GPS 電波遮蔽觀測技術已被應用於研究全球性電離層電子濃度的觀測。GPS/MET 系統之低軌道衛星(LEO)接收由GPS 傳播的相位訊號，在傳播的過程中，電波經過電離層時而有所影響，在球對稱的假設下，藉由Abel 積分轉換，我們可以得到從1995年4月到1997年2月，超過40000筆的電離層垂直電子濃度分布資料。而經由這些資料所繁衍的foF2值將可用在產生數值映射(numerical mapping)上。在本文中，我們選擇座標函數為球面諧波函數(spherical surface harmonic) 之線性組合，並分成下列三大群組：(1) cos 的次數代表主要緯度趨向，沒有經度的變化。(2) 級數為1之扇形(sectorial)和等軸(tesseral)諧波並包含級數為1的經度變化。(3) 級數為2之扇形(sectorial)和等軸(tesseral)諧波並包含級數為2的經度變化。所衍生出的數值映射(numerical mapping)，將與原始IRI 映射做比較。 Radio occultation technique of the Global Positioning System(GPS) has been applied to obtain vertical profile of electron density in the Ionosphere. Global Position System/Meteorology (GPS/MET) uses a satellite in low Earth orbit(LEO) to receive the phase signals of GPS radio wave affected by ionosphere while propagating through it. By using Abel integral transform under the assumption of spherical symmetry, we caculated more than forty thousands of vertical profiles of the ionospheric electron density from April 1995 to Feb 1997. And the retrieved foF2 values have been used to produce numerical maps. In this study, we choose our coordinate functions as a particular set of the functions which are linear combinations of surface spherical harmonics and are divided into three groups according to different orders from 0 to2. And the derived numerical maps will be compared with the original IRI maps.
