空載重力觀測系統

Abstract

在本論文的工作中，發展了一套適用於台灣地區空載重力測量的空載重力測量軟體，而台灣地區空載重力測量計畫中將使用一台 LaCoste air/sea空載重力儀與一台 Beechcraft雙螺旋槳飛機，關於 此重力儀與飛機的特性於本論文中有詳細說明。飛機承載重力儀在空中的軌跡是由GPS觀測量以動態定位的放式求解得，而飛機的速度與加速度則是先計算一多項式並使之符合於飛機的軌跡後以數值微分的方式求得。吾人利用飛機的速度與位置來計算 效應的改正部分，而重力儀的觀測量修正需要飛機垂直方向的加速度。在本論文中，使用一個時間相關的方式來偵測GPS時間與重力儀時間之間的常差，GPS時間是較為準確的並應為相對應於重力儀觀測量之時間。由於GPS所決定的飛機軌跡含了高頻的變化，因此我們使用一高斯濾波器來降低雜訊。經由兩條長度各約30km與60km基線的GPS靜態觀測資料，吾人測試了GPS動態定位軟體與濾波器的處理後，結果顯示兩條基線的加速量在經過濾波處理過後，皆能達到mgal的精度，符合空載重力測量的要求。經整合GPS定位模組、重力值約化模組、時間常差偵測模組後便完成了未來將使用於台灣地區空載重力測量計畫的軟體。

In this work, a computer package for airborne gravimetry is developed for airborne gravity survey in Taiwan. The LaCoste air/sea System II gravimeter and a Beechcraft airplane will be used in Taiwan’s airborne gravity missions, and their characteristics are presented. The trajectory of the aircraft carrying the gravimeter is determined using GPS observations in a kinematic positioning approach. The velocities and accelerations of the aircraft are determined by numerical differentiations that are based on a polynomial fitting of the trajectory. The velocities and positions of the aircraft are used to compute the effect. The vertical components of acceleration are needed to correct for the measurements from the gravimeter. A time-correlation approach is used to determine the shift between the GPS time and the gravimeter time. The GPS time is more accurate and is used to tag the gravity measurements. The raw trajectory as determined by GPS contains high-frequency variations, which are reduced by a Gaussian filter. Two static baselines of 30 km and 60 km in length are used to test the GPS positioning software and the effect of filtering. The results show that, at both baselines the accuracy of filtered accelerations is at mgal level and satisfies the need of airborne gravity reduction. The GPS positioning module, the gravity reduction module and the time-shift module are integrated in a computer package that will be used in future airborne gravity missions in Taiwan.