在GPS定位測量所必備的條件中，最重要的一點就是測點必須具有良好的對空視野來接收太空中的GPS衛星訊號，但是近年由於過度地都市化，高樓大廈林立，使得吾人在都市地區進行GPS測量時，常因高樓大廈林立，而造成GPS衛星遮蔽效應。當接收到的衛星顆數低於4顆，就會因為觀測量不足而造成定位的失敗，而虛擬衛星最大的好處就是提供了額外的觀測量，並對都市的遮蔽問題提供了解決之道。另外由於虛擬衛星採用與GPS相同之頻率與訊號格式，故不需要大幅度更改，可利用現有GPS接收器為之。

本文以相對定位為基本概念，乃採用行進中(On-the-fly)求解之多層次最小二乘搜尋法( Muti-stage Least Squares Search, MSLSS,羅正方,1998 ) ，利用虛擬衛星(pseudosatellitec或pseudoite，簡稱PL)輔助GPS觀測量對單一時刻求解，在台南鄭子寮重劃區進行實驗。由三個實驗中證明了虛擬衛星所提供的額外觀測量大大地提昇平面跟高程的定位精度，且解決了衛星遮蔽的問題，並且改善了定位之精度因子(dilution of precision,DOP)，且對測量之整體性(integrity)，連續性及妥善率均能有所提昇。

Among the requisitions for GPS positioning measurement, the most important is that, to effectively receive the signals from the GPS satellites, the observation stand must be sited at a location without much shelter. But due to over-urbanized for the past years, the cities are stood in great numbers of large buildings and thus caused the shelter effect when we try to take the GPS measurements.On condition that the GPS could only receive signals from less than 4 satellites, the positioning will lose efficacy because of not enough measurements. To solve this problem, the greatest advantage of PL is to provide extra measurement for GPS, and provide a solution to the shelter effect in cities. In additional, thanks to the PL adapts same frequency and signal format as satellite, we can use the existing GPS receiver no need of much modification.

This article adopts the DGPS as basic concept, and the On-the-fly solution of Multi-stage Least Squares Search [Lo, 1998], to proceed with experiment at Tainan Zheng-Zi-Liao Consolidation Area, aiming at in one epoch exercising PL to assist GPS measurement to bring out a solution. By way of observation of three separate experiments, it proves that PL could provide extra measurement to greatly improve plane and height positioning accuracy, resolving the satellite shelter problem, improving the positioning dilution of precision, and also the measurement integrity, continuity and property.

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