以T/P-family與Cryosat-2測高衛星觀測北美地表高程變動

Abstract

監測地表高程變動為現在大地測量中重要的議題，然而本研究區(北美加拿大附近區域)受限於地理環境，不易使用傳統之監測方法，因此本研究利用四個雷達測高衛星，包含TOPEX/Poseidon、Jason-1、Jason-2 (1992-2016年)與Cryosat-2 (2010-2016年)衛星資料來監測北美之地表高程變動。進行地表高程變化計算時，須先進行波形重定改正來改善高程觀測精度，並透過最小二乘方式進行時間和空間的擬合，再進行地形梯度修正，最後組成時間序列，即可計算獲得地表高程變化速率。結果顯示，於北美五大湖區域T/P-family與Cryosat-2測高衛星計算之湖水位變化速率皆呈現上升的趨勢，其兩者結果與湖水位站資料的相關係數都高達0.9以上。於陸地計算之結果顯示，在哈德遜灣附近大部分也都呈現出抬升的趨勢，並計算T/P-family測高衛星的七個不同軌跡交叉點之變化速率差異，差異最小可達0.6 cm/yr，差異最大則為3.1 cm/yr，且與二個實測GPS觀測高程相比(垂直變化速率為1.50和1.05 cm/yr)，速率差異皆為0.01 cm/yr。最後探討測高衛星高程的季節性變化與雪深變化之相關性。

Monitoring land surface height changes is an important issue in geodesy, particularly in North America, where many factors can contribute to height changes. In North America, monitoring height changes by conventional methods can be very difficult because of the region’s vastness and inaccessibility. In this study, we determine land surface height changes in North America using radar altimeter data from the missions TOPEX/Poseidon, Jason-1, Jason-2 (1992-2016) and Cryosat-2 (2010-2016). The subwaveform retracking is carried out to improve height accuracy and then determine surface height changes. The height changes are represented by a function containing space and time parameters, which are then least-squares estimated to reduce the raw height observations to those at common reference positions. The altimeter-derived lake levels in the Great Lakes region are rising and are highly correlated with lake-gauge measurements (correlation coefficients>0.9). In the land region, the altimeter-derived heights are mostly on the rise. Our quality analysis shows that: (1) at selected crossovers of the T/P-family altimeters, the differences in altimeter-derived vertical rate reach 0.6 cm/yr, (2) at two GPS stations, the differences between the vertical rates from GPS and T/P-family satellite altimeters are about 0.1 cm/yr. The correlations between the seasonal changes of altimetry-derived heights and snow depth variations are discussed.