NATIONAL REPORT OF THE FEDERAL REPUBLIC OF ... - IAG Office

14
Introduction
Celestial Reference Frames & Interaction
with Terrestrial Reference Frames
In geodesy, there are two types of celestial reference
frames: (1) positions of extragalactic objects and (2)
dynamic realizations by ephemeris (positions and velocities
or orbital elements) of the planets, the moon and artificial
earth orbiting satellites. The first ones are often called
inertial, the second ones quasi inertial (see SCHUH et al.,
2003). While satellite orbits are in general discussed in
Commission 4 “Satellite Orbit Modelling”, this chapter
concentrates on (quasi) inertial frames as such and their
interaction with terrestrial reference frames.
Towards ICRF2
In January 1998, the VLBI-determined ICRF (608 radio
positions of extragalactic objects) replaced the optical FK5
as the celestial reference frame. Since then, it was extended
twice, 1999 by 59 (ICRF-Ext1) and 2002 by 50 sources
(ICRF-Ext2). To keep the ICRF homogeneous throughout
the extensions, the same VLBI analysis setup was kept as
used for the first solution in 1995. With continued applicable
VLBI observations and improvements in analysis a
better realization of the ICRF is now possible and an even
better realization is feasible in the foreseeable future. So
the IAU, the IERS, as well as the IVS aim at a new realization
of the ICRS in the next years. It is planned to be
completed concurrent with the 2009 IAU General Assembly.
The IAU as well as the IVS have working groups related
to ICRF2. BKG and DGFI actively take part in the IVS
Working Group for ICRF2, which was founded 2006 in
Prague during the IAU General Assembly. The result of
this working group will be submitted to the IAU Working
Group. This IAU working group will then validate the
ICRF2, and, in case of positive evaluation, be engaged in
the formulation of resolutions to be adopted by the IAU.
Both, DGFI and BKG submit catalogues, source position
time series and other relevant results.
Effect of various analysis options on VLBIdetermined
CRF
In 2006, the effect of various analysis options on VLBIdetermined
CRF was investigated at DGFI (TESMER et al.,
2006a, 2006b, and TESMER, 2007):
– different troposphere mapping functions and gradient
models,
V. TESMER 1
– choice of the data set (neglecting sessions before 1990
and 21 astrometric sessions),
– handling of sources that may not be assumed to have
time-invariant positions,
– handling of the station network (estimate the station
positions per session, as positions and velocities over 20
years, or fix them to a priori values).
The biggest, clearly systematic effects in the estimated
source positions up to 0.5 mas were found to be due to
different gradient models (esp. the selection of the a priori
values and the constraints). The choice of the data set does
generally not have a significant influence. This holds also
(with several exceptions) for different options how to treat
sources which are assumed to have time-invariant positions.
Furthermore it turned out that fixing station positions to
values not consistent to the solution itself can noticeably
affect CRF solutions.
Interaction between CRF and TRF
At DGFI, a VLBI solution with a TRF, the EOP and a CRF
being estimated simultaneously was established applying
a non-biasing NNR and NNT datum for the TRF and NNR
for the CRF (TESMER et al., 2004). Using such minimum
datum conditions, biases were avoided which are due to
fixed reference frames or other relevant parameters of the
observation equations. HEINKELMANN et al. (2006) presents
a similar solution and gives more technical details.
TESMER (2006) summarizes the results of a research project
“consistent realization of reference systems by VLBI”,
supported by DFG (Deutsche Forschungsgemeinschaft,
DR143-11). In this context, most interesting is: (1) The
sparse southern VLBI observing network implicates a non
sufficiently redundant observing geometry. This is why
some parameters of southern sources and stations are
significantly correlated in CRF and TRF solutions (like
O’Higgins, Antarctica or Hobart, Australia). (2) This also
holds for sources and stations, which were not observed in
varying network constellations (like Crimea, Ukraine or
Saint-Croix, Virgin Islands, USA).
Source position time series
Presently, if CRF solutions are computed with VLBI, one
position is estimated for the whole data span (suitable data
exists since 1984). This assumes the apparent position of
the sources to be constant in time. But, there are some
sources, for which today a constant model of the position
1 Volker Tesmer: Deutsches Geodätisches Forschungsinstitut (DGFI), Alfons-Goppel-Straße 11, D - 80539 München, Germany, Tel.
+49 - 89 - 2 3 0 31 11 98, Fax +49 - 89 - 2 3 0 31 12 40, e-mail tesmer@dgfi.badw.de