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.     

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°.     

天测与测地VLBI资料处理中的剩余钟行为建模

利用实测资料和模拟数据，分析了在天测与测地VLBI数据处理软件CALC／SOLVE中，剩余钟行为连续分段线性拟合的局限性和周期函数拟合的可行性．对CALC／SOLVE系统增配了周期函数拟合模块，并选取了1990年至2001年的1567次天测与测地VLBI实验，进行了剩余钟行为两种拟合模型情况下的实测资料综合解算．结果表明，在获得同等水平的时延拟合残差加权均方根(wrms)的情况下，对剩余钟行为采用周期函数拟合，使得解算参数的个数相比于连续分段线性拟合有显著减少，提高了解算的自由度．虽然在两种模型情况下的台站和射电源坐标以及地球定向参数等的解算精度无显著差异，但周期函数拟合存在可以识别的优势，85％以上的台站坐标和74％以上的源坐标以及51％以上的EOP精度均有所提高，均值分别为0．02mm、0．3μas和1．5μas．     

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).     

New consideration of atmospheric refraction in laser ranging data

In this paper we reconsider the formulae of tropospheric refraction correction for the Satellite Laser Range technique. From the expansion of the complementary error function, a new continued fraction form of the mapping function at optical frequencies is derived. The correction terms related to the operation frequency of the laser beam are considered in both the zenith delay and the mapping function. The correction for low-elevation satellites is briefly reviewed. The theoretical accuracy of the new mapping function has been analysed via the ray tracing integrals under the standard atmospheric profile. With respect to the radiosonde data, the deviations of the new mapping function are investigated in an elevation range down to near 1°, which is comparable with the results of the Marini–Murray formulae .     

基于Lomb算法的非等间距VLBI观测资料频谱分析研究

研究非等间距采样频谱分析中所发生的虚假信号问题．利用Lomb非等间距频谱分析方法对准多周期采样的模拟资料和VLBI实测资料进行频谱分析,得出了准多周期采样会产生虚假谱峰的结论．通过对频谱特征的分析,给出了虚假谱峰的位置分布的经验公式并从统计学角度证明了该公式．利用该公式可以确认准多周期采样频谱分析中的虚假谱峰位置,同时对公式的具体应用进行了讨论．其结论是基于VLBI的观测资料采样特征得到的,但同样适用于一般的准多周期采样资料的频谱分析．     

中性大气掩星标准反演技术

结合JPL和GFZ的中性大气标准反演算法，发展了上海天文台中性大气掩星标准反演算法流程．对CHAMP观测资料进行中性大气反演，并将反演结果与ECMWF(the European Center for Medium Range Weather Forecasting)的预报大气数据进行比较．作为讨论，指出将来的发展方向．     

Potential of the VLTI for linking stellar frames to ICRF

The question of positioning the optical counterparts of the ICRF quasars is outlined in the perspective of future space astrometry missions, which ultimately will bring a new realization of the ICRS in the optical range. Ground-based interferometry with a dual-field observing mode (PRIMA/VLTI),together with the missions DIVA and FAME, will have a key role in building an extragalactic reference frame in the optical/near-IR range with about the same accuracy as that of the present (VLBI) primary frame. This revised version was published online in July 2006 with corrections to the Cover Date.     

Photometric detection of high proper motions in dense stellar fields using difference image analysis

The difference image analysis (DIA) of the images obtained by the Optical Gravitational Lensing Experiment (OGLE-II) revealed a peculiar artefact in the sample of stars proposed as variable by Woźniak in one of the Galactic bulge fields: the occurrence of pairs of candidate variables showing anti-correlated light curves monotonic over a period of 3 yr. This effect can be understood, quantified and related to the stellar proper motions. DIA photometry supplemented with a simple model offers an effective and easy way to detect high proper motion stars in very dense stellar fields, where conventional astrometric searches are extremely inefficient.     

Study of a Bistatic Radar System Using VLBI Technologies for Detecting Space Debris and the Experimental Verification of its Validity

Space debris are increasing around the Earth. The observation of space debris is a key issue for the investigation and monitoring of space environment. But the observation opportunities and the detection ability are limited in existing monostatic radar systems. This paper proposes a bistatic radar which is composed of a transmitting station and a receiving-only station. A carrier wave modulated by PN-PSK signals is used in combination with a VLBI (Very Long Baseline Interferometry) recorder for range measurement between space debris and stations. The receiving radio wave is processed on the basis of VLBI techniques. Accordingly, the system is shown to have significant advantages over a monostatic radar. We actually formed a bistatic radar system, and observed a satellite in order to experimentally verify the validity. The configuration of the system, data analysis and the experimental results are described.     

太阳选址全国日照条件分析

利用国家气象信息中心气象资料室整编的中国756个基本、基准气象站在1971～2000年间的日照时数和日照百分率年值、月值数据集,研究了全国范围的日照条件,结合1951～2008年间我国云量、水汽要素资料,分析了影响日照的因素.研究结果表明,我国日照时数和日照百分率的分布形势一致,东南少而西北多,从东南向西北递增.在我国东部地区日照时数由南向北逐步递增,西部地区日照条件整体好于东部,符合常规.藏西南狮泉河一带日照条件最优;40°N纬度带附近和藏南地区次之,江南华南、四川盆地以及云贵高原东部地区的日照条件较差;其余地区日照条件居中.日照时数受云量、水汽要素的影响,呈负相关关系.     

利用地基GPS技术反演武汉地区大气可降水分

利用武汉地区的探空资料和GPS实测数据，对对流层干分量延迟、对流层加权平均温度进行了检验分析．结果表明，对于武汉地区而言，常用的大气干分量延迟模型(SAAS Hopfield and Black)存在着1-2cm的系统误差，这在利用GPS资料估算大气可降水分(PWV)时会引入2-3mm的误差；对流层加权平均温度与常用的Bevis公式也存在着一定的差异，但这种差异对PWV结果影响很小．为此，提出了校正对流层干分量延迟的方法，并利用实测数据对该方法进行了检验．实践证明，这种校正方法基本上可以消除常用干分量模型的系统误差。     

Correcting systematic effects in a large set of photometric light curves

We suggest a new algorithm to remove systematic effects in a large set of light curves obtained by a photometric survey. The algorithm can remove systematic effects, such as those associated with atmospheric extinction, detector efficiency, or point spread function changes over the detector. The algorithm works without any prior knowledge of the effects, as long as they linearly appear in many stars of the sample. The approach, which was originally developed to remove atmospheric extinction effects, is based on a lower rank approximation of matrices, an approach which has already been suggested and used in chemometrics, for example. The proposed algorithm is especially useful in cases where the uncertainties of the measurements are unequal. For equal uncertainties, the algorithm reduces to the Principal Component Analysis (PCA) algorithm. We present a simulation to demonstrate the effectiveness of the proposed algorithm and we point out its potential, in the search for transit candidates in particular.     

An optimal extraction algorithm for imaging photometry

This paper is primarily an investigation of whether the 'optimal extraction' techniques used in CCD spectroscopy can be applied to imaging photometry. It is found that using such techniques provides a gain of around 10 per cent in signal-to-noise ratio over normal aperture photometry. Formally, it is shown to be equivalent to profile fitting, but offers advantages of robust error estimation, freedom from bias introduced by mis-estimating the point spread function, and convenience. In addition some other techniques are presented, which can be applied to profile fitting, aperture photometry and the 'optimal' photometry. Code implementing these algorithms is available at http://www.astro.keele.ac.uk/~timn/.     

An autonomous adaptive scheduling agent for period searching

We describe the design and implementation of an autonomous adaptive software agent that addresses the practical problem of observing undersampled, periodic, time‐varying phenomena using a network of HTN‐compliant robotic telescopes. The algorithm governing the behaviour of the agent uses an optimal geometric sampling technique to cover the period range of interest, but additionally implements proactive behaviour that maximises the optimality of the dataset in the face of an uncertain and changing operating environment. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Automated spectral classification using template matching

An automated spectral classification technique for large sky surveys is pro-posed. We firstly perform spectral line matching to determine redshift candidates for an observed spectrum, and then estimate the spectral class by measuring the similarity be-tween the observed spectrum and the shifted templates for each redshift candidate. As a byproduct of this approach, the spectral redshift can also be obtained with high accuracy. Compared with some approaches based on computerized learning methods in the liter-ature, the proposed approach needs no training, which is time-consuming and sensitive to selection of the training set. Both simulated data and observed spectra are used to test the approach; the results show that the proposed method is efficient, and it can achieve a correct classification rate as high as 92.9%, 97.9% and 98.8% for stars, galaxies and quasars, respectively.     

Single star scidar: atmospheric parameters profiling using the simulated annealing algorithm

In this paper, we present a new method to estimate, for each turbulent layer labelled i , the horizontal wind speed   v ( h _i )  , the standard deviation of the horizontal wind speed fluctuations  σ _v ( h_i )  and the integrated value of   C ² _n   over the thickness  Δ h_i   of the turbulent layer   C ² _n ( h_i )Δ h_i   , where   h_i   is the altitude of the turbulent layer. These parameters are extracted from single star scintillation spatiotemporal cross-correlation functions of atmospheric speckles obtained within the generalized mode. This method is based on the simulated annealing algorithm to find the optimal solution required to solve the problem. Astrophysics parameters for adaptive optics are also calculated using   C ² _n ( h_i )  and   v ( h_i )  values. The results of other techniques support this new method.     

The astroclimatological comparison of the Paranal Observatory and El Roque de Los Muchachos Observatory

The new extremely large telescope projects need accurate evaluation of the candidate sites. In this paper, we present the astroclimatological comparison between the Paranal Observatory, located on the coast of the Atacama Desert (Chile), and the Observatorio del Roque de Los Muchachos (ORM), located in La Palma (Canary Islands). We apply a statistical analysis using long-term data bases from Paranal and Carlsberg Automatic Meridian Circle (CAMC) weather stations. The monthly, seasonal and annual averages of the main synoptical parameters in the two sites are computed. We compare the long-term trends in order to understand the main differences between the two sites. Significant differences between the two analysed sites have been found. Temperature has increasing trends in both observatories with somewhat higher evidence at the ORM. Seasonal variations of pressure at Paranal have been highly decreasing since 1989, and we do not see the same phenomenon at the ORM. The two sites are dominated by high pressure. In cold seasons, relative humidity (RH) is lower than 60 per cent at CAMC and 15 per cent at Paranal. In warm seasons, RH is lower than 40 per cent at CAMC and 20 per cent at Paranal. The analysis of the dew point has shown better conditions at Paranal with respect to CAMC in winter, autumn and spring before 2001, while the two sites are becoming similar afterwards. Winds at the ORM are subject to pronounced local variations.     

Detection of Faint Asteroids Based on Image Shifting and Stacking Method

In order to improve the ability to find the faint and small celestial bodies in the solar system, a method of shifting and stacking images which improves the detection efficiency of faint moving objects is applied to process the sequential optical images. This method determines the existence of moving objects by using the method of false position to pre-estimate the apparent velocities of moving objects, then determines iteratively the accurate positions of moving objects based on the SNR (Signal-to-Noise Ratio) and elongation of stellar image. Using the sequential images of the China Near Earth Object Survey Telescope (CNEOST), we carry out an experiment and succeed in detecting asteroids fainter than 21 magnitude which are invisible on a single image. Thus, the feasibility of this method is verified.