电离层掩星反演的正则最大熵法

电离层掩星数据现已成为电离层观测数据的重要来源,对掩星数据的反演研究一直是掩星研究的热点.传统采用的改正TEC(1btal Electron Content)的Abel变换反演法为线性反演法,它会把测量误差带入反演结果中.为改善反演效果受测量误差的影响,引入两种非线性的反演方法一正则化和正则最大熵反演法.随后设计模拟试验,对3种方法进行验证、比较,得到正则最大熵反演法可以很好地减小测量误差的影响.     

火星电离层无线电掩星探测仿真研究

对中俄联合火星星-星电离层掩星技术体制进行了分析和介绍,采用三维射线追踪方法对电离层掩星事件的电波观测值进行了模拟计算,并利用模拟的掩星观测数据进行了电子密度廓线反演,结果说明仿真算法可靠.利用仿真的方法,分别对掩星电波相位观测误差和卫星轨道误差等带来的反演误差进行了个例计算和分析,结果得到:5%周的相位测量误差对白天电离层掩星探测结果的影响可以忽略,而夜间电子密度测量的绝对误差小于4×108 m-3;卫星轨道误差对掩星的主要影响是导致电离层高度抬升或下降.结果表明,中俄联合火星电离层掩星探测技术体制先进,可望获得高精度的电子密度廓线;其技术体制也可以用于月球电离层环境的探测.     

An Emulation Research on the Radio Occultation Exploration of Martian Ionosphere

The technical system of the Sino-Russian joint satellite-to-satellite Mars ionosphere occultation is analyzed and introduced. The analogue computation of the observed values of the radio waves of the ionosphere occultation event is carried out by adopting the three-dimensional ray tracking method and the electron density profile inversion is conducted by means of the simulated occultation observational data, with the result showing that the emulation algorithm is reliable. By taking advantage of the emulation method the case computation and analysis of the inversion errors caused by the observational error of the occultation radio wave phase and the satellite orbital error are respectively carried out, and it is obtained from the result that the effect of the phase measuring error of the 5% circle on the result of the daytime ionosphere occultation exploration may be neglected, while the absolute error of the night electron density measurement is less than 4 × 108 m^?3, and the main effect of the satellite orbital error on the occultation leads to the lifting or falling of the ionospheric height. The result shows that the technical system of the Sino-Russian joint Mars ionosphere occultation exploration is advanced. It can be expected that the high accuracy electron density profile is obtained and the technical system can be applied to the exploration of the lunar ionospheric environment.     

TEC Compensated Inversion Method of Ionospheric Occultation

For the inversion of ionospheric occultation data, the Abel inversion method of calibrated TEC is usually adopted, but the spherical asymmetry of the real ionosphere will bring errors to the inversion result of electron densities. This paper studies a TEC compensated inversion method. By combining with a background field, this method may eliminate the effect of the horizontal variability of electron density on the TEC, and improve the applicability of the assumption of spherical symmetry as well as the inversion accuracy. Applying this method to the inversion of simulative occultation events, the result demonstrates that compared with the traditional Abel transform inversion, the TEC compensated inversion method can reduce the inversion error. Comparing the various background fields obtained in different ways, it is shown that the higher the coincidence level between the background field and the real field, the better the inversion result.     

GPS/LEO掩星方法中的电离层延迟量对太阳射电辐射流量的响应

对无线电信号在电离层中的传播路径进行了数值模拟;针对掩星观测中的五组实际卫星轨道数据(包括GPS和LEO卫星),给出了在太阳射电辐射流量(FLUX)分别为0,70,160和240等几种情况下的电离层延迟量对10.7cm波长的太阳辐射流量的响应结果;这分别对应着无太阳辐射、太阳射电辐射处于活动低谷、平静期和高峰期等几种情形。从中可以发现,掩星观测中电离层延迟量对10.7cm波长的太阳射电辐射流量的响应表现为如下特性:电离层延迟同参数FLUX的大小有明显的对应关系,即FLUX越大,则掩星观测中的电离层延迟越大;对于上升掩星情况而言,掩星观测中电离层延迟量起先逐渐增加,然后达到某一峰值,其后逐渐下降;在掩星观测的初期和中期,太阳射电辐射处于活动低谷、平静期和高峰期等几种情形之间的电离层延迟量差异都较为显著,而在掩星观测的后期,几种情形相互间电离层延迟量的差异都比较小。     

Statistical analysis of the results of champ occultation data processing at Shanghai astronomical observatory

With the development of the CPS (Global Positioning System) it has become possible to retrieve accurate profiles of atmospheric temperature, pressure and moisture from CPS occultation data. Using the inversion module developed in Shanghai Astronomical Observatory, (SHAG), we obtained atmospheric profiles from more than 2700 CHAMP occultation events observed in the period 2002 Aug. 1–17. The retrieved profiles are compared with the data of the European Center for Medium-range Weather Forecasts (ECMWF), and their errors are analyzed. An optimal method of statistical analysis is proposed and applied. The statistical results show that GPS occultation data may contribute valuably to numerical weather forecast and long-term monitoring of the earth's climate.     

On the singular points of the Abelian integral transformation in the GPS/LEO occultation technique

In the inversion terrestrial atmosphere technique the inversion of theAbelian integral is one of the most often used methods for deriving the refractive index profile of the terrestrial atmosphere from GPS/LLEO radio occultation data. There exists the problem of singular point in the Abelian integral. Different methods for solving this problem are discussed and a method of finding an analytic solution of the Abelian integral after a variable transformation is proposed. The accuracies of the various methods are compared by means of simulation calculations.     

大口径射电望远镜电磁兼容评估技术研究

电磁兼容性是设备或系统的重要性能指标, 也是保障系统的工作效能和提高系统可靠性的重要因素. 大口径射电望远镜运行阶段, 台址周围无线电业务及内部潜在的电磁干扰会降低观测系统灵敏度、影响天文观测的质量. 本论文针对拟建的新疆110 m全向可动射电望远镜(Qi Tai raido Telescope, QTT)开展了系统电磁兼容评估技术及控制方法研究, 具有重要的工程应用价值. 首先, 依据现有电波环境测量方法的不足, 深入分析了仪器设备的关键参数配置方法及测量时间计算方法, 采用Y因子法校准测量数据, 提出一种准实时电波环境测量方法. 面向高重复性宽带频谱, 分析了宽带频谱信号和噪声特征, 结合标准差理论, 提出一种基于邻值比较的信噪分离方法, 并采用邻值统计方法优化关键参数, 提高信噪分离精度. 针对QTT台址, 开发了自动化电波环境监测系统, 该系统6 GHz以下频段系统增益大于40 dB, 系统噪声系数小于2 dB, 测量不确定度小于1.49 dB, 具有极高的系统灵敏度和测量精度; 分析了频谱监测数据流, 设计了基于HDF5 (Hierarchical Data Format version 5)的数据存储格式, 开发了自动化电波环境测量和监控软件及数据处理软件. 依据QTT台址长期监测数据, 评估分析了台址电磁环境、主要干扰源特征及其影响. 其次, 提出大口径射电望远镜馈源口面干扰电平限值量化方法, 建立了基于台址地形的电波传播模型, 分析了现有电波传播模型的优缺点及适应性, 结合QTT台址实际地形及地质特征, 采用Longley-Rice和Two-Ray电波传播模型, 预测分析了QTT台址潜在干扰区域电磁干扰达到射电望远镜的电波路径衰减, 结合大口径射电望远镜天线增益量化方法, 提出设备所在位置干扰电平限值量化方法, 运用该方法对QTT台址潜在干扰区域的干扰电平限值进行量化. 依据设备所在位置干扰电平限值, 调研分析了国内外军用、民用电磁兼容测量标准, 结合电磁干扰对射电天文观测的影响, 提出一种大口径射电望远镜电磁兼容控制方法, 解决了现有电波暗室测量系统无法直接测量评估电子设备电磁兼容的问题, 该电磁兼容控制方法计划应用于QTT建设及运行阶段, 确保系统拥有良好的电磁兼容性. 最后, 依据QTT台址潜在干扰区域干扰电平限值, 结合典型电子设备电磁辐射频谱, 分析了QTT电磁兼容设计需求, 提出电磁兼容设计初步方案. 另外, 针对台址建筑设施内的中低电磁辐射干扰源, 提出一种低成本建筑屏蔽方法, 应用于QTT台址现有建筑.     

On the reduction of occultation light curves

In the first two sections of this paper, the two basic methods of reducing occultation light curves—curve fitting and inversion—are reviewed and compared. It is shown that the curve fitting methods have severe problems of nonuniqueness. In addition, in the case of occultation curves dominated by spikes, it is not clear that such solutions are meaningful. The inversion method does not suffer from these drawbacks. Methods of deriving temperature profiles from refractivity profiles are dealt with in the third section. It is shown that, although the temperature profiles are sensitive to small errors in the refractivity profile, accurate temperatures can be obtained, particularly at the deeper levels of the atmosphere. The final section contains a brief discussion of the ambiguities that arise when the occultation curve straddles the turbopause.     

Inversions of satellite to satellite electron content: Simulation studies with NeUoG-plas

The signals of Global Navigation Satellites have found a large number of uses in atmospheric and ionospheric research. Reception of the signals from a satellite in a Low Earth Orbit (LEO) leads regularly to occultation of the signals by the surface of the Earth. Before an occultation the signals traverse the ionosphere with rays with decreasing height of their perigees. Satellite electron content observed prior to ‘setting’ occultations or after `rising' occultations can be used as input data for inversion. The inversion procedure gives horizontally averaged height profiles of electron density.Assessment studies are needed to find out under which conditions the profiles from inversions are representative for ‘true’ electron density profiles above the Earth occultation point.A great number of such studies have been carried out using the ionosphere/plasmasphere model NeUOG-plas for forward and backward modelling. Different transmitter-receiver scenarios have been investigated.We describe the assessment procedure and report on results showing the most interesting cases and statistics.     

Analysis of stellar occultation data: Effects of photon noise and initial conditions

A new inversion technique for obtaining temperature, pressure, and number density profiles of a planetary atmosphere from an occultation light curve is described. This technique employs an improved boundary condition to begin the numerical inversion and permits the computation of errors in the profiles caused by photon noise in the light curve. We present our assumptions about the atmosphere, optics, and noise and develop the equations for temperature, pressure, and number density and their associated errors. By inverting in equal increments of altitude, Δh, rather than in equal increments of time, Δt, the inversion need not be halted at the first negative point on the light curve as required by previous methods. The importance of the boundary condition is stressed, and a new initial condition is given. Numerical results are presented for the special case of inversion of a noisy isothermal light curve. From these results, simple relations are developed which can be used to predict the noise quality of an occultation. It is found that fractional errors in temperature profiles are comparable to those of pressure and number density profiles. An example of the inversion method is shown. Finally, we discuss the validity of our assumptions. In an appendix we demonstrate that minimum fractional errors in scale height determined from the inversion are comparable to those from an isothermal fit to a noisy isothermal light curve.     

Turbulence in planetary occultations: III. Effects on atmospheric profiles derived from intensity measurements

The intensities of radio and optical signals observed during spacecraft and stellar occultations by planets scintillate due to atmospheric turbulence. The combined effect of turbulent fluctuations in refractivity and the average atmospheric gradient are found to produce slightly smaller signal intensity scintillations than the homogeneous case when there is no gradient, in contrast to a prediction that the scintillations would be markedly increased. Profiles of atmospheric temperature and pressure derived from intensity measurements are found to have much larger errors due to turbulence than do the corresponding profiles derived from radio Doppler frequency measurements. However, such errors are still small in the limit of weak scattering, which is assumed here. Radio and optical occultation experiments tend to be complementary since the generally shorter distances involved in the former mean that the radio experiments can probe relatively deeply into the atmosphere, while the optical experiments are limited to tenuous atmospheric regions. Because the radio experiments generally have a much greater dynamic measurement range, they are more likely to encounter conditions where strong scattering occurs than will the optical occultation experiments, provided the rms turbulent refractivity increases with depth approximately as the refractivity of the quiescent atmosphere.     

GPS无线电掩星数据处理系统

GPS无线电掩星技术能提供高精度、高分辨率、全球覆盖的地球电离层和中性层大气剖面。它具有全天候、低费用、无系统长期漂移等优点。从1995年4月至1997年3月,首次GPS／MET试验的成功显示了GPS掩星技术对监测全球大气能够发挥重要的作用,从而成为当前空间探测技术的研究热点之一。该文主要叙述了无线电掩星技术的数据处理系统的有关情况,并介绍了美国UCAR的掩星数据处理系统CDAAC的概况,可作为我国开展无线电掩星计划有关工作的借鉴。     

GPS掩星振幅反演中的天顶补偿项

推导了GPS无线电掩星振幅观测反演地球大气技术中Abel积分变换中的天顶补偿项 ,定性地说明了天顶补偿项对计算过程和反演剖面的影响。     

Ionospheric Ground-based Measurement Networks

Critical to the ionospheric monitoring and modelling processes whether they are related to the theoretical investigations, the space weather prediction services or the planning and operation of radio systems is a modern network of ground-based synoptic instruments making regular measurements. A short review of the principal types of ionospheric ground-based measurement networks currently available in Europe is followed by brief survey of the particular problems associated with a timely and user-friendly fashion of data delivery for assimilation purposes.     

Argon in the Martian atmosphere: Do the results of Mars 6 agree with the optical and radio occultation measurements?

Mars 6 discovered an inert gas (probably argon) in the Martian atmosphere. An analysis is carried out for the available spectroscopic observations, radio occultation results, and other data with the aim of determining the maximum argon content with which they are consistent. Possible seasonal variations of pressure are taken into account. It is shown that 25–30% of argon is consistent with all these data. The presence of argon slightly increases the mean pressure estimates; 25% argon increases pressures derived from radio occultation data by 0.5 mbar.     

GPS/LEO掩星观测的变分同化技术

在简单介绍GPS/LEO掩星探测大气的发展历史和科学意义之后,详细阐述了反演的基本原理;分析了标准反演中存在的问题,并说明一维变分同化(1DVAR)在反演方法中的重要性;给出了一维变分同化中价值函数的求解,以及各种同化因子;简单介绍了对当前气象学中普遍使用的四维变分同化(4DVAR);重点讨论了各种同化方法,以及使用各种同化因子的优缺点。最后,通过CHAMP卫星的观测实例分析,验证了GPS数据在数值天气预报(NWP)中的作用,以及相对于标准反演法一维变分对气象要素的改进。     

Turbulence in planetary occultations: II. Effects on atmospheric profiles derived from Doppler measurements

Turbulence in planetary atmospheres leads to both fluctuating and systematic errors in atmospheric profiles derived from Doppler measurements during radio occultation. If the upper atmospheres of Venus and Jupiter are about as turbulent as the earth's troposphere, we deduce rms fractional errors in temperature and pressure of less than ～ 10^?2 for the Mariner 10 and Pioneer 10/11 occultation experiments. Fractional systematic errors are typically of the order of 10^?6. These estimates depende rather weakly on quantities characterizing the atmosphere and the occultation, and it is conjectured that turbulence-induced errors in atmospheric profiles derived from Doppler measurements are always very small in the weak scattering limit     

Thermal structure of the atmosphere of Venus from Pioneer Venus radio occultations

Eighty-seven measurements of the thermal structure in the atmosphere of Venus between the altitudes of about 40 and 85 km were derived from Pioneer Venus Orbiter radio occultation data taken during four occultation seasons from December 1978 to October 1981. These measurements cover latitudes from ?68 to 88° and solar zenith angles of 8 to 166°. The results indicate that the characteristics of the thermal structure in both the troposphere and stratosphere regions are dependent predominantly on the latitude and only weakly on solar illumination conditions. In particular, the circumpolar collar cloud region in the northern hemisphere (latitude 55 to 77°) displays the most dramatic changes in structure, including the appearance of a large inversion, having an average magnitude of about 18°K and a maximum of about 33°K. Also in this region, the tropopause altitude rises by about 4.8 km above its value at low latitudes, the tropopause temperature drops by about 60°K, and the pressure at the tropopause decreases by an average of about 240 mbar. These changes in the collar region are correlated with observations of increased turbulence and greater amplitude of thermal waves in the region, which is located where the persistent circulation pattern in the Venus atmosphere changes from zonally symmetric retrograde rotation to a hemispherical circumpolar vortex. It was shown that the large zonal winds associated with this circulation pattern are not likely to produce distortions in the atmosphere of a magnitude that could lead to temperature errors of the order of the mesosphere inversions observed in the collar region, but under certain circumstances zonal wind distortion could cause errors of 3–4°K.     

上海天文台CHAMP掩星资料处理结果的统计分析

随着GPS全球定位系统的发展,已经有可能使用GPS掩星技术反演精确的大气温度,压强和湿度剖面．在概要地描述上海天文台(SHAO)开发的GPS掩星数据资料反演地球大气流程模块以后,分析了2002年8月1日至2002年8月17日期间2700多次CHAMP掩星数据资料的结果,并且与欧洲中尺度天气预报分析(ECMWF)资料进行比较和统计分析,讨论了CHAMP掩星数据的质量,并分析了CHAMP掩星数据的观测误差和ECMWF的分析模型误差．提出了一种优化的统计分析方法,它能更客观地反映GPS掩星技术的外部符合．统计分析结果表明GPS掩星数据将有可能成为数值天气预报和长期监测地球气候的非常有价值的数据资料．