LEO卫星轨道误差对无线电掩星反演大气参数的影响

采用数值方法估计了LEO卫星轨道误差对无线电掩星反演大气参数的影响。并将其应用于1995年10月11日的某次掩星事件的观测资料处理和分析，得到了LEO卫星轨道误差对无线电掩星反演大气参数影响的定量结果。     

地基行星大气掩星观测资料流程和数据整理

在中国火星探测萤火一号(YH-1)计划中, 包括了地基掩星观测反演火星大气的科研任务. 观测资料整理是反演流程的第一步. 本文描述了地基火星大气掩星观测处理软件系统的观测数据流程和观测资料整理模块,并详细介绍了观测资料整理模块的结构和功能, 其中包括时间系统转换、历表插值、坐标系变换、信号时延改正以及掩星平面建立. 利用行星数据系统公布的火星快车无线电科学数据和由SPICE得到的地球、火星历表以及火星快车的轨道数据, 结合本文的算法, 得到了一些实验结果.      

基于GPS掩星平均弯曲角的折射率反演研究

利用Abel积分变换通过掩星弯曲角计算折射率需要对高层弯曲角进行统计优化.目前由于所使用的背景场资料和具体反演方法不同,导致所发布的掩星大气数据气候统计值存在一定差异.本文使用2008年1,4,7,10月共4个月的COSMIC大气掩星附加相位数据,从纬圈平均弯曲角廓线反演相应月平均折射率,对反演结果进行比较分析.研究表明,利用掩星折射数据进行气候研究时无需逐一对掩星探测廓线进行统计优化,在40 km以下高度基于平均弯曲角的反演方法与传统统计相比能够获得几乎一致的月平均折射率,在50 km以上高度基于平均弯曲角的反演结果更加接近ECMWF资料统计.      

基于X射线掩星探测反演行星大气密度

X射线掩星是一种常见的天文现象，基于X射线掩星探测的大气密度反演是一种涉及学科交叉的新方法，其通过处理高能X射线天体辐射源的掩星观测数据实现大气密度的反演，基本原理为X射线在大气中传播时，X射线光子被大气中的原子（包括分子中的原子）吸收和散射，从而导致X射线强度发生衰减，根据衰减后X射线信号的强度反演对应的密度廓线。本文根据X射线掩星探测的应用需求，论证了基于X射线掩星实现大气密度反演的新方法，重点介绍了光变曲线拟合和能谱拟合两种地球中高层大气密度反演算法，分析了X射线掩星探测反演大气密度的研究进展和研究方法，对基于X射线掩星反演大气密度的优点进行分析和讨论，进而对X射线掩星探测的应用场景进行展望。结果表明，作为一种新型中高层大气密度测量手段，X射线掩星探测可对中高层大气密度实现有效探测，弥补了目前中高层大气密度实测数据的不足。     

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GRO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.      

基于小波分解与重构方法研究电离层偶发E层

电离层不规则体对卫星导航、通信、雷达系统等有重要影响.通过数值模拟及与实测数据的对比,论证基于小波分解与重构方法实现利用掩星数据反演电离层不规则体的可行性.以电离层偶发E层为例,利用国际参考电离层(IRI)模拟背景电离层电子密度分布,利用掩星探测的水平电子密度总含量δht反演不规则体信息,并与模拟数据进行比较.对200...     

电离层掩星反演的TEC修正反演法

电离层掩星数据反演的传统方法是采用改正TEC的Abel 变换反演法, 实际电离层的非球对称性会给电子密度的反演结果带来误差. 文中研究了利用TEC修正方法结合背景场来剔除TEC 受电子密度水平变化的影响, 改善球对称假设适用性, 提高反演精度, 并应用此方法于模拟掩星事件的反演. 结果表明, 与传统的Abel 变换反演相比, TEC 修正反演法能够减小反演误差. 用TEC 修正反演法对不同方法获取的背景场的反演结果比较表明, 背景场与实际场吻合的程度越高, 反演效果越好.      

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30km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2K.      

风云三号C星GNOS北斗掩星电离层探测初步结果

利用风云三号卫星C星GNOS掩星探测仪电离层数据,分析了2013年10月FY-3C GNOS探测的北斗掩星电离层廓线分布,将2013年10月1日至2015年10月10日期间FY-3C GNOS观测的F₂层峰值电子密度（N_mF₂）与地面电离层测高仪观测结果进行对比,验证了FY-3C GNOS北斗电离层掩星的探测精度.结果表明,FY3-C GNOS北斗电离层掩星与电离层测高仪探测的N_mF₂数据相关系数为0.96,平均偏差为10.21%,标准差为19.61%.在不同情况下其数据精度有如下特征:白天精度高于夜晚;夏季精度高于分季,分季精度高于冬季;中纬地区精度高于低纬地区,低纬地区精度高于高纬地区; BDS倾斜同步轨道（IGSO）卫星精度高于同步轨道（GEO）卫星和中轨道（MEO）卫星.FY-3C GNOS北斗电离层掩星与国际上其他掩星电离层数据精度的一致性对GNSS掩星探测资料的综合利用具有重大意义.      

掩星大气探测系统误差源分析

根据掩星大气探测反演原理,研究了影响系统产品精度的误差源,并对多普勒观测误差、天线相位中心变化误差、多路径误差、卫星质心变化误差、导航卫星钟飘、卫星姿态变化误差和精密定轨速度误差进行分析,根据工程实现可行性提出了系统误差分配建议,为掩星大气探测系统设计提供参考.      

Obtaining more accurate electron density profiles from bending angle with GPS occultation data: FORMOSAT-3/COSMIC constellation

Since 1995, with the first GPS occultation mission on board Low Earth Orbiter (LEO) GPS/MET, inversion techniques were being applied to GPS occultation data to retrieve accurate worldwide distributed refractivity profiles, i.e. electron density profiles in the case of Ionosphere. Important points to guarantee the accuracy is to take into account horizontal gradients and topside electron content above the LEO orbit. This allows improving the accuracy from 20% to 50%, depending on the conditions, latitude and epoch regarding to Solar cycle as reported in previous works.     

Measurements of the upper troposphere and lower stratosphere during tropical cyclones using the GPS radio occultation technique

Water vapour transport to the upper troposphere and lower stratosphere by deep convective storms affects the radiation balance of the atmosphere and has been proposed as an important component of climate change. The aim of the work presented here is to understand if the GPS radio occultation technique is useful for characterization of this process. Our assessment addresses the question if severe storms leave a significant signature in radio occultation profiles in the upper troposphere/lower stratosphere. Radio occultation data from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) were analyzed, focusing on two particular tropical cyclones with completely different characteristics, the hurricane Bertha, which formed in the Atlantic Basin during July 2008 and reached a maximum intensity of Category 3, and the typhoon Hondo, which formed in the south Indian Ocean during 2008 reaching a maximum intensity of Category 4. The result is positive, suggesting that the bending angle of a GPS radio occultation signal contains interesting information on the atmosphere around the tropopause, but not any information regarding the water vapour. The maximum percentage anomaly of bending angle between 14 and 18 km of altitude during tropical cyclones is typically larger than the annual mean by 5–15% and it can reach 20% for extreme cases. The results are discussed in connection to the GPS radio occultation receiver which will be part of the Atomic Clock Ensemble in Space (ACES) payload on the International Space Station.     

Global monitoring of tropospheric water vapor with GPS radio occultation aboard CHAMP

The global positioning system radio occultation (GPS RO) technique provides a powerful tool for atmospheric sounding which requires no calibration, is not affected by clouds, aerosols or precipitation, and provides an almost uniform global coverage. The paper deals with application of GPS RO measurements from CHAllenging Minisatellite Payload (CHAMP) for the retrieval of tropospheric water vapor profiles. CHAMP RO data are available since 2001 with up to 200 high resolution atmospheric profiles per day. We introduce a new direct method for water vapor retrieval from GPS RO data. Additionally, a 1Dvar algorithm is used for this purpose. The so derived CHAMP water vapor profiles are validated with radiosonde data on a global scale. Here, both methods come to statistically comparable results revealing a negative bias of less than 0.1 g/kg and a standard deviation of less than 1 g/kg specific humidity in the mid troposphere. Potentials of CHAMP RO retrievals for monitoring the mean tropospheric water vapor distribution on a global scale are presented.     

An alternative Shell inversion technique - Analysis and validation based on COSMIC and ionosonde data

Multi-channel Global Positioning System (GPS) carrier phase signals, received by the six low Earth orbiting (LEO) satellites from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) program, were used to undertake active limb sounding of the Earth’s atmosphere and ionosphere via radio occultation. In the ionospheric radio occultation (IRO) data processing, the standard Shell inversion technique (SIT), transformed from the traditional Abel inversion technique (AIT), is widely used, and can retrieve good electron density profiles. In this paper, an alternative SIT method is proposed. The comparison between different inversion techniques will be discussed, taking advantage of the availability of COSMIC datasets. Moreover, the occultation results obtained from the SIT and alternative SIT at 500 km and 800 km, are compared with ionosonde measurements. The electron densities from the alternative SIT show excellent consistency to those from the SIT, with strong correlations over 0.996 and 0.999 at altitudes of 500 km and 800 km, respectively, and the peak electron densities (N_mF₂) from the alternative SIT are equivalent to the SIT, with 0.839 vs. 0.844, and 0.907 vs. 0.909 correlation coefficients when comparing to those by the ionosondes. These results show that: (1) the N_mF₂ and h_mF₂ retrieved from the SIT and alternative SIT are highly consistent, and in a good agreement with those measured by ionosondes, (2) no matter which inversion technique is used, the occultation results at the higher orbits (∼800 km) are better than those at the lower orbits (∼500 km).     

Atmosphere sounding by GPS radio occultation: First results from TanDEM-X and comparison with TerraSAR-X

On 21 June 2010 the TerraSAR-X satellite was joined by the TanDEM-X satellite. A Global Positioning System (GPS) radio occultation (RO) experiment using the twin satellites has been carried out to estimate the precision of GPS atmospheric soundings. For the Day Of Year (DOY) 330–336, 2011, we analyze phase and amplitude data recorded by GPS receivers separated by a few hundred meters in a low earth orbit and derive collocated atmospheric refractivity profiles. In the altitude range 10–20 km the standard deviation between TerraSAR-X and TanDEM-X refractivity does not exceed 0.15%. The standard deviation is rapidly increasing for lower and higher altitudes; close to the surface and at an altitude of 30 km the standard deviation reaches 0.8% and 0.5%, respectively. Systematic deviations between TerraSAR-X and TanDEM-X refractivity in the considered altitude range (0–30 km) are negligible. The results confirm the anticipated high precision of the GPS RO technique. However, the difference in the retrieved refractivity in the lower troposphere for different Open Loop (OL) signal tracking parameters, altered onboard TanDEM-X for DOY 49–55, 2012, calls for an in depth analysis. At the moment we can not exclude that a potential bias in the OL Doppler model introduces a bias in our retrieved refractivity at altitudes <8$<8$ km.     

Observation of wave structures in the upper atmosphere by means of radio holographic analysis of the radio occultation data

A radio holographic approach, developed by Pavelyev (1998), Hocke (1999), Igarashi (2000), is applied to observation of wave phenomena in the upper atmosphere using Global Positioning System — “Microlab-1” satellite (GPS/MET) radio occultation data. In the current state the radio holography approach uses the radar focused synthetic aperture principle to obtain high spatial resolution, and to remove the interference part corresponding to scattering from the upper ionosphere. High spatial resolution and accuracy of the radio halographic method is validated by means of revealing the weak signal reflected from the sea in the GPS/MET radio occultation data. The radio holographic method gives a new possibility to measure directly the vertical gradient of the electron density altitude profile in the D-layer using the radio occultation signal. The results of the application of radio holographic analysis to two GPS/MET occultation events (07 February 1997, No. 0447, 0158), in the D-region of the ionosphere, are discussed. Wave structures in the electron density concentration with a vertical spatial period of 1.4–6 km, and variations in the electron density gradient from ±5·10⁹ to ±8·10⁹ [1/(m³km)], have been retrieved from the D-layer data. The features observed in the vertical electron density profiles may be connected with breaking of gravity waves in the D-layer of the ionosphere.     

An introduction of mountain-based GPS radio occultation experiments in China

The mountain-based GPS radio occultation is a novel approach to lower atmospheric profiling. The experiments of the mountain-based GPS radio occultation were conducted on the top of Mt. Yaogu (29.38°N, 113.68°E, ∼1240 m) on December 17, 2003, and on the top of Mt. Jiugong (29.39°N, 114.65°E, ∼1550 m) on July 24, 2004. Based on these observation data, the scientific data processing software has been developed and is used to retrieve successfully the atmospheric refractivity profiles. The validation experiment was performed on the top of Mt. Wuling (40.60°N, 117.48°E, ∼2118 m) during August 1–29, 2005. Collocated automatic weather station and the radiosondes nearby were operated simultaneously for the comparison campaign. Results show that the radio occultation technique obtained about 40 profiles every day with the receiver antenna pointing to the south. Comparisons show that the refractivity measured by occultation agree well with those by the radiosondes, but not well with those by the automatic weather station due to their much different geographic locations of measurements. Results of these experiments suggest that the mountain-based GPS radio occultation is an economic reliable novel technique monitoring temporal and spatial variations of local lower atmospheric environments.     

Location of layered structures in the ionosphere and atmosphere by use of GPS occultation data

A method is introduced to locate the layered structures in the atmosphere and ionosphere based on simultaneous observations of radio wave intensity and phase variations in trans-ionospheric satellite-to-satellite links. The method determines location of a tangent point on the trans-ionospheric ray trajectory where gradient of refractivity is perpendicular to the ray trajectory and influence of a layered structure on radio wave parameters is maximal. An estimate of the location of a layer can be obtained from a combination of the phase and intensity variations. This new technique was applied to measurements provided during FORMOSAT-3 and CHAMP radio occultation (RO) missions. For the considered RO events the location of the inclined plasma layer in the lower ionosphere is found and the electron density distribution is retrieved. The method is checked by measuring the location of the tangent point on the ray trajectory in the neutral gas in the atmosphere. The results showed a fairly good agreement.     

Evaluation of uncertainty in gravity wave potential energy calculations through GPS radio occultation measurements

The application of the Global Positioning System (GPS) radio occultation (RO) method to the atmosphere enables the determination of height profiles of temperature, among other variables. From these measurements, gravity wave activity is usually quantified by calculating the potential energy through the integration of the ratio of perturbation and background temperatures between two given altitudes in each profile. The uncertainty in the estimation of wave activity depends on the systematic biases and random errors of the measured temperature, but also on additional factors like the selected vertical integration layer and the separation method between background and perturbation temperatures. In this study, the contributions of different parameters and variables to the uncertainty in the calculation of gravity wave potential energy in the lower stratosphere are investigated and quantified. In particular, a Monte Carlo method is used to evaluate the uncertainty that results from different GPS RO temperature error distributions. In addition, our analysis shows that RO data above 30 km height becomes dubious for gravity waves potential energy calculations.