遊校及遠距校正對衛星基準連續站頻率與定位誤差之比較

Comparison of frequency and positioning errors of CORS using on-site calibration and remote frequency calibration

本研究為維護基本控制點品質，利用遠距頻率校正技術評估衛星基準連續站之頻率偏移與頻率穩定度。結果顯示利用相位法與電碼法解算17處GNSS觀測站之時間偏差量，除NTPU站之外，其餘觀測站的時間準確度在正負15~60 ns之間，此一差值應來自於訊號線的硬體延遲；此外，所有測站的頻率穩定度與頻率偏移量均優於1E-11，（規格數值1E-9），顯示本研究所使用的17處GNSS接收儀之內部時鐘皆有不錯之表現。接收儀型號皆為Leica GR50的墾丁（KDNM）、台西（TASI）、通霄（TASO）三站的頻率穩定度及頻率偏移皆略差於其他站，但仍在規格範圍內。本年度針對YMSM、NTPU、TASI、TASO、LSB0等五站同時進行以銣鐘或銫鐘之直接頻率比對，結果顯示相位法得出的頻率穩定度較優，直接法適合校正外接銣鐘之接收儀。17處GNSS觀測站採PPP解算之平均24小時定位精度在水平向與垂直向分別為3 mm與5 mm，由於各站接收儀頻率誤差皆在規格內，因此不影響解算之定位精度。

To maintain the quality of basic control points, this study uses carrier phase method and pseudorange method to remotely measure the frequency offset and stability of GNSS continuously operating reference stations (CORS). The results showed that in 17 CORSs, the normalized phase offset is ±15~60 ns and frequency stability and frequency offset are both better than 1E-11 (specification is 1E-9). The phase offset is caused by cable delay. All results from the 17 CORSs meet the GPS receiver specification we expect, except for NTPU. The frequency stability and frequency offset of KDNM, TASI, and TASO stations, which use Leica GR50 receivers, are slightly higher than other CORSs, but all are still within the regular uncertainty. We also used rubidium and cesium frequency standards to directly measure the frequency performance of YMSM, NTPU, TASI, TASO and LSB0. The results show that the carrier phase method is the best, while on-site calibration is suitable for the receivers with external rubidium clock. The average positioning accuracy of the 17 CORSs in horizontal and vertical directions is 3 mm and 5 mm, respectively, through PPP. Since the frequency stability of the receivers at each station are within specifications, it does not affect the positioning accuracy.