As discussed in other papers in this
volume, the IAU XXIII General Assembly adopted a
new fundamental celestial reference frame: the
International Celestial Reference Frame (ICRF)
based on VLBI observations of extragalactic radio
sources (Ma et al.,
1997). It is approximately 300 times more accurate
than its predecessor, the FK5. At present, no
other technique has produced a more accurate
celestial frame than VLBI, Since no other
astrometric technique provides an external
standard of accuracy, the VLBI claim of a great
leap forward in accuracy must be verified by
internal consistency tests. This paper addresses
one aspect of internal consistency: the ability of
independent VLBI software packages to reproduce a
celestial frame without significant loss of
accuracy. This is no small task since the software
packages are large - involving on the order of 100
000 lines of code. What does VLBI software do?
Aside from routines designed to collect the data
and extract raw observables which will not be
considered here, its principal task is to model
the differential group delay and phase delay rate
of radio signals received at two widely separated
antennas (Sovers, Fanselow & Jacobs, 1998).
The software then refines this model via a least
squares adjustment of relevant physical parameters
which describe station locations, source
positions, clock offsets, atmospheric refraction,
tidal effects, etc. In the early 1990s, studies
revealed that differences in software
implementation and analyst’s choices of model
options were one of the largest contributors to
differences in independent calculations of VLBI
celestial frames. These differences were of
comparable size to the formal uncertainties of the
celestial frame’s source positions.
The X/Ka-band 2018b Celestial Reference Frame
