Development and applications of a GNSS buoy for monitoring tides and ocean waves in coastal areas

  • 林 演斌

Student thesis: Doctoral Thesis

Abstract

Real-time tide data for estuaries are very important for simulations of river water levels to assess flood risks To design coastal structures wave data are important Tide data in these regions are usually estimated by interpolating or extrapolating tide data from the neighboring tide stations This method may be useful for normal conditions but for extreme sea state conditions such as typhoon-induced storm surges or swells this is not the case because the sea surface in the local area may vary significantly However a simple platform for measuring tide in estuaries and coastal areas is not available Floating data buoys have been verified to be reliable platforms for ocean monitoring and they have been deployed worldwide to provide long-term and real-time meteorological and oceanographic data Nevertheless the previous studies did not measure real-time tides and waves simultaneously using a buoy in estuaries and coastal areas This work was aimed toward developing a Global Navigation Satellite System (GNSS) buoy that observed water surface elevations and provided real-time tide and wave data in estuaries and coastal areas In this work a GNSS buoy that utilized a Virtual Base Station (VBS) combined with the Real-Time Kinematic (RTK) positioning technology was developed to monitor water surface elevations in estuaries and coastal areas The GNSS buoy included a buoy hull a RTK GNSS receiver data-transmission devices a data logger and General Purpose Radio Service (GPRS) modems for transmitting data to the desired land locations Laboratory and field tests were conducted to test the capability of the buoy and verify the accuracy of the monitored water surface elevations For the field tests the GNSS buoy was deployed in the waters of Suao (northeastern part of Taiwan) Wan-li and Small liu-qiu (southwestern part of Taiwan) Tide data obtained from the GNSS buoy were consistent with those obtained from the neighboring tide station The root-mean-square error (RMSE) of the tide data was within 10 cm According to the correction of inclinations of the GNSS buoy the reduction of RMSE was little For Wan-li buoy it was only 1 1 cm The water surface elevations significant wave heights zero-crossing periods one-dimensional wave spectra directional wave spectra and peak wave directions obtained from the GNSS buoy were generally consistent with those obtained from an accelerometer-tilt-compass (ATC) sensor The significant wave heights observed by using GNSS and ATC were nearly identical even when the height of a typhoon-caused swell was as high as approximately 7 m Data were utilized to examine the performance of the GNSS buoy The field tests demonstrated that the rate of effective data was 83% and 74% in measuring tides and waves respectively The data were considered to be effective data when their accuracies reach centimeter-level For the field tests with the GNSS antenna installed at an elevation of 2 26 m as the hourly averaged total inclination of the buoy hull was less than the error in tide caused by the inclination was negligible However as the angle increased up to the uncorrected GNSS tide underestimated the water level by 12 cm Whenever the distance from the antenna to the still water surface was known the relationship between the tide error and the instantaneous total inclination was a cosine-related function The correction of the water level due to instantaneous total inclination did not cause a significant change in the values of significant wave height mean wave period or peak wave direction Thus it is not necessary to incorporate inclinometers in the GNSS buoy for ocean wave observation It is however necessary to incorporate inclinometers in the GNSS buoy for tide observation except when the distance from the antenna to the still water surface is equal to zero or the error is acceptable The field tests demonstrated that the developed GNSS buoy could be used to obtain accurate real-time tide and wave data in estuaries and coastal areas
Date of Award2018 Jan 29
Original languageEnglish
SupervisorChing-Jer Huang (Supervisor)

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