電波掩星法反演電離層E域電子密度誤差移除之研究; Improvement of GPS Radio Occultation Retrieval of Ionospheric E region Electron Density

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题名:	電波掩星法反演電離層E域電子密度誤差移除之研究;Improvement of GPS Radio Occultation Retrieval of Ionospheric E region Electron Density
作者:	曾凱佳;Tseng,Kai-Jia
贡献者:	太空科學研究所
关键词:	福衛三號;電波掩星法;電離層E域;誤差移除;COSMIC/FM-3;Radio occultation;E region
日期:	2013-07-17
上传时间:	2013-08-22 11:41:32 (UTC+8)
出版者:	國立中央大學
摘要:	福衛三號(FORMOSAT-3/COSMIC)搭配電波掩星技術(Radio Occultation)得到的散塊E層資料，有不同取樣頻率獲得的超額相位資料，本文將分析散塊E層的發生機率，並嘗試移除E域電子密度的誤差。使用不同取樣頻率資料(ionPhs，1Hz與atmPhs，50Hz)分析的結果發現皆有觀測到散塊E層有季節性變化，主要發生在夏季半球，但不同取樣頻率在高度上的觀測稍有差異，此外ionPhs-Method1結合了atmPhs在高度上的取樣頻率較高與ionPhs對超額相位的篩選條件，得到了較合理的發生機率。接著再利用此方法，排除散塊E層的資料，並對E域電子密度的誤差移除。電波掩星技術是利用福衛三號接收GPS信號所得到的校正TEC(Calibrated TEC)，根據穿透法(Piercing Method)，反演出電子密度。由於F域之電子密度較大，校正TEC大部分由F域所貢獻，且假設球對稱以利於反演，導致反演出的E域電子密度有相當大的誤差。本研究利用IRI(International Reference Ionosphere)模式，模擬福衛三號的校正TEC值，再由Abel Transform的反演，可得到模擬的E域電子密度。與實際IRI 模式的E域電子密度比較，可計算出電波掩星反演的模擬E域電子密度的誤差值。將福衛三號所觀測的電離層E域電子密度扣除此反演誤差後，再與地面電離層觀測儀(ionosonde)實測的電離層電子密度相比，發現扣除誤差後的福衛三號電離層電子密度觀測值的誤差大幅下降。分析2006年7月至2011年的COSMIC電離層資料顯示，原先還沒扣掉誤差值時，COSMIC反演的電離層E域電子密度平均偏差值比地面電離層觀測儀的實測值大了30%至45%之間。但在扣掉反演誤差之後，在春秋兩季的COSMIC觀測值的平均偏差值降到了-10%以內，而夏冬兩季則降到20%以下。此結果顯示，本研究所提出的方法可有效的降低GPS電波掩星反演誤差。 The thesis try to analyze the ionospheric data retrieved by F3/COSMIC radio occultation (RO) technique. We use the GPS amplitude and excess phase profiles with different sampling rates (1Hz & 50Hz) to select the sporadic E (Es) layer events to obtain the morphology of Es layer occurrence rates. The global distribution of Es layer occurrences shows strong seasonal variation with summer maximum and winter minimum. After extracting Es layer occurrence, we improve the RO retrieval accuracy and precision of ionospheric E region electron density with aid of IRI model. The spherical symmetry assumption of the global electron density distribution used in the Abel inversion is the main error source in the RO process. In order to quantitatively estimate the retrieval error of RO-retrieved E region electron density, the calibrated total electron content (TEC) from the empirical IRI model along GPS ray path detected by the COSMIC satellite is utilized to simulate the RO-retrieved electron density profile in different seasons. The RO retrieval error is obtained by comparing the IRI electron density profile with the retrieved one. The results show that retrieved electron density in E region is overestimated in the equatorial region (-10°~10°) and in the mid-latitudes(±30°~±50°), and underestimates in the low latitude regions(±10°~±30°). In order to eliminate the RO retrieval error, the error in the simulated electron density based on the IRI model is used to remove the COSMIC-measured E region density in different latitudinal regions, and the results are compared with the ionosonde measurements for the data taken from July 2006 to Dec. 2011. Before removing the error, the mean deviation of the COSMIC-measured E region peak electron density is about 30~40%. After the error is removed, the mean deviation is reduced significantly, less than 10% in the spring and autumn and 30% in the summer and winter. These results indicate that much more reliable E region electron density can be obtained with this method and the reliability of COSMIC measurement in the lower ionosphere is improved.
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