對流層效應對於GNSS──R資料估算海水面高度之影響

Assessing Tropospheric Effects on Retrieving Sea Surface Heights from GNSS-R Data

近年來因氣候暖化導致全球海水面快速上升，對沿海低窪地區人民的生命和財產安全產生嚴重威脅。因此，有效且持續監測沿岸海水面變化是一重要的研究課題。一般而言海水面變化可利用衛星測高與潮位站來觀測，然而測高衛星在沿岸易受到陸地訊號干擾造成觀測精度不佳|潮位站觀測則會受到地表垂直運動的影響，無法直接獲得絕對海水面變化。近來全球導航衛星系統反射技術（Global Navigation Satellite System Reflectometry, GNSS-R）之訊噪比（Signal-to-Noise Ratio, SNR）資料證實可有效觀測海水面高度，用來補足及改善前述兩項技術所面臨的問題。本研究取得設於孟加拉吉大港以及臺灣和平港GNSS測站之SNR資料，利用調和分析輔助Lomb Scargle Periodogram（LSP）法估算海水面高度變化，最後與共站或鄰近潮位站資料比較進行精度評估。此外，對流層效應會造成GNSS-R估算海水面高度誤差，因此研究中採用折射改正及對流層模型改正來評估此影響。研究結果顯示吉大港站之GNSS-R成果與潮位站觀測量的差值標準偏差為30.4公分，相關係數為0.97|而和平港站之差值標準偏差為12.8公分，相關係數為0.96。另一方面，吉大港站對流層效應呈現明顯地與衛星仰角相關誤差，進行折射改正和對流層模型改正後，其差值標準偏差分別由30.4公分降低為28.1公分和29.4公分。然而，臺灣和平港站的衛星仰角相關誤差不甚明顯，其主因可能是SNR資料之衛星仰角較高或是天線設置高度較低，因此對流層改正之效果有限。

Due to global warming, global mean sea level has risen rapidly in recent years, causing many serious impacts, especially in the coastal low-lying areas. Thus, effective and continuous monitoring coastal sea level changes is an important research topic. In general, the conventional methods to measure sea level variations are to use satellite altimetry and tide gauges. However, altimetry observations in the coastal regions are inaccurate resulting from complex waveforms caused by land contaminations| tide gauge measurements are affected by vertical land motions and cannot provide geocentric (absolute) sea level changes directly. Recently, signal-to-noise ratio (SNR) data from Global Navigation Satellite System Reflectometry (GNSS-R) have been demonstrated for measuring sea surface heights. This study use SNR data from GNSS stations at Chittagong site in Bangladesh and Heping site in Taiwan to retrieve sea surface heights by Lomb Scargle Periodogram (LSP) aided with tidal harmonic analysis, and the results are compared with the nearby or co-located traditional tide gauges. In addition, the tropospheric effects would cause bias in sea surface heights from GNSS SNR data, so two different tropospheric corrections are applied, including refraction correction and tropospheric model correction. The result in Chittagong station shows the standard deviation (STD) of differences is 30.4 cm, and the correlation coefficient is 0.97, while the STD of differences is 12.8 cm, and the correlation coefficient is 0.96 in Heping station. The obvious elevation-dependent errors observed in Chittagong station are mainly from tropospheric effects. After applying the refraction correction and tropospheric model correction, the STD in Chittagong station is reduced from 30.4 cm to 28.1 cm and 29.4 cm, respectively. However, the elevation-dependent errors in Heping station is not significant. It may result from that the elevation angle of SNR data is higher or the antenna height is lower, so the tropospheric correction is limited.