Towards a new VLBI system for geodesy and astrometry
Johannes Boehm(1), Joerg Wresnik(1), Harald Schuh(1) and Dirk Behrend(2)
(1) Vienna University of Technology, Gusshausstrasse 27-29, 1040 Vienna, Austria
(2) NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, United States
Abstract
Very Long Baseline Interferometry (VLBI) provides highly accurate
and unique products for the realization and maintenance of the
celestial and terrestrial reference frames, ICRF and ITRF, as well as
for the Earth Orientation Parameters (EOP). Concerns about the aging
technology, which has been used for the past three decades, and about
radio interference problems that decrease the number of useable
observations, led to the establishment of the International VLBI
Service for Geodesy and Astrometry (IVS) Working Group 3 'VLBI 2010'.
Its main tasks were to examine current and future requirements for
geodetic VLBI including all components from antennas to analysis, and
to create recommendations for a new generation of VLBI systems. The
results were summarized in a vision paper, which is recommended for
coordination of new developments in VLBI and for plans to invest in new
components by member institutions. This presentation reviews IVS
activities of recent years and gives a perspective about new
developments for meeting future goals, which are set up by the Global
Geodetic Observing System (GGOS) project of the International
Association of Geodesy. These goals are 1 mm measurement accuracy on
global baselines, continuous measurements for time series of station
positions and EOP and turnaround time to initial geodetic results of
less than 24 hours. To reach these requirements the performance of a
new VLBI system has to be optimized in following strategies:
- reduce
the random component of the delay-observable error, i.e., the
per-observation measurement error, the stochastic properties of the
clocks, and the unmodeled variation in the atmosphere,
- reduce
systematic errors, increase the number of antennas and improve their
geographic distribution,
- reduce susceptibility to external
radio-frequency interference,
- increase observation density, i.e. the
number of observations per unit time,
- develop new observing
strategies.
To fulfill all these considerations, along with the need for low cost
of construction and operation, simulations are developed for all
aspects of geodetic VLBI, including equipment, processes, and
observational strategies. At the IGG Vienna simulations are carried out
to evaluate new observing strategies and schedules, to improve modeling
of the troposphere and the clocks, to find the best antenna
configuration and to optimize the network geometry. These are done by a
sequence of software programs. Main part of the simulation studies is a
so-called Monte Carlo simulator which creates artificial observations.
The criteria to evaluate the potential of the VLBI system are: baseline
length repeatabilities, formal errors of EOPs and a quality measure of
the representation of stochastic processes (troposphere, clocks).