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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/5149

    Title: 由GPS信號反演大氣濕折射度之數值模擬;A simulation study on the use of GPS signals to infer 3-D atmospheric wet refractivity structure
    Authors: 江振慶;Chen-Ching Chiang
    Contributors: 太空科學研究所
    Keywords: GPS;斷層掃瞄;數值模擬;大氣濕折射度;tomography;GPS;simulation;atmospheric wet refractivity
    Date: 2003-06-25
    Issue Date: 2009-09-22 09:45:59 (UTC+8)
    Publisher: 國立中央大學圖書館
    Abstract: 水氣含有極高的潛熱能,對於大氣過程影響至鉅。因此,許多科學家畢其一生鑽研觀測水氣含量的技術。而現階段國際上正熱烈探討的新技術為利用GPS訊號觀測大氣中水氣含量的技術,其原理乃藉由水氣分子掌控GPS訊號所密切關聯的〝電〞的特性,即折射度(refractivity),其中水氣所貢獻的部份稱做濕折射度(wet refractivity)。 本研究主要目的為利用GPS訊號反演三維濕折射度分佈,將探討現階段相關反演技術中,各種因子對反演精確度的影響,這些因子包含線性內插法、高層不同大小的模擬區塊、地面與高空約制、整體模擬區塊大小、衛星訊號接收截角、高山接收器、模擬區域內的接收器數量、訊號量、PDOP值與#SV值以及衛星軌道分佈等。 為了掌控天氣變化可能帶來的影響,我們首先假設一個已知濕折射度的大氣分佈環境,使得濕折射度有水平及高程梯度的空間變化,但在時間上沒有變化,目的在於減少誤差因子。之後選定模擬區域為長跟寬各為16公里、高為10公里的立體區域(Cube),再細分小網格(Voxel),每個立體小網格大小則是定長與寬各4公里,高度則為1公里,即將模擬的立體區域分成160個立體小網格,也就是有275個網格點。GPS接收器則是放置在地面每個網格點處,即設置25個GPS接收器在模擬區域內。利用IGS(International GPS Service)精密軌道資料算出在模擬區域內的訊號路徑分佈,再利用濕遲延公式以及已知的大氣環境加上訊號路徑,便可以得到參考大氣的濕遲延量值,該參考值是完全無誤差的。最後使用那些小網格以及所建立的斷層掃描求解模式,將那275個未知網格點的大氣濕折射度(Nw)求出,並且跟參考值來做比較及分析。 研究的結果發現,整體而言,在比較低層的地方,大約是4到5公里以下,反演結果較接近真實大氣環境,而高層部份反演的結果誤差較大。可是模式是使用最小平方法求解,所以高層若是誤差太大也會連帶影響低層的求解。所以為了改善高層的誤差及提昇低層反演結果精確度,吾人將模擬網格的高層加大,使得高層網格內訊號量變多,反演出的結果也有明顯的改進。此外,就其他所探討的因子而言,發現接收截角會影響低層的反演結果好壞;立體網格變小則會影響高層的結果變好,低層結果變差;加入高山接收器會使高層結果變好;而接收器數量變多則是會使整體反演結果變好。 我們知道大氣中極大部份的水氣含量都是集中在距地表5公里以下,而本研究開發的反演模式已經可以正確地反演出距地表五公里以下的濕折射度分佈,即可以掌握到大部份的水氣含量分佈,對於近即時天氣現象的掌握具有正面的影響。 Atmospheric water vapor is a key variable in numerical weather prediction (NWP) models. On the other hand, it is a crucial factor to limit the accuracy of high-precision GPS positioning technique. For both issues, knowledge about the amount of water vapor is extremely important. In this study, we perform a simulation study to employ GPS signals through a developed tomographic scheme to retrieve 3D structure of atmospheric wet refractivity, which may be assimilated into NWP models for advancing forecasting or position calculation for improving GPS positioning accuracy. For the purpose of knowing the absolute accuracy of the developed tomographic method, a well-defined temporal and spatial varying state of atmospheric profile is utilized. Under this idealized circumstance, a series of factors that may influence the retrievals can be easily examined and their impacts may be clearly quantified. They include the size of voxels of the studied volume (area), the cutoff angle of the GPS signals to be used for computation, the values of the positional dilution of precision (PDOP) factors of the GPS signals, the total number of GPS receivers, the influence of additional GPS receivers installed at higher elevations, … etc. Based upon the use of a variety spectrum of adjustable factors, many interesting findings are obtained. For example, the more the number of the observed GPS signals the better the retrievals as expected. Also, the smaller the PDOP value the better the retrievals.
    Appears in Collections:[太空科學研究所 ] 博碩士論文

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