以GPS浮標監測區域性平均海水面之研究

Using GPS Buoy to Monitor the Mean Sea Level of Local Area

本研究之主要目的為設計一適於直接監測平均海水面之GPS（Global Positioning System，GPS）浮標，以及建置一量測水平面可行之方法。通常，平均海水面是由一驗潮站經長時間測量後所決定。但是，這種港口岸邊之驗潮站所測得之數據為區域基準，屬區域高程系統，它可能被當地地殼形變所干擾而改變。以全球觀點視之，此數據應建置於國際地球參考框架（International Terrestrial Reference Frame，ITRF）系統並歸屬於全球高程系統中。一般認為最直接的方法是將GPS追蹤站設於驗潮站附近，即所謂之連續運作GPS參考站（Continuously operating GPS reference station，CGPS），再用水準儀以直接水準測量方式連結高程，以此方式將區域高程系統轉成全球系統。但問題是如果驗潮站與追蹤站位於不同岩塊，又受到板塊運動影響，則其間之相對高程必然改變。為解決此問題，於是設計一種新型之GPS浮標，它包含一圓型版面含漂浮物體、GPS接收儀與天線組、防水裝置、數值傳輸天線、無線傳輸器、導流器等。三個天線以等邊三角形設置，由其幾何關係可證明，中心點之虛擬天線高程等於三個天線高程之平均值。為測試浮標之性質，將其放置於高雄港驗潮站附近，作連續性動態觀測以監測海平面變化，完成後將數據交由POSGPS軟體以動態定位（On The Fly，OTF）模式處理，根據三天觀測數據之處理結果，可獲得海平面零點精度為5mm～7mm，並且證明天線如果設置於浮標平台之幾何中心且不高時，則不須計算與修正浮標之姿態，一樣可以獲取一定精度之高程。

The purposes of this study are to design a suitable GPS buoy to determining a mean sea level directly, as well as to set up a feasible method of measuring the sea level.Usually, the mean sea level is determined by tidal gauge for a long term measurement. However, the record of tidal gauge located on the shoreline near the harbor is belonged to local datum and referred to local vertical reference system. It will be interrupted by local crustal deformation. From global point of view, the record should involve with the ITRF ellipsoid and refer to global vertical reference system. The direct method of measurement is to set up GPS tracking station near the tidal gauge, the so-called continuously operating GPS station and connect to the tidal gauge by leveling measurement, then, the local sea level may be transformed to the global absolute sea level. The problem is that if the tidal gauge and the GPS tracking station are located on different bedrock due to great tectonic activity. Then, a certain amount of relative vertical motion may be changed. For solving this problem, we design a new type of GPS buoy. The buoy includes a circular plate-shaped body, a floating material, three GPS antenna modules, a protection device, a data-transmitting antenna, a pillar body, three GPS receivers, a data-retrieving module, a communication module, and a water deflector. Three GPS antennas are arranged in the order of an equilateral triangle. From the relationship of geometry, it is proved that the height of geometric center is equal to the average height of those three antennas.To test the characteristics of this GPS buoy, we set it above the sea surface near Tide Gauge of Kaohsiung harbor for continuously monitoring sea level change and deal with the dynamic relative positioning of the GPS tracking station, the instantaneous positions of GPS buoy can be solved by the OTF technique of software POSGPS. Based on the result collected from Kaohsiung harbor for three days, the accuracy of the zero point of the sea level is about 5mm~7mm. It is proved that if the antenna is set up at the geometric center of the GPS buoy surface and the antenna height is as short as possible （only antenna is preferred, not tripod）, the accuracy of mean value of ellipsoid height obtained from the solution of instantaneous position is enough. It is not necessary to set up three GPS antennas to modify its attitude for determining the height of the GPS buoy.