Author: Sarah Böckmann, Axel Nothnagel
(boeckmann _at_ uni-bonn.de, nothnagel _at_ uni-bonn.de)
The official contribution of the International VLBI Service for
Geodesy and Astrometry (IVS) to ITRF2008 consists of session-wise
datum-free normal equations of altogether 4,539 daily 24h Very
Long Baseline Interferometry (VLBI) network sessions from 1979.7
to 2009.0 including data of 115 different VLBI sites. It is the
result of a combination of individual series of session-wise
datum-free normal equations provided by seven analysis centers (ACs)
of the IVS. All series are completely reprocessed following
homogeneous analysis options according to the IERS Conventions 2003
and IVS Analysis Conventions. A correction for atmospheric pressure
loading was not applied as it was approved by the IERS Directing Board.
Unknown parameters contained in the normal equation systems are station
positions and EOPs. Zenith wet delays, atmosphere gradients and clock
parameters were (pre-)reduced by the individual ACs.
Source positions are handled by each AC independently under the premisses
that they stay within the error boundaries of the ICRF. They are kept
fixed to these values and are not contained in the normal equation systems.
1) Analysis strategy:
=====================
As for the IVS contribution to ITRF2005, the combination
is performed at the normal equation level. The strategy for
the intra-technique combination mainly consists of three
steps.
1) The session-wise datum-free normal equation systems
of the individual ACs are transformed to an identical
reference epoch and to an equal set of a priori values.
2) Weights for the normal equation systems of each AC
are estimated for each single session within a variance
component estimation procedure.
3) The weighted equation systems of each contributing AC
are stacked to combined session-wise normal equation
systems.
These session-wise combined normal equation systems form
the IVS input to the ITRF2008.
2) ACs Contribution:
====================
7 ACs have contributed:
AC Software Time span #sessions #stations
-----------------------------------------------------------------
BKG Calc/Solve 1984.0–2009.0 4,424 94
GSFC Calc/Solve 1979.7–2009.0 4,592 125
OPA Calc/Solve 1979.7–2009.0 4,489 120
USNO Calc/Solve 1979.7–2009.0 4,250 100
SHAO Calc/Solve 1979.7–2009.0 4,465 120
IGGB Calc/Solve 1979.7–2009.0 4,370 112
DGFI OCCAM(LSM) 1984.0–2009.0 3,184 52
-----------------------------------------------------------------
3) Combined Products:
=====================
AC Software Time span #sessions #stations
------------------------------------------------------------------
IVS DOGS_CS 1979.7–2009.0 4,539 115
------------------------------------------------------------------
Consistency of the individual contributions to the IVS combination:
WRMS values computed from the single-session station position differences
between the estimates of each individual contribution and the combined
series for the north, east and height component.
AC - IVS comb. North (mm) East (mm) Height (mm)
-----------------------------------------------------------------
BKG 1.6 2.1 3.7
DGFI 1.6 2.0 3.1
GSFC 0.9 1.1 1.9
IGG 0.9 1.2 2.1
OPA 0.7 0.9 1.5
SHAO 0.7 0.8 1.5
USNO 1.0 1.4 2.2
-----------------------------------------------------------------
average 1.1 1.4 2.2
WRMS values computed from the single-session EOP differences between the
estimates of each individual contribution and the combined EOP series from
1984 to 2008 for polar motion, dUT1 and their first derivatives as well as
nutation offsets.
AC - IVS comb. X-pole Y-pole dUT1 LOD X-pole Y-pole dX dY
rate rate
(µas) (µas) (µs) (µs) (µas/d) (µas/d) (µas) (µas)
------------------------------------------------------------------------------
BKG 56.2 55.3 3.5 6.2 174.3 170.2 46.0 48.0
DGFI 57.8 55.2 3.4 6.5 174.9 163.6 47.8 48.9
GFSC 34.4 34.2 2.7 3.5 103.5 98.5 28.8 30.5
IGG 37.1 32.7 2.9 4.4 113.9 106.8 32.5 33.2
OPA 30.6 31.2 2.8 3.8 93.5 92.6 29.1 29.9
SHAO 27.9 28.4 2.6 2.9 86.8 80.0 24.7 26.1
USNO 41.7 37.7 3.0 4.2 122.5 111.9 34.2 36.5
------------------------------------------------------------------------------
average 40.8 39.2 3.0 4.5 124.2 117.7 34.7 36.2
For detailed information on the IVS submission to ITRF2008 see:
Böckmann, S., T. Artz, A. Nothnagel (2010)
VLBI terrestrial reference frame contributions to ITRF2008.
J Geod 84:201-219, DOI 10.1007/s00190-009-0357-7.