GPS固定站資料品質及海潮負載影響定位精度之研究

Abstract

高精度GPS相對定位依賴於GPS固定站精確坐標及優良觀測資料品質，因此本研究針對GPS觀測資料品質及海潮負載對於GPS定位精度的影響量進行分析。一般來說GPS測量採用靜態相對定位來消除共同性的誤差，例如時鐘和電離層和對流層延遲引起的誤差。然而，相對定位在長距離定位有不能消除之誤差，例如海潮負載，因此降低GPS定位精度。在本研究詳細分析影響GPS定位精度因素，並著重於GPS資料品質及海潮負載效應。其中使用6個GPS資料品質指標，分別為接收儀內部時鐘頻率穩定度（stability）、接收儀內部時鐘平均偏移量（offset）、週波脫落（o/slps）、觀測資料量(obs)及多路徑效應（mp1&mp2），並發現以接收儀時鐘穩定度（stability）及平均偏移量（offset）影響定位精度為最大。 海潮負載應對於台灣GPS固定站高精度影響較大。為研究海潮負載引起高程變化，我們收集台灣沿海GPS固定站資料並進行小波分析與調和分析，且評估數個國際知名海潮負載，其中台灣本島以NAO.99b 有最佳修正成果，而高程精度有效提高30-45％，也發現台灣西南部GPS站整體精度提升高於台灣東南部，本研究結果預計有助於改進台灣沿海GPS固定站之資料處理及定位之精度，未來可用於高精度地殼監測地殼運動和相對海平面高度上升等應用。

High-accuracy relative GPS positioning relies on precise coordinates of base stations and good GPS data quality. The aim of this study is to improve the quality of GPS observations using a best model of ocean tidal loading effect and by identifying factors that degrade GPS positioning accuracy. In general, common-mode errors, such as errors due to clock and ionospheric and tropospheric delays can be reduced by differential GPS positioning. However, non-common errors, such as ocean loading effect, cannot be eliminated by relative positioning, thereby lowering GPS positioning accuracy. In this study, factors affecting the GPS positioning accuracy are analyzed in detail. The focus is on ocean tidal loading effect, as well as GPS data quality. The following six quality indicators of GPS data quality are investigated: receiver clock drift (stability and offset), cycle slips in the carrier-phase observations (o/slps), multipath on L1 (mp1), multipath on L2 (mp2) and number of observations (obs). The key finding is that the receiver clock drift dominates the positioning accuracy. Ocean tidal loading effect (OTL) has a strong influence on GPS heighting accuracy. To investigate the OTL-induced height variations, we conduct wavelet and harmonic analyses for GPS data collected at Taiwan’s coastal stations. We assess several OTL models and conclude that NAO.99b is the optimal model. OTL corrections improve GPS heighting accuracy by 30-45%. The overall accuracy improvement at the GPS stations in southwestern Taiwan is higher than that at the stations in southeastern Taiwan. The phases OTL-induced height variations are 1.5-2 hours behind those of ocean tidal heights in eastern Taiwan. The result from this study is expected to benefit GPS positioning accuracy at Taiwan’s GPS coastal stations. The dominating factors of GPS positioning accuracy can used to improve GPS data processing for high-accuracy monitoring of crustal motion and relative sea level rise.