使用波形重定提升Geosat/GM測高精度與重力異常：以台灣近海為例

Abstract

此研究焦點在於使Geosat/GM衛星測高資料藉由波形重定提升精度，而實驗區域為台灣海域。與開放海域之測高波形比較，海域之中接近陸地之波形較為複雜。本研究使用改良型門檻值演算法定義閥門改正量，隨之可計算距離改正量與海水面高改正量。重定之成果是藉由大地起伏比較海水面高來評估，該大地起伏是由台灣周圍高解析度大地水準面模型而得。對全區比較，由重定改正之海水面高與大地起伏之間提升8%。在離海岸線10km以內之海域比較則改善了17%且可使用之海水面高數量增加。計算重力異常之方法為inverse Vening Meinesz公式與最小二乘配置法，兩種方法同時使用原始海水面高與重定改正後之海水面高做比較。由測高計算得到之重力異常與船載重力異常比較結果顯示，相較於原始海水面高來說，重定改正後之海水面高得到較好的結果，最好之範例可以達到改善12%。而最好之範例為最小二乘配置法使用重定改正後之海水面高結合空載重力資料計算得到之重力異常。

This research focuses on improvement of Geosat/GM satellite altimeter data by waveform retracking and the experimental area is over Taiwan waters. In comparison to altimeter waveforms over open oceans, waveforms over waters close to land are complicated. This work uses an improved threshold algorithm of waveform retracking to determine gate correction and then to compute range correction and sea surface height (SSH) correction. The performance of retracking is assessed by comparing SSH with geoidal heights from a high resolution geoid model around Taiwan. Over the entire region of comparison, the agreement between retracking-corrected SSH and geoidal heights is improved by 8%. Over waters less than 10 km to the coasts, the improvement is 10% and the number of valid SSH increases. The methods of inverse Vening Meinesz formula and least square collocation are used to compute gravity anomalies from both raw SSH and retracking-corrected SSH. The results show that, in comparison to raw SSH, retracking-corrected SSH yield a better agreement between altimeter-derived gravity anomalies and shipborne gravity anomalies, with a best case of 12% improvement. Least-squares collocation is used to compute gravity anomalies from a combined set of retracking-corrected SSH and airborne gravity data, which agree best with the shipborne gravity anomalies in all cases.