03/000 Effect of the reference radiosource instability on the TRF solution Australian Government Geoscience Australia 4 th General IVS Meeting, 9-13, January, 2006, Chile, Concepcion

Selection of radiosources Geoscience Australia 11 January ICRF (Ma et al., 1998) 2.Feissel-Vernier (2003) 3.Structure index (Fey & Charlot, 1997; 2000) A set of reference quasars to be fixed

ICRF catalogue (1998) Geoscience Australia 11 January defining sources with position accuracy ~0.4 mas candidate sources other sources 608 sources separated into 3 groups

ICRF defining sources Geoscience Australia 11 January 2006

Feissel-Vernier (2003) Geoscience Australia ‘stable’ sources (groups 1 and 2) 2.7 ‘unstable’ sources (group 3) ‘highly unstable or drifting’ sources (group 4) 362 pre-selected sources separated into 4 groups 11 January 2006

Structure index Fey and Charlot (1997, 2000) Geoscience Australia 1.Structure index = 1, if 0 ps  τ < 3 ps 2.Structure index = 2, if 3 ps  τ < 10 ps 3.Structure index = 3, if 10 ps  τ < 30 ps 4.Structure index = 4, if 30 ps  τ <  Radiosources separated into 4 groups by structure index based on median structure correction (in terms of time delay 11 January 2006

Structure index Fey and Charlot (1997, 2000) Geoscience Australia 11 January compact or very compact; Index 1 or extended; Index 3 or ICRF radiosources

The three lists are not consistent !!! Geoscience Australia 11 January ~ one-third of ‘defining’ sources have structure index 3 or ‘stable’ sources are ‘defining’ and 49 ‘other’ 3.53 ‘stable’ radiosources have structure index 3 or 4 (of 164 available)

ICRF source instability (quasar , DEC) Geoscience Australia 11 January 2006 Shift ~1 mas ( ~3 cm ) defining Highly unstable # 1

ICRF source instability ( , structure) Geoscience Australia 11 January 2006

ICRF source instability (quasar , declination) Geoscience Australia 11 January 2006 offset ~12 mas ( ~35 cm ) other stable # 4

ICRF source instability ( , structure) Geoscience Australia 11 January 2006

ICRF source instability (quasar , right ascension) Geoscience Australia 11 January 2006 Instability ~1.5 mas ( ~5 cm ) Defining unstable # 1-2

Problems Geoscience Australia 11 January 2006 ‘ compact’ does not mean stable; extended radiosource can become unstable any time; positions of extended radiosources observed by different networks can be inconsistent; The southern hemisphere is less know than the northern hemisphere

Selection Geoscience Australia 11 January 2006 ‘Hybrid approach’ - long observational history - structure index # 1 or # 2 - statistical analysis of daily positions

Solution Geoscience Australia 11 January 2006 All observed radiosources were split into 3 groups - ‘reference’ (estimated as global parameters with NNR); - ‘unstable’ (estimated as daily parameters); - all other (estimated as global parameters without NNR) Solution 1: 207 ICRF-Ext.2 as ‘reference’; 107 – ‘unstable’ (Fey et al., 2004) Solution 2: 199 ‘stable’ as ‘references’; 163 – ‘unstable’ (Feissel-Vernier, 2003)

Solution Geoscience Australia 11 January 2006 Global solution strategy All station coordinates are estimated as daily parameters imposed by the NNR, NNT-constraints VLBI data

Results Geoscience Australia 11 January 2006 Solution 1 time series of the VLBI site daily positions Solution 2 time series of the VLBI site daily positions “Solution 1 – Solution 2” time series of the VLBI site daily positions differences. Reflect the selection effect of the stable/unstable radiosources only!

Geoscience Australia 11 January 2006 Westford, vertical component difference

Geoscience Australia 11 January 2006 Ny-Alesund, vertical component difference

Geoscience Australia 11 January 2006 Hartrao, vertical component difference

Geoscience Australia 11 January 2006 Hobart26, vertical component difference

Geoscience Australia 11 January 2006 Vertical component difference vs scan number Hobart Onsala

Offset between the reference positions (1997.0) StationLatitude (º) Offset in height (mm) Offset in latitude (mm) Offset in longitude (mm) Ny-Alesund / / /- 0.1 Onsala / / /- 0.1 Urumqi / / /- 1.3 Westford / / /- 0.1 NRAO / / /- 0.4 Kokee / / /- 0.1 Fortaleza / / /- 0.2 Hartrao / / /- 0.3 Santia / / /- 2.0 DSS / / /- 0.6 Hobart / / /- 0.6 O’Higgins / / /- 0..9

Results Geoscience Australia 11 January 2006 Solution 1 time series of the VLBI site daily positions Solution 2 time series of the VLBI site daily positions “Solution 1 – Solution 2” time series of the VLBI site daily positions differences.

Results Geoscience Australia 11 January Uncertainty due to the Solution 1 inadequate selection - Uncertainty due to the Solution 2 inadequate selection - Uncertainty due to all other effects Calculated for all VLBI sites individually - Repeatability of the Solution 1 time series - Repeatability of the “Solution 1 – Solution 2” differences - Repeatability of the Solution 2 time series

Geoscience Australia 11 January 2006 Effect on vertical component

Geoscience Australia 11 January 2006 Effect on latitude component

Geoscience Australia 11 January 2006 Effect on vertical component

Conclusion Geoscience Australia 11 January 2006 Radiosources used as ‘reference’ are not stable sometimes; The temporal instability of the ‘reference’ radiosources results in degradation of the TRF solution accuracy, mostly in the Southern Hemisphere; The number of scans is additional factor for the TRF accuracy degradation

Recommendation Geoscience Australia 11 January 2006 The advanced procedure radiosource selection for the next ICRF is a important factor for the VLBI TRF solution improvement; The Southern Hemisphere needs new high slew rate VLBI site for the both CRF and TRF improvement;