近年來GPS衛星導航已被廣泛的應用在民間，可配合電子地圖、數碼城市等提供精確的定址服務，而導航所使用的硬體除了專門使用的機器外，手機更是目前的主流，在一般的狀況下智慧型手機以GPS功能進行定位並將結果顯示在地圖上供一般民眾使用。但在建物林立的都市區域則有衛星訊號遮蔽的問題，在街道中房屋遮蔽衛星訊號使得平面誤差量到30公尺以上，常令導航結果出錯。
而在街道中，由於街道房屋的遮蔽使得衛星分布受限，且通常橫越街道方向的定位誤差會大於沿著街道方向的定位誤差。本研究為了改善單點定位在都市地區中定位精度不佳的問題，特別是橫越街道方向的定位精度，提出使用遮蔽估算的方法，利用單點定位的解，搭配星曆資料與房屋資訊，利用推估的方法對都市地區中的定位精度做改善。
在成功大學自強校區電機系館頂樓無遮蔽的狀態下收集衛星觀測數據，而房屋資訊則是使用模擬的方式得到資料，以便對街道房屋的條件進行更變，以模擬在不同的街道寬、房屋高度和街道走勢情況下，單點定位的定位精度，並測試遮蔽估算在不同的街道模擬狀態的成效。結果顯示房屋高度和街道寬都對單點定位的精度有直接影響，而房屋高度更是對遮蔽估算有直接的相關，房屋高度越低的狀況下遮蔽估算在橫越街道的定位精度改善成效越佳。

GPS technique has been used in civilian navigation in recent years and it can integrate with other data like digital maps or 3D city models to provide more precise positioning for Location-Based Service (LBS). In addition to other professional instruments, using smartphone to locate positions on digital maps is nowadays a common way.
Since satellite signals may be obstructed of buildings, the poor performance of posi-tioning accuracy for navigation is a main problem in the cities. The positioning accuracy is usually worse in the cross-street direction than that in the along-street direction usually in the cities.
We use the so-called shadow location estimation to improve the poor positioning ac-curacy in urban area, especially in the cross-street directions, by these three data: initial so-lution from Single Point Positioning (SPP), ephemeris and information of buildings.
In order to calculate the positioning accuracy and test the efficiency of shadow loca-tion estimation in different street condition, we acquire the complete GPS data at attic of Department of Electrical Engineering in National Cheng Kung University and simulate the information of buildings.
Results show that building heights and street widths are the main influence factors of the positioning accuracy. Shadow location estimation has a better efficacy to improve the positioning accuracy in cross-street direction when building height is lower.

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