衛星雷射測距技術發展：台灣自主衛星定位及地心座標確定

Abstract

衛星雷射測距 (SLR)技術可用於精密測定衛星軌道、決定絕對坐標參考框架以及時變重力場。目前SLR用於確認低軌衛星軌道之外部精度，在徑向軌道的精度已經可以達到1公分，可檢驗GPS定軌結果。預計將於2014年底發射之福衛七號衛星任務將提供SLR觀測數據，未來可以驗證福衛七號GPS軌道精度。由於地球形狀不斷的改變，因此需建立一全球參考框架 (如ITRF)。 台灣SLR站將屬於此全球參考框架。此參考框架可提供一組地表的參考點位以方便用於測量板塊構造、區域性沉陷負載效應及地球旋轉參數等。SLR站觀測資料可結合IGS、Doppler及VLBI等觀測資料進行聯合求解。地球重力場是地球質量分佈的表現，地球的質量因大氣流動、海洋環流，地下水位變化及融冰等因素而產生時間及空間上的變化，進而引起地球重力場的變化。在適當資料輔助下，更可分別推估大氣結構，海洋質量或地下水位等之變化，對瞭解全球變遷有相當之幫助。本計畫擬逐年發展一套自衛星SLR追蹤資料進行軌道驗證、建立台灣地區絕對坐標框架及計算地球重力場變化之理論及電腦程式。未來將以台灣福衛七號衛星任務SLR資料，配合測高資料、水文、大氣等資料模擬演算地球重力場變化。再者，將結合福衛七號及GRACE GPS追蹤資料以改善福衛七號衛星及重力場之精度，最後將分析地球重力場之時間序列及變化機制。

The technique of Satellite Laser Ranging (SLR) can be used for the determinations of precise orbit, reference coordinate frame and temporal gravity field. The future Taiwan satellite mission - FORMOSAT-7, to be equipped with SLR reflectors, will be launched in late 2014. The accuracy of SLR-derived orbit in the radial direction is better than 1 cm and can be used to assess the accuracy of GPS-determined orbit. The Earth’s shape is constantly changing, and a well-defined global coordinate frame such as ITRF is needed to model the change. The future Taiwan SLR station will be included in the coordinate frame. The reference frame can be used in the studies of plate tectonics, regional subsidence and loading of the Earth due to mass change. SLR can combine other three main geodetic techniques for precise space and ground positioning: the GPS, VLBI, and DORIS. The gravity field of the earth will change in space and time due to factors such as atmospheric circulation, oceanic circulation, ground water-level variation and melting ice. By appropriate techniques, such gravity changes will reveal atmospheric mass variation, oceanic mass redistribution and ground water-level change, which are vital to the understanding of global change. In this research, we will develop theories and computer programs for precise orbit determination (POD), absolute reference frame of Taiwan, and global time-varying gravity fields from SLR observations. We will apply these programs to the FORMOSAT-7 satellite mission with altimeter data, hydrological and oceanographic data. GPS observations from the FORMOSAT-3 and GRACE missions will be combined with SLR data to enhance the accuracy of the estimated temporal gravity fields. Time series of gravity field will be analyzed to understand the mechanisms of gravity variations at different temporal and spatial scales.