利用全球電離層網格縮短GPS精密單點定位之收斂時間

Shortening the Convergence Time of GPS Precise Point Positioning with Global Ionosphere Maps

精密單點定位（Precise Point Positioning, PPP）是衛星定位的最新技術，其優點在於無須使用主站即可獲得定義在國際地球參考框架下的高精度定位成果，因此逐漸成為地籍測量、工程及科學應用常用的技術。由於所有的衛星定位接收儀都支援GPS星系，使用GPS觀測量進行PPP定位（GPS PPP）是最被廣泛接受的技術。然而GPS PPP需要較長的收斂時間才能達到較高的定位精度，目前縮短收斂時間的方法之一是將全球電離層網格（Global Ionosphere Map, GIM）資訊與PPP計算模型相結合。星間一次差分（Between-Satellite Single-Differenced, BSSD）是一種廣泛使用的PPP計算模型，其優勢在於可以透過星間一次差分技術消除觀測量上之接收儀差分電碼偏差及接收儀鐘差影響。但在將BSSD模型與GIM結合時必須也要對GIM資訊進行一次差分處理，這會造成GIM的雜訊因誤差傳播而被放大。國際間尚未有文獻完整探討GIM資訊誤差放大之後對於GPS PPP收斂時間的影響，因此本研究嘗試結合GIM與BSSD模型，探討縮短GPS PPP收斂時間的可行性。於實驗分析中，我們分別使用了國際GNSS服務組織（IGS）產製的經驗（empirical）法GIM以及國立成功大學太空天氣研究室產製的物理（physical）法GIM。成果顯示，當設定GIM資訊的一次差分先驗精度為±2 m時，BSSD模型結合上述兩種GIM均可有效縮短GPS PPP的收斂時間。另外，根據地籍測量實施規則對於戶地測量的精度規範，當應用PPP於農地等級數值法戶地測量時，引入GIM資訊將可有效提升其作業效率。

Precise point positioning (PPP) technique can use single receiver to compute centimeter-level positioning results under the international terrestrial reference frame (ITRF). Nowadays, the PPP technique has been one of the commonly used technologies for cadastral survey, engineering and scientific applications. Since GPS constellation is supported by all receivers, all users can use GPS measurements to achieve PPP (GPS PPP); however, the positioning results of GPS PPP requires a long convergence time, decreasing the practice efficiency. Combining the information of a global ionosphere map (GIM) with the PPP model has been one effective way to reduce the convergence time. Between-satellite single-differenced (BSSD) model is a popular PPP model at present because it can eliminate the receiver clock error and receiver differential code biases through the BSSD technique. However, when combining the BSSD model with the GIM information, the errors of the GIM information will also be amplified according to the theorem of error propagation. Until today, no studies have been conducted to evaluate the impact of amplified errors on the convergence time of GPS PPP. Therefore, this study attempts to combine the BSSD model with the GIM information and evaluates the feasibility about decreasing the convergence time of GPS PPP. In the analysis, we adopted the IGS (empirical) GIM and a National Cheng Kung University (NCKU) physical GIM. The experimental results show that the convergence time of GPS PPP can be reduced for both of the two GIM when the a priori sigma of the BSSD ionospheric delays is given with±2 m. Moreover, according to the regulation of cadastral survey in Taiwan, when the PPP technique is applied in agricultural regions, introducing the GIM information can effectively improve its efficiency.