河口即時潮位資料對於河川水位模擬以評估淹水風險非常重要，同時，海岸波浪資料對於海岸構造物的設計非常重要。河口潮位資料常以鄰近潮位資料內差或外插推估而得，此方法在一般情況下或許有用，但在極端海況條件下，例如颱風引起的暴潮或湧浪，可能因局部海表面顯著的變化而失準。然而，在河口海域欠缺穩固平台以架設傳統潮位計，浮動式的資料浮標已被國際及國內廣泛採用，且已被證實在海洋觀測上相當可靠，能提供長期且即時的海氣象資料。但是，以往國際上尚未研究在資料浮標同時觀測即時的潮位與波浪。因此，本研究目的為發展能在河口與海岸同時觀測即時潮位與波浪之全球衛星導航系統(Global Navigation Satellite System, GNSS)浮標，並研究其效能。
本研究結合內政部國土測繪中心之虛擬基準站即時動態定位(Virtual Base Station Real-Time Kinematics, VBS-RTK)技術發展GNSS浮標，GNSS浮標由殼體、RTK GNSS接收器、資料傳輸裝置、資料擷取裝置等組成，在實驗室與現場測試GNSS浮標性能，並驗證觀測資料準確度，現場測試分別在臺灣東北部的蘇澳海域與西南部的灣裡及小琉球海域進行。證實GNSS浮標與鄰近潮位站所觀測潮位相當一致，均方根誤差皆小於10 cm，在灣裡測試期間，此誤差以瞬時傾角修正後，改善幅度僅1.1 cm。波浪方面，GNSS浮標觀測之水位時序列(water surface elevation)、示性波高(significant wave height)、零上切週期(zero-crossing period，或稱為平均週期)、一維波譜(one-dimensional wave spectrum)、方向波譜(directional wave spectrum)與主波向(dominant wave direction)，皆與加速度-傾角-電羅經(accelerometer-tilt-compass, ATC)波浪儀之觀測結果一致。即便在颱風引起波高接近7 m之情況下，兩儀器之示性波高仍極為一致。
本研究亦探討GNSS浮標效能，試驗結果顯示，GNSS浮標觀測潮位與波浪之有效資料（解算成功率達門檻值以上的資料）比例分別大於83%及74%。在GNSS天線安裝高度為2.26 m的條件下，當浮標逐時平均總傾角小於 時，潮位誤差基本上可忽略，而當逐時平均總傾角達 時，未修正的潮位低估12 cm。當已知天線安裝高度時，潮位誤差與瞬時總傾角之關係為餘弦相關函數。而以瞬時總傾角修正水位高度與否，兩者之間的示性波高、平均週期與主波向差異皆極微小。由此可知， GNSS若為了觀測波浪，則不需傾角計；若為了觀測潮位，則需要傾角計，除非天線安裝高度為0，或能容許上述的潮位誤差。根據本研究試驗分析結果，證實GNSS浮標能直接觀測而獲得公分等級之高度資料，且能被使用於河口與近岸海域，同時觀測即時的潮位與波浪。

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.

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