| 摘要: | 本研究之目的為利用福爾摩沙衛星三號觀測資料，建立可描述L頻段全球電離層閃爍指數的經驗模型。現有的電離層閃爍或不規則體模型大多由有限之地面觀測或單顆衛星探測建構而成，因此，其適用地區和時間解析度十分受限。雖然亦有結合多種不同頻段之地面和衛星觀測資料以擴大建構模型的適用範圍，然而結合多種儀器得到的多頻率觀測結果卻又難以針對某一波段完整描述電離層閃爍之發生。此外，大多數既有的電離層閃爍模型是以地面接收特高頻(30-300MHz)衛星訊號資料建構而成，此與現今使用的全球導航衛星系統(Global Navigation Satellite System,GNSS)的L頻段(1-2GHz)明顯脫節。故利用全球長期觀測資料建立L頻段的閃爍模型以供相關科技應用參考是為當務之急。福爾摩沙衛星三號(FORMOSAT-3/COSMIC, F3/C)為台美雙邊合作計畫，在2006年4月15日成功發射至低軌道運行。六顆相同的微衛星酬載GPS掩星觀測實驗儀器，對全球電離層垂直閃爍指數進行探測。福衛三號星系平均一日提供超過2000筆觀測90至800km高度之間的閃爍指數廓線資料，大量且密集的福衛三號閃爍指數觀測開創了電離層閃爍天氣研究的新紀元。本研究即利用在2007到2014年間福衛三號超過一千兩百萬筆閃爍指數廓線的最大值(S4max)建立閃爍經驗模型。已知地面閃爍指數具有日變化、月變化、隨地理位置變化以及隨太陽活動變化。令這些變化同時存在且獨立，因此，建模時可將其視為彼此相互正交的函數，各變量函數相乘結果則可代表地面閃爍指數。比對模型模擬與福衛三號觀測統計結果、衛星探測不規則體發生機率分布、地面GNSS 相位閃爍與地面振幅閃爍記錄皆相互吻合，證實模型之穩定和可用性。惟因使用S4max，建構之模型必須乘上校正係數以符合地面觀測結果。
進一步以福衛三號完整的閃爍指數廓線資料，計算不同強度地面閃爍指數的發生機率。廓線資料轉換的地面閃爍指數與實際地面GNSS接收機觀測閃爍強度相仿，廓線資料不須校正可直接使用，且可用以計算地面S4閃爍發生機率。比對同時同地點資料發現當S4>0.15與S4>0.2，兩者發生機率吻合，福衛三號廓線資料可用來建構模型以估計地面閃爍發生機率。根據獲得的閃爍發生機率與其強度之高度相關性，結合先前建構之閃爍模型，可使模型具有模擬不同強度閃爍發生機率能力。結果顯示，模型模擬各強度閃爍發生機率結果與地面實際觀測閃爍發生機率相符。整體而言，長時間連續且綿密均勻的福衛三號全球閃爍觀測資料，提供人類首次建構L頻段地面S4閃爍指數強度及發生機率，以作為衛星通訊、定位與導航應用之參考。
;This study establishes an empirical model to describe ionospheric scintillations of the L-band S4-index. Existing scintillation models have been developed by limited data of a single satellite and/or ground-based radar and satellite receiver observations over the continent, which are restricted by both coverage and temporal resolution. To resolve the above short coming, some models are constructed by combining various satellite, receiver, and radar observations. However, frequency bands from various observations seriously result in the scintillation property being difficultly studied. Note that most of the existing scintillation models have been developed based on VHF (30-300 MHz) frequencies. Today, the most popular frequency band for satellite positioning, negation, and communication applications is the L-band (1-2 GHz, the GNSS (global navigation satellite system) broadcasting frequency). Therefore, it is essential to develop an L-band scintillation to meet and support daily usages and applications. Six microsatellites of the joint Taiwan-US satellite constellation mission, termed FORMOSAT-3/COSMIC (F3/C), were successfully launched in to a circle low Earth orbit at 01:40 UTC on 15 April 2006. Each satellite houses a GPS occultation experiment payload globally deriving the vertical profile of scintillation S4 index in the ionosphere. This constellation provides instantly more than 2000 ionospheric scintillation S4 index profiles 90-800 km altitude. Dense global scintillation S4 index probing brings a new era for studying the space weather in the ionosphere. The empirical model is developed to simulate the S4 index on the ground by using about the maximum value (S4max) on each 12 mega F3/C S4-index profiles during 2007-2014. Since the scintillation inhabits prominent diurnal, seasonal, geographic, and solar activity variations, the variations are treated to be independent to each other, and the model can be constructed by multiplying the variations altogether. The constructed model could well reproduce the F3/C S4-index probing, and yield good agreements with results of in-situ irregularity observations by satellites and ground-based receiving satellite signals. These confirm that the model can be used to forecast global L-band scintillations on the ground and in the near surface atmosphere.
Instead of the S4max, when the whole information of the F3/C S4 index profile is used, the model is further able to predict the occurrence probability for given a scintillation intensity observed on the ground. The S4 index intensity on the ground converted from the whole profile is similar to that observed by co-located ground-based GNSS receivers, while for S4>0.15 and S4>0.2, the occurrences of the two are also highly correlated. Based on the F3/C S4-index profiles, the empirical model is constructed to predict the intensity and the occurrence probability of the L-band S4 index on the ground of the globe, which shall benefit satellite communication, positioning, and navigation applications. |
