福衛三號及其後續衛星計畫之近即時自動化定軌系統：低軌衛星軌道誤差及鐘差對掩星計算成果之影響

Abstract

福衛三號衛星已於2006年4月發射升空，以星座掩星方式，對大氣進行觀測，目前運轉已滿7年，遠超過原先設計之2年以上任務壽命(衛星設計壽命5年以上)；福衛七號是福衛三號的後續計畫，預定於2016年及2018年分兩批發射，該計畫將提升福衛三號衛星本體與掩星酬載的性能，建立任務型星系，俟福衛七號星系部署完成後預計可提供每日8000 點全球均勻分佈的大氣資料。本研究之目的在於發展一套自動化的近即時定軌系統，利用建置於Linux系統的叢集電腦（PC Cluster）使用平行處理運算，並搭配Bernese v5.0定軌軟體，本研究成功自主研發國內第一套自動化近即時定軌系統。本系統利用有限的地面站觀測資料加上EHRI演算法計算高取樣率GPS鐘差，並使用單點定位法求得低軌衛星減動力軌道及低軌衛星鐘差等近即時產品。利用福衛三號觀測資料進行系統自動化定軌測試運算，目前系統延遲時間約17分鐘，另透過實驗比較，平行化的獨立計算系統設計較連續性的計算系統約可提高系統時效約64%(速度加快近三倍)。根據軌道較差分析，本自動化系統求得之福衛三號軌道精度約為10 cm、速度精度約為0.168 mm/s，其相應之掩星偏折角誤差約為0.051μrad，符合福衛三號反演計算精度需求；求得之近即時GPS鐘差與後處理鐘差較差均方根值約1.427 ns，時鐘穩定度約6.1E-12，其相應之掩星偏折角誤差約3.75 μrad，目前此GPS鐘差誤差略大，但透過使用完整GPS地面網(全球幾何分布均勻)之地面站資料，預計可與UCAR系統之高取樣率GPS鐘差達到相同等級精度；另，求得之近即時福衛三號鐘差與後處理福衛三號鐘差較差均方根值約為3.742 ns，在掩星計算中吾人可透過相位觀測量的一次差分法消除此低軌衛星鐘差之誤差。本自動化近即時系統預計提供福衛七號近即時軌道產品於掩星計算時使用，並可同時作為福衛三號掩星資料之驗證平台。

The COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) mission, a constellation of six Low-Earth-Orbit (LEO) satellites, was launched on 15 April 2006. It is the world’s first demonstration constellation using radio occultation (RO) signals from GPS satellites. The mission reached the end of its five-year design life in 2011. COSMIC-2 mission is a follow-on mission of COSMIC with an upgraded payload for improved RO applications. It will contain 12 satellites, providing 8000 profiles per day. The 12 satellites are planned to be launched and deployed in two clusters of 6 satellites into the orbits of both low and high inclinations in 2016 and 2018, respectively. The objective of this dissertation is to develop a near real-time (NRT) orbit determination (OD) system, called NRT National Chiao Tung University (NCTU) system, to support COSMIC-2 in atmospheric applications and verify the orbit product of COSMIC. We develop an automatic system under a Linux environment in a PC cluster and this system is capable of automatic determinations of NRT GPS clocks and LEO orbit and clock. The core processing software in our system is Bernese GPS Software Version 5.0. To assess the NRT (NCTU) system, we use eight days of COSMIC data (March 24-31, 2011), which contain a total of 331 GPS observation sessions and 12,393 RO observation files. The parallel scheduling for independent GPS and LEO estimations and automatic time matching improves the computational efficiency by 64% compared to the sequential scheduling. Orbit difference analyses suggest a 10-cm accuracy for the COSMIC orbits from the NRT (NCTU) system, which is at the same level of accuracy as the NRT (UCAR) system. The NRT University Corporation for Atmospheric Research system, called NRT (UCAR) system, is used in the Taiwan Analysis Center for COSMIC (TACC) of the Central Weather Bureau, Taiwan. The mean velocity accuracy from the NRT orbits of COSMIC is 0.168 mm/s, corresponding to an error of about 0.051μrad in bending angle. The RMS difference in NRT GPS clocks between the NRT (NCTU) and the post-processing products is 1.427 ns and the mean RMS value of frequency stabilities is 6.1E-12, corresponding to an error of about 3.75 μrad in bending angle. We expect that our system will deliver a much improved GPS clock solution when the full set of available GPS ground stations is used. The RMS difference in NRT COSMIC clocks between the NRT (NCTU) and the post-processing products is 3.742 ns, which is reduced by using single-differenced phase observables.