掩星觀測為利用低軌道衛星接收GPS衛星穿過大氣層的訊號，依訊號的都卜勒效應，計算大氣參數和電離層電子濃度剖面。台灣即將發射六顆低軌道衛星接收GPS訊號以觀測地球大氣結構，運作之後將提供涵蓋全球的觀測資料，預計將提供國內電離層和大氣層科學研究和天氣方面相當多的資料。目前掩星觀測反演大氣參數技術主要由美國UCAR開發，本論文研究目的為開發本土性的反演掩星參數軟體。期待從掩星觀測的研究上，發展自行的反演掩星能力，除了學習國外的反演技術外，也能精益求精，開發出更好更精準的反演大氣參數技術。
掩星反演由低軌道衛星量測到的GPS訊號都卜勒平移量，換算成訊號因為大氣電離層電子濃度梯度和大氣中性折射率梯度造成的訊號偏折角，再由偏折角反演出大氣電子濃度和大氣折射度，以及大氣壓力和溫度隨高度的剖面。訊號在介質傳遞時，會因為大氣介質的不規則分佈，如水氣變化、重力波、電離層擾動或閃爍現象等造成訊號的多路徑效應及訊號震幅的擾動。另外反演技術和理論的不盡理想、觀測技術的誤差和觀測資料解析度上的限制，都會造成反演精度上的誤差。本文除了介紹掩星反演技術外，也將利用訊號路徑覓跡法，模擬掩星觀測訊號震幅和相位，藉此探討掩星訊號傳遞特性和影響反演精度的因素，並且改善大氣反演技術。在本文中，模擬的誤差包括觀測解析度對反演的影響、多路徑效應、繞射現象、超折率現象等。模擬的結果說明多路徑效應的確為影響反演精度的主要因素，在訊號震幅非常弱時，繞射現象影響將變大。在超折率發生時，在超折率現象以下的區域，反演的精度都將受到影響。
在反演大氣參數時，電離層對訊號的影響將變成反演大氣參數的誤差。傳統的電離層校正方式，可消除與載波頻率平方成反比的遲延量，但是幾何遲延和更高階的電離層遲延仍無法完全校正。本論文提出一新的電離層校正方式，根據電離層對幾何遲延和路徑遲延相對載波頻率的特性，分別對幾何遲延和路徑遲延做校正。由模擬的結果和實際的掩星觀測資料都獲得更好的校正結果。
在本文比較了幾何光學反演法和全波譜反演法反演結果，在幾何光學反演法上，本文改進了訊號平滑的方式。在掩星訊號越接近地表時，訊號強度因穿過的大氣層厚度較厚，訊號偏折也較大，所以訊號強度較弱，容易受到噪音影響或訊號接收技術的限制。由我們改進的技術，比較適合處理掩星訊號是受到噪音影響的狀況，全波譜等等利用傅利業轉換的反演方法，較適合處理多路徑影響的掩星觀測訊號。目前的掩星觀測解析度，只能解出較大空間尺度的多路徑效應，對於小尺度的多路徑效應仍無法求解。
有本文反演的大氣參數與UCAR反演的結果，趨勢上大致相符。未來將持續改進反演技術，期待在福爾摩沙三號衛星升空之後，能夠對掩星觀測資料做精準的反演和分析。

Radio occultation technique has been used in planetary science to obtain reliable and accurate temperature profiles of the other planets’ atmosphere for decades. It relies on the fact that radio waves are bent and delayed due to the gradient of atmospheric refractivity along-ray-path. With the advent of Global Positioning System (GPS), it becomes possible to retrieve the refractivity and temperature profiles of the Earth’s atmosphere from the occultation data. After the successful mission achieved by the Microlab 1 launched in 1995, Taiwan will implement her own space occultation mission called Formosat-3 to be launched in 2006. In this dissertation, retrieval algorithms to retrieve atmospheric variable profiles from radio occultation data will be discussed with the emphasis on the issue of ionospheric influence.
There are systematical errors associated with ionospheric influence in retrieving key atmospheric parameters from radio occultation (RO) soundings. In order to obtain better-quality retrievals, we develop a new method, thereafter called National Central University Radio Occultation (NCURO) scheme, to reduce the ionospheric influence. The excess phase is divided into two parts, namely geometric delay and path delay (delay along ray path due to refractivity effect). An excess phase equation is presented and implemented in the NCURO scheme whose performance is evaluated through comparisons with model simulation and experimental data. The model simulation is based on the use of the ionospheric model IRI-2001 and atmospheric model NRLMSISE-00. Results show that the NCURO scheme significantly reduces the ionospheric influence at altitudes above 70 km as does the scheme presented in the literature, and provides better corrections for the atmospheric profile.
The occultation data are simulated by ray tracing method for discussing the influence of multipath and superrefraction. The accuracy of RO is reduced by superrefraction below superrefraction region. The amplitude and excess curves of observation RO data are similar to our simulation results of multipath and diffraction. The multiapth seems to be the dominant error source while the water vapor enhance below 7 kilometer. The diffraction influence is stronger while the amplitude of the GPS signals is weaker.

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