IERS1996规范在参考系方面的改进

对ＩＥＲＳ１９９６规范中有关参考系方面作了简单而系统的介绍，重点叙述了与ＩＥＲＳ１９９２标准相比ＩＥＲＳ１９９６规范在参考系方面的主要改进：天球参考架中的基本源从５７颗增加到２３６颗；动力学参考架采用ＪＰＬ ＤＥ４０３／ＬＥ４０３历表代替ＤＥ２００／ＬＥ２００历表；采用ＮＵＶＥＬ ＮＮＲ－１Ａ板块运动模型代替ＮＵＶＥＬ ＮＮＲ－１模型；变更了９个基本常数值；给出了天极坐标的观测和理论间差的经验模型     

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。     

进行被掩源初选的一种有效方法及其应用

以JPL提供的太阳系天体数值历表DE200/LE200和美国海军天文台提供的岁差、章动改正程序为出发点,本文介绍为计算大行星、月球掩源(恒星或射电源)而进行的被掩源初选的一种有效方法。用该法本文给出了1995-2050年各大行星及月球可能掩的FK5星。     

掩星情报站

三月份仅有两次适合观测的月掩星现象,但这两颗被掩星均亮于四等,分别是月掩2．9等的天蝎σ星和月掩3．5等的双子δ星,这两次掩星现象分别发生在下弦和上弦前后,所以极其适合观测。小行星掩星适合观测的有八次,掩食时间最长的一次长达15秒。其中被掩星亮于6等的就有三次,为我们观测小行星掩星提供了难得的观测练习机会。     

CHAMP观测资料的振幅反演初步结果

在几何光学和薄相位屏的假设下，给出GPS／LEO无线电掩星反演地球大气技术中振幅反演的计算方法．从产生接收信号振幅变化的机制出发，提出产生信号振幅变化的几何衰减和物理衰减两个不同的源．利用CHAMP卫星观测资料的个例，用观测信噪比序列进行了大气弯曲角、大气折射率、压力、温度剖面的反演；并与相位反演结果进行比较和讨论．     

上海天文台GPS掩星技术研究现状

简要地介绍了GPS／LEO卫星无线电掩星技术的基本原理、意义以及国际上的最近动态，列举了几个主要的LEO卫星计划；重点分析了GPS掩星探测地球大气技术中需要注意的若干问题，并推出可能采取的改进方案和相应的技术路线。上海天台在GPS／LEO空基气象学中已经成功地提出：通过通约LEO／GPS轨道方法，实现掩星地面观测点控制的新思路。作为一个应用实例，针对东中国海洋上空大气状况对中国东部经济发达的地区特别是上海地区的天气有着重要影响的背景，提出利用通约LEO／GPS卫星可对该地区大气实施监控。为了充分利用GPS／LEO掩星观测资源，上海天台开展了振幅反演的计算方法研究；提出了在振幅反演中存在着无线电信号几何衰减和物理衰减两种不同的衰减机制，并考虑了它们的数学模型以及对反演结果的影响。简要地叙述了掩星质量因子的定义和计算方法。作为国内天地球动力学研究中心，上海天台将与国内外一些合作单位考虑建立GPS／LEO掩星技术观测处理的软件系统。作为空基GPS气象学的推广，还考虑进行山基和飞机载掩星观测实验，论证用这些方法监测局部大气剖面的可行性以及它们在国民经济和科学研究上的作用。     

掩星情报站

在七月份我国东北的北部地区观测完月掩昴星团的天象后,随后八月份在我国西北的新疆地区和部分青海、甘肃地区也可欣赏到这次月掩昴星团的天象,这次月掩昴星团时月相正好在下弦月附近,非常适合观测,尤其以新疆地区观测条件最佳。而本月有多达十次的小行星掩星适合我国爱好者观测。这十次小行星掩星的被掩星亮度全部在7～9等之间,其中有九次发生在最后十天之内,对于我国爱好者来说是一次观测小行星掩星难得的良机!     

~(444)Gyptis小行星掩SAO138868A恒星的观测

依照David W.Dunham博士寄来的补充预报,我们于1982年6月24日对(444)Gyptis小行星掩星天象,进行了光电观测。被掩恒星是SAO138868A,目视星等为8~m.9,光谱型是K_2。所用仪器为本台的60cm反光望远镜,配有单通道光电光度计。光电倍增管为EMI9502B,用双笔自动记录仪可同时记录光信号和以秒记录的时标     

用α—修正平均平滑月掩星观测资料

因为短的积分时间（0^s.001）和月球附近很强的背景光这度，月掩星观测资料通常需要进行某种处理以除去噪声，而α-彩好的消除噪声能力，可以用来平滑月掩星观测资料，很少甚至完全不影响掩星的衍射图象。本文介绍了α-修正平均方法并指出对α=0，ι=9的α-修下平均是对月掩星观测资料的最佳滤波器。     

掩星驿站

金秋十月是个收获的季节,对掩星爱好者来说,同样是个观测掩星的良好时机,因为本月适合观测的掩星事件有十次之多,其中月掩星和小行星掩星事件分别各有五次,均适合爱好者进行观测。在五次月掩星事件中,有四次发生在凌晨,被掩星最亮的达2．3等。     

Forthcoming close angular approaches of planets to radio sources and possibilities to use them as GR tests

During close angular approaches of solar system planets to astrometric radio sources, the apparent positions of these sources shift due to relativistic effects and, thus, these events may be used for testing the theory of general relativity; this fact was successfully demonstrated in the experiments on the measurements of radio source position shifts during the approaches of Jupiter carried out in 1988 and 2002. An analysis, performed within the frames of the present work, showed that when a source is observed near a planet’s disk edge, i.e., practically in the case of occultation, the current experimental accuracy makes it possible to measure the relativistic effects for all planets. However, radio occultations are fairly rare events. At the same time, only Jupiter and Saturn provide noticeable relativistic effects approaching the radio sources at angular distances of about a few planet radii. Our analysis resulted in the creation of a catalog of forthcoming occultations and approaches of planets to astrometric radio sources for the time period of 2008–2050, which can be used for planning experiments on testing gravity theories and other purposes. For all events included in the catalog, the main relativistic effects are calculated both for ground-based and space (Earth-Moon) interferometer baselines.     

On the possibility of investigating the galactic center region with the integral observatory by the method of lunar and terrestrial occultations

The possibility of investigating the sky region near the Galactic center with instruments of the INTEGRAL orbital astrophysical gamma-ray observatory by the method of its occultation by the Earth and the Moon is considered. Existing engineering constraints on the observing conditions, such as the admissible orientation of the INTEGRAL satellite relative to the direction to the Sun and the performance of measurements only outside the Earth??s radiation belts, are taken into account. Long time intervals during which the lunar occultation center passes at angular distances of less than 2° from the Galactic center have been found. Such events occur under the adopted constraints two or three times per year without any correction of the INTEGRAL satellite orbit. The orbit can be corrected to reduce the angular distance between the Moon and the Galactic center in occultation events. The required velocity impulses do not exceed several meters per second. The possibility of the Galactic center being occulted by the Earth has been analyzed. In this case, to perform measurements, the admissible (in radiation exposure) height of the working segment of the orbit should be reduced to 25 000 km, which can be problematic. At the same time, part of the Galaxy??s equatorial region is shadowed by the Earth for a time long enough to carry out the corresponding experiments.     

Optimization of lunar occultations for determining the positions of point-like X-ray sources

In view of the scheduled satellite mission EXOSAT (European X-Ray Observatory Satellite) of ESA (European Space Agency) the lunar occultation technique to determine the position of point-like X-ray sources is investigated. An error analysis for the source coordinates resulting from this technique is presented and an occultation strategy is proposed to achieve optimum lunar occultations. The analysis takes into account the errors of the space coordinates of the satellite and the Moon, the unevenness of the lunar surface, the intensities of source and background, the apparent angular velocity of the Moon as seen from the satellite, the finite sizes of the preoccultation position error boxes of the X-ray sources and the inaccuracies in the satellite orbit correction manoeuvres necessary to achieve the occultations.     

The radio occultation method for the study of planetary atmospheres

Planets, the Moon and the Sun have a number of ‘atmospheres’ which may be measured by the radio occultation method, using radio links to spacecraft which are being occulted by the body as seen from Earth. Molecular atmospheres, ionospheres, magnetospheres, particulate atmospheres and several general-relativistic atmospheres can all affect radio signal charecteristics. Measured Doppler frequencies, signal amplitudes and wave polarizations contain information on these atmospheres. From such occultation measurements it is possible, for example, to derive profiles which are related to the changes with height of temperature, pressure, density, free electron concentration and the fractional volume occupied by particulate matter. Important clues for identifying molecular or particulate constituents can also be obtained. Even though one type of measurement may be sensitive to several different atmospheric characteristics, these characteristics can often be separated due to differences in their dependence on radio wavelength or on height above the surface.     

Future possibilities for lunar occultation studies of faint X-ray sources

Lunar occultation can be considered of interest for future missions dedicated to X-ray astronomy only if instruments with a large enough collecting area are used. In this case, observations of the numerous, faint X-ray sources occulted by the moon during a typical satellite lifetime of several years, can, in principle, add good source positioning as a free bonus to a high-throughput, medium-imaging mission. In the practical case of the EPIC instrument aboard the XMM ESA satellite, a simple calculation shows the potential usefulness of this technique.     

Photometry of occultation candidate stars: II. Uranus 1985–1990 and Saturn 1986–1991

V and I magnitudes are presented for 20 stars to be occulted by the Uranian rings during the period 1986–1990 and for 15 stars to be occulted by Saturn's rings during 1986–1991. K magnitudes for seven Uranus stars are presented. The occultation stars are up to one magnitude brighter in the K bandpass than one would predict on the basis of V-I colors alone. This effect can be explained by typical amounts of interstellar reddening.     

The improvement of the lunar ephemeris

At present the fundamental lunar ephemeris is based on Brown's theory of the motion of the Moon with improvements based on the bypassing of Brown's Tables, the removal of the great empirical term, the substitution of the relevant constants of the IAU system of astronomical constants and the retransformation of Brown's series in rectangular coordinates to spherical coordinates. Even so this ephemeris does not represent adequately the recent range and range-rate radio observations, and it will be inadequate for use in the analysis of laser observations of corner reflectors on the Moon. Numerical integrations for these purposes have already been made at the Jet Propulsion Laboratory, but improved theoretical developments are also required; new solutions of the main problem are in hand elsewhere. Work at H.M. Nautical Almanac Office is aimed at obtaining improved values of the constants of the lunar orbit by a rediscussion of occultation observations made since 1943 and at the redevelopment of the series for the planetary perturbations using more precise theories of the motion of the Sun and planets. The techniques and preliminary results of exploratory numerical integrations were briefly described.Presented at the Conference on Celestial Mechanics, Oberwolfach, Germany, 17–23 August, 1969.     

The 7 and 25 June 1985 Neptune occultations: Constraints on the putative Neptune “arc”

Stellar occultations by Neptune on 7 and 25 June 1985 were observed in the K band from Sutherland (SAAO) to search for conforming evidence of the ring-like “arc” reported by Hubbard et al. (W.B. Hubbard, A. Brahic, B. Sicardy, L.-R. Elicer, F. Roques, and F. Vilas (1986). Nature 319, 636–640). A binary star was occulted on 7 June 1985, and since both components were occulted by the planet, their relative positions could be precisely determined. A single sharp dip, of high signal-to-noise ratio, was observed in the post-emersion occultation trace. If this feature were caused by material near Neptune, its corresponding projected equatorial plane radius is either 62,600 ± 160 km or 63,760 ± 120 km, depending on which of the binary star pair was occulted. The equatorial radius, width, and optical depth of the 7 June feature are similar to those described by Hubbard et al. The absence of a corresponding post-emersion dip due to the occultation of the companion star suggests that the ring-like material is discontinuous over a scale of several thousand kilometers in ring circumference. No ring-like features were observed during pre-immersion. The 25 June 1985 occultation was also successfully observed, including atmospheric occultation profiles for both immersion and emersion. No evidence for ring-like material was found in the region probed by this occultation during post-emersion, which included the entire range of equatorial radii over which “arc” events have been previously reported.     

Developing an Optical Lunar Occultation Measurement Reduction System for Observations at Kaau Observatory

We present here a detailed explanation of the reduction method that we use to determine the angular diameters of the stars occulted by the dark limb of the moon. This is a main part of the lunar occultation observation program running at King Abdul Aziz University observatory since late 1993. The process is based on the least square model fitting method of analyzing occultation data, first introduced by Nather et al. (Astron. J. 75:963, 1970).     

A high speed near infrared photometer for lunar occultation studies

A recently developed near infrared high speed photometer intended for lunar oc-cultation studies is described. The primary scientific objective is to reach milli arc second levels of angular resolution so that circumstellar structure of the occulted sources can be resolved. Near infrared sky brightness close to the lunar limb is also studied. Angular diameter derived from the observed occultation of IRC +20169 is presented and system performance discussed.