to the Karst hydrology (Braitenberg et al., 2019; Pivetta et al. 2023). The Northern Adriatic region is of particular interest due to its unique geomorphological and hydrodynamic features, where shallow waters amplify tidal effects and produce complex loading signals. By analyzing these variations, we aim to refine models of OTL (Tan et al., 2021) and better understand the interactions between oceanic forces and Earth's solid structure. The stable environment of the Grotta Gigante cave, with minimal external noise, and proximity to the coast, provides an ideal location for conducting such long-term gravimetric and tilt observations. These initiatives mark a significant advancement in geodetic infrastructure within the Trieste area, enabling multidisciplinary research and offering valuable insights into geophysical and environmental processes, with a particular focus on the unique challenges posed by the northern Adriatic region.