Abstract
In this study, sea level variation observed by a 1-Hz Global Positioning System (GPS) buoy system is verified by comparing with tide gauge records and is decomposed to reveal high-frequency signals that cannot be detected from 6-minute tide gauge records. Compared to tide gauges traditionally used to monitor sea level changes and affected by land motion, GPS buoys provide high-frequency geocentric measurements of sea level variations. Data from five GPS buoy campaigns near a tide gauge at Anping, Tainan, Taiwan, were processed using the Precise Point Positioning (PPP) technique with four different satellite orbit products from the International GNSS Service (IGS). The GPS buoy data were also processed by a differential GPS (DGPS) method that needs an additional GPS receiver as a reference station and the accuracy of the solution depends on the baseline length. The computation shows the average Root Mean Square Error (RMSE) difference of the GPS buoy using DGPS and tide gauge records is around 3 - 5 cm. When using the aforementioned IGS orbit products for the buoy derived by PPP, its average RMSE differences are 5 - 8 cm, 8 - 13 cm, decimeter level, and decimeter-meter level, respectively, so the accuracy of the solution derived by PPP highly depends on the accuracy of IGS orbit products. Therefore, the result indicates that the accuracy of a GPS buoy using PPP has the potential to measure the sea surface variations to several cm. Finally, highfrequency sea level signals with periods of a few seconds to a day can be successfully detected in GPS buoy observations using the Ensemble Empirical Mode Decomposition (EMD) method and are identified as waves, meteotsunamis, and tides.
Highlights
Quantifying sea level change is desirable because sea level change has a direct impact on societal well-being on a scale of time over decades and centuries
The accuracy of sea level variations observed by the Global Positioning System (GPS) buoy, whose coordinates are estimated by differential GPS (DGPS), compared to the tide gauge reaches 5 cm
In the study, a 1-Hz geocentric sea level time series is successfully observed by a GPS buoy using the Precise Point Positioning (PPP) technique wherein the accuracy is properly assessed by comparing the tide gauge records and the solution derived by the DGPS technique
Summary
Quantifying sea level change is desirable because sea level change has a direct impact on societal well-being on a scale of time over decades and centuries. Chen et al (2004) estimated the coordinates of GPS buoys using precise ephemerides and satellite clock error data of the International GNSS Service (IGS) rapid product, whose latency is 17 hours, with kinematic PPP technique and found that the root mean square error (RMSE) is around 10 - 20 cm compared to the positioning solution derived by DGPS. We assessed the accuracy of GPS buoy positioning processed by PPP and DGPS techniques with a goal to see if a GPS buoy can be used to measure sea level variation data to obtain high-frequency ocean signals. The high-frequency GPS buoy observed sea level time series were subjected to an examination using the empirical mode decomposition (EMD) method in order to analyze ocean signals
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