衛星測高及GPS測高對近岸重力之貢獻

Abstract

鑑於衛星測高於近岸測距精度不佳，致重力異常模型解算成果受限，本研究結合船載GPS測高與衛星測高資料，以台灣離島綠島及蘭嶼二區周邊海域為研究區域，改善近岸重力異常解算精度。其中GPS測高資料係以「臺灣本島與離島高程連測計畫」–台灣鄰近海域船載重力測量作業蒐集的GPS相位觀測資料（取樣間隔為1秒）為實驗數據，衛星測高資料採Cryosat-2、Geosat及ERS-1等觀測數據。 有關測高觀測量處理方面，Geosat及ERS-1測高衛星資料採次波形門檻值演算法（門檻值30％）進行波形重定處理並搭配高斯濾波去除雜訊，相較於未進行波形重定之觀測量解算重力異常之成果，提升達11.12％之精度；另船載GPS海面高使用Bernese 5.0動態定位解算並以120秒之罩窗濾波取樣之數據，計算二者之殘餘梯度代入Inverse Vening Meinesz計算，再與船測重力比較，其差值之方均根作為重力異常解算之精度指標。 結合GEOSAT、ERS-1及Cryosat-2衛星測高資料與GPS測高觀測量，其二離島之重力異常解算精度皆有提升：於綠島區域提升0.13mgal，約改善2.08％，蘭嶼區域提升0.43mgal，約為4.42％，另以岸邊距離探討重力異常精度變化，於近岸處有顯著提升；再利用DTU10 MSS以內插方式求得GPS測高數據處理後之資料點位相對應的海面高資料，結合衛星測高資料解算重力異常，研究成果顯示以GPS測高資料相較於DTU10內插資料進行解算，於蘭嶼地區10-20公里處即有4.5％之精度提升，綠島地區之10公里內海域，更高達8.76％之提升。

Altimeter-derived sea surface heights (SSHs) in coastal areas can be seriously degraded because of the corrupted altimeter radar waveforms and poor geophysical corrections. The study is aimed at improving coastal gravity around Lanyu and Lutao by combine GPS-derived SSHs and retracked altimeter-derived SSHs here. GPS-derived SSHs at 1 Hz sampling frequency are from the shipborne gravity campaigns from 2007 to 2010, and satellites altimeter measurements are from the missions Cryosat-2,Geosat and ERS-1. The sub-waveform threshold retracker is used to determine improved range tracking gates in the waveform data of Geosat and ERS-1. The Bernese 5.0 GPS computer package determines the positions of ships and therefore the SSHs, which are then filtered and resampled for gravity determination. The Inverse Vening Meinesz (IVM) formula is used to derive gravity anomalies from the retracked altimeter SSHs and from the GPS-derived SSHs. The resulting gravity anomalies are compared with shipborne gravity measurements, and the role of GPS-derived SSHs is assessed. Compared to the altimeter-only gravity field, the accuracy of the combined altimeter and GPS gravity field is improved by 0.13mgal around Lutao, and 0.43 mgal around Lanyu, corresponding to improvement percentages of 2.08% and 4.42% in the two regions. This shows the value of including GPS SSHs. Compared to the use of DTU 10 SSHS, the GPS-derived SSHS improve the gravity accuracy around Lanyu by 4.50% within 10-20 km to the coast, and by 8.76% within 10 km to the coast. This shows the value of including GPS-derived SSHs.