利用CHAMP GPS資料決定衛星軌道及地球重力模型

Abstract

本論文結合GPS Prep.和GEODYN II由GPS相位資料採用動力法解算CHAMP軌道，求得之軌道與GFZ公佈之CHAMP精密軌道於X、Y、Z三方向的差異量分別為1.454m、3.467m、2.925m，整體而言解算軌道的精度為公尺量級。地球重力場模型是採取兩步法利用德國Muenchen技術大學所提供之CHAMP軌道來進行求解，所得之重力場模型稱為NCTU，其中以GRACE衛星數據所得之重力場模型GGM01S當做真值，分別減去EGM96和NCTU得到大地起伏差異量、重力異常差異量，結果不管於大地起伏亦或重力異常皆顯示所解算的NCTU比EGM96更接近真值，證明了本文所解算的NCTU相較於EGM96其精度確實有改善。時變研究利用一年之CHAMP軌道，每個月選取三天共12組軌道資料，分別以低階的5階、10階重力場模型來看 、 的變化與地心運動，兩個階次的 、 時間變化圖形相似，皆有趨近季節性變化，但以10階的圖形較平滑；而地心運動時變圖形顯示5階和10階之1階球諧係數曲線走勢差異性不大。

With the CHAMP GPS phase data, GPS Prep. and GEODYN II are used in a dynamic method to compute CHAMP satellite orbits. The RMS differences in the X, Y, and Z directions between the computed orbits and the GFZ precise orbits are 1.454 m, 3.467 m, 2.925 m, respectively. In general, the accuracy of the computed orbits in this work is at the meter level. A two-step method with data from the Muenchen technical university of Germany is also used to calculate CHAMP satellite orbits and to estimate the earth’s gravity fields. The resulting gravity models are named NCTU models. Both geoid undulations and gravity anomalies from the NCTU models agree better with those from the GGM01S gravity model than those from the EGM96 model. This shows that the NCTU models outperform EGM96. Using one year of CHAMP satellite’s orbital data in a degree-5 solution and a degree-10 solution, the time-variations of J2, J3 and geocenter motions are obtained. The time-variations of J2 and J3 show secular and seasonal variations. The difference in geocenter motion from the degree-5 and degree-10 solutions is marginal.