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The 5

th

IVS General Meeting Proceedings, 2008, p. 275­278

Interpretation of VLBI Results in Geodesy, Astrometry and Geophysics

Comparison and Combination of CRF Catalogues
Julia Sokolova, Zinovy Malkin
Pulkovo Observatory RAS Contact author: Julia Sokolova, e-mail: jrs@mars.hg.tuwien.ac.at

Abstract. In 2007, a joint IERS/IVS Working Group has been established to consider practical issues of creating the next ICRF generation, ICRF2. The goal of the WG is to seek after ways to improve the existing ICRF. In this study we investigate a p ossibility of ICRF improvement by means of using combined ICRF catalogue instead of a catalogue computed in a single analysis centre, even though using most advanced models and software. In this work, we present a new version of Pulkovo combined catalogue of radio source positions computed using the metho d proposed in [5]. Radio source catalogues that were submitted in 2007 in the framework of the WG activity were used as input for mutual comparison and combination. Four combined catalogues have b een calculated: Two first of them provide a stochastic improvement of the ICRF, and last two of them allow us to account also for systematic errors in the current ICRF version.

1. Intro duction
The celestial reference frame (CRF), as realized by a set of coordinates for selected celestial ob jects, is widely used for numerous astronomy, navigation, time and other measurements. The CRF accuracy and stability are all-important for successful solutions of all these tasks. After publishing of the first VLBI radio source catalogue (RSCs), attempts were made to improve the accuracy of radio-band CRF by means of constructing combined catalogues, as it was customary for optical astronomy, where fundamental catalogues served as an international standard for astrometry and other measurements on the sky. Different methods were used to obtain a combined RSC, e.g. [1, 2, 3, 4] Also, up to 1995, IERS (International Earth Rotation Service, now International Earth Rotation and Reference Systems Service) used Derived combined RSC for maintenance of the IERS Celestial Reference Frame. In 2007, a joint IERS/IVS Working Group has been established to consider practical issues of creating the next ICRF generation, ICRF-2. The goal of the WG is to seek 275


after ways to improve the existing ICRF. Large experience accrued by optical astrometry over centuries shows that combining catalogues of the star positions leads to better random and systematic accuracy than individual catalogues. In this work, we present a new version of Pulkovo combined catalogue of radio source positions computed using the method proposed in [5]. Radio source catalogues that were submitted in 2007 in the framework of the WG activity were used as input for mutual comparison and combination. Four combined catalogues have been calculated: 1. The first two catalogues (the one has been calculated using all input catalogues and the second one using only catalogues obtained with CALC/SOLVE software) provide a stochastic improvement of the ICRF 2. and the last two of them one allow us to account also for systematic errors in the current ICRF version. All computations have been done for the set of 194 ICRF defining sources included in all input catalogues.

2. Input Catalogues and Comparisons
Table 1. Information about input catalogues IVS Centre AUS, Australia BKG, Germany DGFI, Germany CGS, Italy USNO, USA GSFC, USA MAO, Ukraine IAA, Russia Soft Time span OCCAM 11/1979 Calc/Solve 01/1984 OCCAM 01/1980 Calc/Solve 08/1979 Calc/Solve 08/1979 Calc/Solve 08/1979 SteelBreeze 04/1980 QUASAR 08/1979 (mon/yr) # of delays 04/2007 2647809 10/2007 5156489 01/2005 3650771 07/2006 4640972 05/2007 5238056 03/2007 5510462 05/2007 5194922 05/2007 5116010 # of sources 1515 (212 def ) 1076 (212 def ) 686 (199 def ) 625 (161 def ) 923 (212 def ) 923 (212 def ) 2541 (26 def ) 907 212 def )

BKG

DGFI CGS

USNO GSFC

MAO

IAA

ICRF
300

BKG

DGFI CGS

USNO GSFC

MAO

IAA

ICRF

AUS 139 125 151 166 139 112 149 279 BKG DGFI CGS USNO G SF C M AO IAA 58 67 69 70 97 97 35 76 70 255 82 324

AUS 181 155 202 196 161 163 208 283
250

BKG
250

53

71 54 59 74 81

44 67 67 38

84 66 82 71

208 272
200

59 71 57 73

DGFI
200

92 318
150

CGS USNO
100

77 82 268 88 76 84 64 242

150

70 114 105 286 65 68 261 99 289 272

100

G SF C M AO IAA

216
50

50

108 260 244

0

0

Figure 1. WRMS of the intercomparison of the sources coordinates by (left) and by (right), For 194 common sources. Unit: Microarcseconds

276


Weighted root-mean-square (WRMS) differences of the radio source coordinates between the input catalogues and ICRF are shown in Fig. 1 (Unit microarcsecond). One can see from Fig. 1 WRMS differences have the least values for catalogues computed with Calc/Solve software, both for intercomparison of these catalogues and their comparison with ICRF. The latter most probably is caused by the fact that the ICRF was constructed using Calc/Solve software. Moreover it can be clearly seen that all the input catalogues demonstrate rather large differences with the ICRF, which may indicate significant systematic errors in the latter.

3. Combined catalogues
The systematic differences between the input catalogues and the ICRF found by the LF method were applied to all the input catalogues in order to transform them to the ICRF system. After that, the coordinates of all sources in transformed catalogues were averaged with weights depending on the formal errors of coordinates. In result, the combined catalogue PUL08C01a and PUL08C01b (only input catalogues constructed using CALC/SOLVE were used) were constructed.

10 0 -10

10 0 -10

50 Dec 0 -50 0 100 200 r. a.

300

50 Dec 0 -50 0 100 200 r. a.

300

Figure 2. PUL08C01a - ICRF, by (left) and by (right), Unit: mas

10 0 -10

10 0 -10

50 0 Dec -50 0 100 200 r. a.

300

50 Dec 0 -50 0 100 200 r. a.

300

Figure 3. PUL08C01b - ICRF, by (left) and by (right), Unit: mas

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Figure 2 show the systematic differences between the combined catalogue PUL08C01a and the ICRF. One can see that the catalogue PUL08C01a represents the ICRF system at a level of about 10 microarcseconds. Figure 3 shows the systematic differences between the combined catalogue PUL08C01b and the ICRF. The catalogue PUL08C01b represents the ICRF system at a level of about 5 microarcsec.

4. Short summary
Four combined radio source catalogues have been constructed. The first two of them, PUL08C01a and PUL08C01b can be considered as stochastic improvement of the current realization of the ICRF. The final combined catalogues PUL08C02a and PUL08C02b, provides both stochastic and systematic improvement of the ICRF.

References
[1] Walter, H. G. 1989, A&A, 210, 455 [2] Walter, H. G. 1989, A&AS, 79, 283 [3] Yatskiv, Ya. S. & Kur'yanova, A. N. 1990, in: Inertial Coordinate System on the Sky, Proc. 141th IAU Symp., ed. J. H. Lieske & V. K. Abalakin, 295 [4] Kur'yanova, A. N. & Yatskiv, Ya. S. 1993, Kinemat. fiz. nebesnyh tel, 9, 15 [5] Sokolova, Ju. & Malkin, Z. 2007, A&A, 474, 665-670

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