Coastal tides measurement in Indonesia using GNSS-Reflectometry

Abstract

The important role of coastal areas in human life and their vulnerability to natural hazards such as sea level rise, make continuous monitoring of coastal ecosystem rehabilitation concerning the local sea level is urgently required. Conventional tide observation using a tide gauge sensor measures a local sea level relative to the land. Unfortunately, the effects of deformation and land subsidence around tidal stations make local tide observations irrelevant to measure absolute sea level rise. Currently, an alternative method is being developed accordingly. The method utilizes Global Navigation Satellite Systems (GNSS) signal, known as GNSS-Reflectometry (GNSS-R), to observe the tide. This research aimed to observe the sea level from the reflection of the GNSS signal. Tidal observation using GNSS-R was carried out in four Geospatial Information Agency’s tidal stations, i.e., Morotai, Piru, Tehoru, and Pasangkayu. The tide was determined based on the dominant frequency of a multipath signal, which is obtained by separating the composite signal from the direct one using 3rd order polynomial fitting. The combination of GPS and GLONASS at the L1 and L2 frequencies resulted in tide observations with an RMSE value of 18 cm. The result is 90% correlated with tidal observation using tide gauge sensor. The residual of main tidal constituent vector in both semidiurnal and diurnal components ranged from 0.01 to 0.02 m. This means that the tide prediction generated from the GNSS-R observation is statistically similar to the prediction generated from the tide gauge sensor.