姚宜斌,張 順,孔 建

1. 武漢大學(xué)測(cè)繪學(xué)院，湖北 武漢 430079; 2. 武漢大學(xué)南極測(cè)繪研究中心，湖北 武漢 430079

GNSS空間環(huán)境學(xué)研究進(jìn)展和展望

姚宜斌1,張 順1,孔 建2

1. 武漢大學(xué)測(cè)繪學(xué)院，湖北 武漢 430079; 2. 武漢大學(xué)南極測(cè)繪研究中心，湖北 武漢 430079

對(duì)流層和電離層是地球近地空間環(huán)境中兩個(gè)重要的組成部分，是靠近地球表面且與人類生活聯(lián)系最密切的大氣圈層。全球?qū)Ш叫l(wèi)星系統(tǒng)技術(shù)的快速發(fā)展，為GNSS空間環(huán)境學(xué)的研究提供了良好的契機(jī)。本文介紹了現(xiàn)有GNSS空間環(huán)境學(xué)中在對(duì)流層和電離層方面的研究現(xiàn)狀和進(jìn)展。在GNSS對(duì)流層研究方面，主要集中于GNSS對(duì)流層關(guān)鍵參數(shù)建模和水汽反演兩部分；在GNSS電離層研究方面，主要包括GNSS二維/三維電離層建模和區(qū)域/全球電離層監(jiān)測(cè)。

對(duì)流層；大氣可降水；電離層；GNSS空間環(huán)境學(xué)

對(duì)流層是地球近地空間環(huán)境的重要組成部分之一，是與人類生活聯(lián)系最密切的大氣圈層。作為對(duì)流層中一種非常重要的溫室氣體，水汽在其變化過(guò)程中會(huì)吸收和釋放大量潛熱，直接影響地面和空氣溫度，進(jìn)而影響大氣垂直穩(wěn)定度和對(duì)流天氣系統(tǒng)的形成與演變，在全球大氣輻射、能量平衡、水循環(huán)中都扮演了極其重要的角色。水汽是降水、蒸發(fā)和濕度平衡的結(jié)果，它是底層大氣圈相關(guān)天氣過(guò)程中的一個(gè)重要指標(biāo)，是天氣、氣候變化發(fā)生和發(fā)展的主要驅(qū)動(dòng)力，是災(zāi)害性天氣形成和演變的重要因子。大氣中的水汽受季節(jié)、地形及其他全球氣候條件等因素的影響，具有空間分布不均勻、隨時(shí)空變化較快等特性。因此，研究掌握全球水汽變化的時(shí)空特性有助于了解全球水汽循環(huán)路徑，可為監(jiān)測(cè)和預(yù)報(bào)暴雨、寒流、臺(tái)風(fēng)等多種惡劣天氣和重大旱澇災(zāi)害災(zāi)前信息獲取與災(zāi)害預(yù)警提供數(shù)據(jù)支持，對(duì)于研究全球氣候變化和改善氣象預(yù)報(bào)水平具有重要的科學(xué)和現(xiàn)實(shí)意義。

作為地球近地空間環(huán)境的另外一個(gè)重要組成部分，電離層的變化，特別是空間暴的發(fā)生，對(duì)航天安全、無(wú)線電通信、定位與導(dǎo)航等有破壞性影響，近年的研究發(fā)現(xiàn)，一些自然災(zāi)害(如地震、臺(tái)風(fēng)、海嘯、火山噴發(fā)等)的孕育和發(fā)生過(guò)程及一些人為活動(dòng)(如火箭發(fā)射等)都有可能引起電離層異常，很可能成為預(yù)報(bào)重大自然災(zāi)害和監(jiān)測(cè)人類活動(dòng)的一種潛在手段。利用現(xiàn)代科技手段進(jìn)行日地空間特別是地球空間的探測(cè)，掌握電離層的基本結(jié)構(gòu)和變化規(guī)律，不僅有利于提高測(cè)速、定位、授時(shí)、通信和導(dǎo)航等系統(tǒng)的精度，而且對(duì)于研究高空大氣各層之間的相互關(guān)系和作用，特別是對(duì)全球性的電離層擾動(dòng)及不規(guī)則變化的發(fā)生機(jī)理的研究等具有重要的科學(xué)意義。這項(xiàng)工作已引起不少國(guó)家的學(xué)者甚至是政府部門的重視，在電離層監(jiān)測(cè)及其應(yīng)用研究方面已取得不少成果。

傳統(tǒng)水汽和電離層探測(cè)手段時(shí)空分辨率低，受天氣影響，GNSS的出現(xiàn)提供了新的技術(shù)手段。利用GNSS信號(hào)經(jīng)過(guò)電離層、對(duì)流層時(shí)受到的延遲影響，可以高時(shí)空分辨率地反演出電離層電子密度和對(duì)流層水汽信息，監(jiān)測(cè)這兩方面的空間環(huán)境的變化，由此衍生出GNSS空間環(huán)境學(xué)這一新的學(xué)科方向。

下面分別對(duì)GNSS空間環(huán)境學(xué)中的關(guān)鍵技術(shù)進(jìn)行介紹，主要包括對(duì)流層關(guān)鍵參量建模、GNSS對(duì)流層水汽反演、GNSS電離層監(jiān)測(cè)和建模方面的研究。

1 對(duì)流層關(guān)鍵參量建模

對(duì)流層在全球大氣輻射、能量平衡、水循環(huán)中都扮演了極其重要的角色，氣溫、氣壓、水汽壓、天頂濕延遲(zenith wet delay，ZWD)、天頂靜力學(xué)延遲(zenith hydrostatic delay，ZHD)與水汽等都是對(duì)流層中重要的參量，也是研究全球氣候變化、極端天氣產(chǎn)生機(jī)理等的參考指標(biāo)。當(dāng)前主要有3類關(guān)鍵參量建模方法：對(duì)流層關(guān)鍵參量經(jīng)驗(yàn)?zāi)Ｐ汀⒒趯?shí)測(cè)氣象參數(shù)的對(duì)流層延遲模型和基于GNSS觀測(cè)數(shù)據(jù)的對(duì)流層延遲模型。這3類模型建模成本(時(shí)間、人力、物力)依次增加，但相對(duì)應(yīng)的對(duì)流層模型精度也逐漸提高。

1.1 基于經(jīng)驗(yàn)?zāi)Ｐ偷膶?duì)流層關(guān)鍵參量建模進(jìn)展

對(duì)流層關(guān)鍵參量經(jīng)驗(yàn)?zāi)Ｐ椭荚诮鉀Q無(wú)任何輔助信息下通過(guò)模型直接獲取高精度的對(duì)流延遲，文獻(xiàn)[1—2]最初為美國(guó)廣域增強(qiáng)導(dǎo)航系統(tǒng)的應(yīng)用建立了UNB系列模型，用來(lái)估計(jì)所需的氣象參數(shù)，UNB3模型在北美地區(qū)估計(jì)的對(duì)流層天頂延遲的平均誤差約為2 cm。EGNOS模型[3]對(duì)UNB3模型進(jìn)行了簡(jiǎn)化，但是氣象參數(shù)的估計(jì)公式不同，已被用于歐洲和日本等地區(qū)的衛(wèi)星導(dǎo)航增強(qiáng)系統(tǒng)[4-5]。文獻(xiàn)[6]利用美國(guó)國(guó)家環(huán)境預(yù)報(bào)中心(National Centers for Environmental Prediction，NCEP)的數(shù)字天氣模型(numerical weather model，NWM)產(chǎn)品建立了水平分辨率為1°×1°的TropGrid模型，其與EGNOS相比全球平均精度提高了25%。文獻(xiàn)[7]利用由歐洲中期天氣預(yù)報(bào)中心(European Centre for Medium-Range Weather Forecasts，ECMWF)提供的NWM產(chǎn)品ERA-40建立了全球氣壓和溫度經(jīng)驗(yàn)?zāi)Ｐ?global pressure and temperature，GPT)，GPT模型在實(shí)際中得到了廣泛的應(yīng)用[8-14]。文獻(xiàn)[15—16]利用NCEP資料建立了全球?qū)α鲗友舆t經(jīng)驗(yàn)?zāi)Ｐ虸GGtrop系列模型。文獻(xiàn)[17]針對(duì)GPT模型的部分不足之處進(jìn)行了改進(jìn)和優(yōu)化，構(gòu)建了新的經(jīng)驗(yàn)?zāi)Ｐ虶PT2。文獻(xiàn)[18]對(duì)TropGrid模型進(jìn)行優(yōu)化升級(jí)，建立了新的經(jīng)驗(yàn)?zāi)Ｐ蚑ropGrid2，模型能提供氣溫、氣壓、大氣加權(quán)平均溫度及天頂對(duì)流層濕延遲等對(duì)流層關(guān)鍵參數(shù)估值。文獻(xiàn)[19]建立的GPT2w模型，相比GPT2模型增加了水汽遞減率和大氣加權(quán)平均溫度這2項(xiàng)估計(jì)參數(shù)。GPT2系列模型一經(jīng)發(fā)布，便有不少學(xué)者都對(duì)其精度及應(yīng)用效果進(jìn)行了評(píng)估，結(jié)果表明其具有很高的精度[20-22]。文獻(xiàn)[20]考慮到GPT2和TropGrid2模型所提供參數(shù)種類的優(yōu)缺點(diǎn)，提出了對(duì)流層誤差改正模型ITG(improved tropospheric grid model)，該模型的建模對(duì)象包括地表氣溫、氣壓、ZWD及氣溫直減率。

1.2 基于實(shí)測(cè)氣象參數(shù)的對(duì)流層關(guān)鍵參量建模進(jìn)展

基于實(shí)測(cè)氣象參數(shù)的對(duì)流層延遲模型利用氣象參數(shù)來(lái)計(jì)算ZTD，通常將ZHD與ZWD分開(kāi)進(jìn)行計(jì)算。文獻(xiàn)[23]提出了基于氣象參數(shù)的對(duì)流層延遲模型，通過(guò)利用測(cè)站高度及氣溫、氣壓、水汽壓來(lái)計(jì)算ZTD，Hopfield模型的ZHD和ZWD可以分開(kāi)計(jì)算。文獻(xiàn)[24]也提出了基于氣象參數(shù)的對(duì)流層延遲模型，通過(guò)利用測(cè)站緯度、高度，以及氣溫、氣壓、水汽壓即可計(jì)算出ZTD，Saastamoinen模型中ZHD與ZWD也可分開(kāi)進(jìn)行計(jì)算。文獻(xiàn)[25]則提出了利用氣象參數(shù)計(jì)算ZWD的模型，模型輸入的參數(shù)包括測(cè)站處的氣溫、氣壓及水汽壓。文獻(xiàn)[26]也提出了一種計(jì)算ZWD的模型，模型所需的氣象參數(shù)包括大氣加權(quán)平均溫度、水汽壓及水汽壓遞減率。但是不少學(xué)者的研究結(jié)果[27]都表明：Hopfield、Saastamoinen等模型通過(guò)氣象參數(shù)計(jì)算的ZTD與經(jīng)驗(yàn)?zāi)Ｐ拖啾染壬蠜](méi)有優(yōu)勢(shì)，甚至還更差。這在一定程度上不僅使得對(duì)基于氣象參數(shù)的對(duì)流層延遲模型研究熱度降低，也使得該類模型的應(yīng)用偏少。

1.3 基于GNSS觀測(cè)數(shù)據(jù)的對(duì)流層關(guān)鍵參量建模進(jìn)展

基于GNSS的對(duì)流層延遲模型即利用GNSS觀測(cè)數(shù)據(jù)進(jìn)行ZTD解算，然后再建模。目前在利用GNSS觀測(cè)數(shù)據(jù)估計(jì)ZTD時(shí)，都需要通過(guò)對(duì)流層映射函數(shù)將斜路徑的延遲轉(zhuǎn)換為天頂方向的延遲。對(duì)流層映射函數(shù)從發(fā)展至今已經(jīng)日趨成熟穩(wěn)定，映射函數(shù)通常采用連分式[28]。文獻(xiàn)[29]通過(guò)擬合10個(gè)北美探空氣球站的觀測(cè)數(shù)據(jù)，首次建立了基于實(shí)測(cè)大氣的映射函數(shù)，并且把連分式系數(shù)從與氣象參數(shù)相關(guān)改成與溫度和地理位置相關(guān)。文獻(xiàn)[30]利用26個(gè)北半球無(wú)線電探空測(cè)站一年的數(shù)據(jù)建立了NMF映射函數(shù)，在NMF中連分式系數(shù)只與測(cè)站緯度、高度和年積日相關(guān)。由于NMF誤差大小依賴于緯度的變化及對(duì)經(jīng)度不敏感，許多學(xué)者開(kāi)始利用NWM建立投影函數(shù)，如IMF[31]、VMF1[32]。其中VMF1被認(rèn)為是目前最高精度的全球范圍對(duì)流層映射函數(shù)模型，已經(jīng)被GAMIT、Bernese等高精度GNSS數(shù)據(jù)處理軟件所采用[33]?？紤]到非在線用戶無(wú)法獲取VMF1產(chǎn)品的情況，文獻(xiàn)[34]建立了經(jīng)驗(yàn)映射函數(shù)模型GMF，僅需測(cè)站坐標(biāo)及年積日即可提供全球范圍的映射函數(shù)系數(shù)，該模型使用簡(jiǎn)單方便且與VMF1具有很好的一致性。

限于雙差精密定位技術(shù)在實(shí)時(shí)估計(jì)天頂對(duì)流層延遲時(shí)需要引入500 km以外GNSS參考站的問(wèn)題[35]，精密單點(diǎn)定位技術(shù)(precise point positioning, PPP)較雙差技術(shù)相比具有更大的優(yōu)勢(shì)。文獻(xiàn)[36]研究了利用JPL實(shí)時(shí)軌道、時(shí)鐘產(chǎn)品和PPP技術(shù)估計(jì)天頂對(duì)流層濕延遲，結(jié)果表明其精度可達(dá)13 mm。文獻(xiàn)[37]研究了利用PPP技術(shù)估計(jì)對(duì)流層延遲的精度，其結(jié)果表明利用PPP技術(shù)估計(jì)對(duì)流層延遲可以獲得很高的精度。文獻(xiàn)[38]利用了CNES的實(shí)時(shí)改正數(shù)和近實(shí)時(shí)PPP估計(jì)了對(duì)流層延遲，與事后處理的結(jié)果相比大概存在6.5 mm的偏差，RMS為13 mm。

在區(qū)域?qū)α鲗咏７矫妫壳耙呀?jīng)存在著諸多線性內(nèi)插模型，如反距離內(nèi)插模型、線性內(nèi)插模型、最小二乘配置模型、線性組合模型等，但是文獻(xiàn)[39]認(rèn)為這些模型基本類似并沒(méi)有明顯的區(qū)別。文獻(xiàn)[40]提出了含高程因子的對(duì)流層內(nèi)插模型。文獻(xiàn)[41]按照經(jīng)驗(yàn)研究分析了幾種不同形式的對(duì)流層內(nèi)插模型。

2 GNSS對(duì)流層水汽反演

GNSS對(duì)流層水汽反演技術(shù)具有連續(xù)運(yùn)行、全天候、高精度、高時(shí)空分辨率等優(yōu)點(diǎn)，且測(cè)站布設(shè)成本低，投入使用快，可實(shí)現(xiàn)大范圍高密度的實(shí)時(shí)水汽監(jiān)測(cè)，該技術(shù)的出現(xiàn)是傳統(tǒng)水汽探測(cè)技術(shù)的強(qiáng)有力補(bǔ)充，它不僅可以得到對(duì)流層中大氣可降水量(precipitable water vapor, PWV)的二維空間分布，也可以通過(guò)層析成像技術(shù)(tomography technique)重構(gòu)大氣水汽在垂直方向上的三維廓線信息，已逐漸成為獲取對(duì)流層中大氣水汽最具有潛力的手段之一。根據(jù)GNSS水汽反演產(chǎn)品不同，可分為二維水汽產(chǎn)品和三維水汽時(shí)空分布信息。下面分別對(duì)二維水汽和三維水汽反演進(jìn)展進(jìn)行介紹。

2.1 二維對(duì)流層水汽(PWV)反演研究進(jìn)展

在二維PWV反演方面，文獻(xiàn)[42]首次利用GPS觀測(cè)數(shù)據(jù)估計(jì)得到測(cè)站天頂方向的PWV，這促進(jìn)了一門全新的學(xué)科，即GNSS氣象學(xué)(GNSS Meteorology)的發(fā)展。國(guó)內(nèi)外眾多學(xué)者對(duì)獲取PWV的可行性和精度進(jìn)行了大量研究。通過(guò)與探空數(shù)據(jù)(radiosonde)、水汽輻射計(jì)(water vapor radiometer, WVR)和甚長(zhǎng)基線干涉(very long baseline interferometry, VLBI)對(duì)比發(fā)現(xiàn)，基于地基GNSS反演的PWV精度在1～1.5 mm[35,43-50]。

在斜路徑水汽含量(slant water vapor, SWV)精度評(píng)定方面，眾多學(xué)者對(duì)SWV的計(jì)算方法進(jìn)行改進(jìn)，提出了顧及雙差殘差、星間單差等反演水汽的方法[48-49]，并將結(jié)果與微波輻射計(jì)對(duì)比發(fā)現(xiàn)，GPS反演SWV的精度在4 mm[51]。近年來(lái)，隨著我國(guó)北斗衛(wèi)星導(dǎo)航系統(tǒng)的迅猛發(fā)展，相關(guān)學(xué)者也對(duì)北斗衛(wèi)星系統(tǒng)獲取PWV的精度進(jìn)行檢驗(yàn)。文獻(xiàn)[52]基于上海市氣象局的北斗氣象站數(shù)據(jù)反演PWV，并與GPS和探空數(shù)據(jù)計(jì)算結(jié)果進(jìn)行對(duì)比，發(fā)現(xiàn)其均方根誤差分別小于3.5和3.6 mm。文獻(xiàn)[53]對(duì)北斗衛(wèi)星探測(cè)PWV的性能進(jìn)行分析，并與探空數(shù)據(jù)計(jì)算結(jié)果進(jìn)行對(duì)比，發(fā)現(xiàn)北斗反演PWV與探空數(shù)據(jù)計(jì)算結(jié)果有很好的一致性，但與GPS反演的PWV有2～3.3 mm的系統(tǒng)誤差。

2.2 三維水汽反演研究進(jìn)展

在對(duì)流層層析領(lǐng)域，文獻(xiàn)[54]首次提出了利用區(qū)域觀測(cè)網(wǎng)重構(gòu)對(duì)流層水汽結(jié)構(gòu)的概念。文獻(xiàn)[55]首先實(shí)現(xiàn)了利用層析技術(shù)得到區(qū)域GPS網(wǎng)的四維濕折射率圖像，證明了利用層析技術(shù)監(jiān)測(cè)對(duì)流層時(shí)空變化的可行性。隨后，眾多學(xué)者對(duì)三維水汽層析方法進(jìn)行大量驗(yàn)證和改進(jìn)[56-59]，提出了有限先驗(yàn)信息非約束、改進(jìn)卡爾曼濾波、蒙特卡羅等水汽反演方法。

在多系統(tǒng)數(shù)據(jù)和多源數(shù)據(jù)聯(lián)合反演水汽方面，試驗(yàn)證明了利用多系統(tǒng)觀測(cè)數(shù)據(jù)可以在一定程度上提高水汽反演結(jié)果的精度和可靠性[60-61]。此外，也有相關(guān)研究聯(lián)合地基和空基GNSS觀測(cè)數(shù)據(jù)聯(lián)合反演水汽的方法[62]。近年來(lái)，一些學(xué)者也相繼提出了利用合成孔徑雷達(dá)(interferometric synthetic aperture radar, InSAR)和GNSS觀測(cè)值聯(lián)合反演三維水汽信息的思路[63-65]。重構(gòu)的三維水汽信息可用于氣象方面的研究，例如對(duì)冷鋒路徑的探測(cè)[66]、改善不同尺度數(shù)值預(yù)報(bào)結(jié)果[67-71]及災(zāi)害性天氣的研究[47]。

在層析模型求解和算法改進(jìn)方面，文獻(xiàn)[66]提出了阻尼最小二乘方法對(duì)觀測(cè)方程進(jìn)行求解。文獻(xiàn)[72—73]采用擴(kuò)展的序貫逐次濾波方法，克服了解算結(jié)果敏感性的問(wèn)題。文獻(xiàn)[74]給出了一種新的節(jié)點(diǎn)參數(shù)化水汽反演方法。文獻(xiàn)[75—76]提出了基于卡爾曼濾波的三維水汽層析算法。文獻(xiàn)[77]提出了基于代數(shù)重構(gòu)算法層析三維水汽的方法。文獻(xiàn)[78]為了克服水平約束方程權(quán)值選取不合理對(duì)層析結(jié)果造成的影響，提出了選權(quán)擬合法進(jìn)行層析解算的方法。文獻(xiàn)[79—82]在層析方程解算方面分別提出了自適應(yīng)卡爾曼濾波方法、聯(lián)合迭代重構(gòu)算法、三維分布數(shù)值積分方法和抗差-方差分量估計(jì)的水汽反演算法。文獻(xiàn)[83]對(duì)代數(shù)重構(gòu)算法在水汽反演中的應(yīng)用進(jìn)行討論，并通過(guò)實(shí)驗(yàn)證明該算法能夠滿足三維水汽反演的要求。

在對(duì)層析網(wǎng)格劃分，約束信息選取方面，文獻(xiàn)[84]對(duì)三維水汽層析中網(wǎng)格大小、水平和垂直分辨率選取、觀測(cè)噪聲及不同衛(wèi)星系統(tǒng)對(duì)層析結(jié)果的影響進(jìn)行了詳細(xì)分析。文獻(xiàn)[85]對(duì)國(guó)內(nèi)外層析水汽網(wǎng)格劃分方法進(jìn)行描述。文獻(xiàn)[86]對(duì)不同層析垂直分辨率及層析區(qū)域選擇方法進(jìn)行研究，提出了一種優(yōu)化的區(qū)域網(wǎng)格劃分方法。文獻(xiàn)[87—89]針對(duì)側(cè)面穿出射線利用問(wèn)題提出了引入水汽單位指數(shù)和比例因子等一系列反演技術(shù)。

3 電離層監(jiān)測(cè)和建模方面

3.1 二維電離層建模研究進(jìn)展

隨著全球?qū)Ш叫l(wèi)星系統(tǒng)(GNSS)技術(shù)的快速發(fā)展，地基GNSS的全球電離層TEC(total electron content)監(jiān)測(cè)與建模已成為當(dāng)前的研究熱點(diǎn)之一[90-96]。目前，IGS(International GNSS Service)電離層工作組下設(shè)7個(gè)電離層分析中心，分別是歐洲定軌中心(CODE)、美國(guó)噴氣推進(jìn)實(shí)驗(yàn)室(JPL)、歐空局(ESA)、西班牙加泰羅尼亞理工大學(xué)(UPC)，馬薩諸塞大學(xué)(UML)、中國(guó)科學(xué)院(CAS)和武漢大學(xué)(WUH)。不同機(jī)構(gòu)在二維模型的處理方法上有所差異，JPL在電離層單層模型(single layer model, SLM)假設(shè)的基礎(chǔ)上，以三角格網(wǎng)內(nèi)插和雙三次樣條函數(shù)內(nèi)插的方法建立電離層模型[97]。UPC則是在基于雙層電離層假設(shè)，以逐基準(zhǔn)站準(zhǔn)層析的方式建立電離層模型，對(duì)于無(wú)觀測(cè)值區(qū)域采用克里金插值的方法進(jìn)行合理外推[98]。CODE、ESA和WHU均采用15階次的球諧函數(shù)(spherical harmonic, SH)在全球范圍內(nèi)建模[99-100]，得到時(shí)空分辨率為2 h×2.5°(緯度)×5°(經(jīng)度)的全球電離層VTEC格網(wǎng)。CAS電離層產(chǎn)品首先采用廣義三角級(jí)數(shù)函數(shù)逐基準(zhǔn)站地建立局部電離層模型，然后采用球諧函數(shù)建立全球電離層 TEC 模型用于保證無(wú)觀測(cè)區(qū)域內(nèi)電離層 TEC 的合理外推[94-95]。

現(xiàn)階段，地基GNSS仍是電離層探測(cè)最重要的技術(shù)手段之一，但GNSS基準(zhǔn)站大多分布在陸地，南半球海洋和高緯區(qū)域幾乎沒(méi)有基準(zhǔn)站分布，使得模型在這些區(qū)域精度有限?？栈婋x層探測(cè)技術(shù)具有精度高、全球均勻覆蓋等優(yōu)點(diǎn)，因此聯(lián)合地基與空基等多源數(shù)據(jù)進(jìn)行電離層建模的研究具有重要意義。文獻(xiàn)[101]結(jié)合GNSS數(shù)據(jù)和衛(wèi)星測(cè)高數(shù)據(jù)進(jìn)行全球電離層建模，結(jié)果表明衛(wèi)星測(cè)高數(shù)據(jù)可以有效提高模型的精度。文獻(xiàn)[93]首次聯(lián)合地基GNSS、LEO掩星及衛(wèi)星測(cè)高數(shù)據(jù)進(jìn)行建模，結(jié)果表明模型的 RMS 降低了0.1TECU。2011年，文獻(xiàn)[102]利用地基 GNSS、LEO掩星觀測(cè)值、海洋測(cè)高衛(wèi)星數(shù)據(jù)和甚長(zhǎng)基線干涉 VLBI 電離層觀測(cè)值建立區(qū)域電離層模型。文獻(xiàn)[96,103]利用地基GNSS觀測(cè)值、海洋測(cè)高衛(wèi)星、COSMIC及DORIS觀測(cè)值建立全球電離層模型，并利用赫爾默特方差分量估計(jì)對(duì)不同觀測(cè)值精確定權(quán),模型在海洋地區(qū)的精度和可靠性進(jìn)一步提高。

3.2 三維電離層層析研究進(jìn)展

電離層二維模型具有估計(jì)模型簡(jiǎn)單、精度高等優(yōu)點(diǎn)，但是通常假定所有電子集中在一個(gè)薄層上，不能反映電離層的空間結(jié)構(gòu)變化。為此，文獻(xiàn)[104]在國(guó)際上首先提出了電離層層析成像(computerized ionosphere tomography, CIT)的概念，其實(shí)現(xiàn)手段主要借助于快速飛行的極軌衛(wèi)星在短時(shí)間內(nèi)對(duì)待探區(qū)域的一次斷層掃描反演信號(hào)傳播路徑的TEC經(jīng)度-高度方向分布信息。此后，國(guó)內(nèi)外許多電離層研究者先后在理論和方法上對(duì)三維電離層層析技術(shù)進(jìn)行了深入研究，建立了多種電離層層析模型。目前，這些模型大致可分為兩類：一類是函數(shù)基電離層層析模型[105-109]；另一類是像素基電離層層析模型[110-123]。

在函數(shù)基方面，文獻(xiàn)[124]早在1992 年就提出用經(jīng)驗(yàn)正交函數(shù)展開(kāi)表示電離層垂直模式，用球諧函數(shù)表示電離層水平模式。文獻(xiàn)[125]最早明確給出函數(shù)基電離層模型的公式，并利用WAAS (wide area augmentation system)系統(tǒng)的觀測(cè)數(shù)據(jù)和隨機(jī)反演方法，反演了80～580 km高度范圍內(nèi)電子密度的空間分布。文獻(xiàn)[126]將函數(shù)基層析模型的反演高度范圍擴(kuò)展到整個(gè)電離層高度，并利用GPS觀測(cè)數(shù)據(jù)和Kalman濾波重構(gòu)了電離層結(jié)構(gòu)的時(shí)空分布。文獻(xiàn)[108]基于GPS觀測(cè)數(shù)據(jù)，利用B樣條基函數(shù)和正交函數(shù)建立了函數(shù)基層析模型，并重構(gòu)了電離層電子密度的時(shí)空分布。文獻(xiàn)[127—128]研究了一種基于B樣條基函數(shù)的三維電離層建模方法。文獻(xiàn)[129]提出了一種基于Chapman函數(shù)的射線追蹤層析算法。文獻(xiàn)[130]提出了一種附加投影函數(shù)的函數(shù)基電離層層析算法。

在像素基方面，常用的反演算法有ART(algorithm reconstruction technique)、MART(multiplicative algorithm reconstruction technique)和SIRT(simul-taneous iteration reconstruction technique)[131-132]。為了克服觀測(cè)信息不足給層析結(jié)果帶來(lái)的不利影響，國(guó)內(nèi)外很多電離層研究者提出了改進(jìn)的方法。文獻(xiàn)[133]聯(lián)合28個(gè)站的GPS/MET掩星數(shù)據(jù)和IGS提供的全球160個(gè)站的觀測(cè)數(shù)據(jù)，利用Kalman濾波方法實(shí)現(xiàn)了真正意義上的三維層析。文獻(xiàn)[134]提出了一種參數(shù)化電離層模型輔助的Kalman濾波法。文獻(xiàn)[135—136]提出三維變分?jǐn)?shù)據(jù)同化算法，并在2004年利用該方法，開(kāi)發(fā)了一套電離層電子密度分析程序，該程序可以同化GNSS衛(wèi)星和測(cè)高儀等多手段觀測(cè)數(shù)據(jù)。文獻(xiàn)[137]提出了廣義奇異值分解算法。文獻(xiàn)[138]提出了Sobolev正則化約束的SIRT算法。文獻(xiàn)[139]提出了融合GPS觀測(cè)數(shù)據(jù)和測(cè)高儀數(shù)據(jù)的層析方法。文獻(xiàn)[121]提出了一種兩步法電離層層析算法。文獻(xiàn)[123]發(fā)展了一種自適應(yīng)的聯(lián)合迭代重構(gòu)算法，通過(guò)自適應(yīng)地調(diào)整松弛因子和加權(quán)參數(shù)，能夠有效地反演電離層電子密度。文獻(xiàn)[140]提出過(guò)一種附加雙網(wǎng)格約束和速度圖像的電離層層析算法。文獻(xiàn)[141]提出了顧及電離層變化的層析反演新算法，提高了電子密度反演精度。

3.3 電離層監(jiān)測(cè)和應(yīng)用進(jìn)展

GNSS二維/多維建模具有常規(guī)電離層探測(cè)手段(如電離層測(cè)高儀)無(wú)法比擬的優(yōu)勢(shì)。其探測(cè)時(shí)間和空間分辨率高，精度可靠，所以在電離層監(jiān)測(cè)和預(yù)報(bào)領(lǐng)域具有廣闊的應(yīng)用前景。電離層的不規(guī)則擾動(dòng)對(duì)航天安全、無(wú)線電通信、導(dǎo)航定位等有重要的影響，因此監(jiān)測(cè)異?？臻g天氣下的電離層擾動(dòng)具有重要意義。早在1996年，文獻(xiàn)[142]就利用60臺(tái)GPS觀測(cè)站求取的GIM圖像，對(duì)磁暴期間電離層異?，F(xiàn)象進(jìn)行了研究，文獻(xiàn)[143]利用層析技術(shù)監(jiān)測(cè)了磁暴期間不同高度方向電離層響應(yīng)機(jī)制，GNSS電離層反演手段的出現(xiàn)和發(fā)展促進(jìn)了磁層-熱層-電離層耦合機(jī)制的研究[144-147]。地震電離層異常，包括震前電離層異常(pre-earthquake ionospheric abnormal，PEIA)和同震電離層異常(coseismic ionospheric disturbances，CID)是近十幾年來(lái)研究的一個(gè)熱點(diǎn)之一。地震的電離層前兆第一次引起人們的注意是1965 年，文獻(xiàn)[148—149]首次對(duì)1964 年Alaskan M9.0地震震區(qū)上空電離層擾動(dòng)進(jìn)行了研究。早期地震電離層異常的研究主要集中在統(tǒng)計(jì)性研究[150-154]，隨著電離層信息更加多元化和研究的不斷深入，地震電離層異常的研究向更加精細(xì)的方向發(fā)展。文獻(xiàn)[155]在研究Chi-Chi地震時(shí)利用格網(wǎng)搜索的方法不僅估計(jì)出CID傳播速度而且確定了CID觸發(fā)點(diǎn)的地面位置。文獻(xiàn)[156]引入了電離層地震學(xué)的概念，并從理論上總結(jié)了目前地震電離層異常擾動(dòng)物理機(jī)理的研究成果，同時(shí)指出該學(xué)科將是未來(lái)幾十年具有挑戰(zhàn)性的熱門研究課題。隨著GNSS連續(xù)運(yùn)行站在全球范圍內(nèi)數(shù)量的不斷增加及電離層層析算法的不斷完善和發(fā)展，GNSS電離層監(jiān)測(cè)必將在空間物理研究等領(lǐng)域發(fā)揮更加積極的作用。

4 GNSS空間環(huán)境學(xué)未來(lái)研究和展望

4.1 對(duì)流層建模展望

在對(duì)流層建模方面，基于氣象參數(shù)的對(duì)流層延遲模型與利用GNSS觀測(cè)數(shù)據(jù)建立的對(duì)流層延遲模型相比，精度仍然存在著一定的差距，主要是在ZWD的計(jì)算方面精度不足。倘若能夠繼續(xù)提高利用實(shí)測(cè)氣象參數(shù)計(jì)算ZWD的精度，使其接近于GNSS能夠獲取的精度，對(duì)于大范圍、密集、高精度監(jiān)測(cè)對(duì)流層或水汽具有重大的意義，能夠節(jié)省大量的成本。另外，目前還未開(kāi)展對(duì)不同對(duì)流層觀測(cè)值的實(shí)時(shí)融合研究，利用廉價(jià)的氣象觀測(cè)設(shè)備來(lái)加密GNSS網(wǎng)從而對(duì)對(duì)流層實(shí)現(xiàn)更密集的監(jiān)測(cè)，所以研究最優(yōu)的融合算法，將不同精度、不同數(shù)據(jù)源的對(duì)流層觀測(cè)值進(jìn)行融合得到精度更優(yōu)、水平分辨率更高的對(duì)流層模型產(chǎn)品具有重要的意義。

4.2 對(duì)流層水汽反演及應(yīng)用展望

基于GNSS對(duì)流層水汽反演已經(jīng)較為成熟，但對(duì)于其在氣象等方面的應(yīng)用仍待研究。對(duì)于二維水汽信息進(jìn)行降雨預(yù)報(bào)來(lái)講，一方面可以對(duì)某區(qū)域多個(gè)測(cè)站的觀測(cè)數(shù)據(jù)進(jìn)行聯(lián)合處理，以期得到更為準(zhǔn)確、全面的預(yù)報(bào)結(jié)果；另一方面，大氣水汽與溫度息息相關(guān)，可以通過(guò)分析降雨前后水汽與溫度的相關(guān)關(guān)系，建立一個(gè)更為合理和精確的多因子短臨降雨預(yù)報(bào)模型。在全球范圍內(nèi)通過(guò)融合多源觀測(cè)數(shù)據(jù)(地基和空基GNSS觀測(cè)數(shù)據(jù)、無(wú)線電探空儀數(shù)據(jù)、COSMIC數(shù)據(jù)和ECMWF再分析資料等)對(duì)大尺度中長(zhǎng)期的二維水汽信息進(jìn)行分析和研究，探究全球水汽變化演變機(jī)理，識(shí)別重大氣候?yàn)?zāi)害致災(zāi)因子，對(duì)中長(zhǎng)尺度氣候?yàn)?zāi)害事件進(jìn)行監(jiān)測(cè)和預(yù)報(bào)。在三維水汽產(chǎn)品方面，可將GNSS三維對(duì)流層產(chǎn)品與WRF模式數(shù)值同化，對(duì)WRF模式中數(shù)據(jù)同化系統(tǒng)模塊進(jìn)行改進(jìn)，彌補(bǔ)地表常規(guī)資料和高空探測(cè)資料的不足，進(jìn)一步提高WRF模式的預(yù)報(bào)能力。

此外，大量研究均已表明基于GNSS反演水汽的能力及優(yōu)越性，如何進(jìn)一步拓展GNSS水汽產(chǎn)品在氣象學(xué)上的應(yīng)用也是重點(diǎn)研究方向之一。

4.3 電離層監(jiān)測(cè)和建模展望

在精化全球電離層模型方面，多系統(tǒng)多源數(shù)據(jù)融合將成為下一步研究的重點(diǎn)。在建立電離層模型時(shí)，筆者認(rèn)為應(yīng)進(jìn)一步考慮伽利略系統(tǒng)、北斗系統(tǒng)等多系統(tǒng)數(shù)據(jù)對(duì)建模的貢獻(xiàn)，同時(shí)要考慮不同系統(tǒng)及不同頻率間的組合定權(quán)問(wèn)題。另外，電離層高階項(xiàng)與磁場(chǎng)分布強(qiáng)度密切相關(guān)，而全球磁場(chǎng)的分布不一致，因此電離層高階項(xiàng)對(duì)模型精度的影響也有待進(jìn)一步的研究。電離層層析算法方面，基于1 Hz及50 Hz的高頻、多系統(tǒng)GNSS數(shù)據(jù)進(jìn)行高時(shí)間分辨率的電離層層析反演，進(jìn)一步結(jié)合非相干散射雷達(dá)、測(cè)高儀及InSAR技術(shù)進(jìn)行三維層析的優(yōu)化，優(yōu)化層析算法提高電離層層析模型的可靠性和精度，特別是提高層析模型空間分辨率和時(shí)間分辨率。在電離層應(yīng)用研究方面，在現(xiàn)象提取、統(tǒng)計(jì)的基礎(chǔ)上，應(yīng)深入研究電離層異常觸發(fā)、傳播的物理機(jī)制。如地震電離層異常研究，可以結(jié)合破裂面分析、巖石圈-大氣層-電離層的耦合機(jī)制進(jìn)行綜合研究，地震通過(guò)瑞利波、聲重波、海嘯波等對(duì)電離層產(chǎn)生作用，今后的研究中將結(jié)合此類觀測(cè)數(shù)據(jù)對(duì)電離層異常進(jìn)行相關(guān)分析，驗(yàn)證相關(guān)理論的準(zhǔn)確性。

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(責(zé)任編輯：陳品馨)

Research Progress and Prospect of GNSS Space Environment Science

YAO Yibin1,ZHANG Shun1,KONG Jian2

1. School of Geodesy and Geomatics, Wuhan University, Wuhan 430079, China; 2. Chinese Antarctic Center of Surveying and Mapping, Wuhan University, Wuhan 430079, China

Troposphere and ionosphere are two important components of the near-earth space environment. They are close to the surface of the earth and have great influence on human life. The developments of Global Navigation Satellite System (GNSS) over the past several decades provide a great opportunity for the GNSS-based space environment science. This review summarizes the research progress and prospect of the GNSS-based research of the Earth’s troposphere and ionosphere. On the tropospheric perspective, modeling of the key tropospheric parameters and inversion of precipitable water vapor (PWV) are dominant researching fields. On the ionospheric perspective, 2D/3D ionospheric models and regional/global ionospheric monitoring are dominant researching fields.

troposphere; PWV; ionosphere; GNSS space environment science

The National Natural Science Foundation of China (No. 41574028); The Natural Science Foundation for Distinguished Young Scholars of Hubei Province of China (No. 2015CFA036)

YAO Yibin(1976—), male, PhD, professor, majors in geodetic data processing and GNSS space environment science.

姚宜斌,張順,孔建. GNSS空間環(huán)境學(xué)研究進(jìn)展和展望[J].測(cè)繪學(xué)報(bào)，2017,46(10)：1408-1420.

10.11947/j.AGCS.2017.20170333.

YAO Yibin,ZHANG Shun,KONG Jian.Research Progress and Prospect of GNSS Space Environment Science[J]. Acta Geodaetica et Cartographica Sinica,2017,46(10):1408-1420. DOI:10.11947/j.AGCS.2017.20170333.

P228

A

1001-1595(2017)10-1408-13

國(guó)家自然科學(xué)基金(41574028)；湖北省杰出青年科學(xué)基金(2015CFA036)

2017-06-22

修回日期： 2017-09-04

姚宜斌(1976—)，男，博士，教授，研究方向?yàn)闇y(cè)量數(shù)據(jù)處理理論與方法、GNSS空間環(huán)境學(xué)。

E-mail： ybyao@whu.edu.cn