| 摘要: | 飛行高度低於400 km 的地球軌道稱為極低地球軌道（VLEO）。能夠在VLEO運作的人造衛星的優勢包含較低的發射價位、短暫的通訊間隔時間、高解析度的地球觀測、以及在不容易進行持續性原地觀測的大氣區域進行高價值的科學觀測。但是，VLEO人造衛星的研發及操作仍然有許多跟高層大氣脫離不了關係的挑戰必須克服。VLEO 較大的空氣阻力會導致衛星再入大氣層前的壽命萎縮，亦會增加衛星的加熱。VLEO 較快的軌道速度結合現有軌道推算阻力模型的不確定性對追蹤與地面通訊也形成很大的挑戰。最後，VLEO 飛行高度的主要空氣成分是形成非常有效率氧化劑的氧原子，對衛星材料的侵蝕不容忽視。本計畫有三個主軸與新加坡、美國合作：一方面與法國電動推進廠商 ThrustMe 合作開發兩顆 VLEO 衛星開發、整測、操作。另一方面，要運用前述及現有平台進行攸關 LEO 太空環境的高層大氣科學觀測，以及地球環境遙測。VLEO衛星將以小型電離層探測儀（CIP）、即將開發的氧原子感測器、光學影像/超光譜儀、以及 GPS接收器作為酬載。兩顆衛星會另外加裝離子推進器，可讓 6U立方衛星在中度太陽活動下 250 km 維持11個月。本計劃將在台灣建立航太界睽違已久的 VLEO 衛星開發、操作能力。VLEO 衛星搭載 CIP 及氧原子感測器可在以前僅限於短暫探空火箭觀測的區域提供寶貴的長期電離層、熱氣層季節變化觀測。該觀測可延伸福衛五號/AIP、INSPIRESat-1、IDEASSat三個衛星在不同高度、地方時間的電離層觀測以研究垂直耦合機制，並與資料同化模型比較。兩個VLEO衛星的 GPS 軌跡將結合其他低軌道衛星GPS軌跡量化現有軌道推算器阻力經驗模型及資料同化物理模型的軌道預測誤差，並以機器學習方式產生校正方法。這個成果將針對太空天氣研究成果作業化的目標推進一大步。我們也會量化VLEO觀測對資料同化模型的影響。最後，第二顆VLEO衛星將搭載本校開發中的超光譜儀，以觀測及辨視不同來源的PM2.5氣溶膠空氣污然。這項成果將凸顯VLEO衛星的重要優勢，亦會針對重要公共衛生及環保研究提供重要觀測數據。本計劃的目標會將高層大氣太空環境研究成果應用於受航太界及使用者睽違以久的開發目標，讓VLEO作業環境的觀測與應用能力大幅提升。 ;Very Low Earth Orbits (VLEOs) are defined as having altitudes less than 400 km. VLEO spacecraft are of great interest due to the potential for lower launch costs, low latency, enhanced Earth observation resolution, and sustained in-situ observations in an undersampled atmospheric region. However, several challenges exist to the utilization of VELO spacecraft, many of which are closely related to the upper atmosphere in this region. Enhanced drag results in shorter spacecraft lifetimes and increased heating. Tracking and telemetry are complicated by the higher velocities combined with uncertainties in orbit propagation drag models. The dominant atmospheric constituent at these altitudes is atomic oxygen, which is a corrosive oxidization agent acting upon spacecraft materials.We propose a three pronged approach to developing VLEO spaceflight capacity and utilizing such spacecraft for scientific and Earth observation purposes, in conjunction with existing missions and models along with colleagues in Singapore and the U.S.. This will result in the development of two spacecraft in collaboration with French electric propulsion firm ThrustMe, to be launched in 2020 and 2022. The spacecraft will carry the Compact Ionospheric Probe (CIP), a to be developed atomic oxygen sensor capable of detecting atomic oxygen fluxes, optical or hyperspectral imagers, as well as GPS receivers. Both spacecraft will carry ion thrusters that are capable of sustaining a 6U CubeSat at 250 km altitude for 11 months under moderate solar conditions. This effort will establish the capacity and expertise needed for VLEO spacecraft design and operations in Taiwan.CIP and the atomic oxygen sensor will allow for the seasonal variation of thermosphere/ionosphere composition to be explored at VLEO altitudes, which in the past, has been limited mostly to sporadic sounding rocket observations. This will extend the ionospheric observations of FORMOSAT-5/AIP, INSPIRESat-1, IDEASSat, and FORMOSAT-7/COSMIC-2 to provide in-situ observations at multiple altitudes and local times. The combined dataset will be compared to results from data assimilation models and used to understand vertical coupling effects. The GPS ephemeris from the two VLEO missions, as well as from other LEO missions, will be used to quantify the orbit propagation error when using current empirical drag models, as well as the physics-based models driven by data assimilation. The orbit propagation errors will be further analyzed by machine learning algorithms to produce empirical models for error correction. This will facilitate the application of physics-based models to spacecraft tracking and orbit propagation - one of the key objectives of operationalizing space weather research results. The effect of assimilating in-situ observations from VLEO on forecast ability will also be explored. Finally, the hyperspectral imager being developed by NCU will be carried aboard the second spacecraft, and will be used to identify and track PM2.5 aerosol pollutants. This will elucidate and leverage the strengths of a VLEO Earth observation platform, while also providing observations crucial to public health and environmental studies. Taken together, this project will allow for the operationalization of aeronomy research results to support the development of aerospace systems in a challenging, but promising, operational environment. |
