全球定位系統(Global Positioning System, GPS)被廣泛應用在大地控制網的建立、追蹤站位移速度估計等高精度的應用中。傳統上，往往是以相對定位的方式獲得高精度之追蹤站位移速度，但所直接得到的也是各追蹤站間的相對速度；而以IGS(International GNSS Service)產品精密星曆、精密時鐘產品定位的精密單點定位(Precise Point Positioning, PPP) 技術則具有可直接獲得在國際參考框架下(International Terrestrial Reference Frame, ITRF)的絕對位置解等優勢。但由於上述IGS產品為各分析中心之弛約制解的加權平均，其原點和地心之間存在差異，連帶影響了PPP的定位成果。因此以PPP所算得的追蹤站位移速度之精確度，也就值得探討。
本研究以均勻分布全球的33個IGS 追蹤站於2002年至2010年的觀測資料，以PPP 方式估計其位移速度，並與在ITRF 下的位移速度相比。成果顯示，在ITRF2000 時期，位移速度之差異在E、N、H三方向各為0.22±1.34mm/yr、-0.93±0.89mm/yr、4.82±2.20mm/yr，藉由Helmert七參數轉換得以消除系統性的差異，差異在三方向各降至-0.07±1.29mm/yr、-0.25±0.57mm/yr、-0.00±2.22mm/yr。而在ITRF2005時期，其差異在三方向各為-0.07±0.73mm/yr、-0.28±0.53mm/yr、2.50±2.04mm/yr ， 經過Helmert七參數轉換， 其差異為0.05±0.64mm/yr、-0.33±0.42mm/yr、0.00±2.01mm/yr，僅在高程方向差異有所改善。代表隨著參考框架更新，IGS 產品的品質也有所提升，因此在ITRF2005時期以PPP方式估計追蹤站的之絕對水平位移速度其精確度已在1mm/yr以內。

Global Positioning System (GPS) has been widely used in various applications that require high-precision positioning results, such as geodetic control networks and site velocity estimation. Traditionally, site velocities have been determined using relative GPS positioning for its higher accuracy than that of point positioning. However, the obtained velocity results are thus relative in nature. In recent years, the precise point positioning (PPP) technique, which uses International GNSS Services (IGS) products -- precise satellite ephemerides and clocks -- to directly estimate site positions in the International Terrestrial Reference Frame (ITRF), has become a promising tool for absolute site velocity estimation. Nevertheless, the PPP-derived site positions have been reported to be partially biased as a result of the fact that the above IGS products are based on the loosely constrained IGS analysis centre (AC) solutions. So it is an interesting issue to examine the accuracy of PPP-derived site velocities. In this research, we computed a set of PPP-derived velocity solutions for 33 evenly distributed IGS global tracking stations from 2002 to 2010, and compared them with their respective quantities defined in the ITRF. During the ITRF2000 period, the velocity differences in the east, north, and up directions are 0.22±1.34, -0.93±0.89, 4.82±2.20 mm/yr, respectively. After performing the 7-parameter Helmert transformation to absorb the systematic biases between the PPP results and the ITRF, the differences are significantly reduced to -0.07±1.29, -0.25±0.57, 0.00±2.22 mm/yr, respectively. During the ITRF2005 period, the velocity differences in the east, north, and up directions are -0.07±0.73, -0.28±0.53, 2.50±2.04 mm/yr, and 0.05±0.64, -0.33±0.42, 0.00±2.01 mm/yr after Helmert transformation. Only the vertical component is notably improved by the transformation. This indicates that the quality of IGS products improves with time so the biases existed in the PPP solutions are gradually diminishing. It is concluded that under the current ITRF2005, one can reliably use the PPP technique to obtain highly accurate, on the level of sub-mm/yr, absolute horizontal site velocities.

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