高解析且高程相依之海潮負載效應於近岸超導重力站及衛星追蹤站之研究：重力與位移變量之模式、驗證及改正

Abstract

本研究特別考慮與測站高程相依之OTL格林函數，除使用不同解析之區域、全球海潮模型，分別積分計算測站近﹙內圈﹚、遠﹙外圈﹚區網格之牛頓引力﹙Newtonian﹚及彈性﹙elastic﹚效應，並針對內圈範圍大小及網格間距進行效能測試，且積分所需海陸遮罩網格來自於高解析之海岸線資料庫及數值地形模型，以提高解算精度。本研究已成功地利用FORTRAN電腦程式語言，開發一套推估海潮負載﹙OTL﹚效應之數值模式，特別是針對海潮變化較大的近岸測站，評估其計算成果精度，可以獲得具體的改善。根據本文以新竹超導重力站及部分離島絕對重力觀測資料驗證結果，本研究開發之SGOTL模式與國外部分既有模式﹙GOTIC2、g7.0及BS﹚相比表現較佳，OTL重力效應於新竹超導重力站可達0.1 μgal級精度，且各分潮效應合量約11 μgal。 此外，本研究開發DISOTL模式推估OTL位移效應，以內政部新竹及馬祖衛星追蹤站﹙GPS﹚資料驗證結果，其與國外既有模式﹙GOTIC2及BS﹚相比，OTL效應可達1 mm級精度。另分析13個鄰近台灣的IGS站區域特性，西太平洋KWJ1站（位於馬紹爾群島）之徑向分量合計可達8.5 cm，而台灣海峽週邊GPS站則以馬祖站之徑向合量效應最強約5.5 cm；因此OTL位移效應對於高精度之定位測量技術，確實產生不小影響，有予改正必要。本文以近岸及部分離島地區之GPS連續實測資料，進行OTL位移效應改正測試，其連續坐標解之標準偏差值明顯下降，最大可獲得35%改善幅度。

With consideration of the height dependent Green’s functions for the Newtonian and elastic effects, this research utilizes a regional and a global tide model to account separately for near (inner) and far (outer) zone contributions, and optimizes the inner-zone region and the grid interval for numerical convolution. A set of computer programs coded in FORTRAN, has been successfully developed to model the gravity and displacement effects due to ocean tide loading (OTL), especially for coastal stations with large ocean tides. The coastline is not only defined by the full-resolution shoreline but optionally a digital elevation model (DEM). We model the gravity effect due to OTL in a computer program SGOTL. A case study using gravity observations at the Hsinchu superconducting gravity station and some offshore islands around the Taiwan Strait suggests that SGOTL outperforms some selected global OTL programs (GOTIC2, g7.0 and BS). The gravity due to OTL at the Hsinchu superconducting gravity station can be up to 11 μgal in amplitude and achieves an accuracy of 0.1 μgal . In addition, the horizontal and vertical displacements due to OTL can be modeled in a computer program DISOTL. Based on the GPS sites of MOI (at HCHM and MZUM), OTL-induced displacements from DISOTL, GOTIC2 and BS model differ at 1 mm level in amplitude. The displacements at 13 IGS stations in the western Pacific can be up to 8.5 cm in amplitude (KWJ1, Marshall Islands); At stations around Taiwan, the vertical displacements can be up to 5.5 cm (at MZUM). Such large OTL effects (over 1 cm) will have a profound influence on the precise positioning techniques such as VLBI (Very Long Baseline Interferometry), SLR (Satellite Laser Ranging) and GPS (Global Positioning System). A case study at coastal and offshore-island GPS continuous stations suggests that DISOTL can model the OTL corrections and reduce the coordinate variations by up to 35% .