ION GNSS 2012
Session A3: Geodesy, Surveying & RTK for Civil Applications

Title: Using a New Type of GPS Buoy for Determining the Mean Sea Level
Author(s): P-H. Hsu, Chia-Nan University of Pharmacy and Science, Taiwan; C-L. Tseng, National Cheng-Kung University, Taiwan
Room: 103/104 (NCC)

The surface of mean sea level can be determined by tidal gauge, satellite altimetry and GPS buoy. Usually, to maintain the national vertical datum is based on the support of tidal gauge that provides an accurate and precise height record. Nowadays in Taiwan area the main data set of height datum is almost gathered from tidal gauge record. Ideally, the height of the primary tidal bench mark located on the stable solid earth is based on the result obtained from the long term tidal gauge record and it can be referred to all bench marks around the island. However, the height difference between the tidal gauge and primary tidal bench mark is relative. In Taiwan, unstable ground is the problem for tide gauge constructed in area of surface bedrock. Due to storms, harbor accidents, earthquakes or other reasons, the accurate relation might be destroyed. Hence, two situations must be considered. First, if they are located on same plate with same amount of rise or subsidence, their relative position may not changed but the position of the sea surface with respect to a global datum may be altered. Second, if the amount of vertical movement is different, the zero point of tidal gauge may be changed and causes datum inconsistency. Taiwan is located on the convergent boundary of the Eurasian Plate and Philippine Plate. It provides a spectacular example of active collision and mountain building in a narrow area between major tectonic plates and subduction zones. The models of the West Pacific geodynamic subduction have two types, towards the continent and towards the ocean. No matter what type it is, the balance between subduction and collision is necessary. According to the former research, the relative plate convergence is approximately 7 cm per year with an azimuth of 55*. This oblique collision involving a reversal of subduction from East dipping South of Taiwan, to West dipping North of Taiwan, results in intense seismicity. Therefore, the continental collision is the main tectonic process and it is the reason for the large numbers of earthquakes occurring in these areas. From precise levelling surveys along the Longitudinal Valley and coast, and tidal observation around the Coastal Range, a rapid vertical crustal movement has been demonstrated. Since 1984 several re-levelling measurements along these routes have revealed that the Coastal Range uplift, with respect to the Longitudinal Valley floor, is at a rate of about 3 cm per year. From above description, it is realized that Taiwan is located on a great tectonic activity region; the position of the tidal gauge is extremely unstable. Since the technique of space geodesy has been developed rapidly, it can achieve absolute sea level accurately. That it is possible to collect data using GPS buoy to determining the mean sea level is the purpose of this research. When the GPS buoy drifts on the surface of the sea, the antennas of GPS cannot continuously perpendicular to the buoy plate due to up and down of the wave. Usually, the movements of airplanes or ships can be referred to three angular rotations about each of three mutually perpendicular axes i.e. x rotation, called pitch; y rotation, called roll; z rotation, called yaw. It causes error of ellipsoid height measured from the GPS receiver. To avoid the error, this article will discuss the design of GPS buoy, the principle of buoy gesture measurement, the derivative of correction formula and the test of field survey. The new type of GPS 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. The GPS antenna modules are located on the circular plate-shaped body of vertices of a regular triangle. The protection device is used to protect the GPS antenna modules to avoid dampening by water. The water-proof pillar body is inserted into the plate-shaped body. The data-retrieving module is electrically connected to the GPS receiver to receive the coordinate data of the buoy. The communication module is electrically connected to the data-retrieving module and the data-transmitting antenna so as to use the data-transmitting antenna to transmit the coordinate data of the buoy to a monitoring station. The water deflector is used to keep the pillar body to limit region at which the body floats around. To establish GPS buoy above the surface near Tide Gauge of Kaohsiung harbor for continuously monitoring sea level change and dealing with the dynamic relative positioning of the GPS tracking station integrated with the Tide Gauge, the instantaneous positions of GPS buoy can be solved by the OTF technique. Since GPS tracking stations are connected to the IGS network, the height of the sea surface can be referred to the global datum i.e. the ITRF ellipsoid. Then based on ellipsoid height of each point and time series, the curve of height distribution is drawn. The curve can represent the instantaneous position of the sea surface. Due to wave moving up and down irregularly, the height collected from buoy must be filtered properly in order to remove the noise. According to the result corrected 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 geometry 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 gesture for determining the height of the GPS buoy.

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