以全球定位系統構建台灣地區電離層模型之研究(I)

Abstract

在精密全球定位系統(GPS)定位及導航應用方面，電離層延遲為 GPS 觀測量的主要誤差來源之一。電離層延遲的大小，與從 GPS 人造衛星至接收機電磁波路徑沿線之全電子含量(Total Electron Content, TEC)有密切關係。而 TEC 的變化量又跟很多變數有關，例如，太陽黑子的數目、磁場的活動狀況、觀測的季節、觀測的時間、觀測的地點及方向等。由雙頻 GPS 接收機的觀測量，經過處理後，可以估計當時的 TEC。因此，利用此特性，可以將多部的雙頻 GPS 接收機佈設在一特定區域，即時的估計電離層延遲，並構建區域性的電離層模型，提供單頻 GPS 接收機使用，以消除電離層延遲，提高單頻 GPS 接收機導航、定位之精度。由雙頻接收機的 L1 和 L2 觀測量，可以估計由接收機與各被觀測 GPS 衛星間的 TEC。然而，如果欲求得無偏差之 TEC 絕對估值，必須事先估算各顆 GPS 人造衛星與各接收機之 L1/L2 差分延遲。因此，如欲構建區域性的電離層模型，除了必須事先估算 GPS 人造衛星與接收機之 L1/L2 差分延遲外，還要驗證 GPS 測 TEC 的效能、探討構建區域性電離層模型的演算法。因此，本研究案的主要目的就是利用台灣地區的 GPS 資料，以研究構建台灣區域電離層模型有關的議題：（1）以一種屬於單站模型技術的演算法，估計每一顆 GPS 人造衛星與各接收機之差分延遲量的總合；（2）用改正後的 GPS 估算之 TEC 值，與其他方法獲取之 TEC 觀測值（如電離層探測儀，ionosonde）或預測值（如國際參考電離層，IRI）比較分析，以驗證 GPS 測 TEC 的效能；（3）以改良的網格式演算法，構建台灣地區域的電離層模型。

The ionospheric delay is one of the main sources of error in precise GPS positioning and navigation. The magnitude of the ionospheric delay is related to the Total Electron Content (TEC) along the radio wave path from a GPS satellite to the ground receiver. The TEC is a function of many variables, including long and short term changes in solar ionising flux, magnetic activity, season of the year, time of day, user location and viewing direction. A dual-frequency GPS receiver can eliminate (to the first order) the ionospheric delay through a linear combination of L1 and L2 observables. However, the majority of civilians use low-cost single-frequency GPS receivers that cannot use this option. Consequently, it is beneficial to estimate ionospheric delays over the region of interest, in real-time, in support of single-frequency GPS positioning and navigation applications. A dual-frequency GPS receiver can eliminate the ionospheric delay through a linear combination of the L1 and L2 observations. One of the challenging problems in making absolute ionospheric delay measurements using the dual-frequency observations to the GPS satellites is to estimate satellite and receiver L1/L2 differential delays. In addition, the issues, such as the verification of GPS-based TEC accuracy, the selection of suitable algorithm to generate a regional ionospheric modeling need to be studied in order to generate a regional ionospheric modeling over a region. Hence, the main object of this proposal is to study the related issues to generate a real-time ionospheric modeling over Taiwan region: (1) an algorithm which can estimate the sum of the satellite and receiver L1/L2 differential delays of each tracked GPS satellite using a single-site modeling technique, (2) verifying the performance of GPS-based TEC values with those values derived from ionosonde or IRI, (3) an alternative grid-based algorithm to improve the performance of real-time regional ionospheric modeling over Taiwan region.