Comparison of COSMIC Radio Occultation Refractivity Profiles with Radiosonde Measurements

In recent years, radio occultation （RO） technology making use of global positioning system （GPS） signals has been exploited to obtain profiles of atmospheric parameters in the neutral atmosphere. In this paper, the RO refractivity profiles obtained from the Constellation Observing System for Meteorology, Ionosphere and Climate （COSMIC） mission are statistically compared with the observations of 38 radiosonde stations provided by the Australian Bureau of Meteorology during the period from 15 July 2006 through 15 July 2007. Different collocation criteria are compared at first, and COSMIC RO soundings that occur within 3 hours and 300 km of radiosonde measurements are used for the final statistical comparison. The overall results show that the agreements between the COSMIC refractivity profiles and the radiosonde soundings from the 38 stations are very good at 0-30 km altitude, with mean absolute relative refractivity deviations of less than 0.5%. Latitudinal comparisons indicate that there are negative refractivity deviations in the lower troposphere over the low latitude and middle latitude regions and large standard deviations exist in the lower troposphere of low latitude regions, which can reach up to ～6%. The comparisons of COSMIC RO refractivity profiles and radiosonde observations for 3 polar stations in four different seasons indicate that the accuracy of GPS RO profiles is better in the Austral summer and autumn than in the Austral spring and winter during the year from September 2006 to August 2007.

Retrieval of Water Vapor Profiles with Radio Occultation Measurements Using an Artificial Neural Network

A new method applying an artificial neural network （ANN） to retrieve water vapor profiles in the troposphere is presented. In this paper, a fully-connected, three-layer network based on the backpropagation algorithm is constructed. Month, latitude, altitude and bending angle are chosen as the input vectors and water vapor pressure as the output vector. There are 130 groups of occultation measurements from June to November 2002 in the dataset. Seventy pairs of bending angles and water vapor pressure profiles are used to train the ANN, and the sixty remaining pairs of profiles are applied to the validation of the retrieval. By comparing the retrieved profiles with the corresponding ones from the Information System and Data Center of the Challenging Mini-Satellite Payload for Geoscientific Research and Application （CHAMP-ISDC）, it can be concluded that the ANN is relatively convenient and accurate. Its results can be provided as the first guess for the iterative methods or the non-linear optimal estimation inverse method.

Variability of the Martian ionosphere from the MAVEN Radio Occultation Science Experiment

The Martian ionosphere is produced by a number of controlling processes, including solar extreme ultraviolet radiation (EUV) and X-ray ionization, impact ionization by precipitating electrons, and day-to-night transport. This study investigates the structural variability of the Martian ionosphere with the aid of the radio occultation (RO) experiments made on board the recent Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. On the dayside, the RO electron density profiles are described by the superposition of two Chapman models, representing the contributions from both the primary layer and the low-altitude secondary layer. The inferred subsolar peak electron densities and altitudes are 1.24×10^5 cm^-3 and 127 km for the former, and 4.28×10^4 cm^-3 and 97 km for the latter, respectively, in general agreement with previous results appropriate for the low solar activity conditions. Our results strengthen the role of solar EUV and X-ray ionization as the driving source of plasma on the dayside of Mars. Beyond the terminator, a systematic decline in ionospheric total electron content is revealed by the MAVEN RO measurements made from the terminator crossing up to a solar zenith angle of 120°. Such a trend is indicative of day-to-night plasma transport as an important source for the nightside Martian ionosphere.

A Novel Tool for the Determination of Tropopause Heights by Using GNSS Radio Occultation Data

The tropopause is a transitional layer between the troposphere and the stratosphere. The exchange of chemical constituents of the atmosphere (namely masses of air, water vapor, trace gases etc.) and energy between the troposphere and the stratosphere occurs through this layer. We suppose that just exchanges that are taking place at the tropopause heights are strongly influenced by the Global Change forcing. For this reason it is particularly urgent to accumulate temporal data the most accurate possible and with a certain continuity series to understand comprehensively what is happening to our climate. It is well known that Radio Occultation technique applied using Global Navigations Satellite Systems (GNSS-RO) is a powerful tool to detect the tropopause heights. It can be done working on the level 2 data provided by GNSS-RO payload: i.e. atmospheric profiles of pressure and temperature. We propose to measure tropopause using GNSS-RO level 1 data;i.e. the bending angles (BA) of the GNSS signal through the atmosphere. We fit the BA profiles applying in the integral relationship of BA as refractivity profile of background the Hopfield dry model of atmosphere which depends on the fourth degree of the height above the Earth. Through the layers in which tropopause is contained, the residuals between the background model and the observed BA have an anomalous trend. The residuals in this zone form anomalous non-gaussian bumps that we have exploited just to determine the relevant parameters of the tropopause. Such bumps are due to the wrong theoretical assumption made by Hopfield for the re-construction of the dry refractivity that the temperature lapse rate of the atmosphere is constant. But we know that the definition of tropopause according the World Meteorological Organization (WMO) is just the height where a sudden change of the temperature lapse rate usually occurs. Thus in the present work we have determined tropopause heights with new algorithms which exploit the bumps occurring along the BA profiles achieved by GNSS-RO. We have used the huge amount of data provided by several space missions devoted to GNSS-RO (namely COSMIC, METOP, etc.) for tuning the algorithms, performed a validation and provided a robust statistical soundness. The same GNSS-RO observations are helpful also to reconstruct the Mapping Function commonly applied in geodetic applications. Global mapping functions built with GNSS-RO and their evolution in time can be an interesting parameter helpful for climate investigations as well.

Determination of Total Electron Content at Equatorial Region—Thailand Using Radio Occultation Technique

This paper investigates the total electron content (TEC) which is a major challenge to space and related industries, most especially in an equatorial region like Thailand within the geographical latitude 07°35'N - 20°17'N. This research was achieved using radio occultation data from COSMIC mission. The monthly, seasonal and annual TEC and electron density variation monitoring conducted during increasing solar activity from 2010 to 2013 due to changes and instability of ionospheric parameters. It was observed that electron density and TEC was predominant in summer season. Summer has the highest electron density and TEC values all through and the annual mean values keep on increasing within the period under consideration. In conclusion, ionospheric fluctuations and perturbations were observed to be at pick between the months of March and May. The results of the study demonstrated that ionospheric irregularities were steadily on the increase, confirming 24th solar cycle prediction by NASA and depended on many factors among the major ones which are located on the latitude, season and solar activity.

Optimization of the Mars ionospheric radio occultation retrieval

Electron density is a key parameter to characterize Martian ionospheric structure and dynamics. Based on the ephemeris and auxiliary information derived from the Spacecraft, Planet, Instruments, C-matrix, and Events(SPICE) toolkit, we calculated the bending angle of signal path from the frequency residuals measured by the Mars Express Radio Science Experiment(Ma RS) of the Mars Express(MEX) mission under the assumption of a spherically symmetric ionosphere. We stratified the ionosphere into layers and assumed a linear change of bending angle between layers, to derive profiles in radial distance of refractivity with the optimized parameters of upper integral limit of 4890 km and baseline correction boundary of 3690 km. Meanwhile, we also compared the retrieved electron density profiles between the frequency residuals of the single-frequency and differential Doppler of the dual-frequency. In total, ~640 electron density profiles of Martian ionosphere between April 2004 and April 2015 were retrieved successfully. There are 24 profiles identified manually that exhibit an additional sporadic layer occurrence below the normal two-layers. We also found that the peak altitude of this layer increases with the main peak altitude.

Gravity wave activities in Tibet observed by COSMIC GPS radio occultation

The Gravity waves activities in the Tibetan Plateau are very complex with different effects and natures that are not clear due to the lack of high resolution data from space and also from ground. In this paper,using COSMICGPS radio occultation data from 2006 to 2014, the atmospheric gravity waves activities and climatologic behaviors in Tibetan stratosphere are studied and analyzed, which show different characteristics. Most of the gravity waves with potential energy(Ep) at altitude of 17-24 km are associated with mountain waves. A good correlation between gravity wave activities and zonal wind flow is found. The distribution of gravity wave(GW) activities in Tibet is strongly connected with zonal wind variation and topography. GW activities are enhanced in winter seasons and decreased in summer seasons since strong western winds persist at all heights of the Tibetan troposphere. The gravity waves generated in the Tibetan area are mostly related to the orography of the area. The vertical wavelengths of GWS are shorter.Gravity waves in the Northwest have different vertical wavelengths in the Southeastern part of Tibetan Plateau, and dominant wavelengths are 3-5 km in the Northwest and 2-3 km in the Southeast,respectively. In the summer, the Northwestern part is the main source of wave generation while in the winter the GW is generated almost from all peaks of the Tibetan Plateau. Gravity waves in the region are clearly related to deep convection, which can also be proved by the inverse relation of Outgoing long wave radiation(OLR) and potential energy(Ep).

Impact of GNSS radio occultation bending angle data assimilation in YH4DVAR system

Global Navigation Satellite System （GNSS） radio occultation measurements have been assimilated into the four- dimensional variational data assimilation system （YH4DVAR） using a one-dimensional bending angle operator （GBAO） as a new type of observation. For the sake of verifying the impact of GNSS radio occultation （RO） measurements to the data assimilation system, three experiments have been conducted. The statistical results of the analysis error experiment and forecast skill experiment show that the GNSS RO measurements have an impact on the analysis system. The typhoon forecast experiment shows the impact on the important weather process. They all have a positive impact on the weather forecast. Lastly, we look forward to future work on the observation system simulation experiment （OSSE） to investigate the impact of GNSS RO measurements as a function of observation number, which is an effective method to estimate the saturation of the observation number.

Global Statistical Study of Ionospheric Waves Based on COSMIC GPS Radio Occultation Data

Extracting and parameterizing ionospheric waves globally and statistically is a longstanding problem. Based on the multichannel maximum entropy method（MMEM） used for studying ionospheric waves by previous work, we calculate the parameters of ionospheric waves by applying the MMEM to numerously temporally approximate and spatially close global-positioning-system radio occultation total electron content profile triples provided by the unique clustered satellites flight between years 2006 and 2007 right after the constellation observing system for meteorology, ionosphere, and climate（COSMIC） mission launch. The results show that the amplitude of ionospheric waves increases at the low and high latitudes（~0.15 TECU） and decreases in the mid-latitudes（~0.05 TECU）. The vertical wavelength of the ionospheric waves increases in the mid-latitudes（e.g., ~50 km at altitudes of 200–250 km） and decreases at the low and high latitudes（e.g., ~35 km at altitudes of 200–250 km）.The horizontal wavelength shows a similar result（e.g., ~1400 km in the mid-latitudes and ~800 km at the low and high latitudes）.

卫星微波高层探测通道定标精度评估

全球/区域数值预报同化系统所需要的重要资料来源之一是卫星微波遥感的亮温数据。微波遥感仪器的高层大气(平流层中上部)探测通道亮温的定标精度,受多种因素影响,存在一定的偏差。由于高层大气高精度探测资料相对比较匮乏,因此对微波遥感的高层大气亮温的精度检验评估成为重要研究内容。本文对在轨运行的微波温度遥感器AMSU-A、ATMS和FY-3D MWTS-2在2020年观测数据进行了定标精度检验评估,综合使用了三种评估方法进行了较为全面的分析,即基于掩星资料、再分析资料的辐射传输模拟比较和同类仪器交叉定标比较。三种检验评估方法从不同角度揭示了各仪器高层大气探测通道观测资料的误差特征,分析了对比误差的可能来源。三种评估方案给出的高层通道亮温的偏差存在差异,但基本结果变化趋势是一致的,即高层通道相比中低层通道,存在较大的噪音。除了交叉定标方法,其它两种方法都检验出了高层资料偏差的季节变化;整体而言,AMSU-A表现要优于MWTS-2和ATMS。本研究评估的高层亮温偏差的时间、空间变化特征可为微波高层大气资料的同化、气候应用提供参考。

火星无线电掩星探测进展

火星是地球的姊妹星,研究火星对了解火星、地球乃至太阳系的演化具有重要意义.自从1964年美国水手4号发射,首次成功地运用无线电掩星技术探知到火星的环境特征之后,国际上不少的火星任务都开展了掩星实验,取得了重要进展.本文依据采用了无线电掩星技术进行勘探的火星探测器发射时间顺序展开调研,针对具有开创性的水手系列、火星全球勘测者、火星快车、火星大气挥发演化探测器、天问一号等,梳理分析和总结了各任务的火星无线电掩星方式以及所获取的廓线数量、位置分布、获取方式等产品信息,以及部分相关的研究结果.本文还分析了当前火星无线电掩星探测方式存在的局限性,并探讨了可能的对策.火星无线电掩星后续可重点考虑多颗星-星掩星结合星-地掩星方式形成掩星星座;并通过选用适当的信号探测频率、改进反演算法等方式进一步提高掩星质量;火星掩星探测手段还可与火星顶部探测雷达、直接探测等手段相结合,发展火星多源数据融合技术.随着探测方式的不断改进,无线电掩星探测将是火星探测的重要手段.未来会有数量越来越多、时间与空间覆盖越来越全面、精度越来越高的掩星数据用于火星的整个空间环境研究,包括大、中尺度乃至小尺度结构的特征与演化规律都将被人类掌握.

QX-1 GNOS M掩星数据质量分析

QX-1 GNOS M是首台在轨实现北斗、GPS和Galileo三系统兼容的小型商业化全球导航卫星掩星探测仪. 2021年10月14日18:51 LT,气象一号(QX-1)卫星发射并入轨.自发射以来,该卫星已收集大量观测数据.基于QX-1 GNOS M的结构组成及性能,统计分析了2022年8月17日全天的掩星事件及其全球分布情况.通过将8月17日至9月3日的掩星数据与NCEP再分析模式对比,评估QX-1掩星事件的探测穿透深度和折射率精度,同时检验Galileo掩星数据的可靠性和一致性.初步分析结果表明, QX-1 GNOS M在实现三系统兼容后,掩星事件数量相较于仅使用GPS系统的情况增加了约1.5倍.这一结果进一步证明,不同全球导航卫星系统(GNSS)所提供的掩星数据在精度上具有一致性.此外研究显示,在多系统兼容的背景下, QX-1 GNOS M能够提供更为丰富和精确的气象数据.

GRO和LRO掩星事件模拟研究

GRO(Global Navigation satellite system Radio Occultation)和LRO(Low Earth Orbit Radio Occultation)联合组网探测地球大气是无线电掩星探测技术的主要发展方向.本文根据掩星事件的数学判据,仿真分析了LEO卫星主要轨道参数对CRO和LRO掩星事件数量和全球分布情况的影响.研究表明:卫星轨道越低GRO掩星事件越多:轨道倾角在30°和75°之间时,GRO掩星事件较多.全球覆盖率也较大;利用极轨卫星进行LRO掩星探测时.LRO掩星事件较均匀地分布在各纬度带.研究成果对GRO和LRO联合星座设计具有参考价值.

星基GNSS掩星事件预报方法分析

针对全球卫星导航系统(GNSS)星基掩星观测时长通常仅有几十秒,特别是在升星掩星观测中,需要快速实现负高度角卫星信号的捕获跟踪,才能获得及时有效的掩星观测时长的问题,基于先验轨道信息辅助的开环跟踪技术,提出一种基于切线高度的掩星事件快速预报方法,以辅助掩星接收机快速捕获和跟踪GNSS掩星信号,从而保证掩星观测数据质量和有效观测时长:对地球局部曲率中心进行修正,得到更为精准的切线高度;然后设置掩星事件的判决条件,采用极值的方式快速预报出符合阈值的掩星事件;最后对全球内曲率中心和曲率半径进行分析。实验结果表明,曲率中心的修正距离最大值可至45 km,曲率半径波动范围可达到65 km,足以影响掩星探测的准确预报。基于气象、电离层和气候星座观测系统(COSMIC-2)实测数据的仿真结果表明,掩星预报的全球定位系统(GPS)掩星漏报率低于3%,格洛纳斯卫星导航系统(GLONASS)掩星漏报率约14.5%,且预报掩星开始时间大多提前5~10 s;证明基于切线高度的掩星预报方法能够有效地应用于星基掩星事件的预报,以辅助接收机准确捕获和跟踪信号,提升掩星观测资料的数据量和有效观测时长。

基于二级MZM相干调制实现高速长距离RoF通信

提出并计算机仿真证明了基于二级光电双臂相干调制及双二进制光信息编码技术实现超高速超长距离光纤微波传输(Radio over Fiber)的系统设计方案.通过第一级光电双臂相干调制构建光双二进制信号,其信号占用频谱带宽比NRZ码降低50%.通过第二级光电双臂相干调制加载40 GHz超高频载波信号,实现光纤微波传输.计算机仿真结果分析表明:CW激光光源发射功率为0 d Bm,经过多级EDFA光信号放大与色散补偿光纤,在色散系数为17 ps/(nm·km),衰减系数为0.2 d B/km的标准单模光纤中单通道传输,系统码元传输速率可达40 Gb/s、传输距离1 500 km以上.

ROPP反演软件算法及其精度分析

介绍了ROPP反演软件中无线电掩星反演的算法与精度分析.采用COSMIC卫星2008年1月1日全天的附加相位数据,反演得到折射率、温度、压强与湿度等参数,并与CDAAC相应结果进行对比实验结果表明,在30 km高度以下,折射率、压强和湿度的相对误差在2%以内,温度误差不超过2 K.

ROPP掩星数据反演软件及其精度分析

GPS掩星探测技术已成为地球大气和电离层探测的重要手段。介绍了无线电掩星数据反演地球大气参数的原理方法以及处理软件ROPP的功能和模块构成;并详细阐述了模块的算法原理。依据掩星探测几何关系,在球对称模式下利用ROPP软件进行参数配置,对COSMIC掩星数据进行预处理,由阿贝尔积分反演得到大气折射率廓线;并利用一维变分反演方法得到大气参数如温度、湿度和压强的廓线。反演结果图示说明,与CDAAC处理数据进行了比较和误差分析,分析表明折射率、温度、压强和湿度的吻合程度较好,30km以下折射率误差约在5%范围内;同时给出了误差的来源及分析。

Temporal distribution characteristics of GNSS ionospheric occultation data and its effects in earthquake-ionosphere anomaly detection

The temporal distribution characteristics of COSMIC occultation data are analyzed in detail, and the limitations in earthquake-ionosphere anomaly detection caused by the temporal distribution characteristics of COSMIC occultation data are discussed using the example of the Wenchuan earthquake. The results demonstrate that there is no fixed temporal resolution for COSMIC occultation data when compared with other ionospheric observation techniques. Therefore, occultation data cannot currently be independently utilized in research studies but can only be used as a complement to other ionospheric observation techniques for applications with high temporal resolution demands, such as earthquake-ionosphere anomaly detection.

“天问一号”无线电掩星观测试验及特征分析

中国“天问一号”探测器环火飞行为星–地火星无线电掩星观测提供了必要条件,可有效支持火星大气、电离层反演探测。重点介绍国内首次基于“天问一号”的火星无线电掩星观测试验。利用“天问一号”预报轨道,预报地面深空站掩星观测弧段;组织深空站,对“天问一号”进行火星入掩全过程观测,采集与记录探测器原始下行信号;利用自研深空开环测量软件,对“天问一号”下行信号进行处理分析,提取火星无线电掩星特征。结果表明:信号中幅度衍射斑纹与频率翘尾特征被成功提取,印证了此次星–地火星无线电掩星事件被有效观测,为后续基于“天问一号”的火星大气探测科学研究,积累了重要技术经验与测量数据。

风云卫星的掩星干大气温度廓线精准度特征

风云三号气象卫星C星/D星/E星的掩星接收机接收GPS和北斗导航卫星信号,形成掩星事件,进而反演得到大气温度廓线。利用2016—2022年FY-3C/3D/3E GPS和北斗掩星干大气温度廓线,通过与ERA5再分析资料对比,分析掩星干大气温度的精准度特征。结果表明:掩星干大气温度廓线在200 hPa至20 hPa间精度最高,标准偏差约为1 K,且GPS与北斗掩星的误差特征相近。在稳定度方面,FY-3C GPS掩星干大气温度平均偏差线性变化趋势为-0.0055 K·a^(-1),与国外同类掩星资料稳定性相当。因2021年初掩星天线多径效应订正,FY-3D GPS掩星干大气温度与ERA5再分析资料的平均偏差出现明显跳变,平均偏差值从-0.154 K降为-0.007 K,负偏差显著减小。总体上,FY-3C GPS掩星干大气温度廓线长序列稳定度较好,北斗掩星干大气温度廓线精度能够达到或优于GPS,风云极轨卫星序列的GPS和北斗掩星在长序列稳定性上具有良好的应用前景。