以福衛三號及GRACE低軌衛星GPS資料推算時變地球重力場

Abstract

本論文內容是結合福衛三號及GRACE衛星高-低衛星追蹤資料反衍時變地球重力場。為了估算時變重力地位係數，吾人已成功發展兩種重力反衍方法:軌道擾動解析法及殘餘加速度法，此兩種方法分別應用殘餘軌道擾動量(動態軌道及動力軌道之差值)及殘餘加速度(觀測加速度及參考加速度之差值)兩種不同觀測量，分別建立與時變重力地位係數之線性關係後進行估算時變重力地位係數。 吾人首先使用 Bernese 5.0軟體計算福衛三號及GRACE衛星公分級動態軌道。此後，以標準力模式進行計算作用於福衛三號及GRACE衛星上之各種擾動力，此部分使用之主要計算軟體為NASA Goddard研發之 GEODYN II軟體。福衛三號表面擾動力如大氣阻力、輻射壓及其他微小表面擾動力須以力模式進行求解，並於每飛行一圈即解算一組適當表面力參數。所解算之原始5秒一筆之六顆福衛三號衛星及10秒一筆之兩顆GRACE衛星動態及動力軌道重新取樣為一分鐘一筆之軌道位置資料，及後以數值微分得到加速度資料分別以為重力場反衍之用。 吾人以2006年8月一個月福衛三號及GRACE動態及動力軌道資料求解時變重力地位係數，分別使用軌道擾動解析法及殘餘加速度法處理福衛三號單一解及合併GRACE成果解，福衛三號及GRACE平均動態及動力軌道差異量分別約為7.5公分及6.5公分。福衛三號單一解可解出某些已知的時變重力訊號，但仍含有雜訊，合併解則可看出某些程度提升了GRACE單一解某些區域時變重力訊號。 此外，吾人處理自2006年9月至2007年12月共16個月的福衛三號及GRACE精密定軌資料進行每月低階時變重力係數求解至5階，使用軌道擾動解析法及殘餘加速度法所得到之大地起伏變化將與CSR RL04解進行比較分析，15階之合併GRACE解也將進行求解。吾人使用軌道擾動解析法及殘餘加速度法所得到之低階帶諧係數之變化與SLR及CSR RL04解同期觀測比較，發現四者變化趨勢極為相似。

This dissertation is aimed at temporal gravity field recovery from the analyses of the high-low satellite-to-satellite tracking (hl-SST) data from the COSMIC and GRACE satellite missions. In order to estimate the time-varying geopotential coefficients, two efficient methodologies, the analytical orbital perturbation (AOP) approach and the residual acceleration (ACC) approach, are developed in the research. With the reference orbits removed, orbital perturbations (difference between kinematic and reference orbits) and residual accelerations (difference between observed and reference accelerations) from the residual orbits are linear functions of the time-varying geopotential coefficients. Such linear functions enable convenient establishments of observation equations to estimate geopotential coefficients. The Bernese 5.0 software is used to compute the cm-level kinematic orbits of COSMIC and GRACE. The NASA Goddard’s GEODYN II software is used to compute the perturbing forces acting on COSMIC and GRACE satellites based on the standard models of orbit dynamics. The accelerations due to the atmospheric drag, solar radiation pressure and other minor surface forces are estimated by some relevant model parameters over one orbital period from COSMIC’s kinematic and reduced dynamic orbits. The 5s kinematic and dynamic orbits from six COSMIC and the 10s orbits from two GRACE satellites are re-sampled into 1 minute normal point positional data and then converted to acceleration data by numerical differential for gravity recovery. To validate the theories and computer programs associated with the AOP and ACC approaches, some experimental solutions of time-varying geopotential coefficients are carried out using one-month (August 2006) of COSMIC and GRACE kinematic and dynamic orbits. The average RMS in RTN directions of reduced COSMIC and GRACE (1 minute) between kinematic orbits and dynamic orbits are about 7.5 and 6.5 cm. The COSMIC solutions reveal several well-known temporal gravity signatures, but contain artifacts. The combined COSMIC-GRACE solutions enhance some local features in the GRACE solutions. The monthly COSMIC and GRACE precise orbit data from September 2006 to December 2007 (16 months) are processed to recover monthly low-degree (up to degree 5) geopotential coefficients by the AOP and ACC approaches. The geoid variations from such low-degree geopotential coefficients are compared with the CSR RL04 solutions. Two combined solutions by the AOP and ACC approaches (up to degree 15) are also carried out. The monthly variations of the zonal geopotential coefficients, the AOP and ACC solutions (degree 5) closely resemble the SLR-derived and CSR RL04 solutions.