大氣層由各種物質所組成，水是其中一種物質，其三相變化過程中的蒸發(凝結)、融化(凝固)會吸收(放出)能量，尤以大氣層中的水蒸氣蘊含大量潛熱。此亦是熱門議題：溫室效應、全球暖化的一關鍵影響份子。水氣相較於大氣層中其他的氣體，其變化相當大，並且對人類的生活帶來諸多影響，如高濕度的空氣導致不舒服感；茂盛的水氣對流風起雲湧，成雲致雨；地表植物透過蒸散將熱量轉送。
因此，若能在最快速的時效下獲得大氣中水氣含量的資訊，對於天氣狀態的研究與分析以及氣象的預報會有相當的幫助。全球衛星定位系統(Global Positioning System, GPS)之雙頻電碼/載波相位觀測量在近十年中成功地應用於測量大氣中的水氣含量，即可降水量(Precipitable Water, PW)。
在本研究中，以GPS近即時的求解可降水量有兩關鍵要點：效率與精度。效率方面是透過使用預報星曆來達成快速解算的目的；而精度則透過GPS觀測檔的結合來求取。本文利用瑞士伯恩大學(University of Bern, Switzerland)所發展之Bernese GPS Software 5.0來處理GPS觀測資料，並利用其Bernese Process Engine的自動化功能來達到自動化與近即時求解的目的。本研究所針對的地區為台灣各地之GPS測站，利用內政部之GPS測站資料，與中央氣象局的探空資料來研究與分析近即時求解可降水的可行性。
本研究的結果顯示，在效率方面最佳可達到5分鐘內求解；而在精度方面於穩定天氣可達到1.6 mm的近即時求解精度，在擾動的天氣可達到2.0 mm的精度。

Water vapor in the atmosphere is an influential factor of the hydrosphere cycle, which exchanges heat through phase change and is essential to precipitation. Because of its significance in altering weather, the estimation of water vapor amount and distribution in near real time is crucial to determine the precision of the weather forecasting and the understanding of regional/local climate.
There are two key points for estimating PW in near real time precisely: using ultra-rapid ephemeris provided by International GNSS Service (IGS), the other is the combination of current observations and previous observations of a certain period. In this study, the GPS data process had been done by Bernese GPS Software 5.0 which is a software developed by University of Bern, Switzerland. The GPS data used in this study are from Ministry of Interior (MOI) and IGS, and MOI sites are capable of surface meteorological measurements. The radiosonde data from Central Weather Bereau were used to develop Taiwan-specified conversion factors.
The precision of the result is 1.6 mm in general weather condition and 2.0 mm in turbulent weather condition. The general latency of near real time PW estimates is 5 minutes.

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