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PUL Analysis Center

Central Astronomical Observatory at Pulkovo of Russian Academy of Sciences

PUL IVS Analysis Center Rep ort 2007
Zinovy Malkin, Julia Sokolova, Natalia Miller
Abstract
The rep ort briefly presents the PUL IVS Analysis Center activities during 2007 and plans for the coming year. Main topics of investigation in that p eriod were comparison and combination of catalogues of radio source p ositions, analysis of VLBI EOP series, analysis of radio source p osition and zenith trop osphere delay time series.

1. General Information
The PUL IVS Analysis Center was organized in Septemb er 2006 and is located at the Central Astronomical Observatory at Pulkovo of Russian Academy of Sciences (Pulkovo Observatory). The main topics of our activity are: · Improvement of the International Celestial Reference Frame (ICRF), including investigations of radio source catalogues, constructing combined catalogues, investigation of the ICRF stability, and investigation of radio source p osition time series. · Computation and analysis of EOP, station p osition, baseline length and zenith trop osphere delay time series. · Investigation of Free Core Nutation (FCN). · Comparison of VLBI results with other space geodesy techniques. The PUL AC's Web page is available at http://www.gao.spb.ru/english/as/ac vlbi/.

2. Scientific Staff
PUL team consists of three scientists: 1. Zinovy Malkin (70%) -- team coordinator, computation and analysis of EOP, station coordinates and baseline length, development of algorithms and software for data processing and analysis; 2. Julia Sokolova (100%) -- global data analysis for deriving radio source catalogues and p osition time series, comparison and combination of radio source catalogues, time series analysis, development of algorithms and software for CRF studies; 3. Natalia Miller (20%) -- investigation of FCN and zenith trop osphere delay; application of new mathematical methods to time series analysis.

3. Activities
The activities of the PUL IVS Analysis Center during 2007 included: · Investigations in the framework of the IERS/IVS Working Group on the Second Realization of the ICRF were continued. Main directions of this activity are comparison and combination of radio source catalogues, computation and investigation of source p osition time series. The main results obtained in 2007 are the following.
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Central Astronomical Observatory at Pulkovo of Russian Academy of Sciences

PUL Analysis Center

­ Two combined radio source catalogues have b een constructed and investigated [1]. The first of them provides a stochastic improvement of the current ICRF realization, and the second one allows us to account also for systematic errors in the ICRF. Comparison of the celestial p ole offsets obtained from the processing of VLBI observations using ICRF and the combined catalogue has shown improvement of the results. Further improvement is exp ected after refining the comparison and combination procedures. ­ A new method of assessment of the CRF accuracy based on the scatter analysis of the celestial p ole offset (CPO) time series is prop osed [2]. Several scatter indices based on residual analysis of CPO series with resp ect to a FCN model and Allan deviation technique and its extensions, which allow the treatment of unequally weighted and multidimensional observations, are investigated. Application of these criteria to several radio source catalogues showed their ability to p erform a preliminary assessment of the quality of the CRF realizations. ­ Several radio source catalogues and source p osition time series from a global analysis of VLBI data with the OCCAM 6.2 software were calculated using different modes. Comparison of four lists of reference sources showed significant inconsistencies in source selection. The impact of radio source instability on celestial p ole offset estimates was investigated. Analysis showed that variations of radio-source coordinates affect the celestial p ole offset estimates (Fig. 1, [6]). These results mainly were obtained during a 6-month visit of Julia Sokolova at the Institute of Geodesy and Geophysics (IGG), Vienna in March­August 2007. ­ The first version of a new list of the optical characteristics of geodetic radio sources is compiled. Further development is b eing p erformed in coop eration with Geoscience Australia Analysis Center and Pulkovo optical astronomers.
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Figure 1. Variations of source 2145+067 positions (left) and differences between two CPO time series (right). For the first CPO series, all the observations were used. For the second CPO series, the observations of 2145+067 were excluded. Source and station positions were fixed in both cases.

· The IVS combined CPO time series was analyzed using several statistical tools, such as sp ectrum analysis, singular sp ectrum analysis, or wavelet analysis to investigate b oth trend and (quasi)p eriodical comp onents. The results are presented in [4] · A new VLBI network geometry index, the volume of network, is examined as an indicator of the quality of the EOP obtained from VLBI observations [5]. It has b een shown that b oth EOP precision and accuracy can b e describ ed by a p ower law = aV b , where V is the volume
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PUL Analysis Center

Central Astronomical Observatory at Pulkovo of Russian Academy of Sciences

of network, in a wide range of network size from domestic to global VLBI networks. The dep endence found can b e used for comparison of results obtained from different observing programs. · Several very long and very dense zenith trop osphere delay time series provided by the IGG IVS Sp ecial Analysis Center as an IVS trop osphere product were analyzed by means of the method of Singular Sp ectrum Analysis (SSA) in b oth one-dimensional and multi-dimensional modes. The structure of the time series including regular, quasiregular (p eriodical) and irregular (trend) comp onents was obtained and investigated. Using SSA allowed us to derive nonlinear trends in zenith trop osphere delay, and also to research the variation of amplitude and phase of season comp onents with time. · Regular computation of two refined Free Core Nutation (FCN) time series started in 2006 were continued. They were briefly describ ed in the PUL IVS Analysis Center Rep ort 2006. More detailed description and comparison with other models are given in [3] · Development of algorithms and software for data processing and analysis was continued. · PUL archive of VLBI data and products was originated in the end of 2006. At present, all available databases and NGS cards have b een stored along with main IVS and IERS products. · PUL staff memb ers participated in activities of several IVS pro jects, Working Groups and Committees.

4. Outlook
Plans for the coming year include: · Continuation of the IVS related studies. · Development of algorithms and software used for data processing. · Supp ort of the PUL archive of data and products.

References
[1] Sokolova Ju., Malkin Z. On comparison and combination of catalogues of radio source positions. Astron. Astrophys., 2007, 474, No. 2, 665­670. [2] Malkin Z. J. Geod., On accuracy assessment of celestial reference frame realizations. 2007, DOI: 10.1007/s00190-007-0181-x [3] Malkin Z.M. Empiric Models of the Earth's Free Core Nutation. Solar System Research, 2007, 41, No. 6, 492­497. [4] Malkin Z., Miller N. An analysis of celestial pole offset observations in the free core nutation frequency band. In: Proc. 18th EVGA Working Meeting, Vienna, Austria, 12-13 Apr 2007, 93­97. [5] Malkin Z. On dependence of EOP precision and accuracy on VLBI network. In: Proc. 18th EVGA Working Meeting, Vienna, Austria, 12-13 Apr 2007, 75­78. [6] Sokolova, J. Effect of the reference radio source selection on VLBI CRF realization. Presented at Journґes e 2007, Paris, September 17­19, 2007.

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