全球定位系統(GPS)定位精度受許多誤差的影響，目前最主要的誤差來源為對流層誤差和電離層誤差，其中電離層誤差對於RTK GPS 定位具有決定性的影響。電離層的影響量可以由全電子含量(TEC)和電磁波頻率計算求得，並可由雙頻GPS觀測量加以反推求出TEC。一個全球性的電離層模式已由CODE分析中心計算且提供其成果供各界使用，但全球性的電離層模式不能完全描述區域性的電離層變化。因此，本研究嘗試建立一個區域性的電離層模式，實驗資料為2003年10月1日到2004年9月30日台灣GPS追蹤站觀測資料。本研究由區域性電離層模式中分析電離層變化，並和IGS電離層資料比較，以及探討對GPS定位精度的影響。實驗成果顯示：夏季台灣地區電離層高峰出現時間較其他季節約晚了將2個小時。在2003年10月底受磁暴的效應，TEC攀高至全年最大值。比較本研究所得的台灣地區區域性電離層模式和全球性電離層資料位於(120°E,22.5°N)、(120°E,25°N)及(121°E,23.5°N)，兩者差異的標準偏差分別為6.3 TECU、6.9 TECU與6.5 TECU。另外，GPS網平差結果顯示：利用區域性電離層模式可以比全球性電離層模式得到較高之定位精度。

　The accuracy of Global Position System (GPS) is affected by several kinds of error sources. Nowadays, the tropospheric and ionospheric delays are the main error sources in the GPS positioning. Especially for the GPS RTK-Positioning ,ionospheric delays are the most critical factors. The magnitude of ionospheric delays may be determined with the total electron content(TEC) and with GPS dual-frequency observations. The data analysis center CODE has calculated a global ionospheric model and provided the results to all users. Global ionospheric models may not describe regional ionospheric activities clearly. In the study,an ionospheric model for Taiwns is developed by using Taiwan GPS tracking data over the interval Oct.1,2003 to Sep.30,2004. The local ionospheric variations are analyzed. The results of our experiment have shown that the high peaks in summer delay around two hours. The largest amount of TEC was on Oct.,2003, due to the magnetic storms. Comparing our regional ionospheric model with the IGS global ionospheric model at (120°E,22.5°N),(120°E,25°N),and (121°E,23.5°N), we obtain the rms values at the levels 6.3 TECU, 6.9 TECU and 6.5 TECU, respectively. The results from a GPS network adjustment also show that the better positioning accuracy can be achieved by using the regional ionospheric model.

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