GPS軌道誤差特性及基線重複性分析

Abstract

軌道誤差是影響GPS定位精度的重要因素之一，如何將軌道誤差減至 最低，而有 快速、精度足夠之衛星星曆供使用是本文所關心之一。 本文使用IGS全球追蹤站之觀 測資料，加上ITRF94精確座標，利用11 個基準站(Fiducial Sites)調整衛星軌道後， 計算基線長重覆性之結 果，台北-成功213km之基線長重覆性(Baseline Repeatability) 優 於0.01ppm，其結果比使用IGS精密星曆計算基線長重覆性0.02ppm之結果 要好。 造成軌道誤差的因素中，本文對錯誤的初始向量，太陽 輻射壓模式、地位模式不完善，造成軌道誤差的三個因素，進行分析。其 結果顯示，軌道誤差有週期性與隨時間而增加二個主要特性，而與時間相 依之因素造成軌道誤差急速增加。處理資料之結果，調整廣播星曆與IGS 星曆之軌道與原始IGS軌道，比較兩軌道在垂直軌道面(Cross-Track)、沿 軌道(Along-Track)、徑向(Radial)三方向之差值，可看出GPS衛星軌道受 擾動引起之誤差量以沿軌道分量最大，徑向分量最小。此三方向軌道誤差 均可被一解析軌道誤差模式改正至1~2cm。 Orbit error plays an important role on GPS positioning accuracy. This work is concerned with GPS orbit error reduction and providing an fast, accurate ephemeris for GPS users. The GPS measurements made on IGS tracking sites and the accurate coordinates in the ITRF94 frame were used in this work. Data from 11 fiducial sites were used in the orbit relaxation method that corrects for GPS orbits and the resulting baseline repeatability betweenTaipei to Cheng Kun (213km) achieves 0.01 ppm, which is better than the 0.02 ppm repeatability achieved with IGS ephemeris. This work also analyzes GPS orbit errors due to errors in the initial state vector, the solar radiation pressure model, and the geopotential model.Results show that GPS orbit errors contain both periodical and resonant effects. The resonant effect makes orbit error increase with time. The broadcast ephemeris and the IGS ephemeris were corrected by the relaxation method and the corrected and uncorrected ephemeris were differenced to obtainthe cross-track, along-track, and radial components of the orbit error. The smallest error occurred in the radial direction, while the largest one occurred in the along-track direction. All the orbit error components can be modeled with an analytical formula which reduces the errors to 1-2 cm.