Evaluation of the Precision of Three Tropospheric Delay Correction Models

Using the annual GPS observational tropospheric zenith delay data and meteorological data obtained at 36 globally distributed IGS stations in 2003, we have estimated the accuracy and range of application of two often-used models of tropospheric delay correction (the Hopfield and Saastamoinen models), and of a more recent model (the EGNOS model). We point out some defects in the Hopfield model and show that the EGNOS model can be used as the correction model of the tropospheric zenith delay for the GNSS real-time positioning and navigation.     

Improvement on Atmospheric Hydrostatic Delay Correction at Low Elevation Angles

Observation made at low evelation angles is the trend of development of GPS (Global Positioning System) meteorology, wherein the development of highly accurate atmospheric hydrostatic delay corrections at low elevating angles is the main key technique. The comparison among three methods for calculating the atmospheric hydrostatic delay correction of the radio waves from space to the ground-based receiver is made: (1) the atmospheric hydrostatic delay obtained from the path integration of the sounding balloon data under the assumption of atmospheric spherical symmetry, (2) the atmospheric hydrostatic delay acquired from the reanalyzed data of the NCEP (National Centers for Environmental Prediction) under the assumption of atmospheric spherical symmetry and (3) the atmospheric hydrostatic delay got from Niell's atmospheric hydrostatic mapping function. The results of the comparison of them with the atmospheric hydrostatic mapping function obtained from the calculation carried out by taking advantage of the data acquired at 89 sounding balloon stations in China in 2001 show that the accuracy of the method of the path integration of the reanalyzed data of NCEP at low elevating angles (bellow 5°) is about 5 times better than that of the Niell mapping function model.     

提高中性大气折射延迟改正精度的可能途径

针对空间大地测量技术对中性大气折射延迟改正精度的要求,阐述了折射延迟改正值应随测站和随方位而异的必要性．指出,在尚不能直接测定天文大气折射值的情况下,现有的各种改正模型对大气分布模型的依赖性,不能达到预期的精度和降低观测的截止角．根据云南天文台低纬子午环的特殊结构,和测定大气折射的实践,提出了提高折射延迟改正精度的新方法,即：利用各观测站不同方位从天顶附近直到低地平高度角的天文大气折射实测数据,求解得到折射率差和映射函数的参数,从而建立随测站和随方位而异的大气折射延迟改正模型．这一新方法的实施,将能在不需采用大气分布模型的情况下,把天顶延迟的改正精度提高到1 mm以内,低地平高度角的折射延迟改正精度提高到厘米级,并且把截止高度角压缩到5°以内．     

The Method of Differential Measurement of Astronomical Refraction and Results of Trial Observations

Because of the influence of atmospheric refraction the astronomical observations of the objects with the angles of elevation below 15° are generally avoided, but for the sake of the complete theoretical research the atmospheric refraction under the condition of lower angles of elevation is still worthy to be analyzed and explored. Especially for some engineering applications the objects with low angles of elevation must be observed sometimes. A new idea for determining atmospheric refraction by utilizing the differential method is proposed. A series of observations of the starry sky at different heights are carried out and by starting from the zenith with a telescope with larger field of view, the derivatives of various orders of atmospheric refraction function at different zenith distances are calculated and finally the actually observed values of atmospheric refraction can be found via numerical integration. The method does not depend upon the strict local parameters and complex precise observational instrumentation, and the observational principle is relatively simple. By the end of 2007 a simply constructed telescope with a larger field of view at Xinglong Observing Station was employed to carry out trial observations. The values of atmospheric refraction at the true zenith distances of 44.8° to 87.5° were obtained from the practical observations based on the differential method, and the feasibility of the method of differential measurement of atmospheric refraction was preliminarily justified. Being limited by the observational conditions, the accuracy of the observed result was limited, the maximal accidental error was about 6” and there existed certain systematic errors. The value of the difference between the result obtained at the zenith distance of 84° and that given in the Pulkovo atmospheric refraction table was about 15”. How to eliminate the cumulative error introduced due to the integration model error is the key problem which needs to be solved in future.     

A Possible Means of Improving the Accuracy of Refraction Delay Correction of Neutral Atmosphere

The space geodetic technology requires an accurate model of correction of refraction delay by the neutral atmosphere that varies from one observing station to another, and from one azimuth to the next. It is pointed out that under the present condition the astronomical refraction can not yet be directly determined, any correction model because of its high dependence on the assumed atmospheric distribution, is incapable of achieving the required accuracy or of improving the cut-off altitude. In this paper, based on the special properties of the lower latitude meridian circle at Yunnan Observatory and our experience of determining atmospheric refraction therewith, a new method is proposed for improving the accuracy of refraction delay correction. Namely, the measured data of astronomical refraction of an observing station from near zenith to low altitudes in different azimuths are used to evaluate the refractivities and the parameters of the mapping functions, thereby establishing a model of atmospheric refraction delay correction that varies with the observing station and the azimuth. Since it is unnecessary for the new method to adopt any atmospheric distribution model, application of this new method will improve correction accuracy of refraction delay to better than 1mm at zenith and to centimeters at low altitudes, and improve the cut-off altitude to below 5 degrees.     

海潮对天顶延迟测定的影响

GPS气象学是GPS应用的一个新领域,天顶延迟的精确测定是其中的一个关键.由于GPS测站的垂直位移与天顶延迟存在很强的相关性,所以海潮对精密测定天顶延迟的影响不可忽略.本文对中国地壳运动观测网络2001年第010天到040天的GPS数据进行了处理分析,结果显示这种影响大约在几毫米.以位于近海的上海测站和位于内陆的武汉测站为例,最大影响可达±7 mm和±1 mm.     

Image Field Deformation of LAMOST due to Differential Atmospheric Refraction

A vectorial expression of the image field deformation of LAMOST due to the differential atmospheric refraction is presented. The calculated results are compared with those from previous analyses based on the traditional spherical trigonometric formulas. It is demonstrated that different tangential displacements of star images during the observation tracking given by various authors are simply due to different reference points adopted. It is pointed out that the observational celestial pole is the center of the apparent diurnal motion, that, by referring to the observational celestial pole, the effect of the differential refraction on the image field of LAMOST during the 1.5-hour tracking period is approximately equivalent to a constant rotation of -13.65\" for all declination belts. It is therefore unnecessary to design a particular tracking velocity for each observation, and this will be obviously advantageous to the observation implementation. If the maximum tracking error of the fibers is 0.2\", then the fibers are required to be able to re-position during observational tracking for sky regions south of declination 20°and north of declination 60°.     

电离层延迟改正模型综述

电离层延迟改正模型通常可以分为广播星历用的预报模型、广域差分用的实时模型、后处理模型3类,不同应用要求需要选择不同的模型。主要比较分析了几种常用的电离层延迟改正模型: 用于广域差分中生成格网模型的三角级数模型、多项式模型、低阶球谐函数模型等都可以获得很好的改正效果,且这3种模型基本上是等价的;电离层延迟谐函数展开模型可以用来分析电离层长时间系列的变化特征;国际电离层参考模型IRI的改正精度一般可以达到60％的效果;而GPS 星历采用的Klobuchar模型的参数设置存在一些不足,对此提出了一些改进措施。     

Propagation of Electromagnetic Waves in Pulsar Magnetospheres

We have developed a numerical code for the propagation of different electromagnetic modes in a pulsar magnetosphere filled by a relativistic, streaming electron-positron plasma in a strong, curved magnetic field. We determine the trajectories, limiting polarization and damping of the waves leaving the magnetosphere. This revised version was published online in July 2006 with corrections to the Cover Date.     

An Approach of Tropospheric Correction for VLBI Phase-Referencing using GPS Data

The dominant source of error in VLBI phase-referencing is the troposphere at observing frequencies above 5 GHz. We compare the tropospheric zenith delays derived from VLBI and GPS data at VLBA stations collocated with GPS antennas. The systematic biases and standard deviations both are at the level of sub-centimeter. Based on this agreement, we suggest a new method of tropospheric correction in phase-referencing using combined VLBI and GPS data.     

An Approach of Tropospheric Correction for VLBI Phase-Referencing using GPS Data

The dominant source of error in VLBI phase-referencing is the troposphere at observing frequencies above 5 GHz. We compare the tropospheric zenith delays derived from VLBI and GPS data at VLBA stations collocated with GPS antennas. The systematic biases and standard deviations both are at the level of sub-centimeter. Based on this agreement, we suggest a new method of tropospheric correction in phase-referencing using combined VLBI and GPS data.     

测定天文大气折射和建立折射延迟实测模型的必要性

根据经典天体定位测量受天文大气折射的影响和现代空间大地测量对中性大气折射延迟改正的要求,分析了这些修正没有达到预期精度要求的原因:主要在于不能直接测定天文大气折射;文章针对这些影响量对大气分布模型的依赖性,改正值应随着不同的观测站和不同方位而异的要求,提出了提高这两种改正精度的有效途径:在各观测站不同方位的各天顶距,测定天文大气折射值,分别建立不同方位的大气折射实测模型,并利用实测数据,求解出折射率差和映射函数的参数,建立和采用随着观测站、随着方位而异的折射延迟改正模型。这一新方法的实施,将能在避免采用大气分布模型的情况下,把较低高度角的折射延迟改正精度从现在的米级提高到厘米级,并且把截止高度角压缩到5°以内。文章还论述了在各观测站多方向测定天文大气折射值的可能性。     

由束流不稳定性直接放大电磁波的弱磁化修正

本文在文［３］的基础上，考虑背景电磁波色散方程的弱磁化修正，在１＜ωｐｅ／Ωｅ＜１０的范围内（主委适用于太阳高日冕、行星际空间及星系射电喷流等），计算了由束流不稳定性直接放大正常模和反常模的增长率。增长率随着频率、ωｐｅ／Ωｅ、电磁波传播角以及非热电子入射角等参数的变化规律，和忽略磁场时的结果与文［３］大体相同，但是反常模的增长，则随着ωｐｅ／Ωｅ的增大而受到明显的抑制，因而在本文参数范围内，束流不稳定性所放大的电磁波以正常模为主，可以解释太阳ＩＩＩ型爆发和星系射电喷流的偏振性质。     

Determination of proper motions for AC stars: First results

Proper motions of the stars of the Astrographic Catalogue are being derived, using the Hubble Space Telecope Guide Star Catalogue as second epoch. Results on the San Fernando and Cordoba AC zones are presented. Identification with GSC stars (i.e. determination of proper motion) was successful for 97 percent of all AC stars. Comparison of the proper motions thus derived with those of Preliminary PPM South shows that the accuracy is about 0.8 to 0.9 arcsec per century. Thus we can derive proper motions for about 4 million stars, with an accuracy higher than that of the SAO Catalogue.     

Contribution of Vacuum Field to Angular Deviation of Light Path and Radar Echo Delay

The discovery of ‘twin quasistellar objects’ arose interests among astronomers and astrophysicists to study gravitational lensing problems. The deviation of light from its straight line path is caused by two sources according to the general theory of relativity: (i) the presence of massive objects, i.e. the presence of gravitational field and (ii) the presence of a ‘vacuum field’ which arises because there is a non-zero cosmological vacuum energy. Recently, the research on the relationship between cosmological constant and gravitational lensing process is rather active (see reference [1, 2, 3]. According to the Kottler space time metric, we have deduced an explicit representation of the angular deviation of light path. The deviation term is found to be simply , where M is the mass of the ‘astronomical lens’, r_min is the distance between the point of nearest approach and the centre of M, other symbols have their usual meaning. The presence of this term may be meaningful to the study of cosmological constant using the concept of gravitational lensing; however more sophisticated analysis awaits. Consider a signal radar to be sent from one planet to another. We have found that the radar echo delay contributed by the existence of the cosmological constant Λ is expressible as This revised version was published online in July 2006 with corrections to the Cover Date.     

The problem of the sighting line stabilization for axial meridian circles

The paper deals with the method based on treating the axial meridian circle and a long-focus light mark in the prime vertical as a single virtual instrument that has properties of meridian circle. A horizontal axis, a sighting line, and a micrometer cross-wire of the instrument are created by optical means. The key function here is performed by the light mark that ensures a reference direction used for formation of instrumental principal planes and directions. Stability of instrumental azimuth, inclination, and collimation is expected to be about ±0.″01. Besides, an accuracy of observations is not affected by any temperature and weight deformations of telescope mechanics. Owing to the last circumstance, the design is cheap and the telescope aperture may be increased significantly, to about 1m. An estimate of accuracy of observations depends on the adopted model of image motion, and for sites with excellent seeing and bright stars is ±0.″03 to ±0.″05 (rms).     

研究大气反常折射的迫切性

就地球大气对天体测量观测的影响作了分类，介绍了对各类影响的传统处理方法，也介绍了低纬子午环的分别对待方法，特别强调了短周期的和不规则的反常折射影响对提高观测精度的限制，并提出了测定这些影响的可能途径。     

测定天文大气折射和建立电磁波折射延迟实测模型(Ⅵ)-瞬时大气折射值和折射率差的取得

根据测定天文大气折射的原理,叙述了利用相应的观测值获得瞬时大气折射测定值和建立大气折射实测模型的途径,并从各种测定值与最后结果之间的关系,指出了这里对数据处理的要求;文章介绍了对测定值进行波长改正和建立折射延迟实测模型的处理方法,分析了改正模型对天文大气折射测定值的分布要求,给出了观测数据随天顶距的增大而加密的分布模型。     

时间序列数字滤波后自由度的确定--应用于日长变化与南方涛动指数的相关分析

原始观测时间序列进行数字滤波后，其自由度将大大降低，对各种数字滤波器都可以通过Monte Carlo方法模拟获得对应置信水平的相关系数临界值，从而得到时间序列滤波后的自由度，如果滤波后的频段宽度与Nyquist频率之比为Z，那么对于理想的单边和双边滤波器，时间序列滤波后的自由度为原始值的Z／2和Z倍，非理想滤波器一般不符合此规律。