即時「全球導航衛星系統」定位服務發展

The Development of the GNSS Real Time Positioning Services.

「精密單點定位」是一種絕對定位的技術，對應於相對定位，觀測誤差不藉助於兩個觀測站間觀測數據的差分方法消除，亦因此沒有基站之需求。但是同時，「精密單點定位」精度取決於誤差模型化消除的細緻度，迄今挑戰仍大，尤其是即時動態解。在「精密單點定位」作業型態中，依照定位的時間可分為「後處理」與「即時」，依照接收載體是否在運動狀況，可分為「動態」與「靜態」。而就導航應用而言，即時動態是主要的需求。
在「精密單點定位」項下，依照資料處理的考量，由技術面可以歸納出「即時精密單點定位」(RT-PPP)、「整數解精密單點定位」(PPP-AR)、「即時動態精密單點定位」(PPP-RTK)的類別。依照定位服務的角度，相對定位技術中的「網形即時動態定位」(nRTK, Network RTK)可以達到即時、動態之需求。但是該技術服務面的特性，使得同步服務的對象數目受到極大限制，這是因為nRTK採用「觀測空間表述」(Observation Space Representation)，產生、傳送改正量。「精密單點定位」因為使用「狀態空間表述」(State Space Representation)，受限較小。
本文謹就此三種技術，介紹描述，並探討實務面向的關鍵因子。

The Precise Point Positioning (PPP) is an absolute positioning technique. Compared with relative positioning, PPP has no need of reference station because the way of removing or reducing observation error is not relying on the differential technique applied to the observations collected among stations. Instead, the accuracy of PPP depends on the quality of error models. The challenge is still large today, particularly for real time kinematic solutions. Based on the time required for the solution, PPP could be classified into post-processing and real-time. Based on the state of the platform, two categories, static and kinematic, are identified. For navigation purposes, real-time kinematic is the common requirement.
Under the umbrella of PPP, there are three groups of technique. Namely, Real -Time PPP (RT-PPP), PPP-Ambiguity Resolution (PPP-AR), and PPP-Real-Time Kinematic (PPP-RTK). From the view of service types, Network RTK, a relative positioning method, could provide real-time and kinematic solutions. However, the number of concurrent users is limited for Network-RTK, because that it generates and transmits the correction based on Observation Space Representation. While PPP is based on State Space Representation, the limitation is mitigated.
This article introduces PPP, including RT-PPP, PPP-AR, and PPP-RTK. The key factors for practical services are also discussed.